SQA Advanced Higher Biology
Biology: cells and proteins
4 Detecting and amplifying an environmental stimulus
In archaea, bacteriorhodopsin molecules generate potential differences by absorbing light to pump protons across the membrane. In plants, the light absorbed by photosynthetic pigments drives an electron flow that pumps hydrogen ions across the thylakoid membrane of the chloroplast.
Biology: organisms and evolution
Identification and taxonomy
AQA GCE Biology
3.1 Biological molecules
…has strong cohesion between water molecules; this supports columns of water in the tube-like transport cells of plants and produces surface tension where water meets air.
3.2.1 Cell structure
184.108.40.206 Structure of eukaryotic cells
- structure and function of chloroplasts
- cell wall
3.3 Organisms exchange substances with their environment
3.3.2 Gas exchange
Adaptations of gas exchange surfaces, shown by gas exchange:
- by the leaves of dicotyledonous plants (mesophyll and stomata).
Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants.
220.127.116.11 Mass transport in plants
Xylem as the tissue that transports water in the stem and leaves of plants. The cohesion-tension theory of water transport in the xylem. Phloem as the tissue that transports organic substances in plants. The mass flow hypothesis for the mechanism of translocation in plants. The use of tracers and ringing experiments to investigate transport in plants.
3.5 Energy transfers in and between organisms (A-level only)
3.5.1 Photosynthesis (A-level only)
The light-dependent reaction in such detail as to show that:
- chlorophyll absorbs light, leading to photoionisation of chlorophyll
- some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP
- the production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory)
- photolysis of water produces protons, electrons and oxygen.
The light-independent reaction uses reduced NADP from the light-dependent reaction to form a simple sugar. The hydrolysis of ATP, also from the light-dependent reaction, provides the additional energy for this reaction.
The light-independent reaction in such detail as to show that:
- carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco
- ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate
- some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
- some of the triose phosphate is converted to useful organic substances.
Students should be able to:
- identify environmental factors that limit the rate of photosynthesis
- evaluate data relating to common agricultural practices used to overcome the effect of these limiting factors.
Students could devise and carry out experiments to investigate the effect of named environmental variables on the rate of photosynthesis using aquatic plants, algae or immobilised algal beads.
Required practical 7: Use of chromatography to investigate the pigments isolated from leaves of different plants, eg, leaves from shade-tolerant and shade-intolerant plants or leaves of different colours.
Required practical 8: Investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts.
3.5.3 Energy and ecosystems (A-level only)
In any ecosystem, plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide. Most of the sugars synthesised by plants are used by the plant as respiratory substrates. The rest are used to make other groups of biological molecules. These biological molecules form the biomass of the plants.
Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area per given time. The chemical energy store in dry biomass can be estimated using calorimetry.
Gross primary production (GPP) is the chemical energy store in plant biomass, in a given area or volume, in a given time.
Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account, ie NPP = GPP – R where GPP represents gross productivity and R represents respiratory losses to the environment.
This net primary production is available for plant growth and reproduction. It is also available to other trophic levels in the ecosystem, such as herbivores and decomposers.
The net production of consumers (N), such as animals, can be calculated as:
N = I – F + R
where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment.
Students should be able to appreciate the ways in which productivity is affected by farming practices designed to increase the efficiency of energy transfer by:
- simplifying food webs to reduce energy losses to non-human food chains
- reducing respiratory losses within a human food chain.
3.5.4 Nutrient cycles (A-level only)
The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants.
The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock.
3.6 Organisms respond to changes in their internal and external environments (A-level only)
Plants control their response using hormone-like growth substances.
3.6.1 Stimuli, both internal and external, are detected and lead to a response (A-level only)
18.104.22.168 Survival and response (A-level only)
Organisms increase their chance of survival by responding to changes in their environment.
In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.
The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.
Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment.
Edexcel GCE Biology
1 Biological molecules
1.6 Inorganic ions
i Understand the role in plants of:
- nitrate ions – to make DNA and amino acids
- calcium ions – to form calcium pectate for the middle lamellae
- magnesium ions – to produce chlorophyll
- phosphate ions – to make ADP and ATP.
2 Cells, viruses and reproduction of living things
2.5 Sexual reproduction in plants
i Understand how a pollen grain forms in the anther and the embryo sac forms in the ovule.
ii Understand how the male nuclei formed by division of the generative nucleus in the pollen grain reach the embryo sac, including the roles of the tube nucleus, pollen tube and enzymes.
iii Understand the process of double fertilisation inside the embryo sac to form a triploid endosperm and a zygote.
