Importance of Hydolysis
Lipids are a group of organic compounds that are insoluble in water. They include fats, phospolipids, waxes, steroids & certain vitamins. Lipids have particularly two important functions in living organisms. They are used to make the selectively permeable membrane that surrounds cells & they are used to store energy.

Fats:
Fats (Triglycerides) are used to store energy lubricate the skin insulate against the cold & to cushion or protect certain organs. While carbohydrates & proteins each provide 16-17 kilojoules or energy per gram. 1 gram of fat provides 37 kilojoules. This means that more energy can be stored in the body as fat than by the same weight of carbohydrates or proteins.

Structure:
Fat molecules are made of the element Carbon, Hydrogen & Oxygen. Each consists of a Glycerol (C3H8O3) sub unit joined to three fatty acid sub unit. The Glycerol unit is the same in all fats, but the fatty acids can vary resulting in fats with different properties. Fatty acids contain a carboxyl group (COOH) attached to a long chain of Carbon & Hydrogen atoms (A Hydrocarbon Chain). The fatty Acids are joined to Glycerol molecules by a type of reaction called condensation, which results in three molecules of water (H2O). The reverse reaction called hydrolysis occurs when fats are broken down during digestion.

This condensation reaction shows how three Fatty Acids molecules and one Glycerol molecule can join together to make a single molecule of Fat.

Saturated Fat & Unsaturated Fats:
In above diagram the Carbon atoms in the Fatty Acid are joined by single bond & each is joined to the maximum possible number of Hydrogen atoms. The resulting Fat molecule is described as Saturated Fat.

In some Fat, the Fatty Acid sub units contain some double bonds between Carbon atoms, which reduce the number of hydrogen atoms present in the molecule. Such Fat molecules are called Unsaturated Fat.

The double bonds in Unsaturated Fats produce kincks in some of the Fatty Acid side chains, which reduces the tendency of the Fat to solidify. Such Fats are usually the Oils & are liquids at room temperature.

Human food contains three types of Fats

• Saturated Fat (Found mostly in Meat & Dairy Products)
• Monounsaturated Fat (Found in Olive Oil & Avocados)
• Polyunsaturated Fats (Found in Fish & Vegetables Oil).

Phospholipids:
Phospholipids are similar to Fat, but a Phosphates group (PO4) replaces one of the Fatty Acid side chains.

Phosphate group is polar (Hydrophilic), and this makes these molecules to perform an important function of Cell unit formation. This property enables Phospholipids to form the bilayer structure found in Cell Membrane.

Other Useful Lipids:
Lipids have other uses besides being Membrane components & providing a form of Energy storage.

• Waxes, for e.g. are large Lipid molecules that provide a waterproof covering on skin for Leaves, Fruits & the Cuticle (Outer Coat) of Insects.
• Steroids are Fat-soluble compounds made of Carbon rings. The best-known Steroids are the Sex Hormones Testosterone, Estrogen & Progesterone & their precursor Cholesterol.
• Cholesterol also plays an important role in Cell Membranes where it helps to maintain fluidity.
• Related to the Steroids are another group of Lipids, called Terpenes, which are slightly smaller. These include the Fat-soluble Vitamins A, E & K.