Lipids are the third major type of biochemical molecule found in humans. They have several important roles in our body including storing energy, signaling and acting as structural components of cell membranes.
Fun Fact - Do you know earwax is a lipid?
In this lesson, we will learn:
Lipids are organic compounds containing hydrogen, carbon and oxygen atoms, which form the framework for the structure and function of living cells. In the human body, these molecules can be synthesized in the liver.
A general overview of types of lipids:
Lipids are the polymers of fatty acids that contain a long, non-polar hydrocarbon chain with a small polar region containing oxygen.
There are four main groups of lipids.
This is a group of stored lipids and includes fats and oils. Triglycerides are composed of a single molecule of glycerol bound to three fatty acids. Glycerol is three-carbon alcohol with three -OH groups that serve as binding sites.
Fatty acids are long-chain hydrocarbon molecules with a carboxyl group (COOH) at one end which is free to bind to one of the OH groups of the glycerol, thus forming a bond called ester bond.
The fats may be saturated or unsaturated. If the carbons in the chain are single-bonded, the fat is saturated; if there is at least one C=C bond in the chain, it is unsaturated. The structure of fatty acids is responsible for the physical nature of fats and oils (liquid fats) which are greasy and insoluble.
In general, solid fats are saturated, and oils are unsaturated. In most cells, triglycerides are stored in concentrated form as droplets or globules for long term use.
This is a class of lipids that serve as a major structural component of cell membranes. Although phospholipids are similar to triglycerides in containing glycerol and fatty acids, there are some significant differences.
Phospholipids contain only two fatty acids attached to the glycerol, while the third glycerol binding site holds a phosphate group. This phosphate is in turn bonded to alcohol. These lipids have both hydrophilic and hydrophobic regions due to a change in the phosphoric acid/alcohol ‘head’ of the molecule and lack of a charge on the long ‘tail’ of the molecule (formed by the fatty acid).
When exposed to an aqueous solution, the charged heads are attracted to the water phase and the non-polar tails are repelled from the water phase. The way lipids naturally assume single and double-layered (bilayer) configuration makes them a valuable component of the primary framework of cell membranes.
When two single layers of polar lipids come together to form a double layer, the outer hydrophilic face of every single layer will orient itself towards, the solution and the hydrophobic portion will become immersed in the core of the bilayer.
The structure of the lipid bilayer helps the membrane in functions such as selective permeability and fluid nature.
These are complex compounds commonly found in cell membranes and animal hormones. The best known of these is the sterol called cholesterol which reinforces the structure of the cell membrane in animal cells and in an unusual group of cell wall deficient bacteria called mycoplasmas. The cell membrane of fungi also contains a sterol called ergosterol.
Waxes are esters formed between long-chain alcohol and saturated fatty acids. This material is typically pliable and soft when warm but hard and water-resistant when cold e.g. paraffin. Fur, feather, fruits, leaves, human skin and insect exoskeleton are naturally waterproofed with a coating of wax. Waxes are produced by animals and plants and are typically used for protection. Plants use waxes to help prevent water loss. Humans have wax in our ears to help protect our eardrums.
Lipids can serve a diverse range of functions within a cell, including