4 Exchange and transport
4.3 Gas exchange
ii Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces in the leaf and lenticels.
4.7 Transport in plants
i Understand the structure of xylem and phloem tissues in relation to their role in transport.
ii Understand how water can be moved through plant cells by the apoplastic and symplastic pathways.
iii Understand how the cohesion-tension model explains the transport of water from plant roots to shoots.
iv Understand how temperature, light, humidity and movement of air affect the rate of transpiration.
v Understand the strengths and weaknesses of the mass-flow hypothesis in explaining the movement of sugars through phloem tissue.
5 Energy for biological processes
5.5 Anaerobic respiration
iv Understand how anaerobic respiration in plants results in ethanol formation.
5.6 Photosynthetic pigments
i Understand what is meant by absorption and action spectra.
ii Understand why many plants have a variety of different photosynthetic pigments.
9 Control systems
9.3 Chemical control in plants
i Understand that chemical control in plants is brought about by plant growth substances such as auxins, cytokinins and gibberellins.
Core practical 14: Investigate the effect of gibberellin on the production of amylase in germinating cereals using a starch agar assay.
ii Know that auxin has several effects, including cell elongation, suppression of lateral buds (apical dominance) and promoting root growth.
iii Understand that plant growth substances often interact with each other as shown by the antagonistic actions of cytokinin and auxin on apical dominance.
iv Understand how phytochrome controls flowering and photomorphogenesis.
OCR GCE Biology
3 Exchange and transport
3.1.3 Transport in plants
(a) the need for transport systems in multicellular plants
(i) the structure and function of the vascular system in the roots, stems and leaves of herbaceous dicotyledonous plants
(ii) the examination and drawing of stained sections of plant tissue to show the distribution of xylem and phloem
(iii) the dissection of stems, both longitudinally and transversely, and their examination to demonstrate the position and structure of xylem vessels
(i) the process of transpiration and the environmental factors that affect transpiration rate
(ii) practical investigations to estimate transpiration rates
(d) the transport of water into the plant, through the plant and to the air surrounding the leaves
(e) adaptations of plants to the availability of water in their environment
(f) the mechanism of translocation.
4 Biodiversity, evolution and disease
4.1.1 Communicable diseases, disease prevention and the immune system
(a) the different types of pathogen that can cause communicable diseases in plants and animals
(b) the means of transmission of animal and plant communicable pathogens
(c) plant defences against pathogens
4.2.2 Classification and evolution
(f) the different types of variation
5 Communication, homeostasis and energy
5.1.5 Plant and animal responses
(i) the types of plant responses
(ii) practical investigations into phototropism and geotropism
(b) the roles of plant hormones
(c) the experimental evidence for the role of auxins in the control of apical dominance
(d) the experimental evidence for the role of gibberellin in the control of stem elongation and seed germination
(e) practical investigations into the effect of plant hormones on growth
(f) the commercial use of plant hormones
6 Genetics, evolution and inheritance
6.2.1 Cloning and biotechnology
(i) natural clones in plants and the production of natural clones for use in horticulture
(ii) how to take plant cuttings as an example of a simple cloning technique
(i) the production of artificial clones of plants by micropropagation and tissue culture
(ii) the arguments for and against artificial cloning in plants
WJEC GCE Biology
Biodiversity and physiology of body systems
2 Adaptations for gas exchange
(j) the structure of the angiosperm leaf
(k) the role of leaf structures in allowing the plant to function and photosynthesise effectively
(l) the role of the leaf as an organ of gaseous exchange, including stomatal opening and closing
Energy, homeostasis and the environment
2 Photosynthesis uses light energy to synthesise organic molecules
(a) the distribution of chloroplasts in relation to light trapping
(b) chloroplasts acting as transducers converting the energy of light photons into the chemical energy of ATP
(c) the process of light harvesting and the absorption of various wavelengths of light by chlorophyll and associated pigments and the energy transfer to reaction centres
(d) the basic features of Photosystems I and II
(e) cyclic and non-cyclic photophosphorylation as sources of electrons for the electron transport chain
(f) photolysis as a source of electrons for Photosystem II
(g) the reduction of NADP by the addition of electrons and hydrogen ions in the stroma maintaining the proton gradient
(h) reduced NADP as a source of reducing power and ATP as a source of energy for the following reactions: the light-independent stage and the formation of glucose; uptake of carbon dioxide by ribulose bisphosphate to form glycerate-3-phosphate catalysed by Rubisco
(i) the reduction of glycerate-3-phosphate to produce triose phosphate (carbohydrate) with the regeneration of ribulose bisphosphate
(j) the production of other carbohydrates, lipids and amino acids from the triose phosphate (no details of the chemistry of these processes is needed)
(k) the concept of limiting factors in relation to photosynthesis
(l) the role of inorganic nutrients in plant metabolism as illustrated by the use of nitrogen and magnesium
Variation, inheritance and options
2 Sexual reproduction in plants
(a) the generalised structure of flowers to be able to compare wind and insect pollinated flowers
(b) the development of pollen and ovules, including examination of prepared slides of anther and ovary
(c) cross and self-pollination
(d) the process of double fertilisation
(e) the formation and structure of seed and fruit as shown by broad bean and maize
(f) the process of germination of Vicia faba (broad bean)
(g) the effect of gibberellin
International Baccalaureate Diploma: Biology
- Photosynthesis is the production of carbon compounds in cells using light energy.
- Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.
- Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.
- Oxygen is produced in photosynthesis from the photolysis of water.
- Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.
- Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis.
- Light-dependent reactions take place in the intermembrane space of the thylakoids.
- Light-independent reactions take place in the stroma.
- Reduced NADP and ATP are produced in the light-dependent reactions.
- Absorption of light by photosystems generates excited electrons.
- Photolysis of water generates electrons for use in the light-dependent reactions.
- Transfer of excited electrons occurs between carriers in thylakoid membranes.
- Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
- ATP synthase in thylakoids generates ATP using the proton gradient.
- Excited electrons from Photosystem I are used to reduce NADP.
- In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate.
- Glycerate 3-phosphate is reduced to triose phosphate using reduced NADP and ATP.
- Triose phosphate is used to regenerate RuBP and produce carbohydrates.
- Ribulose bisphosphate is reformed using ATP.
- The structure of the chloroplast is adapted to its function in photosynthesis.
9.1 Transport in the xylem of plants
Use models as representations of the real world – mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues. (1.10)
- Transpiration is the inevitable consequence of gas exchange in the leaf.
- Plants transport water from the roots to the leaves to replace losses from transpiration.
- The cohesive property of water and the structure of the xylem vessels allow transport under tension.
- The adhesive property of water and evaporation generate tension forces in leaf cell walls.
- Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Applications and skills:
- Application: Adaptations of plants in deserts and in saline soils for water conservation.
- Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
- Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
9.2 Transport in the phloem of plants
Nature of science: Developments in scientific research follow improvements in apparatus – experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available. (1.8)
- Plants transport organic compounds from sources to sinks.
- Incompressibility of water allows transport along hydrostatic pressure gradients.
- Active transport is used to load organic compounds into phloem sieve tubes at the source.
- High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
- Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
Applications and skills:
- Application: Structure–function relationships of phloem sieve tubes.
- Skill: Identification of xylem and phloem in microscope images of stem and root.
- Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
9.3 Growth in plants
Nature of science: Developments in scientific research follow improvements in analysis and deduction – improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression. (1.8)
- Undifferentiated cells in the meristems of plants allow indeterminate growth.
- Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
- Plant hormones control growth in the shoot apex.
- Plant shoots respond to the environment by tropisms.
- Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
- Auxin influences cell growth rates by changing the pattern of gene expression.
Theory of knowledge:
- Plants communicate chemically both internally and externally. To what extent can plants be said to have language?
- Micropropagation is used for rapid bulking up of new varieties of plant.
Nature of science: Paradigm shift – more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species. (2.3)
- Flowering involves a change in gene expression in the shoot apex.
- The switch to flowering is a response to the length of light and dark periods in many plants.
- Success in plant reproduction depends on pollination, fertilization and seed dispersal.
- Most flowering plants use mutualistic relationships with pollinators in sexual reproduction
Applications and skills:
- Application: Methods used to induce short-day plants to flower out of season.
- Skill: Drawing internal structure of seeds.
- Skill: Drawing of half-views of animal-pollinated flowers.
- Skill: Design of experiments to test hypotheses about factors affecting germination.
- Students should understand the differences between pollination, fertilization and seed dispersal but are not required to know the details of each process.
- Flowering in so-called short-day plants such as chrysanthemums, is stimulated by long nights rather than short days.
