I Grassi in INGLESE (Fats)

Lipids play many vital biological functions and mediate biological processes upon which we all depend for survival. The three most important types of lipids in biological systems are:

1.       Steroids

2.       Triacylglycerols

3.       Glycerolphospholipids

1. Steroids

The basic skeleton of a steroid is a series of fused alkyl rings. An array of functional groups can be added to the basic steroid skeleton to generate compounds with a range of functions (e.g. Sex hormones testosterone and oestradiol and cholesterol.

Cholesterol is a non-polar sterol, which is insoluble in water and other water-based substances such as blood; it is synthesized in the liber, and is needed in tissue throughout the body. As it is insoluble in the blood, it is transported in the bloodstream when associated with a soluble protein named lipoprotein. There are two key members of the lipoprotein family: Low-density lipoprotein (LDL) and high-density lipoprotein (LDL), which form two distinct complexes with cholesterol:

1.       LDL-cholesterol: directs the transport of excess cholesterol to the liber, where it can be broken-down safely (good cholesterol).

2.       HDL-cholesterol: directs the transport of cholesterol to the arteries, where is deposited in the artery walls. The depositing of cholesterol may contribute to the condition atherosclerosis, the narrow hardening of the arteries and disrupt the blood flow. (bad cholesterol)

However cholesterol is a necessary component of animal cell membranes, and because of its rigid structure, it inserts between the glycerophospholipids of the membrane and modulates membrane fluidity.

2. Triacylglycerols

The triacyglycerols are a family of organic compounds that are known as fats and oils. They are derived from two compounds:

1.       The three-carbon alcohol glycerol.

2.       Fatty acids (carboxylic acids possessing long carbon backbones.

The carboxyl group of three fatty acids react with the three hydroxyl groups on glycerol to generate three ester groups.

Fatty acids fall into one of three groups:

1.       Saturated (the alkyl chain contains only carbon-carbon single bonds)

2.       Monounsaturated (the alkyl chain contains one carbon-carbon double bond)

3.       Polyunsaturated (the alkyl chain contains more than one carbon-carbon double bond)

A saturated compound has a higher melting point and a higher boiling point than an equivalent unsaturated compound. Saturated compounds are able to pack together more closely than unsaturated compounds as intermolecular forces are higher and melting and boiling points are elevated relative to unsaturated compounds.

Different triacylglycerols are formed from the joining of different fatty acids to the glycerol framework.

-          Triacylglycerols with a high saturated fatty acid content are classed as fats. They have high melting and boiling point and exist as solids at room temperature

-          Triacylglycerols with high unsaturated fatty acid content are classed as oils. They have low melting and boiling points and exist as liquids at room temperature.

Research has found that high levels of saturated fats in the diet are linked with elevated level of the cholesterol-protein complex LDL-cholesterol.

-          Polyunsaturated fats (more than one double bond) have been found to lower both LDL and HDL cholesterol.

-          Monounsaturated fats (just one double bond) represent the ideal situation: they have been found to lower LDL-cholesterol, but preserve levels of HDL-cholesterol

3. Glycerophospholipids

Glycerophospholipids are the central component of cell membranes, they envelope the content of cells and define their exterior boundaries.

The basic framework of the glycerophospholipids comprises a glycerol backbone and, as such, mirrors that of the triacylglycerols however in a glycerophospholipids, one of the fatty acid groups present in a triacylglycerol is replaced with a phosphodiester group (a phosphate group that has been modified by the addition of one of a number of different substituent groups, usually serine, choline or ethanolamine.

The suitability of glycerophospholipids as the central component of the cell membrane is derived from them possessing both polar and non-polar components. The long alkyl tails of the fatty aid groups are non-polar and hence, hydrophobic, while the posphodiester group is polar and hydrophilic.

However the lipid bilayer doesn’t solely comprise glycerophospholipids, but features a range of transmembrane proteins with various roles and cholesterol.