Lipids
1) Describe the chemical reactions involved in the assembly and
break down of triglycerides in living organisms (8 marks)
Triglycerides are made from a glycerol molecule (1) and three molecules
of fatty acids (1). They are formed by condensation reactions (1). These
reactions result in the formation of ester bonds (1) between the fatty acid
and glycerol molecules, with the production of a molecule of water for
each fatty acid added (1). Triglycerides are broken up by hydrolysis
reactions (1), which are the reverse of condensation reactions (1), with
one molecule of water being added for each fatty acid that’s released (1).
2) Describe the differences between a triglyceride and a phospholipid,
and explain how these differences affect the properties of the
molecule (8 marks).
A triglyceride consists of glycerol (1) and three fatty acid molecules (1).
A phospholipid has the same basic structure, but one of the fatty acids is
replaced by a phosphate group (1). Triglycerides are hydrophobic/repel
water (1). This is a property of the hydrocarbon chains that are part of the
fatty acid molecules (1). The phosphate group in a phospholipid is
hydrophilic/attracts water (1), because its ionised (1). This means that the
phospholipid has a hydrophilic ‘head’ and a hydrophobic ‘tail’ (1).
Proteins
1) Describe the structure of a protein, explaining the terms primary,
secondary, tertiary and quaternary structure. No details are
required of the chemical nature of the bonds (10 marks).
Proteins are made from amino acids (1).
The amino acids are joined together in a long (polypeptide) chain (1).
The sequence of amino acids is the proteins primary structure (1).
The amino acid chain / polypeptide coils in a certain way (1).
The way its coiled is the proteins secondary structure (1).
The coiled chain is itself folded into a specific shape (1).
This is the proteins tertiary structure (1).
Different polypeptide chains can be joined together in the protein
molecule (1).
This is the quaternary structure of the protein (1).
Water and Inorganic Ions
1) Relate the structure of the water molecule to its uses in living
organisms.
Water molecules have two hydrogen atoms and one oxygen atom (1).
The hydrogen and oxygen are joined by covalent bonds / sharing
electrons (1).
Water molecules are polar (1).
Polarity leads to the formation of hydrogen bonds between water
molecules (1).
Water is a solvent (1).
Water’s polar nature allows water to dissolve polar solutes (1).
Water transports substances (1).
Substances are transported more easily when dissolved (1).
Water has a high specific heat capacity (1).
This is due to hydrogen bonds restricting movement (1).
This means its difficult to change the temperature of water (1).
This allows cells to avoid sudden changes in temperature (1).
Water has a high latent heat of evaporation (1).
This is due to hydrogen bonding (1).
It means the evaporation of water has a considerable cooling effect (1).
2) Plants deprived of the mineral magnesium tend to be unhealthy and
their leaves are yellow, whilst those deprived of nitrogen have
stunted growth. Suggest reasons for these observations (7 marks).
Magnesium is a constituent of chlorophyll (1).
If there’s a lack of magnesium then chlorophyll cant be made, and the
leaves will be pale or yellow (1).
If there is less chlorophyll, there will be less photosynthesis (1).
Low rates of photosynthesis will result in the plant growing less (1).
Nitrogen is needed for the manufacture of proteins (1).
Proteins are needed for growth (1).
A nitrogen shortage will lead to a shortage of protein, and the plants
growth will be stunted (1).