With the help of enzymes dehydration synthesis links a fatty acid to glycerol by an ester linkage (the term for a bond between a hydroxyl group and a carboxyl group. Glycerol now has two remaining hydroxyls and each can be bounded to a fatty acid, the product is a fat (triglyceride).
Saturated/Unsaturated Fats
Fatty acids vary in length and the number of double bonds. If there are no double bonds between the carbon atoms then the carbon skeleton is bonded to the maximum number of hydrogen atoms. This is therefore saturated.
An unsaturated fatty acid has one or more double bonds formed by the removal of hydrogen atoms. Whenever a double bond occurs, the fatty acid will have a kink in its shape at that location.
Most fats present in animals are unsaturated having fatty acids lacking double bonds, e.g. fats which are solid at room temperature, e.g. butter, lard, bacon, grease.
In contract plant fats are unsaturated usually liquid at room temperature e.g. olive oil & corn oil.
Phospholipids
Phospholipids have only two fatty acids, the third carbon of glycerol is joined to a phosphate group. Phospholipids are structurally related to fats, but they have only two fatty acids rather than three. The third carbon or glycerol is joined to a phosphate group.
The tails of the molecule, consisting of hydrocarbons are hydrophobic and are excluded from water however the phosphate group and its attachments from a hydrophilic head that mixed with water.
Fluid-Mosaic Model of Cell Membrane
Phospholipid molecules are thought to be arranged as the fluid mosaic model. This proposes that the plasma membrane consists of a fluid filled bilayer of constantly moving phospholipid molecules containing patchy mosaic of protein molecules.
Steroids
steroids are classified as lipids, they have a completely different structure from simple lipids and phospholipids. Steroids have a basic structure composed of three 6-carbon rings joined to one 5-carbon ring.
Cholesterol is an example of a steroid. It provides the basic molecular structure from which other steroids such as bile and sex hormones are formed.
Roles of Lipids
Energy store
Fat is deposited in fatty (adipose) tissue. Like glycogen, if is insoluble in water and structurally compact making it a convenient form in which to store energy.
When required to release its energy, a molecule of fat, is broken down into glycerol and fatty acids which become available for use in aerobic respiration.
Glycerol becomes converted into a 3-carbon sugar and then into pyruvic acid. Pyruvic acid enters a mitochondrion and generates ATP in the normal way. Each molecule of fatty acid is broken down into many molecules of acetyl. CoA in the matrix of a mitochondrion. Acetyl CoA enters the Krebs cycle and produces ATP as before.
Fats liberate more than double the quantity of energy released by the same mass of carbohydrate.
