Explain why a thick layer of lipid stored beneath the skin is important for a polar bear surviving in Arctic conditions. Give three reasons in your answer.

1. Lipids are a dense energy store; when food is scarce they are broken down to release glucose and fatty acids, providing the bear with the energy needed for metabolism.
2. Lipids contain about 9 kJ g⁻¹, more than twice the energy of carbohydrates (4 kJ g⁻¹), so a smaller mass of fat supplies the same energy, making it an efficient reserve.
3. Lipids are poor conductors of heat; a thick subcutaneous fat layer insulates the body, reducing heat loss to the cold environment.
4. Lipids are insoluble in water; they remain in the fat layer rather than dispersing in body fluids, keeping the insulation intact.
5. Lipids are composed of glycerol and fatty acids; this structure gives them long hydrocarbon chains that store large amounts of chemical energy and are hydrophobic, aiding insulation and energy density.