Lipids

Lipids are one of the four major groups of organic molecules; the others being proteins, nucleic acids and carbohydrates.
They are hydrophobic or amphiphilic molecules, most of which are soluble in organic solvents but not in water.

Lipids play several crucial roles in the body. They are an important component of the diet, providing a concentrated energy source as they provide about twice the energy content compared to carbohydrates and proteins.
They also serve as insulation, helping keep body temperature regulated.
They are fundamental to cell membrane structure; the hydrophobic lipid bilayer composes the core of the membrane, creating a barrier that protects the cell and controls the movement of substances in and out of the cell.

Triglycerides are the most common type of lipid. They are composed of a glycerol molecule and three fatty acids.
Phospholipids are similar to triglycerides but one of the fatty acids is replaced by a phosphate group. They are key components of cell membranes due to their amphipathic properties.
Steroids include many hormones and cholesterol. Cholesterol is an important component of cell membranes and a precursor for vitamin D and steroid hormones.

The properties of lipids are largely determined by their structure, specifically the fatty acids they contain.
Fatty acids can either be saturated or unsaturated. Saturated fatty acids have no double bonds between carbons, are usually solid at room temperature, and are mostly found in animal fats.
Unsaturated fatty acids have one or more double bonds, are usually liquid at room temperature, and are typically found in plant oils.

Lipids can be metabolised to produce ATP, in a process known as β-oxidation, where fatty acids are broken down in the mitochondria.
Lipids can also be synthesised in the body in a process called lipogenesis, which typically follows a meal when glucose levels are high.
Ultimately, lipid metabolism helps to maintain the body’s energy balance.