The Nature of Lipids

Lipids are a varied group of organic compounds that contain carbon, hydrogen and oxygen. Compared with carbohydrates they contain a much smaller proportion of oxygen and a large proportion of hydrogen, mostly in long hydrocarbon chains.The key feature of lipids is that they are insoluble in water but soluble in organic solvents such as ethanol. Their long hydrocarbon chains are non-polar, so they cannot form hydrogen bonds with water: they are hydrophobic.Lipids are large molecules, but they are not polymers, because they are not built from a chain of repeating monomers.The main types of lipid are:
• triglycerides: fats (solid at room temperature) and oils (liquid)
• phospholipids: the main component of cell membranes
• steroids, such as cholesterol and steroid hormones like testosterone
• waxes, such as the waxy cuticle on leavesRoles of lipids:
• energy storage: lipids release about twice as much energy per gram as carbohydrates when respired
• insulation: fat under the skin reduces heat loss, and the myelin sheath insulates neurones electrically
• protection: fat cushions organs such as the kidneys
• waterproofing: waxy cuticles and oily secretions
• membranes and hormonesWorked example: lipid releases about 37 kJ g⁻¹ and carbohydrate about 17 kJ g⁻¹. To store 740 kJ, an animal needs 740 ÷ 37 = 20 g of fat, but about 44 g of carbohydrate.