Lipids are molecules that are classified as triglycerides, phospholipids, and steroids, all of which play a key role in various biological functions.
In addition to carbohydrates, there are some other important organic molecules that play key roles in cells. These are lipids, proteins, and nucleic acids. A lipid is a long-chain hydrocarbon that is soluble in nonpolar solvents. They have less oxygen than carbohydrates but twice as much hydrogen as carbohydrates. Lipids can be classified into three groups: triglycerides, phospholipids, and steroids. Triglycerides are the most common type of lipid and are composed of glycerol and three fatty acids. They function as a source of long-term energy, insulation, and protection from shock. Triglycerides can be found in oils and fats from plants and animals. Phospholipids are the main components of the cell membrane and are composed of glycerol, two fatty acids, and a phosphate group. Steroids function as hormones and are composed of carbon, hydrogen, and oxygen, arranged in rings. Unlike other lipids, steroids have no fatty acids but rather a rigid backbone of four carbon rings that are fused together. Cholesterol is the most common type of steroid found in animals.
A fatty acid, such as lauric acid, is a long hydrocarbon chain with a tail made of several hydrocarbons and a single carboxylic group (). Fatty acids can be saturated or unsaturated. A saturated fatty acid has only single bonds between the carbon atoms in the backbone of the molecule. An unsaturated fatty acid has one or more double bonds between the carbon atoms in the backbone of the molecule. If there is only one double bond in the molecule, as found in olive oil, it is called a monounsaturated fat. If there is more than one double bond in the molecule, as found in canola oil, it is called a polyunsaturated fat. Saturated fats are found in animal products, such as butter and meat, and are typically solid at room temperature. Saturated fats increase the amount of cholesterol in the body. Unsaturated fats can be found in oils and plant products. They are typically liquids at room temperature, and they decrease the amount of cholesterol in the body. One example of an unsaturated fatty acid is a trans fat, or trans fatty acid. This type of fatty acid rarely occurs in nature and is commonly synthesized to make products such as shortening. Some fatty acids are essential, meaning that they are required for certain biological functions, but not synthesized by the body. Essential fatty acids, such as omega-3 fatty acids, must be consumed in one's diet.