Three Types of Lipids

Substances containing lipids are easy to recognize by touch because they feel greasy. Lipids can be divided into three groups: fats and oils, phospholipids--which help make up the cell membrane of living things--and all other kinds of lipids. This last group includes steroids and cholesterol. Lipid compounds contain carbon and have fatty acid chains. We need lipids for hormones and vitamins, to give us energy, and to build membranes for our cells.

Hydrophobia

• As you might expect from something related to oil, lipids are hydrophobic--that is, they don't dissolve in water. There are many molecules diverse in structure that fit the bill of a lipid because lipids are recognized according to their insolubility. Though it might seem odd, many lipid molecules have one part that is hydrophilic--water-loving--with the other part hydrophobic--water-fearing. Triacylglycerols, or triglycerides--the fat and oil molecules--have no hydrophilic parts. They are formed when three fatty acids join to a glycerol molecule.

Solid Versus Liquid Fat

• At room temperature, animal fat is solid because the molecules are packed tightly. That's not so with plant and fish fatty acid, which is liquid oil at room temperature because the fatty acid chains have kinks that let molecules be loosely packed. Animal fats are saturated, whereas others are either unsaturated, monounsaturated or polyunsaturated. Fats and oils store energy, providing twice the energy of carbohydrates or proteins. You also need fat lipids to absorb vitamins A, D, E and K, which are fat-soluble.

Phospholipids

• While fat and oil lipids have three fatty acid chains joined to a glycerol molecule, phospholipids have two chains joined to the glycerol with a phosphate group, which you might have guessed from the first part of this lipid type's name. Phospholipids are the main constituent of cell membranes. Their structure lets cells be permeable and is part of keeping the cell environment properly regulated.

Other Lipids

• The third kind of lipid group--those that are neither phospholipids nor fats and oils--includes waxes, pigments and sterols, which include steroids that play a role in hormone regulation. Pigments include chlorophyll and carotenoids, which make plants green and pumpkins orange, respectively. Beta carotene, which can produce vitamin A, is a kind of carotenoid pigment. Besides sterols and pigments, other lipids create protective barriers or sheaths against outside elements. These lipids include wax and cutin. They are at work to repel water from feathers and beeswax.

Cholesterol

• Knowing that sterols are lipids, looking at the end of the word "cholesterol" might give away the fact that cholesterol is a lipid. Cholesterol, along with triglycerides, are blood lipids. Our liver produces enough cholesterol for our needs, but we take in extra when we eat certain foods such as meat and eggs. Too much, called hyperlipidemia--notice the word "lipid" in the middle--can lead to problems such as clogged arteries. When your cholesterol is tested by your doctor, the desired result is a maximum total cholesterol level of 200 mg/dl.