B io Factsheet
September 2000 Number 74
The Structure and Biological Functions of Lipids
This factsheet covers the relevant AS syllabus content of the major examining boards.
By studying this Factsheet candidates will gain a knowledge and understanding of:
• the general nature and structure of lipids as triglycerides(fats and oils) and waxes, including ester bonds and their formation.
• the nature of saturated and unsaturated fatty acids and fats and their implications for human health.
• the structure and properties of phospholipids.
• the roles of phospholipids, cholesterol and glycolipids in cell membranes.
Introduction Fig 1. The formation and breakdown of triglycerides
All lipids contain the elements carbon, hydrogen and oxygen but have a
lower proportion of oxygen in the molecule than carbohydrates. With the
exception of glycerol they are insoluble in water, but dissolve in organic CH2OH + HOOC.R condensation CH2O – OC.R
/synthesis
solvents such as ether, chloroform and benzene. They are lighter(less dense)
than water. They show a wide range of molecular structures, properties CH OH + HOOC.R CH O – OC.R + 3H2O
and biological functions and this diversity has been utilised by living hydrolysis
organisms in a large variety of ways. The major functions of lipid types CH2OH + HOOC.R /digestion CH2O – OC.R
referred to in AS syllabuses are shown in Table 1.
glycerol fatty acid an ester bond triglyceride water
Table 1. Major functions of lipids
Function Lipid type
Energy source as soluble, mobile glycerol, fatty acids. Remember – an ester bond is formed by condensation between an
respiratory substrate alcohol group and an acid group. In this case the alcohol groups are
on the glycerol and the acid groups belong to the fatty acids
Energy store as insoluble respiratory fats in animals, oils in
substrate plants(seeds)
Fatty acids contain the acidic (-COOH) group. They have the general
Thermal insulation against heat loss fats formula R-COOH where the side chain, R, is hydrogen or an alkyl-group,
via body surface such as –CH3 or -C2H5., and so on. They increase in molecular length by –
CH2 for each successive member of the series. Most fatty acids have an
Buoyancy in aquatic mammals fats even number of carbon atoms, usually between 14 and 22. These long
hydrocarbon tails mean that fatty acid molecules are insoluble in water (the
Cushioning and protection fats tails are referred to as hydrophobic or ‘water-hating).
against knocks
Biological functions of triglycerides
Waterproofing on surfaces waxes The digestion of triglycerides is catalysed by lipase enzymes which are
of leaves, insects, skin found, for example, in animal digestive systems, in germinating seeds, in
the lysosomes inside cells and in the secretions of saprophytic bacteria and
Solvent, enabling storage and fats and oils fungi. The synthesis of triglycerides takes place in fat storage depots, for
transport of fat soluble vitamins example, in liver cells, in the adipose cells of mammals, which make up
(A,D,E and K) adipose connective tissue (found under the skin, between the skeletal
Major components of cell membranes phospholipids, cholesterol muscles, around the kidneys and in the yellow bone marrow), in the fat
and glycolipids body of insects and also in the food reserves and endosperm of seeds. The
energy content of triglycerides is higher than that of carbohydrate or protein.
1g of fat will yield around 37kJ of energy on complete oxidation whereas
Glycerol, fatty acids and triglyceride structure 1g of carbohydrates yields 16kJ and 1g of protein yields 17kJ. Triglycerides
Glycerol is a three carbon alcohol which contains three hydroxyl (-OH) form compact food reserves which do not upset the osmotic balance of
groups. Its molecular structure can be seen in Fig 1. Each of the three cells. When oxidised triglycerides also yield water which may be useful to
hydroxyl groups can condense with a fatty acid to form an ester. The desert animals such as the kangaroo rat and camel. These store fat specifically
combination of one molecule of glycerol with three molecules of fatty acid for its metabolic water content.
forms a lipid molecule known as a triglyceride(fat). An equation showing Triglycerides are poor conductors of heat so that fat laid down under the
the formation of fat can be seen in Fig 1. skin can help to prevent heat loss. This is particularly important in aquatic
mammals such as whales, seals and walruses which build up huge deposits
Remember – condensation is the joining of molecules by the removal of of blubber to protect them from heat loss in cold Arctic or Antarctic seas.
water and is important in synthetic reactions. The reverse, hydrolysis, Since the blubber is less dense than water it also provides buoyancy enabling
is the splitting of molecules by the addition of water and is important in the animals to stay near the surface.
digestion. A thick layer of fat around the mammalian kidneys helps to protect them
from damaging knocks.
