LIPIDS

Lipids are organic compound insoluble in water but soluble in organic solvent such as alcohol, benzena, ether. They contain carbon, hydrogen and oxygen but the proportion of oxygen is lower than in
carbohydrates. The main group of lipids are
- Triglycerides                         - Steroid
- Phospholipids                       - Waxes


TRIGLYCERIDES



Triglycerides usually known as fats or oils and are the most common lipids. A triglycerides is formed by the ester linkage of three fatty acids to one molecule of glycerol. A glycerol is an alcohol with three carbon atoms and three hydroxyl group which can form ester bond with the carboxyl group of fatty acids. A fatty acids consists of a long hydrocarbon chain , usually 16 or 18 carbon atoms with a carboxyl group at one end. The three fatty acids mat all be in a single triglycerides, like stearic acid in animal fats or they may be different. Saturated fatty acid have no double bonds between carbon atoms. Unsaturated fatty acids have one or more double bonds between carbon atoms which cause kinks of the chain.



PHOSPHOLIPIDS


A phospholipid molecule is formed from condensation of one glycerol molecule with two hydrocarbon fatty acids chains. The third group of glycerol is linked with a phosphate group which carries a negative charge. The phosphate group is polar and hydrophilic. Phospholipids are amphiphatic molecules. The polar hydrophilic phospholipds head point outwards, attracted to water and forms hydrogen bonds with the molecule. The hydrocarbon fatty acids tails are non polar, forming a hydrophobic with one another. Body cells usually have a watery intracellular environment and extracellular environment. This cause the phospholipids molecules to arrange themselves to form a two layer membrane. The phospholipids molecules give the membrane their fluidity and allow lipid soluble substance and small non polar substance to pass through.



STEROID


Steroid have different structures from other lipids although they are insoluble in water and soluble in organic solvents. Steroid have a basic four ring hydrocarbon structure with different double bonds and functional side chains. An example of a steroid is cholesterol. Cholesterol is the precursor of steroid hormones such as testosteron and oestrogen, bile acids, and vitamin D. Cholesterol is a component of cell membrane and helps to maintenance the fluidity the membrane cell. Most of cholesterol in blood are surrounded by a layer of phospholipid in which protein embedded to form LDL and HDL. When cholesterol exceeds the normal levels in the body, they are deposited on the inner walls of arteries causing atherosclerosis.



WAXES


Waxes are esters of fatty acids with long chain alcohols. They usually have a protective function. Example are the beewax, which constituent the honeycomb of bees and the wax layer on some leaves, fruit, and testas. The waxy cuticle layer acts as a protective   layer, reduces evaporation of water from the plant