Glossary

Terms you need to know

Many of these words appear in our Big Picture issues. We try to link from the articles to the glossary when we can, but if you spot one we haven’t linked, email us at bigpicture@wellcome.ac.uk.

A

ABCD1

Gene that codes for the peroxisomal membrane transporter protein, which mediates the import of very-long-chain fatty acids (VLCFAs) across the peroxisomal membrane for degradation.

Abiotic
Non-living.
Abscisic acid

A plant growth factor/hormone produced in parched leaves, roots and developing seeds that causes stomata to close (to save water) as well as seed dormancy. 

Academic
Relating to higher education and study. For example, a researcher at a university may be described as an academic, or as working in academia.
Acetyl CoA
An intermediate formed in the link reaction by the joining of coenzyme A (CoA) and a two-carbon compound. This then enters the Krebs cycle.
Acetylcholine
A neurotransmitter in the central and peripheral nervous system. Drugs to increase acetylcholine are sometimes used to treat Alzheimer’s disease.
Acidification

The dissolving of carbon dioxide in the oceans, which forms carbonic acid, lowering pH.

Actin

One of the main proteins involved in muscle contraction. Actin binds with myosin, and is then pulled by it to cause muscle to contract. Actin is also used in other situations to help move materials within the body.

Action potential

Describes the change in electrical potential in a membrane associated with the movement of an impulse along a neuron.

Active immunity

Immunity that occurs from the production of antibodies or T cells after exposure to an antigen – for example, following infection with a virus, or immunisation. This triggers the production of memory cells, which ensure a rapid immune response if the immune cells re-encounter a given antigen.

Active transport

The movement of molecules or ions across a membrane from a region of low concentration to a region of higher concentration (against a concentration gradient). This process requires energy, usually in the form of ATP, and assistance from an enzyme.

Adaptation
The practice of changing to accommodate and preparing for the consequences of a particular outcome. In the context of climate change this could involve introducing more resilient crops or improving the design of infrastructure.
Adaptive immune system (also specific immune system)
In mammals, this provides long-lasting protection against specific foreign substances. Helper T cells stimulate plasma B cells to produce antibodies. Memory B cells maintain a ‘memory’ of previous infections the organism has fought.
Adenylyl cyclase (adenylate cyclase)

An enzyme that catalyses the formation of cyclic adenosine monophosphate (cAMP) from ATP.

Adherens junction

A cell junction involved in cell–cell connections via cadherin molecules and actin filaments.

Adipocytes

Cells specialised to store energy as fat, mainly as triglycerides. Also known as lipocytes or fat cells. Can be white, brown or beige.

Adipokines
Hormones produced by adipose tissue. They have a variety of effects on the body’s use of energy and storage. New ones are still being discovered.
ADP (adenosine diphosphate)

A molecule found in all living cells that is involved in the transfer of energy. It is produced when a phosphate group is removed from ATP, a process that releases energy.

Adrenoleukodystrophy (ALD)

An X-linked genetic disease characterised by the progressive deterioration of brain signalling and function.

Adverse reaction
A harmful side-effect of a medicine.
Aerobic
Using oxygen.
Alkaline

Having a pH greater than 7.

Allele

Different versions of the same gene.

Allergen
Foreign proteins that can trigger a reaction from the immune system known as an allergy. An example is allergic rhinitis, also known as hay fever.
Amino acid
A compound that contains both an acidic group and an amino group. There are 20 biologically important amino acids that are present in proteins.
Aminoacyl tRNA synthetase
An enzyme that catalyses the binding of an amino acid to a tRNA molecule. There are at least 20 different types of this enzyme in the cell, each specific to a different amino acid.
Ammonia

A compound of one nitrogen atom and three hydrogen atoms (NH3). It occurs as a colourless gas and is thought to have been present in the atmosphere of the early Earth. Ammonia is created as a by-product of decay and decomposition and of animal waste.

AMPK
AMP-activated protein kinase. An intracellular enzyme important in controlling the cell’s energy use. It is the key link between the body’s overall energy status and activity within the cell.
Amygdala
A small, almond-shaped structure within the brain involved in emotions, particularly fear.
Amyloid

A complex protein deposit that occurs in the liver, kidneys or other tissues in certain diseases.

Anabolism
The process by which large molecules are built up from smaller molecules. Anabolism is the opposite of catabolism and part of metabolism. Anabolic reactions require energy, which is frequently provided by ATP. Making skeletal muscle is an example of an anabolic reaction.
Anaerobic
Without using oxygen.
Analogue sites
Places on Earth that have, or once had, similar biological, ecological or geological conditions to a body in outer space. These locations are the focus of intense research – informing how astrobiologists look for and identify life elsewhere in the universe.
Antibacterial
Something that kills or inhibits the growth of microorganisms such as bacteria, fungi, parasites or viruses.
Antibiotic
A substance that is produced by one microorganism and which inhibits or kills another. Antibiotics vary widely in structure. They can act either on a particular microorganism (i.e. they are specific) or a group of microorganisms (i.e. they are broad-spectrum).
Antibody
A protein that is produced by plasma B cells to fight against antigens. All antibodies are immunoglobulins.
Anticodon
A group of three bases at one end of a tRNA molecule that is complementary to an mRNA codon.
Antigen-presenting cells (APCs)
Cells that break up invading particles or cells and then display parts of them – antigens – for other immune cells to inspect. APCs include the macrophages, dendritic cells and B cells.
Antigens
Antigens are non-self markers, often proteins, that alert cells of the immune system to the presence of potential danger. These antigens may pose no threat on their own – they are just components, such as molecules in bacterial membranes, that raise a flag to immune cells.
Antimicrobial
Something that kills or inhibits the growth of microorganisms such as bacteria, fungi, parasites or viruses.
Antioxidant
A molecule capable of absorbing free radicals (highly reactive molecules produced during many metabolic processes that can damage DNA). Whether eating foods high in antioxidants is beneficial to health is contentious.
Apical

Describes the side of the cell in relation to its position in the body; the apical membrane faces the outside surface of the body or the lumen of internal cavities.

Apoptosis

The programmed and controlled cell death associated with ageing of the cell (reaching the Hayflick limit). This system often fails in cancer cells.

Archaea

One of the three domains of life, archaea encompass any unicellular microorganisms that are genetically distinct from bacteria and eukarya, including organisms that inhabit extreme environments.

Arthritis

An inflammatory condition, causing pain, discomfort and stiffness in the joints.

Artificial immunity
Immunity induced by some kind of intervention, for example, vaccination.
Asexual reproduction
When an offspring is genetically identical to its (single) parent.
Assay

An investigative test to measure the activity or presence of a particular substance. For example, scientists might use an assay to calculate how quickly an enzyme breaks down a substrate.

Astrobiology
The study of life in the universe, both on Earth and elsewhere. This branch of biology looks at the origin, evolution, distribution and future of life.
Atherosclerosis
A condition in which fat within the arteries develops over years into fatty plaques. These plaques can eventually cause complete obstruction. Atherosclerosis mainly affects the aorta and the coronary and cerebral arteries. It can lead to cardiovascular disease.
ATP (adenosine triphosphate)
A molecule found in all living cells that is involved in the transfer of energy. Most of a cell’s ATP is made during respiration.
ATP synthase

An enzyme that catalyses the synthesis of ATP from ADP and inorganic phosphate (Pi), which occurs both in oxidative phosphorylation and during photosynthesis. 

Atrophy

The wasting away of tissue, often due to inactivity or disease.

Autoimmune

When the immune system fails to recognise self from non-self. Autoimmune diseases include rheumatoid arthritis, type 1 diabetes and Crohn’s disease.

Autonomic nervous system

The part of the nervous system that controls involuntary bodily functions, such as breathing, heartbeat and digestion. It is part of the peripheral nervous system.

Auxins

A group of plant growth factors/hormones that: enable plants to respond to light, gravity and water; cause them to keep their leaves; and prioritise stem tip growth over side shoot growth. Includes indole acetic acid (IAA).

Axon
The long projection that carries signals away from the nerve cell body towards other neurons. The membrane voltage change from an incoming signal here triggers the opening of channels that allow ions (charged atoms) to flow into the cell from outside. This causes more channels further along the axon to open, creating an electrical signal (action potential) that propagates along it.

B

B cells
Named after the bone marrow, where immature B cells are produced. Types include plasma B cells and memory B cells. A type of lymphocyte.
Bachelor’s degree
A course of study that leads to a qualification such as a BSc (Bachelor of Science) or BA (Bachelor of Arts). Generally, these take three to four years’ full-time study, although medicine degrees can take six years. Some universities, including Oxford, Cambridge and Dublin, award a BA regardless of the subject studied.
Bacterium (plural: bacteria)

One of the three domains of life, bacteria are single-celled prokaryotic organisms.

Balanced diet

A diet that contains a range of foods with nutrients in the correct proportions to help someone be healthy, including having a healthy weight.

Basal

Describes the side of the cell in relation to its position in the body; the basal membrane faces away from the outside surface of the body or the lumen of internal cavities.

Base pair
Two DNA bases paired up, like rungs on a ladder, which produces the double helix structure.
Basophils
Cells involved in allergic and inflammatory responses. Basophils release histamine like mast cells, but unlike mast cells they circulate in the blood. A type of white blood cell.
Beige fat

A type of adipose (fat) tissue. Beige fat cells are brown fat cells that can emerge in ordinary white fat in response to certain triggers, like extreme cold.

Binary fission

A method of prokaryotic asexual reproduction that produces two identical cells.

Biobanks
Databases of genetic, clinical, environmental and lifestyle information on individuals, along with corresponding clinical specimens.
Biodiversity
The diversity of life. The term usually refers to the number of different species in a given area, but can also refer to genetic diversity. An important concept for ecologists.
Bioengineering
The use of engineering techniques to solve medical problems. An example would be the design and production of artificial limbs.
Biofuel

A substance produced from plant material that is used as a fuel. Examples include biodiesel and bioethanol. Biofuels are renewable sources of energy.

Biosignature
A sign that provides evidence of past or present life. This could be an element (such as carbon), a fossil, a molecular structure or biomarker, or a specific composition or compound that suggests a biological process is at work.
Biotechnology company
A smaller company researching new drugs, often using new technologies (often referred to as ‘biotech’).
Biotic
Living.
Bipedal

Using two legs (or limbs) to walk.

