The Role of Lipids In Living Organisms
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The Role of Lipids in Living Organisms Lipids can be defined as "a group of substances that contain carbon, hydrogen and oxygen, as do carbohydrates, but in lipids the proportion of oxygen is much less."(1) They are also defined as "hydrophobic (insoluble in water) yet are soluble in organic solvents like ethanol, chloroform and ether"(2). Lipids are a diverse group of organic compounds and so can be classified as: 1. "Simple lipids, the animal fats, the vegetable oils and the waxes 2. Complex lipids 3. Phospholipids and related compounds 4. Steroids"(2) "Tryglycerols are formed by a condensation reaction that takes place between glycerol (an alcohol) and monocarboxylic acids (fatty acids)" The result of this reaction gives rise to a molecule of triglycerol and is used by living organisms as a source of energy store. When broken down lipids in the form of tryglycerols yield "38 kJg-1 of energy which is twice as much as the amount of energy that carbohydrates yield, 17kJg-1"(3). Another reason for using lipids as an energy store as oppose to carbohydrates is that lipids are more compact and insoluble in water.
Lipids can be made into phospholipids by replacing one of the fatty acid groups with phosphoric acid. This acid is said to be "hydrophilic (attracts water) and this contrasts with the remainder of the molecule which is hydrophobic"(3). This property of phospholipids is why they make up part of the plasma membrane of cells. They form a phospholipid bilayer with the hydrophilic phosphate group facing outwards and the hydrophobic fatty acids facing inwards. This helps to separate the contents of the cell from the outside of the cell. Another type of lipid is involved with the functions of the plasma membrane. These are steroids. The only feature that steroids have in common with other lipids like tryglycerides is that they are "hydrophobic. All steroids are derivatives of a common structure made up of 4 fused carbon rings. Each different steroid contains different chemical groups attached to these rings."(7). As mentioned above steroids form a significant part of the cell membrane but are also involved in the synthesis of hormones such as, "as testosterone, oestrogen, progesterone, and cortisol."(7) The most common steroid is called cholesterol. Cholesterol limits the "uncontrolled leakage of small molecules (for example water and ions)
Lipids also exist in the form of waxes. Waxes are similar to other lipids like fats and oils but differ in the fact that waxes are, "esters of higher fatty acids with long chain alcohols (i.e. not with glycerol)."(5). A major property of wax is that it is waterproof and is therefore used by plants on their leaves (the waxy cuticle). This is done to "reduce water loss by evaporation since water cannot pass through the insoluble lipid layer"(6). Wax is used by other organisms like bees to construct their larval chambers. The scents produced by many plants are known to be, "derivatives of fatty acids"(5). This is done to attract insects to aid pollination. In humans there is a sebaceous gland that is found in the skin. "This gland produces and 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). In birds the "preen gland produces a secretion which performs a secretion that helps to waterproof the feathers."(5) As has been described, this diverse group of organic compounds has many properties and functions in living organisms of all kinds including mammals, birds, fish and plants.
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