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					The occurrence and functions of lipids in plants and animals.

Lipids, like carbohydrates, are important in energy storage and this is the best known
function of lipids. However lipids are used for a lot of other functions in plants and
animals other than energy storage. A lot of these functions are to do with the structure
of lipids and their characteristics.

Lipids are made up of glycerol and fatty acids. Three fatty acid chains are linked to a
glycerol molecule by ester bonds. A lipid molecule on its own is quite small, but they
link together to form globules. These are long chains of lipids and are the reason for
many of the characteristics of lipids, such as being insoluble in water.

Lipids can be divided into two main groups – fats (which are solid) and oils (which
are liquid) although there are other types eg waxes. Fats can be found in most, if not
all animals. It is used for two main reasons: as an energy store where it is contained in
mammals) as sub-cutaneous fat or for heat preservation under the skin. Oils are also
found in animals, such as secreted through the skin or onto hair, although it is more
important in plants. Oil is found in many seeds eg Sunflower seeds. This is used as a
food store for the embryo to start growing as no light can get to it for photosynthesis.
Lipids can also be used for waterproofing and this is the main function of waxes,
found on the cuticle of leaves. Fat is a very important heat preservation for aquatic
mammals such as whales. Most land mammals have fur to keep warm and this is
effective because it traps air, which is a good insulator. However, this would be
useless in water as there is no air to trap. So aquatic mammals have a large layer of fat
beneath the skin to keep them warm. This also helps them keep their buoyancy.

The most important function of lipids in humans is as an energy store. The energy is
stored in a fat called triglyceride. Triglyceride has a very high concentration of energy
per gram, almost twice as much as carbohydrates. It is stored as tiny droplets in the
cytoplasm of cells and when oxidised, a lot of energy is released. It is for this reason
that a lot of energy can be stored as fat and can easily be changed to energy.

408 words
Essay written in approx 25 mins by L6 student Robyn Atcheson, October 2001.

This is a good example of a well written essay. The English is good and the essay is
well organised, and displays a reasonably good amount of scientific content. Some
additional points which probably should have been included are:
   (1) Significance of lipids being insoluble in water (metabolic inertness, no
        osmotic effect etc)
   (2) Formation of ‘metabolic water’ – camels hump etc
   (3) High energy to mass ratio – importance re locomotion, and for seeds which
        may be dispersed by air.
   (4) Protection from impact – kidneys
   (5) Arthropod cuticle.

             File: lipid (disc 6), and Public folder – A lev biol – biochem