Lipids are organic molecules that are hydrophobic or insoluble in water because the non-polar, covalent bonds linking carbon and hydrogen aren’t attracted to the polar bonds of water. The four major categories of lipids include fats, phospholipids, waxes and steroids.
* Fats *
Fats and oils are lipids composed of two kinds of molecules:
* glycerol (a type of alcohol)
* three fatty acids (so known as triglycerides)
The shape of the fatty acid tails, whether they are straight or have kinks in them, influences whether these fats are considered ‘saturated’ or ‘unsaturated’.
Saturated fats: This type of fat is mostly derived from animal sources. There are single bonds between the carbons in their fatty acid tails, and all carbons are bonded to maximum number of hydrogens that is possible. Therefore the hydrocarbon chains in these fatty acids are fairly straight and packed closely together. This is why saturated fats are solid at room temperature.
Unsaturated fats (oils): This type of fat is mostly from plant sources. There are double bonds between some of the carbons in the hydrocarbon tail, causing bends or kinks. These kinks in the hydrocarbon tails prevent the tails from packing closely together. This is why unsaturated fats are liquid at room temperature.
* Phospholipids *
These fats have a hydrophobic, water hating, hydrocarbon tails and hydrophilic, “water loving” phosphate groups on the end. This means that phospholipids are soluble in both water and oil.
Our cell membranes are made mostly of phospholipids arranged in a double layer with the tails from both layers facing inward and the heads facing outward, called the lipid bilayer.
* Waxes *
These molecules are esters of alcohol which are insoluble in water and difficult to hydrolyze, or break down. Wax forms protective and waterproof layers on some plants, bacteria, animal fur and integuments of insects.
* Steroids *
The central core of a cholesterol molecule, consisting of four fused rings, is shared by all steroids. Our bodies make about 2 g of cholesterol per day, and that makes up about 85% of blood cholesterol, while only about 15% comes from dietary sources.
Cholesterol is precursor to our sex hormones and Vitamin D. Our cell membranes contain a lot of cholesterol which helps to keep the membrane flexible and fluid even when our cells are exposed to cooler temperatures.
* Sources *
Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw-Hill