Bipolar disorder

A condition characterised by periods of elevated mood and periods of depression.

Blood–brain barrier

A semi-permeable membrane separating the blood and the cerebrospinal fluid of the brain.

BMI

Body mass index. A measure used to determine whether a person is underweight, normal weight, overweight or obese, calculated by dividing body mass (kg) by height squared (m2). Healthy BMI values range from 18.5 to 25 kg/m2.

Body fat

Body fat (or adipose tissue) is a mass of fat cells in a thick layer under the skin. There are two types of fat: essential fat, which is vital for the general functioning of the body, and storage fat, which is used to release energy. Excess storage fat is also used by hibernating and migrating animals.

Brown fat

A type of adipose (fat) tissue, also known as brown adipose tissue (BAT). It is made up of cells full of mitochondria and is metabolically active, unlike white fat. The brown colour comes from iron that is attached to proteins in the mitochondria. It develops before we are born, in specific regions of the body such as between our shoulder blades.

C

Cadherin

A family of cell adhesion molecules that are involved in cell–cell anchoring.

Calcification

The build-up of calcium salts in body tissue. It is a normal part of bone formation, but it can also occur in the blood vessels, causing cardiovascular disease.

Calorie (kcal)

A unit (technically a kilocalorie) used to measure energy – particularly in food. One kcal is the amount of heat required to raise the temperature of one kilogram of water by one degree Celsius, and is equal to 4.184 kilojoules (kJ), or 4,184 joules.

cAMP (cyclic adenosine monophosphate)
A common second messenger, essential in many signal transduction cascades.
Cannabinoids

Compounds that are chemically similar to cannabis – including endocannabinoids, which are made in the human body.

Carbon cycle
A set of interlinking biological and geological processes in which carbon moves through through the Earth’s atmosphere, oceans and rocks as well as living things.
Carbon fixation

The first stage of the Calvin cycle. The enzyme RuBisCO catalyses a reaction between ribulose bisphosphate (RuBP) and carbon dioxide, producing glycerate phosphate (GP), thus fixing inorganic carbon into an organic compound.

Carcinogen

A substance or chemical that can cause cancer in living tissue.

Cardiovascular disease

A general term that refers to conditions involving constricted or blocked blood vessels. Examples include various forms of heart disease as well as stroke. 

Carotenoids

Pigments found in the chloroplasts of plants (and some other photosynthetic organisms). They help chlorophyll absorb light energy and protect it from overexposure to light. 

Carrying capacity
The maximum population that the environment and all of its resources can support.
Case-control studies
Studies that use people who already have a disease or other condition to see whether there are characteristics of these individuals that differ from matched partners who don’t have the disease.
Catabolism
The process by which large molecules are broken down into smaller molecules. Catabolism is the opposite of anabolism and part of metabolism. Catabolic reactions release energy, some of which is used to synthesise ATP. Glycolysis – the stepwise conversion of glucose to pyruvate – is an example of a catabolic reaction.
Catalyst

A substance that increases the rate of a chemical reaction without itself undergoing any permanent change.

Caudal
Relating to the posterior part of the body.
Cell body (soma)
The part of the neuron that contains many components typically found in other types of cell. This includes DNA (in the nucleus), which holds instructions for producing the proteins that determine the shape and function of the cell.
Cell membrane
A film of fatty molecules (phospholipids) that controls substances entering and leaving the cell.
Cell wall

A structural layer in the cells of bacteria, fungi and plants outside of the cell membrane. In plants, the cell wall is mostly made of cellulose.

Cell-based immune response
The part of the specific (adaptive) immune response that involves B cells and antibodies.
Central nervous system (CNS)

Made up of the brain and spinal cord, the CNS sends and receives information to and from the body (via the peripheral nervous system), controlling its actions.

Centrioles
A pair of organelles that organise microtubules into spindles on which chromosomes are separated when cells divide.
Cerebellum
The ‘little brain’ that controls balance and coordinates movements. It’s normally required for learning motor skills, such as riding a bike, and is involved in thought processes.
Cerebral hemispheres
The two halves of the brain, each of which controls and receives information from the opposite side of the body.
Chaperone proteins

Proteins found in the cytoplasm of eukaryotic and prokaryotic cells that bind to newly created polypeptide (protein) chains and ensure they fold correctly.

Chemosynthesis
The process used by bacteria and some other organisms to form nutrients from carbon dioxide or methane, using energy produced by reactions involving inorganic chemicals such as sulfur.
Chitin

A polysaccharide found in the cell walls of fungi and the exoskeletons of insects and crustaceans. 

Chlorophyll

A family of green pigments found in and around the photosystems of chloroplasts.

Chloroplasts

The pigment-rich organelles where photosynthesis takes place in plant cells. 

Cholesterol

A type of lipid that is made by animals cells and is needed for maintaining the stability of cell membranes, for the insulation of nerve fibres and for the production of steroid hormones, including testosterone and oestrogen. Most cholesterol is made in the body (including in the liver), though some we get from our diet. Cholesterol is transported in the blood in complexes called lipoproteins.

Chromosome
A large package of DNA found in cells. It contains a set of genes and other DNA elements. Humans have 46 (23 pairs).
Cilia

Small hair-like projections on the surface of cells. In animal cells, they are used to move small particles; in some prokaryotes, they are used for movement.

Cis

A molecular arrangement in which two particular atoms or groups are found on the same side of a double bond. For example, in a particular unsaturated fatty acid, the hydrogens can be on the same side of the carbon–carbon double bond (cis) or on opposite sides (trans).

Climate

The weather conditions that generally prevail in an area over a long period of time. These conditions include, but are not limited to, temperature, rainfall (precipitation) and humidity. Climate is a measure of patterns, and can be thought of as the average weather in one given location over 30 years or longer.

Climate change

A change in global or regional climate patterns beyond that of climate variability, especially change caused by increased levels carbon dioxide in the atmosphere as a result of burning fossil fuels.

Climate variability

Changes within the normal range of weather defined by a climate – for example, an unusually cold winter, a wetter than average spring, a summer drought.

Clinical trial
A study of the effects of a drug in people.
Codon

A group of three bases in DNA or mRNA that codes for a specific amino acid. RNA codons are ‘read’ during protein synthesis.

Coenzyme
Small non-protein organic molecules that bind to, and are required for the activity of, their associated enzyme. An example is coenzyme A (CoA), which combines with a two-carbon compound during the link reaction to form acetyl CoA.
Cohort study
In analytical epidemiology, a study in which a population exposed to a presumed cause of a disease is followed over a set period of time to monitor the appearance of the disease. A control group is also monitored to identify the cause and effect of infection. It is also known as a prospective study.
Collagen

A protein that makes up connective tissues such as tendon, bone and cartilage.

Colony (or cell culture)

A group of identical cells, usually derived from a single parent cell, grown on a solid medium.

Combinatorial chemistry

Creates new compounds out of a limited number of ‘parts’ by putting together the parts in different ways. This makes vast compound libraries, which can then be tested as drug candidates.

Combustion
The process of burning a substance in the presence of oxygen, which produces energy; in a biological context, this could refer to the burning of fossil fuels, which produces carbon dioxide.
Commensalism
A type of interaction between species (symbiosis) in which one species benefits without affecting the other.
Community
Two or more populations living together and sharing a habitat.
Competitive exclusion principle
A principle that states that it is impossible for two species in the same ecosystem to coexist if they are competing for the same resources.
Complement

A set of around 30 proteins in the blood plasma that can be activated by the presence of microbes or antibody–antigen complexes. Complement can destroy pathogens and activate phagocytic cells.

Complementary
Matching. Complementary bases are those that pair up in DNA and RNA, such as cytosine (C) with guanine (G).
Concentration gradient

The gradual difference in concentration of a substance. This is especially important for dictating diffusion – molecules will naturally diffuse down their concentration gradient.

Conformational change

An alteration in shape that is a result of binding a substrate molecule.

Congenital

Something present from birth (such as a disease).

Control experiment
An experiment performed alongside a main experiment that is virtually identical, except that the control group does not receive the same treatment – or independent variable – as the experimental group. Performing a control experiment helps to increase the reliability of your experimental results.
Cortex
The thin, folded structure on the outside of the brain.
Cranial nerve nuclei
Clusters of neurons in the brainstem. Their axons form the cranial nerves.
Cuticle

A waxy layer that sits on top of the epidermis of the leaves and stems of plants. It is a waterproof layer that helps limit water loss from transpiration and helps protect the plant from infection.

Cytochrome b6f

An enzyme in the thylakoid membrane that acts as an electron carrier.

Cytochrome c

A component of the electron transport chain in cellular respiration, in which it carries one electron. It differs from other cytochromes in that it is highly water-soluble.

Cytochrome oxidase

The terminal component of the electron transport chain in cellular respiration, which oxidises cytochrome c. This is where oxygen is actually consumed, by acting as the final electron acceptor and being converted to water.

Cytochrome P450
A group of enzymes that are found in the liver. They are responsible for processing drugs.
Cytochromes
Special proteins that are responsible for generating ATP via electron transport.
Cytokines
Proteins that act as messengers between cells. Often released by immune cells, including helper T cells and others, to signal danger or damage.
Cytokinins

A group of plant growth factors/hormones that promote cell division and the growth of side shoots and new roots. Cytokinins often work in opposition to auxins.  

Cytoplasm
Everything in the cell outside the nucleus; a viscous fluid containing proteins, other organic and inorganic molecules, membranes and organelles.
Cytoskeleton

The networks of protein filaments and tubules that give structure and aid intracellular transport. In eukaryotic cells, the main components of the cytoskeleton are microfilaments, microtubules and intermediate filaments.

Cytotoxic T cells

T cells that kill infected cells and cancer cells by releasing toxic chemicals. Also known as CD8+ cells, because of a protein that they express on their cell surface, and as killer T cells. A type of lymphocyte.

D

Decarboxylation

A chemical reaction that releases carbon dioxide (CO2).

Decomposition
The process by which organic matter decays or breaks down into less complex substances.
Dementia

A group of conditions characterised by memory loss and difficulty with thinking and problem solving.

Demography

The statistical study of a population. Characteristics often measured include birth rates and death rates (which can be used to calculate rates of population growth or decline) and the size of different age groups. These data are useful in evaluating and anticipating the occurrence of a disease in a population.

