Lipids

Facts

Types of Lipids

There are two main groups of lipids covered in the AQA A-Level specification:

1. Triglycerides
2. Phospholipids

Triglycerides

Triglycerides are formed through condensation reactions between:

• One glycerol molecule (a 3-carbon alcohol)
• Three fatty acid molecules

Structure and Formation

• Each fatty acid forms an ester bond with glycerol
• General formula for fatty acids: RCOOH
• The 'R' represents a hydrocarbon chain
• Three water molecules are released during formation

Fatty Acids

Fatty acids can be classified into two types:

1. Saturated Fatty Acids

   • No double bonds between carbon atoms
   • Straight hydrocarbon chains
   • Solid at room temperature
   • Found in animal fats
   • Example: Palmitic acid

2. Unsaturated Fatty Acids

   • Contains one or more double bonds
   • Kinked hydrocarbon chains
   • Liquid at room temperature
   • Found in plant oils
   • Example: Oleic acid

Phospholipids

Phospholipids are modified triglycerides where:

• One fatty acid is replaced by a phosphate group
• Results in a molecule with both hydrophilic and hydrophobic regions

Structure and Properties

1. Hydrophilic Head

   • Contains phosphate group
   • Attracted to water
   • Forms outer surface in membranes

2. Hydrophobic Tails

   • Two fatty acid chains
   • Repel water
   • Point inward in membranes

Properties and Functions

Triglycerides

1. Energy Storage

   • Compact energy store
   • Yields more energy per gram than carbohydrates
   • Insoluble in water
   • Does not affect cell water potential

2. Insulation

   • Provides thermal insulation
   • Protects organs
   • Found in adipose tissue

Phospholipids

1. Membrane Formation

   • Forms bilayers in cell membranes
   • Amphipathic nature allows self-assembly
   • Creates selective barrier

2. Membrane Properties

   • Allows selective permeability
   • Fluid mosaic structure
   • Enables membrane protein function

Biochemical Tests

Emulsion Test

1. Add ethanol to the sample
2. Mix thoroughly
3. Add water
4. A white emulsion indicates the presence of lipids

Recall

Practice