Lipids

[Wyoming - Forbes]

Fats and lipids are important components of the body's homeostatic function. Lipids help with some of the most important processes in the body. Lipids are fatty, waxy or oily compounds that are soluble in organic solvents but insoluble in polar solvents such as water. 

Lipids include: Fats and oils (triglycerides), Phospholipids, Waxes, and Steroids.

Fats and oils are esters of glycerol (a 3-carbon sugar alcohol/polyol) and 3 fatty acids. Fatty acids are hydrocarbon chains of varying lengths with varying degrees of saturation, ending in carboxylic acid groups. Additionally, fatty acid double bonds can be cis or trans, resulting in many different types of fatty acids. Fatty acids in biological systems typically contain an even number of carbon atoms, typically 14 carbons to 24 carbons long. Triglycerides store energy, insulate cells, and aid in the absorption of fat-soluble vitamins. Fats are usually solid at room temperature, while oils are usually liquid.

Lipids are important components of cell membranes. The structure typically consists of a glycerol backbone, 2 fatty acid tails (hydrophobic), and a phosphate group (hydrophilic). Therefore, phospholipids are amphiphilic. In cell membranes, phospholipids are arranged in a bilayer that provides cellular protection and acts as a barrier to certain molecules. The hydrophilic part faces outward and the hydrophobic part faces inward. This arrangement helps monitor which molecules can enter and leave the cell. For example, non-polar molecules and small polar molecules, such as oxygen and water, diffuse easily in and out of cells. Large polar molecules, such as glucose, cannot pass freely, so they need the help of transporters.

Another lipid is wax. Waxes are esters made from long-chain alcohols and fatty acids. They provide protection, especially to plants whose leaves are covered in wax. In humans, cerumen, also known as earwax, helps protect the skin of the ear canal.

Another class includes steroids with 4 fused ring structures. An important steroid is cholesterol. Cholesterol is produced in the liver and is the precursor to many other steroid hormones such as estrogen, testosterone, and cortisol. It is also part of the cell membrane, inserting itself into the bilayer and affecting the fluidity of the membrane.