Dendrites
Protrusions from the cell body of a neuron that form branches connecting to other cells. These connections are input synapses, which receive signals from the axons of neighbouring neurons.
Dendritic cells
Phagocytic antigen-presenting cells (APCs) with an important role in alerting T cells to new pathogens. A type of white blood cell.
Depolarisation

The process of reducing the electrical potential of a cell or membrane, an essential part of the transmission of electrical impulses.

Desmosome

A cell junction involved in cell–cell connections via cadherin molecules and intermediate filaments.

Developmental psychology

The scientific study of how and why human beings change over the course of their life. Originally concerned with infants and children, the field has expanded to include adolescence, adult development, aging, and the entire lifespan.

Diabetes

A group of metabolic diseases caused by a lack of insulin production (type 1) or by the inability of the body to produce enough insulin or use it correctly (type 2).

Diaphragm

A muscle separating the thorax and abdomen that is used in breathing. When it contracts, the lungs inflate; when it relaxes, they deflate.

Differentiation

The process of a cell changing into a new cell type – usually from a less specialised cell type to a more specialised type during development.

Digestion

The process in which food is broken down physically (by the teeth or peristalsis movements of the gut) and chemically (by enzymes throughout the digestive tract) into molecules small enough to be absorbed by the body.

Diploid

A cell containing two complete sets of chromosomes, one from each parent.

DNA
Deoxyribonucleic acid, the chemical that carries genetic information. It is made up of two entwined strings (the ‘double helix’) of four chemical units (bases): A, T, G and C.
DNA methylation

An epigenetic mechanism in which methyl (CH3) groups are added to DNA to stop gene expression, allowing cells to switch certain genes off.

Doctorate
A qualification that involves conducting a piece of original research, usually at a university. These courses take at least three years, full-time. The best-known doctorate is a PhD or DPhil – short for Doctor of Philosophy. People with this qualification can be called ‘doctor’, but are not to be confused with medical doctors (who may also choose to take a PhD).
Domains of life

In the Woese classification system, organisms are separated into three domains of life – eukarya, bacteria and archaea – rather than the five kingdoms of life.

Dopamine
A neurotransmitter involved in the control of movement, posture and mood. L-DOPA (the precursor to dopamine and noradrenaline) is used to treat Parkinson’s disease.

E

Ecology

The study of the relationships between living organisms and their environment.

Ecosystem
A community of living (biotic) organisms and the non-living (abiotic) factors of their environment that they interact with.
Ectoparasite

A parasite that lives on the outside of its host. Examples include fleas, lice and ticks.

EEG (electroencephalogram)

EEG scans detect electrical activity in the brain using electrodes that are attached to the scalp.

El Niño Southern Oscillation

Large-scale sea surface and atmosphere fluctuations that significantly affect climate in the Southern Hemisphere. El Niño means ‘the little boy’ – its effects are generally seen at Christmas time. A cold phase during each El Niño episode is known as La Niña (‘the little girl’).

Electrochemical gradient

A gradient of electrical potential, usually in relation to an ion, which dictates its ability to move across a membrane.

Electron carrier

A molecule that can accept one or more electrons and donate them to another in an electron transport chain. They include NAD, FAD and the cytochromes.

Electron transport chain

A process that uses electron transport to power the transport of protons (H+), leading to the production of ATP. It is an important part of cellular respiration (following the Krebs cycle) and photosynthesis.

ELISA (enzyme-linked immunosorbent assay)
A test that uses antibodies to detect the presence of a specific chemical such as an antibody or another kind of protein. One of the commonest methods of detecting HIV is an ELISA.
Embryogenesis

The process of the formation and development of an embryo.

EMG (electromyography)

The recording of the electrical activity of muscle tissue, or its representation as a visual display or audible signal, using electrodes attached to the skin or inserted into the muscle.

Endemic
Established in a region. Often used to describe diseases.
Endocytosis

The process of a cell engulfing materials from the extracellular space by fusing the cell membrane around the material to create an intracellular vesicle.

Endoparasite

A parasite that lives inside its host’s body. Endoparasites can be multicellular or unicellular. Multicellular parasites live inside the cavities of their host’s body; unicellular parasites may do the same, or may live inside their host’s cells.

Endoplasmic reticulum (ER)
An extensive network of membranes. Rough ER is studded with ribosomes and is a site where proteins are made, modified and processed for shipping. The roles of smooth ER include lipid and steroid synthesis and drug detoxification.
Endosymbiotic theory

An evolutionary theory that eukaryotic cells originated from prokaryotic cells acting symbiotically. According to the theory, certain eukaryotic organelles were originally free-existing bacteria, which then moved inside their symbiotic partners.

Enzyme
A protein that catalyses a chemical reaction by lowering the activation energy required, increasing the rate of reaction. Different enzymes work in different parts of the body on specific molecules (substrates). Enzymes are not altered by the reactions they catalyse.
Eosinophils
Phagocytes that also produce enzymes to counteract the inflammatory molecules released by mast cells. A type of white blood cell.
Epidemic

A widespread outbreak of a disease, usually in a particular area and of limited duration.

Epidemiology
The study of the patterns, causes, spread and control of disease in populations.
Epigenetics
The study of heritable changes to DNA in which the genetic sequence remains unchanged but small chemical groups are added to the DNA. The addition of these groups affects how a gene works.
Epigenome

The complete record of heritable, non-DNA changes to a genome, which often affects whether or not a gene will be read.

Epilepsy

A neurological condition, with symptoms such as loss of consciousness and seizures associated with abnormal electrical activity in the brain.

Epithelium (epithelial cells)

One of the major types of animal tissue, it is characterised by layers of closely packed cells covering internal and external surfaces of the body for protection. Also involved in secretion, regulation and absorption.

Ester bond

The type of bond formed during triglyceride synthesis. This is a condensation reaction where one glycerol molecule reacts with three fatty acid molecules to make one triglyceride molecule and three water molecules.

Ethene

Also known as ethylene; a simple hydrocarbon that acts as a growth factor/hormone in plants, stimulating flowering, fruit ripening, leaf shedding and rotting.

Eugenics
Enhancing particular characteristics of human populations through selective breeding.
Eukarya
Cells with a nuclear envelope; one of the three main divisions of life.
Eukaryotes
Organisms whose cells contain a nucleus and organelles such as mitochondria or chloroplasts. Eukaryotic organisms include animals, plants and fungi.
Evolution

The development of species over time, caused by heritable traits in the population changing from generation to generation, often in response to environmental pressures that favour certain characteristics. 

Excitatory neurotransmitter

Neurotransmitters that increase the likelihood that the receiving neuron will produce an action potential.

Exocytosis

The process of releasing material from the cell into the extracellular space by fusing an internal vesicle containing material with the cell membrane.

Exoplanet
A planet that orbits a star other than the Sun.
Extracellular matrix
The material in between cells that holds tissues together, usually made of scaffolding proteins such as collagen.
Extremophile
An organism that ‘loves’ or thrives in extreme environments, such as intense heat or cold, salinity, acid, pressure or radiation. These organisms are particularly useful in the search for life elsewhere in the universe.

F

Facultative parasite

An organism that may resort to being a parasite, but doesn’t have to to complete its life cycle.

FAD (flavine adenine dinucleotide)

A coenzyme that acts as a hydrogen acceptor in respiration. FAD accepts hydrogen in the Krebs cycle and becomes reduced. In the electron transport chain it transfers electrons from this hydrogen.

Family studies
If a large family group affected by a disease exists, researchers can hunt for genetic markers showing the same inheritance patterns as the disease.
Fat tax

An extra charge imposed by government on foods that have a high fat content. Denmark was the first country to introduce a fat tax, in 2011. It was withdrawn in 2012.

Fat-shaming

Singling out, discriminating against or making fun of overweight or obese people. One type of ‘body-shaming’.

Fatty acid

A building block of the fat in our bodies and in the food we eat. During digestion, the body breaks down fats into fatty acids, which can then be absorbed into the blood.

Fermentation

An anaerobic metabolic process that converts sugar into acids or alcohol.

Fibre

The indigestible element of the plant-derived foods we eat. It comes in two forms: soluble and insoluble. 

First messenger

A signal molecule that binds to a receptor on the cell membrane in order to communicate to the inside of the cell.

Flagellum (plural: flagella)

Long ‘whip-like’ projections used mainly by cells or unicellular organisms for movement.

Fluid mosaic model

This model describes the membrane surrounding animal cells. The plasma membrane has two layers (a bilayer) of phospholipids, which are fluid at body temperature. Each layer has a hydrophilic (water-loving) head facing outwards and two hydrophobic (water-hating) tails facing the inside. The proteins and substances such as cholesterol within the bilayer make it resemble a mosaic.

Focal adhesion

A cell junction involved in cell connections with the extracellular matrix via integrin molecules and actin filaments.

Folate

A salt or ester of folic acid.

Folic acid

A water-soluble B vitamin that helps the body break down complex carbohydrates into simple sugars.

Food and Drug Administration (FDA)
The US version of the Medicines and Healthcare Products Regulatory Agency (MHRA), the body that licenses new drugs in the UK.
Fossil fuels

Non-renewable fuels, such as coal, oil and natural gas, formed through the decomposition of dead organisms over millions of years.

Foundation degree
A university-level qualification that equips a person for a particular area of work, typically an FdA (Foundation Degree in Arts) or FdSc (Foundation Degree in Science). These degrees combine learning in the workplace with academic study and are equivalent to the first two years of an honours degree. Formal qualifications are not always required to enrol on a foundation degree course.
Founder effect

An unusually high frequency of an allele in a population, because it was present in a small group that initially gave rise to that population.

Free radicals

Highly reactive molecules, produced during many metabolic processes, that can damage DNA.

Frontal lobe
The area of the brain that performs complex mental tasks such as making abstract judgements and decisions, thinking about the future, paying attention and inhibiting behaviour (so-called ‘executive functions’). It is also involved in short-term memory, language and movement.
Frontal lobe

One of the four major lobes of the brain, located at the front of the brain.

Fruit fly
Drosophila, an insect commonly used in research, particularly genetics.
Functional magnetic resonance imaging (fMRI)

A technique that reveals activity in the brain by measuring oxygen in the blood (more blood flows to the parts of the brain that are active). This makes it possible to show what brain areas are involved in specific mental processes.

Fungus
A type of eukaryotic organism. ‘Fungi’ is one of the five kingdoms found in biological classification, and includes mushrooms, yeasts and moulds.

