![]() As only lipid-soluble molecules to pass through them therefore entering and leaving the cell and water-soluble molecules cannot pass through therefore are prevented from entering and leaving the cell. It is the phospholipids in the membrane that control the movement of the substances into and out of the cell. The role of phospholipid bilayer is to allow the conditions inside the cell to be different to those outside and to control the movement of substances into and out of the cell. The phospholipids position themselves in a bilayer the hydrophobic groups in contact with each other on the inside of the bilayer and the hydrophilic groups on the outsides of the bilayer. Phospholipids are a group of compounds that are similar to triacylglycerols except that one of the fatty acid chains is instead replaced by a phosphate group several other varying small molecules may be attached to the phosphate giving a group of different structures. ![]() Triacylglycerols also can act as thermal and mechanical insulators and in aquatic animals, can supply buoyancy. The fact that they isolate themselves as hydrophobic droplets makes storage easy, for example, storage of energy in cells which can be released when required via enzyme action. Triacylglycerols are referred to as 'natural fats' and their molecules tend to cluster together to form droplets and keep away from water. Long fatty acid chains arrange themselves into a spherical globule called a micelleform which are achieved by the hydrophobic chains associating together while their hydrophilic heads face outwards, interacting with the water such structures can act as various detergents or soaps. ![]() The glycerol backbone is always constant but fatty acids that are attached to the backbone may differ. Triglyceride molecules are composed of a glycerol (glycerol backbone) and three fatty acids. A fatty acid has the chemical formula CH 3(CH 2) nCO 2-, where n can be anything from 1 to 20 or more.
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