CEA GCE Biology
AS 2: Organisms and biodiversity
2.1 Transport and exchange mechanisms
(a) The principles of exchange and transport
2.1.1 Understand the relationship between an organism’s size and its surface area to volume ratio:
- surface area as the total number of cells in direct contact with the surrounding environment
- surface area affects the rate of exchange of materials at exchange surfaces
- volume as the total three-dimensional space occupied by metabolically active tissues
- volume of metabolically active tissue influences the demand for metabolites
- surface area influences the rate of supply of metabolites to tissues
- as an organism’s size increases, its surface area increases less than its volume (many cells are not in direct contact with the surrounding environment)
2.1.2 Understand the features of exchange surfaces which aid passive and active transport:
- methods of increasing surface area
- thin separating surface
- concentration gradients
- examples to include leaf mesophyll, root hairs, capillaries, erythrocytes, alveoli
2.1.3 Understand the principle of mass transport:
- the need for mass transport systems in flowering plants and mammals
- examples to include movement in xylem, translocation in phloem, circulation and ventilation in a mammal
(b) Gaseous exchange
2.1.4 Understand factors affecting the rate of gas exchange:
- large surface area for exchange
- moist surface into which gases dissolve
- diffusion gradients for O2 and CO2
- diffusion path
- appreciate the relationship between the factors shown in Fick’s Law
2.1.5 understand gas exchange in plants:
- exchange of gases involving O2 and CO2, and both the processes of respiration and photosynthesis
- net exchange of gases (mostly photosynthetic midday, only respirational at night) and the compensation point
- mesophyll surface representing a large, moist surface area for exchange of gases
- diffusion path: thinness of leaves; air space system through the spongy mesophyll; open stomata during the day facilitating the uptake of CO2 (low CO2 concentration gradient)
(c) Transport in plants and transpiration
2.1.9 Recognise and understand plant tissues in relation to water (and ion) transport and translocation:
- xylem (protoxylem and metaxylem) vessels and their distinctive lignification patterns
- phloem tissue (sieve tube elements and companion cells)
2.1.10 Understand the uptake of water and mineral ions by root hairs:
- water uptake involving osmosis
- ion uptake involving active transport
2.1.11 Understand the apoplast and symplast pathways through plant tissues:
- the apoplast pathway along cellulose cell walls
- the symplast pathway through protoplasts connected by plasmodesmata
- the apoplast and symplast pathways in the root and leaf
- the role of the endodermis in ensuring the symplast pathway into the stele
2.1.12 Understand transpiration and the factors influencing its rate:
- stomata as the main route of transpiration
- the cuticle as a minor alternative route of transpiration
- internal factors to include leaf surface area, stomatal density, cuticle thickness
- external factors to include light intensity (influencing stomatal aperture), air currents, temperature, humidity and soil water availability
2.1.13 Understand the movement of water (and dissolved ions) through xylem:
- cohesion-tension theory
- transpiration creating a negative pressure within leaf xylem vessels resulting in the transpiration stream
- the cohesive and adhesive forces of water
- the root pressure hypothesis
2.1.14 Understand the translocation of organic solutes through phloem:
- involving energy expenditure and two-way flow
- evidence for the above properties (Theories of translocation NOT required)
2.1.15 Understand the structural adaptations of xerophytes and hydrophytes:
- xerophytic adaptations to include leaf curvature, reduced surface area, cuticular thickening, hairs, sunken stomata, succulent tissue, deep roots, spines
- hydrophytic adaptations to include stomata mainly on the upper surface of leaves and a prominent air space system (aerenchyma)
(a) The variety of life
2.3.8 Appreciate the five kingdom system of classification:
The five kingdoms as:
2.3.12 Describe the features of Plantae:
- the plants as autotrophs (producers)
- possessing chlorophyll in chloroplasts
- possessing a cellulose cell wall
2.3.14 Appreciate factors that have an adverse impact on biodiversity:
- removal of trees and hedgerows for agricultural and development purposes results in:
- increased soil erosion caused by wind and rain
- decreased biodiversity due to removal of habitat/food
- neglect/lack of management resulting in hedgerows changing into rows of trees with gaps
- too frequent/badly timed cutting of hedgerows resulting in poor habitat conditions, development of gaps and consequent reduction in species present
- loss of hedgerow trees through senescence and felling without re-planting
- increased use of monocultures
- increased use of pesticides and slurry removes soil organisms that help improve soil structure
- increased use of pesticides may remove natural predators of pest species
- increased use of herbicides reduces plant species diversity (eg removal of arable weeds in crops) and reduces the variety of food available to a variety of animal species, thereby reducing animal species diversity
- increased use of artificial fertilisers:
- increases soil erosion due to loss of soil crumb structure
- nitrogenous fertilisers promotes the growth of some plant species only, which reduces both plant and animal diversity (loss of species-rich grassland)
- drainage schemes followed by ploughing and reseeding of unimproved pasture reduces biodiversity
- increased stocking rates eg of sheep, results in over-grazing and damage to hedgerows
- species extinction impacts adversely on other species eg in a food chain.
