A Level Biology Lesson 7:

Lipids: The Structure & Formation of Triglycerides.

Task 1. Check Where this Lesson fits into your Exam Specification!

Task 2. Watch the Revision Notes Lesson in Full.

00:00 Intro screen / Learning Outcomes

00:42 Lipids are not polymers

01:04 Glycerol Structure

01:50 Fatty Acid (Saturated and Unsaturated) Structure

04:37 Lipids contain hydrocarbons

05:24 All fatty acids have the functional group COOH (the Carboxyl Group)...

06:10 Condensation reactions build triglycerides...

07:23 Ester Bond

08:39 Summary.

Task 3. 

Download, Print out & complete the accompanying Interactive Worksheet.


A-Level Biology "Lipids: The Structure and Formation of Triglycerides" Lesson 7. Summary.

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.

Check Your Spec!

★ AQA Specification Reference: - 3.1.3 Lipids. Triglycerides and phospholipids are two groups of lipid. Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid. A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond. The R-group of a fatty acid may be saturated or unsaturated. In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. Students should be able to recognise, from diagrams, saturated and unsaturated fatty acids.


★ CIE Specification Reference: - 2 Biological molecules - 2.2 Carbohydrates and lipids. f) describe the molecular structure of a triglyceride with reference to the formation of ester bonds.


★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 1: Lifestyle, Health and Risk. 1.14 i) Know how a triglyceride is synthesised by the formation of ester bonds during condensation reactions between glycerol and three fatty acids. ii) Know the differences between saturated and unsaturated lipids.


★ Edexcel (Biology B) Specification Reference: - Topic 1: Biological Molecules. 1.2 Lipids i) Understand how a triglyceride is synthesised, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids. ii) Know the differences between saturated and unsaturated lipids.


★ OCR (Biology A) Specification Reference: - 2.1.2 Biological molecules. Learners should be able to demonstrate and apply their knowledge and understanding of: (h) the structure of a triglyceride. To include an outline of saturated and unsaturated fatty acids. (i) the synthesis and breakdown of triglycerides by the formation (esterification) and breakage of ester bonds between fatty acids and glycerol.


★ OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. (j) The structure of triglycerides. To include reference to fatty acids, glycerol, phosphate groups, ester bonds and hydrophobic/ hydrophilic properties.


★ WJEC Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds: - (f) the structure, properties and functions of lipids as illustrated by triglycerides and phospholipids.



Darwin T




1Fractured DNA



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