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Describe the role of lipids in living organisms.

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Introduction

Describe the role of lipids in living organisms Lipids can be defined as 'a group of substances that contain carbon, hydrogen and oxygen.'(1) The proportion of oxygen in lipids is less than in carbohydrates. Lipids are non polar and therefore they are insoluble in polar solvents such as water. Lipids are however soluble in non polar solvents such as alcohols and ethers. The chemistry of lipids are also very varied but they are all esters of fatty acids (can be saturated or unsaturated) and an alcohol. Lipids are a large and varied group of organic compounds. They have an intensive role in biological systems. They can be classified as: * Fats and oil (Triglycerides) * Phospholipids * Steroids * Waxes * Fatty acids (The nature of the fatty acid determines the characteristic of any particular fat.) All triglycerides molecules are formed when a molecule of glycerol condenses and combines with three fatty acid molecules. Triglycerides are used in living organisms as a source of energy store. 'Triglycerides are highly concentrated stores of metabolic energy because they are reduced and anhydrous.'(2) Triglycerides form compact food reserves which do not upset the osmotic balance of cells. For example, triglycerides are often found as droplets in cells and in seeds. ...read more.

Middle

Lipids can also be made out of phospholipids. Two fatty acid molecules and one phosphate group molecule are condensed with a molecule of glycerol. The phosphate group molecule is hydrophilic (polar) in contrast to the tail-end of the molecule which is hydrophobic (non-polar). This property of phospholipids, when placed in water, allows them to arrange themselves in a double layer with the hydrophilic phosphate group facing outwards and the hydrophobic fatty acids facing inwards. This double layer is called a phospholipid bilayer and it forms the basis of 'biological membranes such as the cell membrane.' (4) The phospholipids bilayer is dynamic (fluid) since it is capable of movement. This allows pinocytosis, phagocytosis and the membrane to 'self seal if punctured.'(4) The two non-polar fatty acid tails of the membrane prevents the free diffusion of water and polar molecules through the membrane. It acts as a barrier to free movement. Therefore these fatty acids play an important part in allowing the membrane to exert control over what passes through, via channel proteins. The membrane is therefore selectively permeable only allowing specific molecules to pass into or out of the cell. Glycolipids (a polysaccharide attached to a phospholipid) are found on the outside of cell membrane. ...read more.

Conclusion

Waxes have a complex structure because their alcohol is much larger than glycerol. A major property of wax is that it is waterproof and is therefore used in plants on the waxy cuticle of the epidermis of leaves. This is done to 'reduce water loss by transpiration since water cannot pass through the insoluble lipid layer.'(6) They are also found in organisms which have an exoskeleton. These organisms have an epicuticle which is thin and waxy. It forms a waterproof covering to prevent water loss by evaporation. Other organisms keep waterproof in different ways. 'Sea birds preen their feathers with oils to prevent water from entering the air spaces between the feathers.'(5) In humans there is a sebaceous gland that is found in the skin. 'This gland produces an oily fluid known as sebum.'(6) The sebum is generally used to, 'keep skin soft and supple. It also helps the body to retain heat and prevent excessive evaporation.'(6) Wax is used by other organisms like bees to construct their honeycomb. Finally, wax can also be used as a form of storage in a few organisms such as fish. Finally, lipids form basis of scents. Plant scents are 'fatty acids (or their derivatives)' (5) and so aid the attraction of insects for pollination. In conclusion, lipids are a diverse group of organic compounds which have many properties and functions in living organisms of all kinds. ...read more.

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