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.
|