G

GABA (gamma-aminobutyric acid)

The main inhibitory neurotransmitter in the brain.

Gap junction

A cell junction involved in connecting the cytoplasm of two animal cells to allow direct communication.

GDP (guanosine diphosphate)
A molecule converted to GTP during the Krebs cycle.
Gene
The fundamental unit of inheritance. Genes are lengths of DNA, and most contain information for making proteins.
Gene expression
When a protein is made from the information on a gene.
Genetic code
The order of the chemical units of DNA (bases) determines the order of the chemical units of proteins (amino acids). There are 64 three-letter ‘words’ in the code.
Genetic drift
Random changes in the frequency of alleles in a population, not linked to selection pressures.
Genetics
 1. The study of the structure and function of genes. 2. The genetic features that occur in individuals, families and populations.
Genome

The complete genetic information of an organism, found in nearly every type of cell.

Genotype
The genetic characteristics of an individual (e.g. having the gene or genes for red hair). Compare with phenotype.
Gestation

The period during which a fetus develops inside the uterus (ie pregnancy).

Ghrelin
A hormone produced by cells in the stomach lining and pancreas. Ghrelin stimulates hunger, increasing food intake, and is also important for the secretion of growth hormone.
Gibberellins

A group of plant growth factors/hormones that control stem elongation between leaves, seed germination and fruit growth.

Glial cells
Non-neuronal cells found in the brain and other parts of the nervous system. Their roles include supporting neurons and forming the myelin sheath.
Glucose

A hexose sugar and the product of photosynthesis. It is one of the main sources of chemical energy used in cellular respiration. Animals get glucose from the food they eat; plants create it via photosynthesis.  

Glutamate

The brain’s main excitatory neurotransmitter. It is also the precursor to GABA (gamma-aminobutyric acid).

Glycerol

An odourless liquid that can be used as a solvent, antifreeze, pharmaceutical ingredient and sweetener (known as E422). The condensation of one glycerol molecule with three fatty acid molecules produces one triglyceride molecule – which we use to store fats in the body – and three water molecules.

Glycogen

A branched polymer of glucose that acts as a long-term energy store for animal cells.

Glycolipid

A lipid with carbohydrate attached.

Glycolysis
The splitting of sugar. This process, in which glucose is oxidised and converted to pyruvate, takes place in the cytoplasm. It is the first stage of both aerobic and anaerobic respiration.
Glycoproteins

Proteins with covalently attached sugar units.

Glycosidic bonds

Covalent bonds that link sugar molecules to other molecules, including other sugars.

GM (genetic modification)

The practice of altering an organism’s genome, either by modifying its genes or transferring in genes from another organism. 

Golgi apparatus
An organelle and one of the wondrously complex membrane systems in the cytoplasm, which modifies, packages and directs newly made proteins to where they are needed.
Graduate
Relating to someone who has completed a university degree, usually a Bachelor’s degree. Many employers offer graduate training schemes.
Gravitropism

Movement or growth in plants in response to gravity. 

Gravity

The force of acceleration of one object towards another. Gravity holds humans to the earth and keeps the Earth in orbit around the sun. Gravity on Earth – normal gravity or 1 G – is equal to 9.8 ms-2.

Greenhouse gases

Atmospheric gases that trap heat. The most important are:

  • water vapour (has the biggest greenhouse effect, but levels are relatively stable)
  • carbon dioxide (CO2)
  • methane (present at lower levels than carbon dioxide, but has a much stronger greenhouse effect)
  • lower-atmosphere ozone.

Other chemicals, such as nitrous oxide and CFCs (chlorofluorocarbons), are also greenhouse gases, but are present at low levels.

GTP (guanosine triphosphate)
A molecule produced during the Krebs cycle that donates a phosphate group to ADP to make ATP. It is also very important elsewhere in the cell.
Guard cells

Specialised cells on the outside layer (epidermis) of plant leaves that open and close stomata to control gas exchange and water loss.

H

Habitable zone

The region around a star where liquid water is stable (usually where the temperature sits between 0 and 100ºC, though pressure can affect this).

Habitat
Where a population lives.
Haematopoietic stem cells (HSCs)

Multipotent blood cells that can give rise to all other blood cells.

Haemoglobin

The protein found in red blood cells that is responsible for transporting oxygen in the blood.

Haploid

A cell containing a single set of unpaired chromosomes.

Hard and soft skills

Hard skills can be defined as specific, teachable skills that can be defined and measured, for example maths, technological skills. Soft skills are less tangible and harder to quantify, for example public speaking, research skills, teamwork. 

Hayflick limit
The Hayflick limit describes the number of times most human cells can divide within their lifespan.
Heart disease

A general term that refers conditions affecting the heart. These can include problems with the heart muscle, its valves, its rhythm and its surrounding vessels, and can lead to heart attacks, chest pain (angina) and stroke. 

HeLa cells

The first continuously cultured cells, a human cervical cancer strain taken from a woman called Henrietta Lacks (and named after her).

Helminth

A parasitic worm, such as a flatworm or roundworm. 

Helper T cells

T cells that recognise antigens presented by APCs and stimulate T, B and other immune cells. Also known as CD4+ cells, because of a protein that they express on their cell surface. These are the cells depleted by infection with HIV (human immunodeficiency virus). A type of lymphocyte.

Hemidesmosome

A cell junction involved in cell connections with the extracellular matrix via integrin molecules and intermediate filaments.

Hepatic stellate cells

Hepatic stellate cells are found in the liver and account for 5 to 8 per cent of the cells there. In a healthy liver, these cells are quiescent (dormant) and contain vitamin A droplets. In response to injury, the cells are activated. They lose their vitamin A and proliferate, causing fibrosis, a scarring of the liver.

Heredity
Passing on of characteristics from one generation to the next.
Hexose

A six-carbon sugar formed as the product of six turns of the Calvin cycle. Glucose is a hexose.

High-density lipoprotein (HDL) cholesterol

Cholesterol is transported in the blood in complexes called lipoproteins. High-density lipoproteins (HDLs) tend to remove cholesterol from the tissues to the liver for excretion. Low-density lipoproteins (LDLs) are more likely to deposit cholesterol in damaged areas of blood vessels and lead to the formation of atherosclerotic plaques. High levels of LDL cholesterol can lead to heart disease, whereas high levels of HDL cholesterol can protect against heart disease.

High-throughput screening

A technique that can conduct lots and lots of tests very quickly. In a drug development context, it allows scientists to test many potential candidate drugs and measure their effect on the target. However, this technique is also used in genomics.

Hippocampus
A part of the brain. Essential for making new memories and finding your way around.
Histamine

A chemical with a variety of roles in the body. In allergic reactions it is released when mast cells are stimulated. It causes capillaries to become leaky, releasing fluid and white blood cells into tissues, causing inflammation.

Histone proteins

Specialised proteins inside the nucleous of a eukaryotic cell. These proteins help to regulate and structure DNA by allowing the genetic material to wind or wrap around them.

HIV (human immunodeficiency virus)

HIV is a virus that causes AIDS (acquired immune deficiency syndrome), which is also known as late-stage HIV. HIV is a retrovirus, which means it contains RNA as its genetic material, not DNA. 

Homeostasis

An organism or cell’s tendency to regulate its internal conditions (such as temperature, chemistry, blood pressure, resting time) in order to stabilise health and functioning despite changes in the environment.

Hormones

Chemical messengers that carry information from one part of an organism to another to regulate physiology and behaviour. 

Host

In symbiosis, an organism that harbours another, providing it with nourishment or shelter.

Human Genome Project

The international effort that set out to sequence the whole human genome.

Humoral immune response
The part of the specific (adaptive) immune response that involves T cells.
Hybridoma
A cell formed by the fusion of a plasma B cell and a cancer cell. They are used in the production of monoclonal antibodies.
Hydrogen carrier

A molecule that can accept hydrogen atoms or ions and donate them to another carrier. Hydrogen carriers include NAD and FAD.

Hydrolysis
A chemical reaction that uses water to break bonds within molecules. A hydrolytic enzyme, or hydrolase, increases the rate of hydrolysis. Proteases are a type of hydrolase that catalyse the hydrolysis of interior peptide bonds in chains of peptides.
Hydrophilic

Hydrophilic (lit. ‘water loving’) molecules tend to dissolve in water.

Hydrophobic

Hydrophobic (lit. ‘water fearing’) molecules are insoluble in water.

Hyperplasia

The enlargement of an organ or tissue caused by an increased in the production of cells.

Hypertension
Also known as high blood pressure, this medical condition is caused by an increase in the blood pressure within arteries. The extra pressure can cause damage to the arteries or the heart. Hypertension can raise the risk of coronary heart disease or stroke.
Hypertrophy

The enlargement of an organ or tissue caused by an increase in the size of its cells.

Hypothalamus
A small area at the base of the brain found in all vertebrates, this is the interface between the brain and pituitary gland. It is responsible for many aspects of homeostasis and produces hormones that control a variety of bodily functions, including hunger, thirst, body temperature and sleep patterns.

I

IgA

A type of immunoglobulin, or antibody, that is important at the sticky mucosal surfaces where many pathogens try to enter the body, such as in the intestines.

IgD

A type of immunoglobulin, or antibody, that is found on the plasma membrane of immature B cells and also secreted in the blood. Its role is not yet clear.

IgE

A type of immunoglobulin, or antibody, that binds to allergens and mast cells, triggering the release of histamine. It plays a role in various allergic diseases, including food allergies and asthma, and in immunity to certain parasites, including some kinds of worms and the parasite that causes malaria in humans.

IgG

The most abundant immunoglobulin, or antibody, in the blood serum of humans. It plays an important role in the humoral response of the specific immune system. It is the only type of immunoglobulin that crosses the placenta.

IgM

A type of immunoglobulin, or antibody, that exists mostly as a pentamer (five monomers joined together). It is the first immunoglobulin to be made by a B cell when it is exposed to an antigen.

Immune system
This helps a living thing defend itself against infectious organisms and other foreign substances. In mammals, it includes the non-specific (innate) immune system and the specific (adaptive) immune system.
Immunoglobulin (Ig)
Also known as an antibody. This is a Y-shaped globular protein, whose secondary structure is based mostly on beta-pleated sheets. It is produced by plasma B cells to fight against antigens. Types include IgA, IgD, IgE, IgG and IgM.
In vitro fertilisation (IVF)

A procedure in which an egg and sperm are joined in a test-tube to form a zygote, which can then be implanted into a uterus to grow.

