Lipids
Lipids are a diverse group of hydrophobic organic compounds that are composed primarily of carbon, hydrogen and oxygen, although they have a lower proportion of oxygen compared to carbohydrates.
Triglycerides
Triglycerides are the most common type of lipid and serve as the primary form of energy storage. Structurally, a triglyceride consists of a glycerol molecule bound to three fatty acid chains through ester bonds. The ester bond is a covalent bond formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid, with the release of a molecule of water (condensation reaction). This bonding results in a molecule that is hydrophobic, making triglycerides effective at storing energy in a compact, anhydrous form.
The fatty acids in triglycerides can be saturated or unsaturated. Saturated fatty acids have no double bonds between carbon atoms, resulting in straight chains that can pack closely together, making triglycerides solid at room temperature (e.g., animal fats). Unsaturated fatty acids contain one or more double bonds, which introduce kinks in the chains, preventing tight packing and resulting in triglycerides that are liquid at room temperature (e.g., vegetable oils).
Phospholipids
Phospholipids are a major component of cell membranes and differ significantly in structure from triglycerides. A phospholipid consists of a glycerol molecule linked to two fatty acid chains and a phosphate group, which is often further linked to other molecules such as choline or serine. The fatty acid chains are hydrophobic, while the phosphate group and its attached molecules are hydrophilic. This results in a molecule with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.
In a phospholipid bilayer, the hydrophobic tails of phospholipids face inward, shielded from water, while the hydrophilic heads face outward towards the aqueous environments inside and outside the cell. This arrangement creates a semi-permeable membrane that regulates the movement of substances into and out of the cell. The bilayer’s fluid nature allows for membrane flexibility and the movement of proteins and lipids within the layer, which is essential for various cellular processes, including signal transduction and membrane fusion.
Steroids: Steroids, such as cholesterol, testosterone and oestrogen, are lipids characterised by a structure of four fused carbon rings. These lipids function as hormones, which regulate various physiological processes including reproduction, metabolism and immune responses