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AQA A-Level Biology Summary Notes: 3.1.3 Lipids
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Number 42The Structure and Function of Lipids
This Factsheet summarises the structure and function oflipids. Lipids are organic compounds found in every type of plant and animal
cell. They always contain the elements carbon, hydrogen and oxygen, but the reletive amount of oxygen is less than in carbohydrates.
All lipids are polymers - long chains of repeating units joined together in cells in plant roots forces water into the symplast pathway which is
condensation reaction during which water is released. The commonest under the control of the nucleus. This strip therefore gives the plant
lipids are triglycerides - three fatty acids and glycerol linked by an ester control over substances entering the xylem.
bond (Fig 1). Buoyancy
Since fat is less dense than water, fat reserves provide buoyancy for
Fig 1. Structure of a triglyceride aquatic animals.
8. Precursors of many cell constituents
E.g. gibberellins plant growth substances e.g. carotenes,
H ingredients photosynthetic pigments, coloration pigments, steroid hormones e.g.
-0-C-CH, - CH,.....•CH, testosterone.
⑱ H- -CH, -CH,...-CH, +3 How do animals obtain lipids?
Since all cells contain lipids, any animal that eats a plant or animal cell will
Fo - CH, - CH, *..CH,
fatty acid
take in some lipid. Triglycerides and phospholipids are hydrolysed by
lipase into glycerol and fatty acids. Since lipids do not dissolve in water,
ester bond
they are not easy to digest and their digestion is accelerated in vertebrates
by the secretion of bile salts, which emulsify them into smaller particles,
The three fatty acids may be identical but a mixture of fatty acids also greatly increasing the surface area on which lipase can act.
possible. Thus, many different triglyceride structures occur and this in
Fats are hydrolysed by pancreatic lipases into fatty acids, monoglycerides
turn means that they have a range of functions. Saturated fats are those
and cholesterol. Single triglycerides, phospholipids and cholesterol then
which contain single bonds between the carbons in the hydrocarbon chain
diffuse across the brush border of gut epithelial cells (Fig 2).
of fatty acids and these are usually solids at room temperature. Unsaturated
fats, e.g. vegetable oils, are liquid at room temperature and have some/
Fig 2. Uptake of lipids in ileum
many double bonds between the carbons.
Mono-unsaturated fat - one double bond
Functions polyunsaturated fat =
more than one double bond
Lumen of intestine
There are eight major functions of lipids.
1. Energy storage monoglycerides triglycerides -
fatty acids. lymph
Lipids form excellent energy storage molecules, e.g. as lipid deposits in
cholesterol. cholesterol system
the stroma of the chloroplast and as fats in seeus and adipose tissue of
vertebrates. Oxidation of fats to release energy occurs in the
mitochondria. As in the oxidation of glucose, acetylcoenzyme A is
produced in the first stage, but so are many molecules of NAD and
FAD These are reoxidised in the electron traz.port chain and hence
88% triglyceride
oxidation of fat yields more ATP than oxidation of carbohydrates.
8% phospholipid
Weight for weight, they are therefore high-energy molecules, which is 4% cholesterol
important in fruits or seeds that need to be dispersed.
2. Structural components
Phospholipids usually make up 40% of cell membranes where their Once inside the cell, triglycerides are reassembled to form chylomicrons.
amphipathic nature (having a polar and a non-polar end) enable them These, plus cholesterol and phospholipids move from the epithelial cells
to contribute to the spontaneous formation of the bilayer. Cholesterol into the lymphatic system. The reassembly of triglycerides in the epithelial
is also a major component of animal cell membranes. cells keeps the concentration of fatty acids low. This maintains the diffusion
3. Thermal insulation gradient between the ileum and the inside of the epithelial cells. Lipids are
Fat conducts heat slowly and therefore the triglycerides, which are transported in the lymphatic system before entering the bloodstream near
stored as subcutaneous fat in vertebrates, are important for maintaining the heart.
optimum temperature for metabolism.
Mechanical protection
Typical Exam Question
e.g. of delicate organs, such as kidneys. Describe the structure to function of epithelial cells of ileum.
5. Electrical insulation Microvilli on luminal surface increase surface area for absorption.
e.g. Sphingomyelin isa specialised phospholipid in the myelin sheet of Plasma membrane of epithelial cell has protein carriers for active
nerve axons.
transport.
Waterproofing Epithelial cells contain smooth endoplasmic reticulum for reassembly
e.g. The waxy cuticle on the leaf epidermis or as oils on birds' feathers. of triglycerides.
The presence of waxy suberin in the Casparian strip of endodermal
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