Inflammation
When the receptors of cells involved in the non-specific immune system are engaged by pathogens, the cells release molecules that trigger inflammation. Inflammation causes increased blood flow to the area, which leads to swelling, pain and an increase in temperature.
Influenza A(H1N1)pdm09

The name of the virus responsible for the 2009 influenza pandemic. For a while, ‘S-OIV’ (swine-origin influenza virus) was used, but ‘influenza A(H1N1)pdm09’ virus is now its official WHO-approved name.

Inhibitor
A compound, such as a drug, that can prevent an enzyme or other biomolecule from functioning.
Inhibitory neurotransmitter

Neurotransmitters that decrease the likelihood that the receiving neuron will produce an action potential.

Innate immune system (also non-specific immune system)

The first and second lines of defence against invading pathogens. It is short-lived and doesn’t involve any memory of previous infections. Found in all multicellular organisms. It includes barriers such as skin or plant cell walls, and processes like inflammation.

Inorganic phosphate (Pi)

A molecule that plays a key role in respiration and photosynthesis and which, among its functions, combines with ADP to create ATP.

Insoluble fibre
A necessary part of our diet, found in whole grains and vegetables. Insoluble fibre helps speed the transit of food through the intestines and adds bulk to stools.
Insulin
A hormone secreted by the pancreas that regulates blood glucose levels. When blood glucose levels are high, insulin causes glucose to be stored as glycogen in the liver and muscles.
Integral membrane protein

A protein that is permanently anchored within the plasma membrane.

Integrin

A group of membrane molecules involved in cell–cell binding.

Intermediate filaments

Cytoskeleton components that are made of numerous proteins, involved in providing structure and strength to the cytoskeleton.

Ion
An atom or molecule that has lost or gained electrons to become either negatively or positively charged.
Ion channel
A protein or assembly of several proteins in the cell membrane that opens and closes to let ions move in and out of cells.
Ionising radiation
Radiation that has so much energy that it can remove electrons from an atom’s orbit when it comes into contact with it, ‘ionising’ or charging it. These type of radiation – which include X-rays, gamma rays and cosmic rays – are often harmful to humans, causing DNA mutations or cell death.
Isomer

A compound which has the same formula as another, but with different properties due to an alternative arrangement of atoms. 

J

Joule
A unit of energy with the symbol J. The energy found within food is often discussed in kilojoules (KJ, 103 J) or megajoules (MJ, 106 J). One kcal is equal to 4.184 kilojoules.

K

Ketogenic diet
A high-fat, low-carbohydrate diet that mimics prolonged starvation. The use of fat as the body’s preferential energy source leads to a build up of ketone molecules (which contain a carbonyl group, C=O, bonded to two other carbon atoms). This diet is sometimes used to reduce the number of seizures suffered by children with epilepsy.
Ketones

Compounds that are produced in the liver when fat is respired under starvation conditions (when levels of carbohydrate in the blood are low) or when the body has insufficient insulin to trigger cells to take up glucose from the blood (for example, in type 1 diabetes). At high levels, ketones can be harmful, causing vomiting, hyperventilation and breath that smells like pear drops.

Kin selection
Natural selection favouring the survival of relatives rather than an individual itself.
Knockout
A technique that inactivates an individual gene.
Krebs cycle
A step in cellular respiration that occurs in the mitochondrial matrix after the link reaction. It completes the oxidation of acetyl CoA and precedes the electron transport chain.

L

Leptin

A hormone, produced by adipose tissue, involved in the regulation of hunger. Leptin acts on the hypothalamus in the brain, inhibiting appetite by counteracting the effects of other hormones.

Leucocytes (also leukocytes or white blood cells)
Made in the bone marrow, these cells are an important part of the immune system. There are two main types: granulocytes, which have granular cytoplasm and a lobed nucleus, and agranulocytes, which have smooth cytoplasm and a non-lobed nucleus.
Light-dependent reactions

The first stage of photosynthesis. These reactions take place in the thylakoid membrane, forming an electron transport chain that creates ATP and reduced NADP.

Light-independent reactions

Also known as the Calvin cycle, these three reactions – carbon fixation, reduction and regeneration of ribulose bisphosphate (RuPB) – form the second stage of photosynthesis, in which glucose is produced. They occur in the chloroplast stroma.

Limbic system
A set of structures sitting above the brainstem that are involved in emotion, motivation and memory. They include the hypothalamus, the amygdala and the hippocampus.
Link reaction
A step in the process of aerobic respiration that removes hydrogen and carbon dioxide from pyruvate to form acetyl CoA. This process occurs in the mitochondrial matrix.
Lipase

A type of enzyme that breaks down lipids, including dietary fats and fat stored in our bodies as triglycerides, into glycerol and fatty acids. Lipases cause hydrolysis, the process where the bonds between glycerol and fatty acids are broken using water molecules.

Lipid

The chemical name for fats and other related compounds. The high energy value of lipids makes them an important food type that all organisms need. They form part of the membranes that protect cells from their surroundings. Lipids dissolve in ethanol, but not in water.

Lipid bilayer

A cell membrane consisting of two layers of lipid molecules.

Lipoproteins

A soluble group of proteins which transport fat (or other lipids) in the blood plasma. The two main types are high-density and low-density lipoproteins.

Logarithmic scale

A nonlinear scale based on orders of magnitude (101, 102, 103 and so on) often used for plotting data when there is a large range of quantities that would otherwise be difficult to visualise.

Longitudinal studies
Studies examining the same group of individuals at regular intervals.
Lorenzo’s oil

A proposed treatment for a condition called adrenoleukodystrophy (ALD). It is prepared from olive oil and rapeseed oil, and consists of four parts glyceryl trioleate and one part glyceryl trierucate.

Low-density lipoprotein (LDL) cholesterol

Cholesterol is transported in the blood in complexes called lipoproteins. High-density lipoproteins (HDLs) tend to remove cholesterol from the tissues to the liver for excretion. Low-density lipoproteins (LDLs) are more likely to deposit cholesterol in damaged areas of blood vessels and lead to the formation of atherosclerotic plaques. High levels of LDL cholesterol can lead to heart disease, whereas high levels of HDL cholesterol can protect against heart disease.

Lymphocytes
Agranulocyte white blood cells. Types include B cells, T cells and natural killer cells.
Lysosomes
Membrane-bound organelles that are the cell’s rubbish disposal and recycling units. They contain hydrolytic enzymes.

M

Macrophages
Antigen-presenting cells (APCs) that destroy foreign substances by phagocytosis (engulfing them) and activate other immune cells. A type of white blood cell.
Magnetic resonance imaging (MRI)
A medical imaging technique that uses powerful magnets to visualise the inside of the body. It can be used to study the structure and function of body parts, including the brain.
Magnification

The ability to use a microscope to make an object look bigger.

Mast cells
Cells involved in allergic responses, releasing histamine and other inflammatory molecules. Mast cells sit within skin and mucosal tissues. A type of white blood cell.
MC4R (melanocortin 4 receptor)
A protein found on neurons in the hypothalamus. Activation of MC4R leads to a drop in appetite. Mutations in MC4R account for some cases of severe obesity.
Medicines and Healthcare Products Regulatory Agency (MHRA)
The body that licenses new drugs in the UK.
Medulla oblongata
A part of the brainstem; controls vital involuntary functions such as breathing and heart rate.
Meiosis
A form of cell division that results in four non-identical daughter haploid cells.
Memory B cells
Long-lived B cells that ‘remember’ past infections by recognising antigens to provide a secondary immune response. A type of lymphocyte.
Memory T cells
Long-lived T cells that ‘remember’ past infections to provide a secondary immune response. A type of lymphocyte.
Metabolic pathway
A series of reactions that transform one chemical into another.
Metabolism
The sum of the chemical reactions that occur within the body. Metabolism can be divided into anabolism and catabolism. Chemical reactions are usually grouped into metabolic pathways, with enzymes used to control the rate at which they occur.
Metabolites
The chemicals produced by the reactions in a metabolic pathway.
Metagenomics
Mass sequencing of genomes directly from environmental or other samples, without a culturing step. Provides insight into the communities of organisms living in the sampled environment.
Methane
A compound composed of one carbon atom and four hydrogen atoms (CH4) . It is a naturally occurring gas found in the ground, under the sea and in smaller amounts in the atmosphere. Methane is sometimes considered a biosignature, particularly when it’s present with another gas like oxygen.
MHC (major histocompatibility complex)
A series of proteins found on the surface of our cells. MHC proteins indicate that your cells are ‘self’. Apart from identical twins, we all have our own unique set of MHC proteins.
Microbe
A living organism broadly defined as one that is usually too small to be seen by the human eye. Includes all bacteria, some fungi and algae, and even some animals. Viruses are sometimes included, but they are not living organisms.
Microbiome
The total number and diversity of microbes found in, and on, the human body. Different species inhabit different areas. Those found in the gut have several important roles in digestion.
Microfilaments
Smaller than microtubules, these are made from repeating actin subunits. They are responsible for cell movement and changes in cell shape, and make muscle contraction possible.
Microgravity

The condition of perceived weightlessness in outer space. Sometimes called ‘zero gravity’ or 0 G, microgravity is actually equal to 1 x 10-6 G.

Microtubules
Small, tubular assemblies of protein, made from repeating tubulin subunits, which help maintain the cell’s internal structure and move organelles and cytoplasm using molecular motors.
Mitigation
The practice of attempting to reduce the scale or severity of a particular outcome. In the context of climate change this could involve improving energy efficiency or switching to renewable resources in order to stabilise and reduce greenhouse gas levels.
Mitochondria (singular: mitochondrion)

Rod-shaped bodies in the cytoplasm that supply chemical energy to the rest of the cell.

Mitosis

A form of cell division that results in the creation of two identical daughter cells.

Monoclonal antibody
Monoclonal antibodies are produced from the progeny of a single plasma B cell and so ones of the same type are identical to each other. They are created in the laboratory and are made to be specific to a given antigen. They are increasingly being investigated for the treatment of cancer and other diseases, and are used in pregnancy tests.
mRNA (messenger RNA)

RNA molecules that transfer the gene sequence for a protein from the DNA in the nucleus to the ribosome, where it is ‘read’ to make the protein.

