Structure and biological significance of lipids.
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Scott Bissett L6Du Structure and biological significance of lipids Lipids are made up of a wide variety of molecules, but they all contain carbon, hydrogen and oxygen, with a much higher percentage of carbon and hydrogen molecules than oxygen. There are three kinds of lipids in living organisms: triglycerides, phospholipids and steroids (hormones). Triglycerides are made up of a glycerol molecule, with three fatty acid chains attached by ester linkages. Glycerol is an alcohol containing 3 carbon atoms. The fact it is an alcohol means it has an -OH group at one end. Fatty acids are hydrocarbon chains, with a -COOH group at one end. This -COOH group reacts with the -OH group of glycerol, and a condensation reaction occurs, which is what forms the ester linkage.
These are known as fats, whereas saturated fatty acids (or with relatively short fatty acid chains) are known as oils and are liquids at room temperature. Plants generally produce oils such as olive or corn oil, and animals produce fats such as butter and lard, but there are exceptions to the rule. The functions of triglycerides are multiple. The main use is as an energy source; they can be broken down and oxidised in respiration, and the energy produced used to make ATP. They can also be used as an energy store, as 1g of fat has twice the energy of 1g of starch or protein so more energy can be stored in a smaller space, so fat stores are lighter and smaller.
This ion has a negative charge, which attracts water molecules to it, and so it is hydrophilic (water loving). This provides the phospholipid molecule with an odd characteristic, as one of its site id hydrophilic, and the other is hydrophobic. It is this property that is used to form a phospholipid bilayer in cell membranes. The hydrophilic head of the molecule is in the cytoplasm (which is made mostly of water with other substances dissolved in it) either inside or outside of the cell and the tails point away, towards the centre of the membrane. The tails are attracted to one another van der Waals forces and hydrophobic interactions that give the membranes strength. The hydrophobic property of the tails also means that the membrane doesn't allow molecules to pass easily through them, which is why it has certain purpose built pores, which can allow passage to substances.
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