Introduction


Lipids are heterogeneous compounds insoluble in water but soluble in organic solvents like chloroform. Biological membranes are the organized assemblies of lipids and proteins with some amount of carbohydrates. Lipids are mainly classified into storage lipids and membrane lipids.

Storage Lipids


Fatty acids are the components of storage lipids. Fatty acids are mainly classified into saturated (no double bond) and unsaturated fatty acids (double bond present). Some of the polyunsaturated fatty acids are called essential fatty acids.  They are linoleic acid, linolenic acid and arachidonic acid. Fats are the esters of fatty acids with glycerol as the alcohol. Simple fats contain same kinds of fatty acids in all three positions while mixed fats contain two or more different fatty acids. Oils are fats but liquid at room temperature. Waxes are the esters of fatty acids with long chain mono hydroxyl alcohol.

Membrane Lipids


Glycerophospholipids, sphingolipids and cholesterol are the membrane lipids. Glycerophospholipids contain glycerol as the alcohol, fatty acids, a phosphate group and a nitrogenous or non-nitrogenous group. Lecithin, cardiolipin, plasmalogens are some of the important glycerophospholipids. Sphingolipids are the derivatives of sphingosine, which is an unsaturated amino alcohol. Sphingosine bonded with a fatty acid by amide linkage is known as ceramide. Sphingomyelins are the common sphingolipids. Since they have a phosphate group, they are known as sphingophospholipids. Cerebrosides are the simplest sphingoglycolipids, which have monosaccharides as carbohydrate moiety. Gangliosides are the ceramide- oligosaccharide complexes. Cholesterol is a major component of animal plasma membranes. It has a steroid nucleus. Cholesterol is the metabolic precursor of steroid hormones.

Lipid Aggregates


The sphere aggregates of amphipathic lipids are called micelles. In them, polar head groups are on the exterior and nonpolar tails are in the interior. Lipid bilayer is the basic structure of cell membrane. Liposomes are the spherical vesicles of lipid bilayer. They act as carriers for substances like drugs, proteins, enzymes, genes, etc to target organs.

Biomembranes


Plasma membrane envelops a cell. Moreover, the Subcellular organelles are membrane bound. Plasma membrane has got several important functions since it has high selective permeability properties. In the fluid- mosaic model of membrane, peripheral and integral membrane proteins are found.

Lipoproteins


They are lipid-protein complexes. The protein part is called apoprotein. They are classified based on density: Chylomicrons, very low density lipoproteins, low density lipoproteins and high density lipoproteins.

Eicosanoids


They are derived from twenty carbon unsaturated fatty acids. They are mainly of two types:  Prostanoids and leukotrienes. Prostaglandins, prostacyclins and thromboxanes are called prostanoids. Prostaglandins are known as local hormones due to their diverse physiological role.