In This A-Level Biology Lesson “Lipids: Triglycerides structure and formation” we begin with an overview of the learning outcomes as usual. Next we emphasise that Lipids are not polymers! From here we go on to describe the structure of triglycerides and the structure of glycerol. Glycerol is attached to fatty acids (which are either saturated or unsaturated) and it is essential you can describe and identify these molecules which make up lipids (triglycerides). The Hydrocarbon tail of lipids is discussed along with the fact that all fatty acids have the functional group COOH (the Carboxyl Group). A Recap of condensation reactions shows exactly how triglycerides are ‘built’ - again another common and by now familiar concept frequently covered and examined in A-Level biology. Glycerol is linked to fatty acids by an ester bond (you’ll need to know this and sometimes be asked to ‘label it up’ in a biology exam)…

Lipids – Fats and Oils are composed from 2 types of molecule: -

1. Glycerol

2. Fatty acids

Condensation Reactions Form Lipids.

A “free” Hydroxyl group from the glycerol molecule joins to the OH on the COOH end of a fatty acid. This condensation reaction can also be called “Esterification” due to the resulting bond formed between the glycerol and the fatty acid being called an Ester bond.

Fatty Acids can be represented in many ways. You should be able to recognise and describe the type of fatty acid being shown.

Remember, saturated fatty acids have NO C-C double bonds.

However, unsaturated fatty acids may have 1 or more C-C double bonds – hence they are unsaturated, since the hydrocarbon tail is not completely ‘full’ (saturated) with Hydrogens.

Condensation reactions build triglycerides.

1 glycerol + 1 fatty acid = 1 monoglyceride + 1 molecule1 of Water.

1 glycerol + 2 fatty acids = 1 diglyceride + 2 molecules of Water.

1 glycerol + 3 fatty acids = 1 triglyceride + 3 molecules of Water.

If one of the fatty acids is replaced with a phosphate group, the molecule is a phospholipid.