Multicellular

An organism that is made up of more than one cell (usually many cells).

Multipotent

The ability to give rise to many (but not all) but limited cell types. For example, a haematopoietic (blood) stem cell can give rise to many blood cell types but can’t give rise to liver cells.

Mutation

A change in the DNA sequence of an organism. The simplest mutation is the loss or alteration of a single base pair. Mutations can have a positive, negative or neutral effect. We use the term ‘altered’ to describe an allele that has been mutated from a ‘normal’ version. You might see ‘mutated’ or ‘faulty’ used elsewhere rather than ‘altered’.

Mutualism
A type of symbiosis involving close cooperation between species, in which both benefit.
Mycorrhizae

Symbiotic relationships between plants and fungi that inhabit their roots.

Myelin

The white fatty substance that makes up the myelin sheath. This is a multi-layered fatty cell membrane that wraps around an axon and acts as an electrical insulator. The sheath is interrupted at regular intervals (the nodes of Ranvier), where the channels that generate the electrical signal are located.

Myelin sheath
Many neurons are insulated by myelin a multi-layered fatty cell membrane that wraps around the axon. The sheath is interrupted at regular intervals (the ‘nodes of Ranvier’), where the channels that generate the electrical signal are located.
Myofibril

Made up of many subunits called sarcomeres, cylindrical myofibrils bundle together to form muscle fibres.

Myoglobin

A protein which carries and stores oxygen in muscle cells.

Myosin

One of the main proteins involved in muscle contraction. Myosin binds with actin, and then pulls the actin to cause muscle to contract. 

N

NAD (nicotinamide adenine dinucleotide)
A coenzyme that acts as a hydrogen acceptor in respiration. NAD accepts hydrogen in the link reaction and the Krebs cycle and becomes reduced. In the electron transport chain, it transfers electrons from this hydrogen.
NADP (nicotinamide adenine dinucleotide phosphate)

A coenzyme that acts as an electron acceptor in photosynthesis.

Natural immunity
Immunity that occurs as part of the life of an organism. For example, the response of the immune system to a person contracting measles, or from antibodies crossing the placenta.
Natural killer (NK) cells

Cells that kill virus-infected cells and cancer cells. NK cells release chemicals called cytokines, which alert and attract other immune cells. A type of lymphocyte.

Natural killer T (NKT) cells

These cells have the characteristics of T cells and natural killer cells. NKT cells are involved in both the specific immune system and the non-specific immune system.

Natural selection
The preferential survival and reproduction of organisms better adapted to their environment.
Nausea
A feeling of sickness accompanied by the tendency to vomit.
Necrosis

The death of some or all cells in a tissue or organ through disease or injury, often causing an inflammatory response.

Neural tube
The precursor to the central nervous system in vertebrates. Neural tube defects such as spina bifida occur when the tube does not develop properly. Taking folic acid is important in reducing the risk of these defects.
Neurodegeneration

An umbrella term used to describe a range of conditions that affect the neurons of the brain. This includes Alzheimer’s, Parkinson’s and motor neurone disease.

Neurogenesis

The process of growth and/or development of neurons and related tissue.

Neuromuscular junction

The synapse between a skeletal muscle and a neuron.

Neuron (or nerve cells)

Specialised cells that are able to transmit nerve impulses via a synapse.

Neurotransmitter
A chemical that neurons use to communicate with one another. There are many kinds, including serotonin, dopamine and noradrenaline.
Neutrophils
Fast-acting phagocytes that flock to the site of inflammation. A type of white blood cell.
Niche

The specific role and position an organism occupies within its ecosystem. 

Nitrogen cycle

A set of processes in which nitrogen – in various states – moves between the atmosphere, soil and organisms. The flow of nitrogen through the cycle is largely driven by bacteria.

Nodes of Ranvier
Areas of the axon without myelin. The action potential (electrical signal) jumps rapidly from one node of Ranvier to the next, speeding up conduction.
Non-polar

Any molecule in which the bonding electrons are distributed relatively evenly. Non-polar molecules show little reactivity and do not dissolve in water (they are hydrophobic). The fatty-acid ‘tails’ of phospholipids found in the lipid bilayer are non-polar.

Non-self
Anything that can be recognised as not belonging to the organism. When a non-self marker on a cell or particle is detected, the immune system launches an attack. Transplanted organs or blood can trigger such an attack.
Non-specific immune system (also innate immune system)

The first and second lines of defence against invading pathogens. It is short-lived and doesn’t involve any memory of previous infections. Found in all multicellular organisms. It includes barriers such as skin or plant cell walls, and processes like inflammation.

Noradrenaline
A neurotransmitter that influences emotions, sleeping and learning. It is also a hormone, and affects blood vessel contraction and heart rate.
Nuclear envelope
A double membrane that separates the contents of the nucleus from the cytoplasm.
Nuclear pores
Gaps in the nuclear envelope that allow substances to move in and out of the nucleus.
Nucleic acid
A polymer of nucleotides found in all living things. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are well-known examples.
Nucleolus
The part of the nucleus that produces ribosomes.
Nucleotide
A subunit of nucleic acids that includes a nitrogen base, a five-carbon sugar (ribose in the case of RNA and deoxyribose in the case of DNA) and one or more phosphate groups.
Nucleus
Where almost all the DNA is found.
Nucleus accumbens
A small region in the forebrain with ancient evolutionary origins, which helps regulate survival drives like food and thirst.
Nutraceutical
A combination of the words ‘nutrition’ and ‘pharmaceutical’, this term describes foods or food products that are believed by some people to be medically beneficial, although opinion is divided as to whether they really are. Examples include margarines supplemented with processed plant esters (stanols), which claim to lower consumers’ cholesterol levels.

O

Obesity

A condition in which a person is very overweight, with a high proportion of body fat. Obesity has been linked to many health problems.

Obligate anaerobes
Organisms that are killed in the presence of oxygen in its concentration on Earth, which is approximately 21 per cent of the atmosphere. They are ‘obligated’ not to have oxygen and live in environments without it, such as deep-sea hydrothermal vents.
Obligate parasite

An organism that can only complete its life cycle by parasitically using a host.

Occipital lobe
The part of the brain that manages vision, containing dozens of areas that are specialised for processing inputs from the eyes.
Ocean conveyor/thermohaline circulation/meridional overturning circulation

Global-scale ocean currents – including the Gulf Stream, which keeps the UK’s climate relatively mild. Although some have suggested these currents could disappear as a result of climate change, drastically lowering temperatures in the UK, this is currently thought to be unlikely.

OILRIG
A mnemonic for redox reactions – oxidation is loss (of electrons or hydrogen), reduction is gain (of electrons or hydrogen).
Olfactory bulb

A neural structure in the forebrain involved in the sense of smell.

Omnivore

An individual (animal or human) who eats animals (prey) as well as plants.

Oncology
The branch of medicine that deals with cancer, including diagnosis, therapy, care and screening.
Oocyte

An immature (undeveloped) female sex cell (egg).

Opioids

Compounds that are chemically similar to opium, and are often used as pain-relief medication.

Organ

A group of tissues that act together to perform a function.

Organelles

Intracellular structures that have a specialised function, for example chloroplasts and mitochondria.

Organoid

A cell/tissue culture method that aims to mimic organ structure and function in vitro.

Osmosis

The movement of water from an area of high water concentration to an area of low water concentration (ie with a concentration gradient), across a partially permeable membrane. This process does not require energy.

Osteoporosis

A disease in which the bones are weakened, as old bone is broken down by the body at a faster rate than new bone is created. The risk of osteoporosis increases with age, but it can also be caused by vitamin D deficiency and other medical conditions. 

Oxidation
The loss of electrons or hydrogen (called dehydrogenation), or the gain of oxygen by a molecule, atom or ion.
Oxidative phosphorylation

A process in which mitochondria make ATP using the energy released when electrons are transported from reduced NAD and FAD to oxygen in the electron transport chain of cellular respiration.

Oxyhaemoglobin

The substance formed when haemoglobin has bound to oxygen in the blood.

P

Palm oil

A vegetable oil obtained from the fruit of the oil palm, mostly the African oil palm Elaeis guineensis. It is used as a fat in many different foodstuffs and cosmetics.

Pandemic
A large-scale epidemic affecting many countries.
Parasitism
A type of interaction between species (symbiosis) in which one species (the parasite) benefits at the expense of the other species (the host).
Parietal lobe
The part of the brain that processes information from the body and senses, and integrates it to help orient the body and carry out movement in space.
Parkinson’s disease

A neurodegenerative disease caused by a loss of nerve cells in a part of the brain called the substantia nigra, leading to a reduction in the amount of dopamine in the brain.

Passive immunity
Immunity that comes from antibodies from outside of the organism – for example, those made in the laboratory, or those that cross the placenta between a pregnant woman and a fetus. Short-lived, as memory cells are not made.
Pathogen
A disease-causing organism.
Peptides
Short chains of amino acid molecules that join together, via peptide bonds, to create proteins.
Perforin
A protein that causes cells to lyse (burst) by causing pores to form in their plasma membrane. Found in the granules of natural killer cells and cytotoxic T cells.
Peripheral membrane protein

A protein that temporarily associates with the membrane, either directly with lipids or with integral membrane proteins.

Peripheral nervous system (PNS)

Any part of the nervous system outside of the brain and spinal cord. The PNS connects the central nervous system (CNS) to different parts of the body, enabling the CNS to control them. 

Peroxisomal membrane

Membrane of a cytoplasmic organelle specialised for carrying out oxidative reactions.

Phagocyte

Cells that are capable of engulfing and absorbing materials including waste products, microorganisms and foreign bodies.

Phagocytosis

When a living cell engulfs another cell, bacterium or other particle. Performed by phagocytes such as macrophages, neutrophils and eosinophils.

Pharmaceutical company
A large company researching and making new drugs (collectively often called ‘pharma’).
Pharming

A genetic engineering technique in which genes are modified (or inserted) in cells to make them produce pharmaceuticals.

