Triglycerides, Phospholipids and Cholesterol
What are lipids?
- They are any of a class of organic compounds that are fatty acids or a derivative from a fatty acid.
- They make up about 5% of the organic matter of a cell.
- The lipids that are a solid at room temperature are called fats and ones that are liquid at room temperature are called oils.
- Lipids perform lots of different functions within living organisms:
- They can be respired and thus used as a source of energy and also they can be stored as energy or 'fat' in the adipose cells.
- They make up all biological membranes (phospholipids).
- They provide insulation and protection.
- Some hormones are lipids.
- The actual lipid molecules are made up of carbon, hydrogen and oxygen but the amount of oxygen is very low (much lower than the amount found in carbohydrates).
- They are insoluble in water.
All fatty acids have an acid group at one end of the molecule (this is the same as found on an amino acid) The rest of the molecule consists of a hydrocarbon chain which can be anything from 2 to 20 carbon atoms long, however the most common fatty acids have around 18 carbons in their chains.
Saturated fatty acids are thought to be worse for you than unsaturated fatty acids. Unsaturated fatty acids have double bonds in their carbon chain, this means that it can bond with less hydrogen. The more hydrogen atoms that the carbon atom bonds to, the more saturated it is.
The double bonds in the chain causes the shape of the lipid to change. Double bonds cause 'kinks' in the carbon chain, this means that the molecules push away from each other and as a result of this become more fluid. This is why most unsaturated fatty acids are oils and saturated fatty acids are fats.
- A triglyceride molecule is essentially one glycerol molecule that is bonded to 3 fatty acid molecules in a condensation reaction.
- This reaction occurs between the acid group of the fatty acid and the hydroxyl group of the glycerol molecule and forms a monoglyceride and a water molecule.
- The bond between the two molecules is a covalent ester bond.
- The glycerol molecule will then have two remaining hydroxyl groups which will react with two more fatty acids and produce a triglyceride.
- Phospholipids are essentially the same as triglycerides in that they consist of a glycerol molecule bonded to fatty acid molecules in condensation reactions.
- The one difference is that the third hydroxyl group of the glycerol molecule is bonded to a phosphate group instead.
- The phosphate part (or head) of the molecule is hydrophillic whereas the fatty acid (tails) are hydrophobic.The phosphate part of the molecule is what give phospholipids their ability to form membranes (read more about cell membranes and phospholipids here).
Despite the fact that cholesterol is a lipid it is not made from glycerol and fatty acids, unlike triglycerides and phospholipids.
Cholesterol molecules are small and consist of 4 carbon based rings.
Cholesterol's small, narrow and hydrophobic nature makes it perfect for sitting in between the phospholipid hydrocarbon tails in biological membranes.
Its function is to regulate the fluidity of the phospholipid bilayer and give the membrane strength.
Because cholesterol is so vital to living functions lots of cells in our body can make it, especially the liver cells.
Excess cholesterol can cause problems such as:
- Gallstones - in bile, cholesterol molecules can stick together and this eventually causes lumps called gallstones.
- Atherosclerosis - Blood can deposit cholesterol to the inner linings of blood vessels and cause atherosclerosis and a number of different circulatory problems.