Lipids of Biological Importance

Fatty Acids

These are the building blocks of most lipids. They are long chain organic acids having 4 to 24 carbon atoms. They have a single carboxyl (COOH) group and a long non-polar hydrocarbon tail, which makes it non-polar (water insoluble) and greasy in nature. Fatty acids do not occur in free form in cells but are bound tightly in different types of other lipids. Fatty acids are of many types. They vary in chain length and also differ in the presence, number and positions of their double bonds (Table). Saturated fatty acids are those that do not possess any double bonds in hydrocarbon tail. Their general formula is CH,-(CH,)n-COOH. Unsaturated fatty acids on the other hand have one or more double bonds. Natural fatty acids have even number of carbon atoms.

Triglycerides

These are fatty acid esters of glycerol and are most abundant lipids, also often referred as fats. Oils also come in the same category but the only difference in fat and oil is that, at room temperature fat is solid whereas oil is liquid. More or less all animal lipid is fat. A triglyceride has a glycerol with three fatty acids attached to it. Triglycerides act as storage fat in plant and animal cells. In eukaryotic cells, triglycerides form microscopic droplets, which act as energy storehouse. Specialized cells called adipocytes or fat cells store large amounts of triglycerides that almost occupy the cell interior. Why triglycerides are preferred as storage fuel rather than carbohydrates such as starch or glycogen? There are two reasons for this: first, being hydrophobic, triglycerides do not carry extra weight of water which carbohydrates, being hydrophilic, do carry and are not advantageous to the cell. Second, carbon atoms of triglycerides are more reduced (more hydrogen atoms attached) than the carbon atoms of sugars and this gives twice the amount of energy per gram on oxidation than carbohydrates.

Triglycerides stored under the skin also serve as an insulator against low temperatures. Seals, walruses, penguins and many other warm-blooded animals are well padded with triglycerides. In hibernating animals, triglyceride serves both as an energy source as well as an insulator.

Fatty acids attached in triglycerides could be same or different. Triglycerides with unsaturated fatty acids are liquid at room temperature. Such triglycerides are converted into solid fat by adding hydrogen to the double bond of their fatty acids. This process is used in the manufacturing of hydrogenated fats commonly called as "DALDA". Oxidation (addition of oxygen) of triglycerides by air results in rancid fat, which is off-taste and has awful smell. In body, vitamin E, vitamin C and other reducing substances prevent oxidation of fat.

Waxes

These are fatty acid esters of long chain alcohols with 16 to 22 carbon atoms. Fatty acids in waxes may be saturated or unsaturated and contain 14 to 36 carbon atoms. Skin, hair, wool and fur have waxes, which serve as a protective covering and are secreted by skin glands. Waxes are the chief storage form of metabolic fuel in plankton, a free-floating marine microorganism. Waxes also have pharmaceutical and other uses in industries such as ointments, lotions and polishes.

Phospholipids

Biological membranes are rich in phospholipids. They have a highly polar (water loving) "head" group and a hydrophobic (water repelling) hydrocarbon tail. These characteristics favour formation of biological membranes in a double/ bi layer structure with semi-permeable property. As the name implies, these lipids contain phosphorous and are structural components of biological membranes. A phospholipid contains a glycerol with two fatty acids attached to carbon one, two and a phosphate bounded to third carton in the form of phosphoric acid to which a second alcohol is attached. Different phospholipids differ in their head group alcohol. Within the same phospholipid class, say phosphatidylcholine (a choline containing phospholipid), there may exist a number of molecular species differing in fatty acid composition.

In some phospholipids, one of the fatty acids is attached to glycerol in ether, rather than ester linkage. These lipids are called plasmalogens and are present in vertebrate heart tissue and constitute about half of the phospholipid content. Plasmalogens are also present in the membrane of certain bacteria and some invertebrates. The function of plasmalogens is not precisely known. Platelet activating factor, a type of plasmalogen, released by basophils (a type of white blood cells) plays an important role in inflammation and allergic response.

Sphingolipids

These are present in membranes. They do not contain glycerol but possess a long-chain fatty acid and one long-chain amino alcohol called sphingosine and a polar alcohol. The three carbons of sphingosine are structurally analogous to the three carbons of glyceroi found in Phospholipids, nervous system is rich in sphingolipids. There are three types of sphingolipids differing in polar head groups: 

(a) Sphingomyelins: They contain phosphorylcholine or phosphorylethanolamine as the head group and are clubbed with phospholipids.

(b) Glycosphingolipids: As the name indicates they contain one or more sugars as head group. 

(c) Gangliosides: It contains complex sugars.

Sterols

They contain no fatty acids and are important component of membranes. They consist of four fused rings, three with six carbons and one with five. Cholesterol is the major sterol in animal tissues and has a polar hydroxyl group. Sterols serve as precursor for many compounds, for example, steroid hormones, which regulates gene expression; bile acids that facilitates fat digestion in the intestine.