1
September 2000 Number 74
The Structure and Biological Functions of Lipids
This factsheet covers the relevant AS syllabus content of the major examining boards.
By studying this Factsheet candidates will gain a knowledge and understanding of:
• the general nature and structure of lipids as triglycerides(fats and oils) and waxes, including ester bonds and their formation.
• the nature of saturated and unsaturated fatty acids and fats and their implications for human health.
• the structure and properties of phospholipids.
• the roles of phospholipids, cholesterol and glycolipids in cell membranes.
Introduction Fig 1. The formation and breakdown of triglycerides
All lipids contain the elements carbon, hydrogen and oxygen but have a
lower proportion of oxygen in the molecule than carbohydrates. With the
exception of glycerol they are insoluble in water, but dissolve in organic CH2OH + HOOC.R condensation CH2O – OC.R
/synthesis
solvents such as ether, chloroform and benzene. They are lighter(less dense)
than water. They show a wide range of molecular structures, properties CH OH + HOOC.R CH O – OC.R + 3H2O
and biological functions and this diversity has been utilised by living hydrolysis
organisms in a large variety of ways. The major functions of lipid types CH2OH + HOOC.R /digestion CH2O – OC.R
referred to in AS syllabuses are shown in Table 1.
glycerol fatty acid an ester bond triglyceride water
Table 1. Major functions of lipids
Function Lipid type
Energy source as soluble, mobile glycerol, fatty acids. Remember – an ester bond is formed by condensation between an
respiratory substrate alcohol group and an acid group. In this case the alcohol groups are
on the glycerol and the acid groups belong to the fatty acids
Energy store as insoluble respiratory fats in animals, oils in
substrate plants(seeds)
Fatty acids contain the acidic (-COOH) group. They have the general
Thermal insulation against heat loss fats formula R-COOH where the side chain, R, is hydrogen or an alkyl-group,
via body surface such as –CH3 or -C2H5., and so on. They increase in molecular length by –
CH2 for each successive member of the series. Most fatty acids have an
Buoyancy in aquatic mammals fats even number of carbon atoms, usually between 14 and 22. These long
hydrocarbon tails mean that fatty acid molecules are insoluble in water (the
Cushioning and protection fats tails are referred to as hydrophobic or ‘water-hating).
against knocks
Biological functions of triglycerides
Waterproofing on surfaces waxes The digestion of triglycerides is catalysed by lipase enzymes which are
of leaves, insects, skin found, for example, in animal digestive systems, in germinating seeds, in
the lysosomes inside cells and in the secretions of saprophytic bacteria and
Solvent, enabling storage and fats and oils fungi. The synthesis of triglycerides takes place in fat storage depots, for
transport of fat soluble vitamins example, in liver cells, in the adipose cells of mammals, which make up
(A,D,E and K) adipose connective tissue (found under the skin, between the skeletal
Major components of cell membranes phospholipids, cholesterol muscles, around the kidneys and in the yellow bone marrow), in the fat
and glycolipids body of insects and also in the food reserves and endosperm of seeds. The
energy content of triglycerides is higher than that of carbohydrate or protein.
1g of fat will yield around 37kJ of energy on complete oxidation whereas
Glycerol, fatty acids and triglyceride structure 1g of carbohydrates yields 16kJ and 1g of protein yields 17kJ. Triglycerides
Glycerol is a three carbon alcohol which contains three hydroxyl (-OH) form compact food reserves which do not upset the osmotic balance of
groups. Its molecular structure can be seen in Fig 1. Each of the three cells. When oxidised triglycerides also yield water which may be useful to
hydroxyl groups can condense with a fatty acid to form an ester. The desert animals such as the kangaroo rat and camel. These store fat specifically
combination of one molecule of glycerol with three molecules of fatty acid for its metabolic water content.
forms a lipid molecule known as a triglyceride(fat). An equation showing Triglycerides are poor conductors of heat so that fat laid down under the
the formation of fat can be seen in Fig 1. skin can help to prevent heat loss. This is particularly important in aquatic
mammals such as whales, seals and walruses which build up huge deposits
Remember – condensation is the joining of molecules by the removal of of blubber to protect them from heat loss in cold Arctic or Antarctic seas.
water and is important in synthetic reactions. The reverse, hydrolysis, Since the blubber is less dense than water it also provides buoyancy enabling
is the splitting of molecules by the addition of water and is important in the animals to stay near the surface.
digestion. A thick layer of fat around the mammalian kidneys helps to protect them
from damaging knocks.
1
