Introduction

Lipids have a varied structure but all have the following properties in common:- Insoluble in water Soluble in organic solvents

The three main groups of lipids are:- Triglycerides Phospholipids Steroids

Lipids are important in cell membrane structure and also as energy storage molecules and hormones.

Triglycerides

Triglycerides consist of a single glycerol molecule and three fatty acids.

GlycerolFatty acids

Formation of Triglycerides

Triglycerides form by condensation (dehydration) reactions between the hydroxyl (OH) groups of the glycerol and the carboxyl (COOH) group of three fatty acids.

Triglycerides are esters being derived from an alcohol and a fat.

Triglycerides (cont’d.)

Glycerol (blue) is an alcohol derivative of glyceraldehyde and has three hydroxyl groups. It acts as the backbone of the structure.

Fatty acids (red) – there are more than 70 types of fatty acid but they all have long hydrocarbon tails and a terminal carboxyl group (COOH). The variety of fatty acids determine the properties of each triglyceride.

The structure of triglycerides is varied due to the variety of fatty acids. These molecules may be two types which give rise to lipids with different properties:-

Saturated – these have no double C - C bonds e.g. palmitic acid and stearic acid. Triglycerides containing saturated fatty acids are very straight in structure and pack closely together.

Unsaturated – have one or more double C - C bonds e.g. oleic acid or linoleic acid. Lipids containing unsaturated fatty acids are ‘kinked’ in structure and cannot pack so closely together.

Structure of Triglycerides

Triglycerides in Animals

Animals store their energy in triglycerides with high melting points which are solid at room temperature.

These triglycerides are referred to as fats. result from reaction between

glycerol and a saturated fatty acid e.g. stearic acid.

Triglycerides in Animals

Tips to help you remember. In animals triglycerides contain :

Single bonds only in fatty acids

Saturated fatty acids

Straight chained fatty acids

Stearic Acid is an example

Solid at room temperature – high melting point

Store energy Source of metabolic water

Simply amazing as insulation and buoyancy aid

Triglycerides in Plants

Plants store their energy in triglycerides with low melting points which are liquid at room temperature.

These triglycerides are referred to as oils result from reaction between

glycerol and an unsaturated fatty acid e.g. oleic acid.

Triglycerides in Plants

Tips - to help you remember. In plants triglycerides are the ‘opposite of animals’ so learn the animals first and then go through the list and change as follows

Double bonds in fatty acids Unsaturated Kinked chains Oleic Acid is example Liquid at room temperature – low melting point Store energy

Triglycerides in Cells

Triglycerides are insoluble in water because they have no charge i.e. they have covalent bonds. This causes them to form droplets in the cytoplasm.

Energy storage - triglycerides contain twice the energy/gram of carbohydrates or proteins. During aerobic respiration triglyceride is broken into 2C portions which are fed into the Krebs cycle.

Functions of Triglycerides

Source of metabolic water

– water is released on the breakdown of triglycerides and this property is used efficiently is by desert mammals.

Functions of Triglycerides (cont’d.)

Insulation – triglycerides are found in the blubber of whales and other aquatic animals.

Buoyancy – aquatic animals use triglycerides to help them float as they are less dense than water.