Commonly lipids are made of two parts, glycerol and fatty acids. One of the examples of lipids is triglyceride. When a glycerol molecule is attached with one fatty acid, it is called monoglyceride. If it is attached with two fatty acids, it is called diglyceride, with three fatty acids, triglyceride forms. A condensation reaction produces a triglyceride and water molecule when glycerol and three fatty acids react together. As seen on the diagrams below, each hydrogen atom at the right of the glycerol is bonded to a hydroxyl (-OH) group. This releases water, and ester bond is formed, which is called condensation reaction.
One another important type of lipids is phospholipids. They are essential components of all cell membranes. A phospholipid is similar to a triglyceride, but have one fatty acid group replaced by a phosphate group. This phosphate group contains negative charged atoms, which makes the group itself to polar. Phospholipids have polar head with phosphate group and glycerol, and non-polar tails with fatty acids which are not soluble in water. The cell membrane is composed of a bilayer of phospholipids. The hydrophobic tails face each other (pointing inwards) and the hydrophilic heads point outward and are in contact with the aqueous external and internal cellular environments.
Waxes are esters formed from a fatty acid and a complex alcohol (in place of glycerol). Waxes occur as the water-repellent cuticles of the plant epidermis and of the exoskeleton of insects.
Steroids have a structure of complex rings of carbon atoms. Also, Cholesterol, a steroid, is a component of cell membranes. Sex hormones, growth hormone and bile salts are produced from cholesterol.
We have looked the structure, types and simple biological roles of lipids. Then why are lipids taking very important role in the living organisms?
- One of the very important reasons that lipids are necessary is because it is used as a reserve energy source. As I have commented before, lipids have more chemical potential energy. Upon breakdown they yield 38kJ/g of energy, it is nearly twice as much energy on combustion as do carbohydrates, considering carbohydrates yield 17kJ/g. This is due to the fact that a fat has a greater proportion of H atoms to O atoms and is therefore capable of a greater oxidation.
- Much energy is released during cellular respiration of fats. Lipids may act as a source of metabolic water. When respired they yield water and carbon dioxide. Some desert animals are so efficient in their conservation of water that they obtain their water from the foods they eat or from the metabolism of fat (camels).
- Fats and oils are also used as good stores in some plant seeds and fruits because fats and oils are insoluble in water, and are therefore osmotically inactive. This makes them a useful storage form, for example, olives or peanuts.
- Lipids are a component of cell membranes in the form of phospholipids. Phospholipids play an important role in the structure of these plasma membranes. Cholesterol is a steroid that is essential for keeping these cells membranes fluid.
- In mammals just beneath the skin, fat stores the mammal warm by acting as an insulating layer. This is possible since fat is a poor conductor of heat. For example, subcutaneous fat in animals such as whales keeps itself warm.
- Also, in mammals, fat is deposited around delicate organs to protect them from mechanical damage. Organs such as heart and kidneys are protected by fat as well.
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For organisms such as fur-covered birds and animals, the oily secretion produced in the sebaceous glands helps to waterproof the feathers and fur since lipids are insoluble in water. (However, Lipids will wash out by alkalis. This is why the birds like ducks will not be able to swim if we wash them with soaps.)
- Fats form waxes which waterproofs the cuticles of plants as well, reducing the water loss.
- Some hormones, e.g. testosterone, oestrogen and progesterone (sex hormones) are steroids.
- Lipids occur in the Myelin sheath of nerves as an insulating layer around nerve fibres.
- Fats are important for the absorption of fat-soluble vitamins A, D,E and K by mammals.
- In insects, the exoskeleton is also covered in a waxy cuticle, again reducing water loss through the exoskeleton.
There are various types of lipids, fats & oils, steroids, waxes, and phospholipids. Triglyceride is one of the fats and oils, produced by condensation, reacting glycerol and three fatty acids react together to form ester bond and release water. Phospholipid is similar to a triglyceride, but have one fatty acid group replaced by a phosphate group. Waxes are esters formed from a fatty acid and a complex alcohol. Steroids have a structure of complex rings of carbon atoms.
Lipids take very important part in the lives of living organisms. They store and release more energy, easy to store energy and heat since they are insoluble and a poor conductor of heat. Lipids are a component of cell membranes in the form of phospholipids, helps to protect critical organs to reduce mechanical injury. The oil secretion and waxes allows the plant cuticle and insect exoskeleton to reduce water loss and animal furs to be waterproof. Fats are important for the absorption of fat-soluble vitamins A, D,E and K by mammals. Lastly, some (sex) hormones are steroids, and lipids also occur in the sheath of nerves as an insulating layer around nerve fibres.
