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Lipids are made up of the elements carbon, hydrogen and oxygen but in different
proportions to carbohydrates. The most common type of lipid is the triglyceride.
Lipids can exist as fats, oils and waxes. Fats and oils are very similar in structure
At room temperature, fats are solids and oils are liquids. Fats are of animal origin, while
oils tend to be found in plants.
Waxes have a different structure (esters of fatty acids with long chain alcohols) and can be
found in both animals and plants.
These are made up of 3 fatty acid chains attached to a glycerol molecule.
Fatty acids are chains of carbon atoms, the terminal one having an OOH group attached
making a carboxylic group (COOH). The length of the chain is usually between 14 and 22
carbons long (most commonly 16-18).
Three of these chains become attached to a glycerol molecule which has 3 OH groups
attached to its 3 carbons. This is called a condensation reaction because 3 water
molecules are formed from 3 OH groups from the fatty acids chains and 3 H atoms from the
glycerol. The bond between the fatty acid chain and the glycerol is called an ester linkage.
The 3 fatty acids may be identical or they may have different structures.
In the fatty acid chains the carbon atoms may have single bonds between them making
the lipid saturated. These are usually solid at room temperature and are called fats.
If one or more bonds between the carbon atoms are double bonds, the lipid is
unsaturated. These are usually liquid at room temperature and are called oils.
Functions of lipids
1. Storage - lipids are non-polar and so are insoluble in water.
2. High-energy store - they have a high proportion of H atoms relative to O atoms and so
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Production of metabolic water - some water is produced as a final result of respiration.
4. Thermal insulation - fat conducts heat very slowly so having a layer under the skin
keeps metabolic heat in.
5. Electrical insulation - the myelin sheath around axons prevents ion leakage.
6. Waterproofing - waxy cuticles are useful, for example, to prevent excess evaporation
from the surface of a leaf.
7. Hormone production - steroid hormones.…read more