Lipids

Lipids: An Overview

• Lipids are a group of biological molecules that are insoluble in water but are soluble in organic solvents.
• The main biological roles of lipids include energy storage, serving as structural components of cell membranes, and participating in signalling processes.
• Lipids can be subdivided into different categories, such as triglycerides, phospholipids and steroids.

Triglycerides

• Triglycerides are formed from one glycerol molecule and three fatty acids.
• Ester bonds link the fatty acids and the glycerol molecule together. This process is a condensation reaction resulting in the formation of water.
• They are the main form of energy storage in animals due their high energy content.
• Fatty acids can be saturated (no double bonds between carbon atoms) or unsaturated (one or more double bonds present).

Phospholipids

• Phospholipids are similar to triglycerides but one of the fatty acids is replaced by a phosphate group.
• As a result, phospholipids have a hydrophilic ‘head’ and two hydrophobic ‘tails’.
• The dual nature of these molecules forms the basis for the creation of cell membranes, where they arrange into a bilayer with the tails facing inwards.

Steroids

• Steroids are lipids characterised by a carbon skeleton consisting of four fused rings.
• They have diverse functions in the body. For example, cholesterol is a type of steroid that is a key component of cell membranes and is also used to synthesise other steroids.

Tests for Lipids

• The emulsion test is a common test for lipids. Mix the sample with ethanol, then add water. If a milky-white emulsion forms on the top, lipids are present.
• Lipids can also be detected using the sudan IV test, where lipid-containing samples turn red in the presence of the Sudan IV dye.