Important groups of organic chemicals in cells include proteins, nucleic acids, lipids and carbohydrates. Phospholipids and sterols (steroids) fall under the lipid group. Lipids are a group of organic chemicals that are insoluble in water.
A sterol is any group of natural steroid made from plants and animals. Phospholipids are the second-most-common type of fat.
Steroids are widely distributed in the body and perform many functions. Steroids include cholesterol, sex hormones and certain hormones of the adrenal glands. A phospholipid is a structural component of cell membranes with a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tail.
The structure of the phospholipid bilayer of the cell membrane makes it selectively permeable, allowing certain substances to pass through while keeping others out. Cholesterol molecules (a type of steroid) embedded in the interior of the membrane also help make it impermeable to water-soluble substances and help stabilize the cell membrane.
A phospholipid molecule is made up of a two fatty acid tails and a phosphate group bonded to a glycerol molecule. Steroid molecules are complex structures that include connected rings of carbon atoms.
Cholesterol and other fats are hydrophobic and cannot dissolve in the blood. Hypercholesterolemia (high cholesterol) is a key element of coronary heart disease and may result in heart attacks.