LIPIDS - CAPE U1

Lipids 

Lipids are a mixed group of hydrophobic compounds composed of the elements carbon, hydrogen and oxygen.

Structure:

􀀀glycerol (3C alcohol) + fatty acid

􀀀fatty acid = long HC “tail” with carboxyl (COOH) group “head

Triglycerides 

􀀀3 fatty acids linked to glycerol

􀀀ester linkage = between OH & COOH

Triglycerides are commonly called fats or oils. They are made of glycerol and fatty acids.

Glycerol is a small, 3-carbon molecule with three alcohol groups.

Fatty acids are long molecules with a polar, hydrophilic end and a non-polar, hydrophobic "tail". The hydrocarbon chain can be from 14 to 22 CH₂units long, but it is always an even number because of the way fatty acids are made. The hydrocarbon chain is sometimes called an R group, so the formula of a fatty acid can be written as R-COO^-.

• If there are no C=C double bonds in the hydrocarbon chain, then it is a saturated fatty acid (i.e. saturated with hydrogen). These fatty acids form straight chains, and have a high melting point.

• If there are C=C double bonds in the hydrocarbon chain, then it is an unsaturated fatty acid (i.e. unsaturated with hydrogen). These fatty acids form bent chains, and have a low melting point. Fatty acids with more than one double bond are called poly-unsaturated fatty acids (PUFAs).

One molecule of glycerol joins together with three fatty acid molecules to form a triglyceride molecule, in another condensation polymerisation reaction: see animation formation of triglycerides

Triglycerides are insoluble in water. They are used for storage, insulation and protection in fatty tissue (or adipose tissue) found under the skin (sub-cutaneous) or surrounding organs. They yield more energy per unit mass than other compounds so are good for energy storage. Carbohydrates can be mobilised more quickly, and glycogen is stored in muscles and liver for immediate energy requirements.

• Triglycerides containing saturated fatty acids have a high melting point and tend to be found in warm-blooded animals. At room temperature thay are solids (fats), e.g. butter, lard.
• Triglycerides containing unsaturated fatty acids have a low melting point and tend to be found in cold-blooded animals and plants. At room temperature they are liquids (oils), e.g. fish oil, vegetable oils.

Phospholipids

Phospholipids have a similar structure to triglycerides, but with a phosphate group in place of one fatty acid chain. There may also be other groups attached to the phosphate. Phospholipids have a polar hydrophilic "head" (the negatively-charged phosphate group) and two non-polar hydrophobic "tails" (the fatty acid chains). This mixture of properties is fundamental to biology, for phospholipids are the main components of cell membranes.

When mixed with water, phospholipids form droplet spheres with the hydrophilic heads facting the water and the hydrophobic tails facing eachother. This is called a micelle.
Alternatively, they may form a double-layeredphospholipid bilayer. This traps a compartment of water in the middle separated from the external water by the hydrophobic sphere. This naturally-occurring structure is called a liposome, and is similar to a membrane surrounding a cell.

Waxes

Waxes are formed from fatty acids and long-chain alcohols. They are commonly found wherever waterproofing is needed, such as in leaf cuticles, insect exoskeletons, birds' feathers and mammals' fur.

Steroids

Steroids are small hydrophobic molecules found mainly in animals. They include:

• cholesterol, which is found in animals cell membranes to increase stiffness
• bile salts, which help to emulsify dietary fats
• steroid hormones such as testosterone, oestrogen, progesterone and cortisol
• vitamin D, which aids Ca²⁺ uptake by bones.

Terpenes

Terpenes are small hydrophobic molecules found mainly in plants. They include vitamin A, carotene and plant oils such as geraniol, camphor and menthol.