Essential Oils

Phospholipids - the basis of biological membranesEssentially, a phospholipid molecule is almost identical to a triglyceride molecule. It consists of a glyercol molecule with fatty acid molecules bonded by condensation reactions to produce ester bonds. In phospholipids, the third fatty acid is not added to the glycerol molecule. Instead, a phosphate group is covalently bonded to the third OH group on the glycerol. The bonding of the phosphate group occurs by a condensation reaction and so a water molecule is released. The phosphate 'head' of the molecule is hydrophilic, but the hydrocarbon chain fatty acid tails are hydrophobic. The majority of a phospholipid molecule is insoluble in water, like all lipid molecules. The water solubility of the head group gives phospholipids their characteristics in terms of the capacity to form membranes.Phospholipids and membrane fluidityThe fatty acids that make up a phospholipid may be saturated or unsaturated. Organisms can control the fluidity of membranes using this feature. For example, organisms living in colder climates have an increased number of unsaturated fatty acids in their phospholipid molecules. This ensures that membranes remain fluid, despite their low temperatures.Lipids and respirationRespiration of lipids first requires the hydrolysis of the ester holding the fatty acids and glycerol together. This is the reverse of the condensation reaction that joins them together. Both the glycerol and fatty acids can then be broken down completely to carbon dioxide and water. This releases energy, which is used to generate ATP molecules. The respiration of one gram of lipid gives out about twice as much energy as the respiration of one gram of carbohydrate. Because the lipids are insoluble in water, they can be stored in a compact way and they do not affect the water potential of the cell contents. These features make triglyceride an excellent energy storage molecule. The respiration of lipid gives out a great deal more water than the respiration of carbohydrate. This metabolic water is vital to some organisms. Cholesterol and Steroid HormonesCholesterol is a class of lipid. It is not formed from fatty acids and glycerol like triglycerides and phospholipids. It is a small molecule made from four carbon based rings. Cholesterol is found in all biological membranes. Its small narrow structure and hydrophobic nature allow it to sit between the phospholipid hydrocarbon tails and help regulate the fluidity and strength of the membrane.The steroid hormones testosterone, oestrogen and vitamin D are made from cholesterol. The lipid nature of the steroid hormones mean they can pass directly through the phospholipid bilayer in order to reach their target receptor. This is usually inside the nucleus, so they also pass through the lipid bilayer of the nuclear envelope. Cholesterol is vital to living organisms, so many cells can make it,. Excess cholesterol may be a problem in humans because: in bile, cholesterol can stick together to form lumps called gallstones in blood, cholesterol can be deposited in the inner linings of blood vessels causing atherosclerosis, which can result in a number of circulatory problems A condition known as famial hypercholesterolaemia (FHC) is a genetic disorder, where cells manufacture and secrete cholesterol even though there is already sufficient in the blood to provide for the organisms requirements. This happens because cells do not obey the signals to stop cholesterol production, as they lack a particular cell surface receptor.Individuals with this condition in its worst form can suffer heart attacks and strokes by the age of 2 years.Triglyercide: glyercol plus three fatty acids compact energy store, insoluble in water so doesn't affect cell water potential stored as fat, which also has thermal insulation and protective properties Phospholipids: glycerol plus two fatty acids and a phosphate group forms a molecule that is part hydrophobic, part hydrophilic, ideal for basis of cell surface membranes phosphate group may have carbohydrate parts attached - these glycolipids are involved in cell signalling Cholesterol: four carbon based ring structures joined together forms a small, thin molecule that fits into the lipid bilayer giving strength and stability used to form the steroid hormones

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