2. Biological molecules

2.1 Testing for biological molecules

Tests for biological molecules can be used in a variety of contexts, such as identifying the contents of mixtures of molecules and following the activity of digestive enzymes.

1. describe and carry out the Benedict’s test for reducing sugars, the iodine test for starch, the emulsion test for lipids and the biuret test for proteins

2. describe and carry out a semi-quantitative Benedict’s test on a reducing sugar solution by standardising the test and using the results (time to first colour change or comparison to colour standards) to estimate the concentration

3. describe and carry out a test to identify the presence of non-reducing sugars, using acid hydrolysis and Benedict’s solution

2.2 Carbohydrates and lipids

Carbohydrates and lipids have important roles in the provision and storage of energy and for a variety of other functions such as providing barriers around cells: the phospholipid bilayer of all cell membranes and the cellulose cell walls of plant cells.

1. describe and draw the ring forms of α-glucose and β-glucose

2. define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide

3. state the role of covalent bonds in joining smaller molecules together to form polymers

4. state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar

5. describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides

2.2 Carbohydrates and lipids...

6. describe the breakage of a glycosidic bond in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test

7. describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms

8. describe the molecular structure of the polysaccharide cellulose and outline how the arrangement of cellulose molecules contributes to the function of plant cell walls

9. state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds

10. relate the molecular structure of triglycerides to their functions in living organisms

11. describe the molecular structure of phospholipids with reference to their hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails

2.3 Proteins

An understanding of protein structure and how it is related to function is central to many aspects of biology, such as enzymes, antibodies and muscle contraction. Globular and fibrous proteins play important roles in biological processes such as the transport of gases and providing support for tissues. Water is a special molecule with extraordinary properties that make life possible on this planet 150 million kilometers from the Sun.

1. describe and draw the general structure of an amino acid and the formation and breakage of a peptide bond

2. explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins

3. describe the types of interaction that hold protein molecules in shape: • hydrophobic interactions • hydrogen bonding • ionic bonding • covalent bonding, including disulfide bonds

4. state that globular proteins are generally soluble and have physiological roles and fibrous proteins are generally insoluble and have structural roles

5. describe the structure of a molecule of haemoglobin as an example of a globular protein, including the formation of its quaternary structure from two alpha (α) chains (α–globin), two beta (β) chains (β–globin) and a haem group

6. relate the structure of haemoglobin to its function, including the importance of iron in the haem group

7. describe the structure of a molecule of collagen as an example of a fibrous protein, and the arrangement of collagen molecules to form collagen fibres

8. relate the structures of collagen molecules and collagen fibres to their function

2.4 water

1. explain how hydrogen bonding occurs between water molecules and relate the properties of water to its roles in living organisms, limited to solvent action, high specific heat capacity and latent heat of vaporisation