2.1.2 Biological molecules
The cells of all living organisms are composed of biological molecules. Proteins, carbohydrates and lipids are three of the key groups of biological macromolecules that are essential for life. A study of the structure of these macromolecules allows a better understanding of their functions in living organisms.
- how hydrogen bonding occurs between water molecules, and relate this, and other properties of water, to the roles of water for living organisms
- the concept of monomers and polymers and the importance of condensation and hydrolysis reactions in a range of biological molecules
- the chemical elements that make up biological molecules
- the structure of a triglyceride and a phospholipid as examples of macromolecules
- the synthesis and breakdown of triglycerides by the formation (esterification) and breakage of ester bonds between fatty acids and glycerol
- how the properties of triglyceride, phospholipid and cholesterol molecules relate to their functions in living organisms
- how to carry out and interpret the results of the following chemical tests:
1. biuret test for proteins
2. Benedict’s test for reducing and non-reducing sugars
3. reagent test strips for reducing sugars
4. iodine test for starch
5. emulsion test for lipids
2.1.5 Biological membranes
Membranes are fundamental to the cell theory. The structure of the plasma membrane allows cells to communicate with each other. Understanding this ability to communicate is important as scientists increasingly make use of membrane-bound receptors as sites for the action of medicinal drugs.
- the fluid mosaic model of membrane structure and the roles of its components
- the uses of triose phosphate (TP) [To include the use of TP as a starting material for the synthesis of carbohydrates, lipids and amino acids]
- the difference in relative energy values of carbohydrates, lipids and proteins as respiratory substrates
3.1 Biological molecules
Lipids have many uses, including the bilayer of plasma membranes, certain hormones and as respiratory substrates
- Triglycerides and phospholipids are two groups of lipid.
- Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid.
- A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond.
- The R-group of a fatty acid may be saturated or unsaturated.
- In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.
- The different properties of triglycerides and phospholipids related to their different structures.
- The emulsion test for lipids.
- Students should be able to:
1. recognise, from diagrams, saturated and unsaturated fatty acids
2. explain the different properties of triglycerides and phospholipids.
3.2.3 Transport across cell membranes
The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure. Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane. Movement across membranes occurs by:
- simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)
- facilitated diffusion (involving the roles of carrier proteins and channel proteins)
- osmosis (explained in terms of water potential)
- active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP)
- co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes. Students should be able to:
- explain the adaptations of specialised cells in relation to the rate of transport across their internal and external membranes
- explain how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes.
3.3.3 Digestion and absorption
During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes. Digestion in mammals of:
- carbohydrates by amylases and membrane-bound disaccharidases
- lipids by lipase, including the action of bile salts
- proteins by endopeptidases, exopeptidases and membrane-bound dipeptidases.
Mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, to include:
- co-transport mechanisms for the absorption of amino acids and of monosaccharides
- the role of micelles in the absorption of lipids.
- Understand how a triglyceride is synthesised, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids.
- Know the differences between saturated and unsaturated lipids.
- Understand how the structure of lipids relates to their role in energy storage, waterproofing and insulation.
- Understand how the structure and properties of phospholipids relate to their function in cell membranes.
1. Chemical elements are joined together to form biological compounds
Learners should be able to demonstrate and apply their knowledge and understanding of:
(f) the structure, properties and functions of lipids as illustrated by triglycerides and phospholipids
(g) the implications of saturated and unsaturated fat on human health
(j) the production of other carbohydrates, lipids and amino acids from the triose phosphate (no details of the chemistry of these processes is needed)
(ii) Hydrophobic and hydrophilic interactions influence the location of cellular proteins.
The fluid mosaic model. The role of R groups in determining the distribution of soluble, globular, integral membrane and peripheral membrane proteins.
(c) Membrane proteins
Movement of molecules across membranes.
Phospholipid bilayer as a barrier to ions and most uncharged polar molecules.
(v) Viral structure and replication: Some viruses have a lipid membrane surround derived from host cell materials.
Topic 2 Molecular Biology
2.1 Life is based on carbon compounds including carbohydrates, lipids, proteins and nucleic acids
- Fatty acids can be saturated, monounsaturated or polyunsaturated.
- Unsaturated fatty acids can be cis or trans isomers.
- Triglycerides are formed by condensation from three fatty acids and one glycerol.
Option D Human Physiology
D1. Human nutrition
- Essential nutrients cannot be synthesized by the body, therefore they have to be included in the diet.
- Dietary minerals are essential chemical elements.
- Vitamins are chemically diverse carbon compounds that cannot be synthesized by the body.
- Some fatty acids and some amino acids are essential.
D3. Functions of the liver
- Surplus cholesterol is converted to bile salts
About this resource
This resource was first published in ‘Fat’.