Lipids are the fatty or waxy molecules that make up the basic structure of many cells in both plants and animals. They go by other terms depending on where they are found, including triacylglycerol, triglyceride, and glycerolipid, just to name a few.
Lipids serve important functions in the cell. First, they form a support structure for the cellular and nuclear membranes. They also perform a role in the signaling and transport processes of the cell, especially those involving fat-soluble nutrients like vitamins A, E, and K. Finally, lipids are an important way for cells to store energy.
While many people use the terms lipids and fats synonymously, fats are actually just a subgroup of lipids, as there are other kinds.
Fats are probably what most people think of when they hear the term lipids. These fats can include the saturated and unsaturated fats, monounsaturated and polyunsaturated fats, and trans fats, just to name a few. The key difference in these types of lipids is the solidity of the fat, with trans fat having undergone a hydrogenation process to make it even more solid. This solid nature of trans fat is what makes it considered to be rather unhealthy for humans to eat.
Another category of lipids is the steroids, which is not to be confused with the term that refers to artificial performance enhancing chemicals that some athletes may use to gain a competitive edge. These natural steroids are present in the body already, and can include cholesterol, which is the most common type, estrogen, testosterone, bile salts found in the intestinal bile,and the body's own stress-relieving chemical cortisol.
There are a number of vitamins that are called "fat soluble," many of which are stored in the liver or other organs. One of these is vitamin A, which serves an important role in immune function, vision, and reproduction. It's abundant in fruits and vegetables, whole milk, and liver. Vitamin D serves a role in helping the body absorb other nutrients during digestion, such as calcium, zinc, phosphate, iron, and magnesium. While it's found in certain foods, one of the main sources of vitamin D is through exposure to natural sunlight. Next, vitamin E keeps the heart and the body's cells healthy and is found in many nuts, seeds, and vegetable oils. Finally, vitamin K is important for its role in helping blood to clot, and is believed to help with bone strength in older people. It can be found in a number of leafy greens and cruciferous vegetables, and in meat, eggs, and many grains.
Lipids contain another category, the waxes. These waxes form naturally in the body or plant and serve protective roles. Bees create wax to build their hives and encase larva, humans produce ear wax to protect the sensitive ear canal by trapping particles, and leaves of certain plants produce a waxy substance called cutin to trap water and prevent evaporation.