The role of tryglycerols goes beyond energy storage. Tryglycerols, commonly known as fat in animals and oils in plants is used to aid buoyancy of aquatic vertebrates such as sharks, seals and whales. This is done because lipids are less dense than water. “Sharks have extremely fatty livers which make up to 25% of their body volume and contain a lipid, squaline, with a specific gravity of only 0.86.”(2)
Animals such as kangaroos, camels and rats also use fats to provide themselves with “metabolic water”. “When fats are oxidised in respiration water is produced and is used by the above mentioned animals in their daily metabolism”(2). Animals, for insulation, use layers of fat. “This is because fats can only conduct heat slowly and therefore are good insulators. In mammals fat is stored beneath the skin (subcutaneous fat) to retain body heat. In aquatic mammals like whales, seals and manatees, this layer of fat is known as blubber.”(3) Subcutaneous fat has shock absorbing properties and so is used by some animals to also protect the vital organs, for example, the kidney in humans.
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) in and out of the plasma membrane.”(3) The second role it has in the cell membrane is to “pull together the fatty acid chains in phospholipids and restrict their movement without making them solid.”(5)
Cholesterol is also used to make “bile salts, sex hormones and the hormones of the adrenal cortex”(2). The hormones are made from a small amount of cholesterol believed to be between “120-150g”(3). This is maintained by biosynthesis in the liver and intestine. As can be seen a wide variety of hormones are produced from lipids. Cholesterol is also an important constituent of myelin. Myelin is a substance that is found in the nervous system. It is secreted by “Schwann cells”(5) and acts as a layer of insulation over the axons of nerve cells. The insulation provided by myelin, “maintains the ionic charge found in nerve cells over long distances and thus speeds up the transmission of an impulse”(6). Fast transmission is needed during in voluntary reactions carried out by the body. “Examples of myelinated axons are those in sensory nerve fibres and nerves connected to skeletal muscles”(6).
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.
Bibliography
- Oxford Interactive Encyclopaedia 1997
- Advanced Biology – Principles and Applications , Clegg and Mackean, 1994
- Understanding Biology for Advanced Level, Toole and Toole, 1999
- Notes made in Class
- Encarta 98 Encyclopaedia CD ROM