Phenotype
The physical traits of an individual (e.g. having red hair), which are dependent on an individual’s genotype and interactions with their environment. Compare with genotype.
Phenylketonuria
Also known as PKU, this is a genetic disorder in which the enzyme responsible for metabolising the essential amino acid phenylalanine into tyrosine is inactive. This leads to a build-up of phenylalanine, which can cause brain damage. Sufferers need to eat a very strict low-protein diet to prevent excess phenylalanine being consumed.
Phospholipid

A type of lipid with a hydrophilic head made of a glycerol molecule and phosphate group, and two hydrophobic fatty-acid tails. Biological membranes contain a double layer of phospholipid molecules, called the lipid bilayer.

Phosphorylation

The process of a phosphate group being added to a molecule.

Photolysis

A chemical reaction in which a compound is broken down using light energy. Photosystem II uses water photolysis to replace the electrons emitted from its reaction centre.

Photosynthesis
The process used by green plants and some other organisms to form nutrients from carbon dioxide and water, using sunlight as an energy source.
Photosystem I (PSI)

The second photosystem in the electron transport chain of the light-dependent reactions of photosynthesis (but named because it was discovered first).

Photosystem II (PSII)

The first photosystem in the electron transport chain of the light-dependent reactions of photosynthesis (but named because it was discovered second). It uses light energy to emit excited electrons sourced via photolysis of water.

Phototropism

Movement or growth in plants in response to light.

Phytochemicals

Chemicals that occur naturally in plants, such as stanols, sterols, polyphenols and carotenoids. 

Phytoplankton

Microscopic photosynthesising organisms that live in the upper, sunlit areas of bodies of water.

Pituitary gland
The ‘master gland’ of the body, which releases hormones that control growth, blood pressure, the stress response and the function of the sex organs.
Placebo
A dummy treatment used during clinical trials.
Planar polarity

Planar cell polarity refers to the spatial organisation of cells, which often allows them to carry out specialised functions.

Plant growth factors

The name given to the hormones in plants that control growth and development, such as auxins, cytokinins, gibberellins, ethane and abscisic acid.

Plant sterols

Natural steroid compounds found in plants that are thought to work in several ways, including by lowering cholesterol absorption by the gut. They can be consumed in fruit, vegetables and other plant foods, but people with high cholesterol sometimes consume fortified spreads, yoghurts and milk as well.

Plaques

Made up of fats and cholesterol as well as calcium and blood clotting agents, plaques are lesions in the arterial wall which can cause obstruction, resulting in increased blood pressure and cardiovascular disorders such as atherosclerosis. 

Plasma B cells
B cells that have been activated to produce antibodies. One B cell makes only one type of antibody. A type of lymphocyte.
Plasma membrane
A phospholipid bilayer that contains cholesterol and proteins. It surrounds the cell and enables it to communicate with its neighbours and detect and respond to changes in the environment.
Plasmodesmata

A cell junction involved in connecting the endoplasmic reticula of two plant cells, allowing direct communication.

Pluripotent

The ability to give rise to all cell types of the body (but not placental cells).

Polar

Any molecule that has a partial positive charge at one side and a partial negative charge at the other. Polar molecules react with water (they are hydrophilic). The glycerol and phosphate heads of the phospholipids found in the lipid bilayer are polar.

Polyphenols

Phytochemicals made up of multiple phenol groups. They are responsible for a wide range of plant functions, including growth, reproduction, pigmentation and resistance to pathogens.

Polyploidy
The presence of multiple sets of chromosomes in a cell.
Polysaccharides
Carbohydrates consisting of long chains of sugar molecules.
Polyunsaturated fats

Fat molecules that contain two or more double bonds between their carbon atoms.

Population
All the members of a single species living together in the same location or habitat.
Population genetics

The study of the genes of a population, including both the frequency and interaction of genes and their variants (their alleles).

Positive selection

Selection of alleles that are likely to have provided an advantage to organisms.

Postdoctoral (postdoc)
A label used to describe the period after a doctorate, and often used to describe a paid research job someone can do after completing a PhD. Both the positions and the people that do them can be called postdocs.
Postgraduate qualifications
Qualifications that require someone to have a Bachelor’s degree. They include postgraduate certificates, postgraduate diplomas, Master’s degrees and doctorates.
Precursor
A chemical compound that precedes another in a series of chemical reactions. L-DOPA is the precursor of dopamine and noradrenaline, and cholesterol is a precursor of bile acids and steroid hormones.
Prefrontal cortex
A part of the frontal lobe of the brain that is involved in many cognitive functions, including memory, language, planning and decision making.
Primary structure
The amino acids that make up a protein or polypeptide chain, linked by peptide bonds.
Prokaryote

A single-celled organisms that lacks any membrane-bound organelles. Prokaryotic organisms include two domains of life, bacteria and archaea.

Proliferation

Rapid multiplication of similar types of cell.

Promoter DNA

A region of DNA that initiates transcription of a particular gene.

Prospective studies
Studies that look forward in time.
Protein

A large molecule, made of amino acids, that is encoded by a gene or genes and that performs a specialised job in the cell.

Proteoglycans

Proteins bound to polysaccharide groups, often found in connective tissue.

Protist

A type of eukaryotic and (usually) unicellular microscopic organism. Once a very broad classification, today the term refers to protozoans, unicellular algae and simple fungi.  

Protozoa

A type of unicellular eukaryotic organism. An example is Plasmodium falciparum, which causes malaria in humans.

Pseudogene
A disrupted, usually non-functional version of a gene.
Puberty

The period during which humans reach sexual maturity. 

Pyruvate
An organic acid that is the end product of glycolysis, the first step in cellular respiration.

Q

Quaternary structure
The way that different polypeptide chains are arranged if a protein has more than one subunit.

R

Radiation
The transmission of energy in the form of waves through space. Radio waves, visible light and microwaves are examples of non-ionising radiation, which is not considered harmful to human health, while X-rays, gamma rays and cosmic rays are examples of ionising radiation.
Radioactive

The ability of a molecule or material to emit ionising radiation or particles.

Random sampling
A technique used to select a sample to represent a population. Individuals are chosen from the population at random, for example by using randomly generated numbers.
Reaction centres

Complexes of proteins, each containing a special pair of chlorophyll molecules, that sit at the heart of photosystems. Regular chlorophyll molecules channel light energy to these special chlorophyll pairs (called P680 in photosystem II, P700 in photosystem I), which then, when sufficiently excited, shed a highly energised electron.

Receptor
A protein or assembly of several proteins embedded in the cell membrane that a molecule (such as a neurotransmitter, hormone or drug) can bind to.
Redox reaction
A reaction in which one substance is reduced and another is oxidised.
Reduction

The gain of electrons or hydrogen, or the loss of oxygen, by a molecule, atom or ion; the donation of an electron to a molecule.

Regulatory T cells
Potentially immunosuppressive cells that guard against reactions to ‘self’ and dampen down excessive immune responses. A type of lymphocyte
Rejection

Usually following transplant, a failure of the body to accept the introduced object, most often because the immune system sees it as foreign.

Relapse

In relation to drug abuse, when an individual has not used a drug for a period of time but then returns to using it.

Researcher
A general term for someone paid to do research. They may work at a university or for a private company – such as those in the pharmaceutical industry – that develops drugs and therapies for disease. There are different career stages for researchers, such as being a lecturer, professor or fellow.
Resistance (drug)
The ability of a pathogen (or cancer cell) to survive in the presence of a drug that normally kills it.
Resolution

The smallest distance between two points of a specimen image that can be distinguished; a measure of how much detail can be seen.

Respiration

The chemical process of releasing energy from organic compounds. It is a series of enzyme-controlled reactions in which energy is transferred to produce ATP from ADP and inorganic phosphate (Pi).

Retrospective studies
Studies that look back in time.
Retrovirus

A retrovirus contains RNA as its genetic material, rather than DNA. HIV (human immunodeficiency virus) is an example of a retrovirus.

Ribosome
A molecular machine, made of ribosomal RNA and proteins, with a large and small subunit. This is where translation occurs. Ribosomes are found in the cytoplasm and are bound to the rough endoplasmic reticulum.
RNA
Ribonucleic acid. RNA is made up of four chemical units, or bases: A, U, G and C.
RNA world
A stage in the evolution of pre-cellular life dominated by self-replicating RNA molecules.
RuBisCO

Ribulose bisphosphate carboxylase oxidase; an enzyme that catalyses a reaction between carbon dioxide and ribulose bisphosphate (RuBP).

S

Saturated fat
Fat molecules that contain no double bonds between their carbon atoms. Saturated fat is found in large amounts in foods like butter and lard. A diet high in saturated fats can increase the risk of cardiovascular disease.
Second messenger

A molecule that is created intracellularly in response to the binding of a first messenger to a cell membrane receptor. The second messenger is responsible for initiating intracellular responses.

Secondary metabolites

Organic compounds produced by an organism that are not directly involved in its growth, development or reproduction.

Secondary structure

The shape that a chain of amino acids takes. It includes ‘elements’ such as helices and beta-pleated sheets, depending on the type of protein.

Self
How an organism’s own cells are described. Proteins called major histocompatibility complex (MHC) proteins are found on the surface of cells to indicate that they are ‘self’ and should be left alone by the immune response.
Semi-permeable

Refers to a material or membrane’s ability to allow some substances through but prevent the movement of others.

Serotonin
Made in the central nervous system, but mostly found in the gut, this neurotransmitter is involved in controlling body temperature, mood, appetite and sleep. Drugs to increase serotonin levels are used to treat depression.
Sexual reproduction
When an offspring’s genes come from two parents.
Signal transduction cascade

A series of intracellular events that starts with the binding of a first messenger, causing an increase in levels of the second messenger that causes multiple downstream effects. It is called a ‘cascade’ because the binding of one first messenger can create multiple molecules of the second messenger and bring about multiple effects on the cell.

Single nucleotide polymorphism (SNP)
A genetic variation based on a single ‘letter’ difference in the genetic code.
Soluble fibre
Found in foods including fruit, vegetables and oats, this fibre absorbs water, turning into a gel-like substance that slows digestion.
Solute

Any substance(s) dissolved in a solvent.

Somatic cells

All cells of the body – including cells of the organs, skin, bones and blood – that are not gametes (sex cells) are somatic cells.

Somatic nervous system

Part of the peripheral nervous system, the somatic nervous system is made up of nerves that allow the central nervous system to control the voluntary movement of skeletal muscles.

Specialisation

Structural adaptions to a cell that allow it to perform a specific function.

