Roles of Lipids (AQA A-Level Biology): Revision Notes
Roles of Lipids
Lipids are diverse biological molecules that share common characteristics making them well-suited for various roles in living organisms. They contain carbon, hydrogen and oxygen, but have a lower proportion of oxygen compared to carbohydrates. Lipids are insoluble in water but dissolve readily in organic solvents such as alcohols and acetone.
Key roles of lipids
Cell membranes
Phospholipids form the structural basis of all cell membranes, including the cell surface membrane and membranes surrounding organelles. Their unique properties contribute to membrane flexibility and enable the transfer of lipid-soluble substances across membrane boundaries. This selective permeability is essential for cellular function and maintaining distinct environments within cells.
The selective permeability of cell membranes is crucial because it allows cells to control what enters and exits, maintaining different concentrations of substances inside and outside the cell. This is fundamental to processes like maintaining pH balance, nutrient uptake, and waste removal.
Source of energy
When oxidised during cellular respiration, lipids provide an exceptionally concentrated energy source. They yield more than double the energy per gramme compared to carbohydrates, making them highly efficient for long-term energy storage. Additionally, lipid oxidation releases valuable water as a metabolic by-product, which is particularly important for organisms in arid environments.
Lipids provide approximately 38 kJ/g when oxidised, compared to only 17 kJ/g from carbohydrates. This makes them the most energy-dense biological molecules and explains why organisms store excess energy as fat rather than carbohydrates for long-term energy reserves.
Waterproofing
The water-repelling properties of lipids make them ideal for creating protective barriers. Plants and insects produce waxy lipid cuticles on their surfaces to reduce water loss and prevent desiccation. Similarly, mammals secrete oily substances from sebaceous glands in the skin, creating a waterproof coating that helps maintain hydration and protects against environmental moisture.
Insulation
Lipids, particularly fats, act as effective thermal insulators due to their poor heat conductivity. When stored beneath the body surface, they help organisms retain body heat and maintain stable internal temperatures. Fats also function as electrical insulators in the myelin sheaths that surround nerve cells, enabling rapid and efficient nerve impulse transmission.
The insulating properties of lipids serve dual purposes - thermal regulation and electrical insulation. In nerve cells, the myelin sheath can increase the speed of nerve impulses by up to 100 times compared to unmyelinated nerves.
Protection
Fat deposits around delicate internal organs, such as the kidneys, provide crucial mechanical protection against physical impact and damage. This cushioning effect helps maintain organ integrity during movement and protects vital structures from injury.
Physical properties
The physical state of lipids depends on their fatty acid composition and temperature. Fats remain solid at room temperature (10-20°C) due to their high content of saturated fatty acids, while oils stay liquid because they contain more unsaturated fatty acids.
The difference between fats and oils is primarily structural - saturated fatty acids have no double bonds and can pack tightly together (creating solids), while unsaturated fatty acids have double bonds that create kinks in the molecule, preventing tight packing and maintaining liquid state at room temperature.
Key Points to Remember:
- Lipids serve five main biological roles: membrane structure, energy storage, waterproofing, insulation, and protection
- They provide over twice the energy content of carbohydrates when oxidised
- Their water-insoluble nature makes them perfect for waterproofing and membrane formation
- Fat deposits act as both thermal insulators and protective cushions for organs
- The distinction between fats (solid) and oils (liquid) relates to their fatty acid saturation levels