Major Lipids and Their Properties

Fat cells, SEM
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Lipids are a diverse group of fat-soluble biological molecules. Each major type has distinct properties and is found in certain locations.

Triacylglycerols or Triglycerides

The largest class of lipids goes by different names: triacylglycerols, triglycerides, glycerolipids, or fats.​

  • Location: Fats are found in many places. One well-known form of fat is found in human and animal tissue.
  • Function: The primary function of fats is energy storage. Some animals, such as polar bears, can live off their fat stores for months at a time. Fats also provide insulation, protecting delicate organs and generating warmth.
  • Example: Margarine, a butter substitute, is made from vegetable oils and sometimes animal fats (typically beef tallow). Most margarines have a fat content of about 80 percent.

Steroids

All steroids are hydrophobic molecules derived from a common four fused carbon ring structure.

  • Location: Cellular membrane, digestive system, endocrine system.
  • Function: In animals, many steroids are hormones, which enter cells and initiate specific chemical reactions. These hormones include the androgens and estrogens, or sex hormones, along with corticosteroids such as cortisol, which is produced by stress. Other hormones exist as part of various organisms' cellular structures, adding fluidity to cellular membranes.
  • Example: The most common steroid is cholesterol. Cholesterol is a precursor for making other steroids. Other examples of steroids include bile salts, estrogen, testosterone, and cortisol.

Phospholipids

Phospholipids are derivatives of triglycerides that consist of a glycerol molecule with two fatty acids, a phosphate group on the third carbon, and often an additional polar molecule. The diglyceride portion of a phospholipid is hydrophobic, while the phosphate is hydrophilic.

  • Location: Cell membrane.
  • Function: Phospholipids form the basis of cellular membranes, which play a significant role in regulating homeostasis.
  • