Lipids are hydrophobic molecules, meaning they're non-polar, and they don't mix with water. There are three types of lipids: fats, steroids, and phospholipids.
Fat consists mainly of triglyceride. Triglyceride is a product of a glycerol molecule and three fatty acids. The majority of a fatty acid is a long hydrocarbon. In a triglyceride the carboxyl group seen on the left of each fatty acid (COOH) becomes an ester group, where there's COO, but instead of the last H, there's the rest of the molecule. So, a triglyceride is considered an ester. Fats are very efficient at storing energy. In a pound of fat, there's more than twice as much energy as in a pound of starch. The downside to this is that it makes it difficult to lose excess fat. Fats are stored in adipose cells, special reservoirs, which swell and shrink when fat is deposited and withdrawn from them. In addition, adipose tissues cushion vital organs.
There are two types of fatty acids, saturated and unsaturated.
Here, there are as many hydrogen atoms as possible. There are no double bonds anywhere (aside from the carboxyl group). There are a few interesting things about saturated fats. Firstly, they are solid at room temperature. If you want a fatty food to be solid at room temperature, but it naturally exists as a liquid (an unsaturated fat), you have to hydrogenate it, meaning you give hydrogen atoms to the fat in order to make it saturated. Sometimes, this can go wrong, causing it to create a trans fat, a special kind of unsaturated fat. Trans fats are the least healthy, with saturated fats being the 2nd least healthy. This is because they stack easily, possibly clogging veins and arteries and contributing to heart disease. Also, a fat is only saturated if all of its three fatty acids are saturated.
Unsaturated fats are generally the healthiest fats. They don't stack, and are liquids at room temperature. As stated before, the process of turning unsaturated fats into saturated fats, called hydrogenation, can sometimes go wrong. When that happens, a cis unsaturated fat becomes a trans unsaturated fat. Trans fats don't occur naturally in nature, so our bodies have a much more difficult time breaking them down. For that reason, trans fats are by far the least healthy fats. Unsaturated fats that occur naturally, cis fats, are found in vegetable oils and fish oils (omega-3 and omega-6 fatty acids). They are considered healthy and are important parts of a healthful diet. Omega-3 fatty acids, found in nuts and salmon, have been shown to reduce the risk of heart disease, and relieve symptoms of arthritis and inflammatory bowel disease.
Steroids are only classified as lipids, because they are hydrophobic. They are completely different from fats. Cholesterol is the most common kind of steroid and sterol, or steroid alcohol. The definition of a steroid is an organic compound containing four carbon rings (three with 6 carbon, and one with 5). That doesn't really tell us much though.
A sterol just means that the steroid begins with "HO-", because that makes it an alcohol and a steroid hence ster(oid) + (alcoh)ol = sterol.
Steroids are important parts of cell membranes. They strengthen the membrane, along with ensuring the cell membrane fluidity stays constant throughout.
Steroids can act as hormones, including testosterone, and estrogens. Along with those, aldosterone maintains blood pressure and regulates salt in your body. Finally, cortisol is a steroid hormone that suppresses the immune system, increases blood sugar, and aids in the metabolism of fat, protein, and carbohydrates. Hydrocortisone is the medicine version of cortisol, and it suppresses the immune system, reducing inflammation, pain and swelling, and itching.
Cholesterol is the "base steroid", and it's how our bodies produce other steroids, including estrogen and testosterone. It is vital in cell membranes, but too much can cause blood clots.
Estradiol is a specific type of estrogen. Estrogen is an umbrella term for any female sex hormone that helps a woman's body develop. Estradiol is the most common type of estrogen. This is also a steroid, a sterol to be specific (HO term in the bottom left). Estradiol is the most popular kind of estrogen. A few other types of estrogen are estriol and estrone.
Testosterone is very similar to estradiol, however in the bottom left corner, there's a double bond to an oxygen instead of a hydroxyl, meaning it's not an alcohol. So, testosterone isn't a sterol, but it still is a steroid. Bodybuilders inject themselves with variants of testosterone, called anabolic steroids.
Anabolic steroids are synthetic variants of testosterone. Testosterone causes a buildup in muscle and bone mass during puberty. By injecting yourself with a variant of that, bodybuilders can grow their muscles and become better athletes. They are banned in every major sporting event, because of the unfair advantage given to athletes, along with the negative impact to the players' healths, with side effects including mood swings, depression, liver damage, high cholesterol, shrunken testicles, and infertility.
The third kind of lipid is a phospholipid. These are similar to fats, but with only two non-polar fatty acid "tails" and a polar "head" and it makes up the cell membrane.
So, the polar head group is called choline, and it's hydrophilic. The fatty acids are hydrophobic, and they're on the inside. It looks like this:
The choline head is hydrophilic, and it's on the outside. The hydrophobic tail, the fatty acid, is on the inside. This permits various things to go through, usually uncharged or nonpolar molecules can go through.