Biological Importance of Lipids

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The Biological Importance of Lipids

Lipids are organic compounds found in all types of plant and animal cells. They always contain carbon, hydrogen and oxygen, however the relative amount of oxygen is less than the amount of carbohydrates in lipids. Lipids are polymers and the long chains of repeating units are joined together in a condensation reaction where water is also produced. The most common types of lipids are triglycerides, phospholipids and waxes. Triglycerides are formed by three fatty acids and glycerol linked together by an ester bond. The three fatty acids that make up the triglyceride bond may be identical but a mixture of fatty acids is possible therefore many different triglycerides structures can be formed and this means that they have a range of functions (1). Saturated fats contain single bonds between carbon atoms and they are usually solid at room temperature. Unsaturated fats such as vegetable oils are liquid at room temperature and have many double bonds between carbon atoms.

Lipids form excellent energy storage molecules for example as fats in seeds, lipid deposits in the stroma of the chloroplast and adipose tissue of vertebrates. In addition oxidation of lipids yields more ATP than the oxidation of carbohydrates and therefore they are high-energy molecules, which are important for fruits or seeds to be dispersed. This is because just as the oxidation of glucose, acetylcoenzyme A is produced in the first stage, but so are many molecules of NAD and FAD. In lipids these are reoxidised in the electron transport chain and that’s why they provide more than twice the energy as the same mass of carbohydrate. Also saturated fats contain more energy that unsaturated fats there are more Hydrogen’s, therefore more to reduce the NAD/FAD during the respiration process (1). There is no osmotic effect in lipids so they cannot diffuse away and this helps them to be am excellent storage molecule. Furthermore another major biological function of lipids is that they are used for structural components in cells. For example phospholipids make up 40% of cell membranes and as they have an amphipathic nature (having polar and non-polar end) allows the automatic formation of the bilayer. Phospholipids are made up of two fatty acid molecules and one phosphate molecule. The fatty acid molecules repel water and form a hydrophobic tail, which turns itself away from water but mixes readily with fats (2). The phosphate molecule attracts water and forms a hydrophilic head that interacts with water but not with fats. This means that when the phospholipid molecules are placed in water they orientates themselves so that the hydrophilic head is as close towards any aqueous medium and the hydrophobic tails are as far away from aqueous medium. Thus making the phospholipid bilayer a selectively permeable membrane and only allows certain substances to enter or leave the cell and are therefore essential components of cell and organelle membranes.

Moreover the complex lipid cholesterol is also amphipathic and is present in cell membranes between phospholipid molecules. This is thought to make the membranes stronger and less flexible, but may also reduce permeability. Cholesterol forms the basis of all steroids and also is a precursor of bile salts and sex hormones such as oestrogen, testosterone etc. Glycolipids (molecular complexes of lipid and polysaccharides) form hydrogen bonds with water molecules outside the cell membrane, which helps to stabilise the membrane (2). The carbohydrate portion of the Glycolipid is involved in cell-to-cell recognition and communication, particularly during growth and development, also the carbohydrate part of the glycoprotein may be ‘recognised’ by antibodies, hormones, etc. Lipoproteins transport lipids in the blood; low-density lipoproteins transport cholesterol from cells into the blood. High- density lipoproteins remove cholesterol from the blood. Terpenes are hydrocarbon chains of...
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