Lipids: Structure and Function of Phospholipids | A-level Biology | OCR, AQA, Edexcel

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The key points covered in this video include:

1. Structure of Phospholipids
2. Behaviour in Water
3. Structure of the Phospholipid Bilayer
4. Roles of the Phospholipid Bilayer

Structure of Phospholipids

Phospholipids are a type of lipid that are similar in structure to triglycerides. The difference is that one of the fatty acids is replaced by a phosphate molecule. This occurs through a condensation reaction between the hydroxyl (-OH) groups on phosphoric acid and glycerol. This creates a phosphate ester bond and forms a molecule of water. The phosphate group and glycerol molecule is called the “head” of the phospholipid and the fatty acid chains are called the “tails”. This is often displayed more simply like this:

Behaviour in Water

When the phospholipid is surrounded by water, the hydrogen ions dissociate from the phosphoric acid. This forms a phosphate group which has a negative charge. The negative charge means the phospholipid head is hydrophilic. A hydrophilic molecule is a molecule that is attracted to water due to having a charge. However, the fatty acid tails are non-polar and not charged and therefore are hydrophobic. A hydrophobic molecule is a molecule that is repelled by water due to not having a charge. The hydrophilic and hydrophobic ends of the phospholipid cause the phospholipid to have unique behaviour in water: Phospholipids may form a layer on the surface of the water, Phospholipids may also form a monolayer of phospholipids called a micelles, Phospholipids may also form a bilayer.

Structure of the Phospholipid Bilayer

The phospholipid bilayer is an integral part of all membranes in a cell. In a phospholipid bilayer, the hydrophilic heads form two rows on the outside while the hydrophobic tails are sheltered in the middle. The phospholipids can move past each other which keeps the membrane fluid. However, they will never expose the hydrophobic tails to the water- keeping the membrane stable. For eukaryotes, a high percentage of the cell membrane is composed of phospholipids. For prokaryotes, this percentage is lower as there are more proteins in the membrane.

Roles of the Phospholipid Bilayer

The structure of the phospholipid bilayer allows the cell membrane to carry out some important roles: Control of substances in an out of the cell, Electrical insulation. In the phospholipid bilayer, the area in the middle is a non-aqueous environment. Therefore, this creates a barrier to charged molecules which are dissolved in water. Only small, non-polar molecules like oxygen and carbon dioxide can easily pass through the membrane. Therefore, the membrane is partially permeable, allowing it to control what enters and leaves the cell. The partial-permeability of the membrane also allows it to act as an electrical insulator. It insulates against charge because charged ions cannot pass through the phospholipid bilayer.

Summary

Phospholipids are composed of glycerol, 2 fatty acids and a phosphate group joined together in condensation reactions
The phospholipid head is hydrophilic but the phospholipid tail is hydrophobic
Phospholipids in aqueous solution therefore form a bilayer with heads facing outwards and tails facing inwards
The phospholipid bilayer is an essential part of all cell membranes
The phospholipid bilayer is an electrical insulator and is partially permeable so it can control movement of substances