Lipids are organic molecules found in every type of cell they are insoluble in water but solube in organic solvents such as ethanol. The most common that we eat are triglycerides, used as energy stores in plants and animals.
Triglycerides are made up of 3 fatty acides and one glycerol molecule linked by condensation reactions. The bond that forms between each fatty acid and the glycerol is known as an ester bond. Three ester bonds are formed in a triglyceride.
If the fatty acid chains in a lipid contain the maximum number of hydrogen atoms, they are said to be saturated. In a saturated fatty acids the hydrocarbon chain is long and straight.
There are no carbon to carbon double bonds in the saturated fatty acid chain and no more hydrogens can be added to it. Animal fats from meat and dairy products are major sources of saturated fats.
Straight, saturated hydrocarbon chains can pack together closely. The strong intermolecular bonds between triglycerides made up of saturated fatty acids result in fats that are solid at room temperature.
Monounsaturated fats have one double between two of the carbon atoms in each fatty acid chain. Polyunsaturated fats have a larger number of double bounds. A double bound causes a kink in the hydrocarbon chain; these kinks prevent the unsaturated hydrocarbon chains packing closely together. The weaker intermolecular bonds between unsaturated triglycerides result in oilds that are liquid at room temperature. Olive oil is high in monounsaturated fats. Most other vegetable oils, nuts and fish are good sources of polyunsaturated fats.
Other types of lipids
Cholestrol is a short lipid molecule. It is a vital component of cell membranes with roles in their organisation and functioning. Cholestrol is essential for good health. Cholestrol is made in the liver from saturated fats and also obtained in our diet, found associated with saturated fats in foods such as eggs, meat and dairy products. Howeer there are concers that too high a blood cholesterol can be bad for us.
Phospholipids are similar to triglycerides but one of the fatty acids is replaced by a negatively charged phosphate group. Phospholipds are important components of cell membranes.
As well as supplying energy in the diet, fats also provide a source of essential fatty acids, that is, fatty acids that the body needs but cannot synthesis.