2 Biological molecules
This topic introduces carbohydrates, lipids and proteins: organic molecules that are important in cells. Nucleic
acids, another cl... [Show More] ass of biological molecule, are covered in Topic 6. All of these molecules are based on the
versatile element carbon. This topic explains how carbohydrates, lipids and proteins, which have a great diversity
of function in organisms, are assembled from smaller organic molecules such as glucose, amino acids, glycerol
and fatty acids.
The emphasis in this topic is on the relationship between molecular structures and their functions. Some of these
ideas are continued in other topics, for example, the functions of haemoglobin in gas transport in Transport in
mammals (Topic 8), phospholipids in membranes in Cell membranes and transport (Topic 4) and antibodies in
Immunity (Topic 11).
Life as we know it would not be possible without water. Understanding the properties of this extraordinary
molecule is an essential part of any study of biological molecules. Some of the roles of water are in this topic,
others are in Topics 4, 7, 8, 12, 13 and 14.
2.1 Testing for biological molecules Learning outcomes
Candidates should be able to:
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 Learning outcomes
Candidates should be able to:
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
continued
Learning outcomes
Candidates should be able to:
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 Learning outcomes
Candidates should be able to:
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 Learning outcomes
Candidates should be able to:
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 [Show Less]