Species
A group of living organisms that are able to reproduce and create fertile offspring.
Specific immune system (also adaptive immune system)
In mammals, this provides long-lasting protection against specific foreign substances. Helper T cells stimulate plasma B cells to produce antibodies. Memory B cells maintain a ‘memory’ of previous infections the organism has fought.
Spinal cord
A large bundle of millions of nerve fibres, which carries information back and forth between the brain and the body.
Stanols

A class of organic chemicals that occur naturally in plants. Plant stanols are known to block the absorption of cholesterol.

Star
An incandescent ball of hydrogen and helium that generates its own light and is held together by its own gravity. The gravity of some stars is so great that it can pull planets into orbit around them (such as the Sun with the Earth).
Stem cell

An undifferentiated cell capable of producing more stem cells, which can give rise to other cell types through differentiation.

Steroid

A chemical substance that occurs naturally in the body and can also be made. Naturally occurring steroids include the steroid hormones testosterone, oestrogen and aldosterone.

Sterols

A class of organic chemicals that occur naturally in plants, animals and fungi. The most well-known is the animal sterol cholesterol. Plant sterols are known to block the absorption of cholesterol.

Stomata

Pores in plant stems and leaves that can be opened and closed to control gas exchange and water loss.

Stratosphere

The layer above the troposphere. Ozone in the stratosphere is protective, as it absorbs ultraviolet radiation from the Sun.

Stroke

A condition where blood flow to an area of the brain is cut off, causing brain cells to die as they are deprived of oxygen and glucose.

Sub-Saharan
A term used to describe the African regions south of the Sahara desert.
Substantia nigra
Located in the brain, the ‘black substance’ contains cells that produce the neurotransmitter dopamine and the pigment melatonin, giving it a black appearance.
Substrate-level phosphorylation
A process that makes ATP by donating a phosphate group from an intermediate directly to ADP, for example during glycolysis.
Sucrose

A disaccharide sugar composed of two monosaccharide sugars: glucose and fructose.

Sunspots
Regions of relatively low temperature on the surface of the Sun. Sunspots rise and fall in cycles lasting about 11 years; their numbers also vary over long timeframes. Sunspots seem to affect the Earth’s climate – not because less solar energy hits the Earth (sunspots affect less than 0.5 per cent of the Sun’s output), but possibly through their impact on cosmic rays and cloud cover. Sunspot activity is factored into models of climate change, and its effects are thought to be small compared with those of greenhouse gases.
Symbiosis
Close interaction between two or more different species, which includes both mutualism and parasitism.
Synaesthesia
A neurological condition that can be described as a ‘union of the senses’. Synaesthetes might associate particular colours with letters or taste specific flavours when they hear certain words.
Synapse

The junction between neurons. When a nerve signal travelling along an axon reaches a synapse, it triggers the release of a neurotransmitter that diffuses across the synaptic gap and binds to receptor proteins on the surface of the receiving neuron. This binding causes an influx of ions, changing the membrane voltage and initiating an electrical signal in the second neuron.

Synaptic junction

A specialised junction, usually between two neurons, that allows cell-to-cell communication.

Systematic random sampling
A technique used to select a sample to represent a population. Selection is often done using a sampling frame (a list of all those within a population that can be sampled) and a random number table.

T

T cells
Named after the thymus, the organ where T cells mature. T cells have a protein called the T cell receptor on their surface. Types include helper T cells, cytotoxic T cells and regulatory T cells. A type of lymphocyte.
Target
The molecule that a drug acts on.
Telomere

Telomeres are located at the end of eukaryotic chromosomes and act as protective caps.

Temporal lobe
The part of the brain that contains specialised areas for processing sound, language and memory. It is involved in cognitive functions such as mood, appetite, sleep and learning.
Tertiary structure
The unique three-dimensional shape that a protein takes. The shape is held together by interactions between the side chains of amino acids, including hydrogen bonds, disulphide bonds, ionic and hydrophobic interactions, and van der Waals forces.
Testosterone

A steroid hormone, and the principal male sex hormone. It is produced by the testes in men and by the ovaries, in much smaller quantities, in women. It initiates the development of male reproductive organs during early development and stimulates sperm production in adult males.

Thermogenesis

The production of heat in the body. 

Thigmotropism

Movement or growth in plants in response to touch. 

Thymus
A gland found behind the breastbone. It is where T cells mature.
Tissue(s)

A collection of similar cells that work towards a shared function.

Tolerance

The decrease in a person’s reaction to a drug that may come with prolonged use of it – requiring an increase in dose to achieve the same effect.

Totipotent

A term used to describe cells that can divide and produce all of the other cells of an organism (as well as placental cells). Cells with this ability are very rare in animals (occurring only at the zygote stage), but are common in plants.

Trans

A molecular arrangement in which two particular atoms or groups are found on the opposite side of a double bond. For example, in a particular unsaturated fatty acid, the hydrogens can be on the opposite sides of the carbon–carbon double bond (trans) or on the same side (cis).

Trans fat

A certain way of processing vegetable oils produces what are known as trans fats, or partially hydrogenated oils, which extend the shelf-life of foods they are added to. Trans fats have some carbon–carbon double bonds in the trans position. Eating a diet high in trans fats is linked to cause heart disease, diabetes and stroke. Trans fats occur naturally at low levels in some meat and dairy products.

Transcription

The process in which a length of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase.

Transcription factors
Proteins involved in the process of transcription – often activating this process.
Translation
The process of using the genetic information in mRNA to make a protein chain from amino acids. This is performed by ribosomes.
Triglyceride

An unreactive lipid consisting of one glycerol molecule bonded to three fatty-acid molecules. When dietary fat is digested, triglycerides are produced and stored in adipocytes (fat cells).

tRNA (transfer RNA)
The key adaptor molecules needed for translation. Each tRNA molecule recognises a particular codon and ensures the correct amino acid is brought to the ribosome’s active site for protein synthesis.
Tropomyosin

A key protein in muscle contraction. It prevents contraction by binding to actin, stopping it from binding to myosin.

Troponin

A globular protein which moves tropomyosin to allow binding between actin and myosin in muscle contraction.

Troposphere
The layer of the Earth’s atmosphere closest to the ground, where greenhouse gases are found. Ozone in the troposphere contributes significantly to global warming.
Tumour

An abnormal mass of tissue that occurs because of uncontrolled cell division. Tumours can be benign (not cancerous) or malignant (cancerous).

Tumour necrosis factor (TNF)
A chemical made by macrophages and some other immune cells. It has an important role in the immune system, including inducing fever, inflammation and programmed cell death.
Turgor pressure

The pressure inside cells with cell walls (such as plant cells) that pushes the plasma membrane up to and against the cell wall.

Twin studies
Studies looking at non-identical and identical twins, raised together or apart, to determine the environmental and genetic factors involved in their specific development.

U

Undergraduate qualification
A course that leads to a Bachelor’s degree (or equivalent). In England, Wales and Northern Ireland many students start undergraduate degrees at university at 18 after completing A levels or the International Baccalaureate. In Scotland most students start university at 17 and complete an extra year. Someone studying at this level can be known as an undergrad.
Unicellular

An organism that consists of only one cell.

Unipotent

The ability to give rise to only one cell type.

Unsaturated fat
Fat molecules that contain one or more double bonds between their carbon atoms. Although they are considered healthier than saturated fats, they should still be eaten in moderation.
Up-regulation

The increase in production of a chemical in the body, often in response to a specific stimulus. For example, histamine is up-regulated when you are stung by a bee.

V

Vaccination

Giving antigens that are dead, or alive but weakened, to a person to trigger an immune response that will provide active immunity. Vaccinations sometimes contain the products of the pathogen (e.g. a toxin produced by a bacterium), rather than the pathogen itself.

Vacuoles
Internal bags, surrounded by a membrane, that cells use for storage of food or waste.
Vasodilation
A widening of the lumen (the internal space) of blood vessels.
Vector

An organism that does not cause disease directly, but which passes pathogens from one host to another. 

Ventral tegmental area
Found in the midbrain, this area produces dopamine and forms part of one of four major dopamine pathways in the brain.
Virion
A virus particle. It includes the virus’s genetic material (DNA or RNA) and a protein coat.
Virulent

Causing severe illness. The term is not only used in relation to viruses, but also to bacteria and other parasites.

Virus
A small infectious particle with a single type of nucleic acid (either DNA or RNA), which is usually housed within a protein shell. Viruses can replicate only within the living cells of other organisms. A single virus particle is called a virion.
Vitamin

A substance that enables healthy growth and development. We must consume vitamins, in limited amounts, because the body cannot produce them naturally. Each vitamin is soluble either in water or in fat.

Vitamin A

A fat-soluble vitamin also known as retinol. It is essential for growth, vision in dim light, and the maintenance of soft mucous tissue. It is found in milk products, egg yolk, liver and some vegetables, including carrots and cabbage.

Vitamin D
A fat-soluble vitamin that helps regulate the amount of calcium and phosphorus in the body. It is needed to keep bones and teeth healthy. Vitamin D deficiency can cause the disease rickets in children. Food sources include liver, eggs and fish oil, but most vitamin D is produced by the skin following small bursts of direct exposure to sunlight.
Vitamin E

A fat-soluble vitamin that acts as an antioxidant. It also maintains healthy eyes and skin and strengthens the immune system. Vitamin E is fairly widely eaten, so a deficiency is unlikely to occur. Particularly good food sources are vegetable oil, nuts, eggs, butter and wholemeal cereal.

VO2 max

A measurement of an individual’s aerobic capacity; the maximum rate at which the heart, lungs and muscles can use oxygen.

W

White blood cells (also leucocytes or leukocytes)
Made in the bone marrow, these cells are an important part of the immune system. There are two main types: granulocytes, which have granular cytoplasm and a lobed nucleus, and agranulocytes, which have smooth cytoplasm and a non-lobed nucleus.
White fat

A type of adipose (fat) tissue, also known as white adipose tissue (WAT). White fat serves three functions: heat insulation, mechanical cushioning and, most importantly, a source of energy.

Withdrawal

The symptoms that occur after an individual who has a physical or psychological dependence on a medical or recreational drug stops taking it. Symptoms can included anxiety, sweating, nausea, insomnia and fatigue.

Y

Y chromosome
A small sex chromosome normally found only in male cells in humans.

Z

Zoonosis
The transmission of an infectious disease from animals to humans.
Zygote

A fertilised ovum, created by the union of the nuclei from an ovum (egg) and a sperm cell.