BIOLOGICAL MOLECULES: Lipids

Lipids: a group of substances that are soluble in alcohol rather than water. These include triglycerides, phospholipids, glycolipids and cholesterol.

Macromolecule: a very large, organic molecules

Phospholipid: molecules consisting of glycerol, two fatty acid chains and one phosphate group.

Lipids contain large amounts of carbon and hydrogen, and smaller amounts of oxygen. They're insoluble in water because they're not polar, and so don't attract water molecules, but do dissolve in alcohol. 

Triglycerides

Triglycerides are made up of glycerol and fatty acids. There are many types of fatty acids. We can make many of them in our bodies, but some must be ingested - these are called essential fatty acids.

Glycerol

Glycerol has three carbon atoms. It is an alcohol meaning it has free -OH group. There's three of these OH groups which is important in forming triglycerides.

Fatty Acids

Fatty acids have a carboxyl group (-COOH) on one end, attached to a hydrocarbon tail made of only carbon and hydrogen atoms. The carboxyl group ionises into H+ and COO- groups. This structure is therefore an acid as it can produce hydrogen ions.

If a fatty acid is saturated, there are no C=C bonds in the molecule. If a fatty acid is unsaturated, however, there is a double bond between two carbon atoms. A single C=C bond makes a fatty acid monounsaturated whereas multiple C=C bonds makes a fatty acid polyunsaturated. Having one or more C=C bonds changes the shape of the hydrocarbon chain, giving it a king where the double bond is. Because these kinds push the molecules apart slightly, it makes them more fluid. Animal lipids contain lots of saturated fatty acids which are often solid at 20 degrees. If there are more unsaturated fatty acids, the melting point is lower.

Ester Bonds

A trigylceride consists of one glycerol molecules bonded to three fatty acids - as a glycerol molecule has three -OH groups for the fatty acid chains to bond to. A condensation reaction happens between the carboyl group (-COOH) of the fatty acid and the hydroxyl group (-OH) of the glycerol. Because it is a condensation reaction, a water molecule is produced, and the covalent bond formed is called an ester bond. An ester bond is (O-C=O).

Functions of Triglycerides:

- Energy source: triglycerides can be broken down in respiration to release energy and generate ATP. The first step is to hydrolyse the ester bonds and then both glycerol and the fatty acids can be broken down completely to carbon dioxide and water. Respiration of a lipid produces more…