2.3.15 Appreciate the need for strategies to encourage biodiversity:
- management of hedgerows as important habitats (providing shelter and food) which support biodiversity:
- timing and frequency of cutting
- height × width of hedgerow
- leaving hedgerow trees
- hedgerows may act as wildlife corridors for many species including small mammals, birds, reptiles, amphibians and insects, allowing dispersal and migration to other habitats
- promote the use of polyculture instead of monoculture
- maintenance of set-aside areas of farmland to support a wide variety of species
- conservation of existing woodland
- importance of planting native species of tree on land with low species diversity
- increased use of organic fertiliser
- increased use of crop rotation and N2-fixing plants to improve soil fertility
- increased use of species-rich hay meadow (with low soil nitrate levels)
- implementation of integrated pest management schemes to decrease the use of pesticides
- use of predator strips (small areas of rough grass left undisturbed at field perimeters) to encourage natural predators for pest control
4.3 Coordination and control
4.3.1 Understand the role of phytochromes in the control of flowering in long-day and short-day plants:
- phytochromes as pigments found in the leaves of flowering plants
- phytochromes occur in two interchangeable forms:
- P660 which absorbs red light and rapidly converts to P730
- P730 which absorbs far-red light to rapidly convert to P660 and slowly converts to P660 in the dark
- concept of a critical length of night (dark period) required to remove P730
- appreciate that removal of P730 is required for short-day plants to flower
- appreciate that non-removal of P730 allows long-day plants to flower
- appreciate that artificial manipulation of the photoperiod (and the consequent effect on the levels of P660/P730 present in the leaves) allows plants to flower out-of-season
4.3.2 Understand the role of plant growth substances (hormones) in stem elongation:
- auxins promote cell elongation
- cytokinins promote cell division
- gibberellins promote elongation of intermodal regions
4.3.3 Understand the role of auxins in phototropism:
- directional light stimulus results in the lateral displacement of auxin to the non-illuminated side of the shoot
- a differential growth response results in positive phototropism
- appreciate the significance of positive phototropism in the shoots of plants
- appreciate (in outline only) the experimental evidence for the role of auxins in phototropism (Darwin’s experiments, Boysen-Jensen’s experiments, Went’s experiments)
5.2.1 Describe the sites in the chloroplast where the reactions of photosynthesis occur:
- light-dependent stage on the thylakoids
- light-independent stage in the stroma
5.2.2 Understand the light-dependent stage of photosynthesis:
- photoactivation of photosystem I (PSI) and photosystem II (PSII) resulting in the passage of electrons from PSII to PSI (the Z-scheme) coupled with the production of ATP (photophosphorylation) (Cyclic photophosphorylation NOT required)
- the final acceptor of PSI electrons as NADP+ (with H+ from the dissociation of water) producing reduced NADP (NADPH)
- the replacement of PSII electrons from hydroxyl ions (OH-) resulting from the dissociation of water with the concomitant release of oxygen
5.2.3 Understand the light-independent stage of photosynthesis:
- CO2 fixation and reduction in a C3 plant in terms of reaction with ribulose bisphosphate (C5) producing two molecules of glycerate phosphate (C3) which is reduced by NADPH to a triose phosphate with the consumption of ATP
- the recycling of 5/6 of the triose phosphate to regenerate ribulose bisphosphate
- the utilisation of the remaining 1/6 in the synthesis of C6 sugars and other compounds (CAM and C4 metabolism NOT required)
5.2.4 Appreciate that light is absorbed by chlorophyll and associated pigments:
- absorption spectra to show peak absorption by different pigments
- action spectrum showing which wavelengths of light promote the optimum rate of photosynthesis
5.2.5 Understand external factors limiting the rate of photosynthesis:
- photosynthesis measured as CO2 – uptake or O2 – production
- gross photosynthesis, net photosynthesis and the compensation point
- light availability, CO2 availability and temperature limiting the rate of photosynthesis
About this resource
This resource was first published in ‘Plants’.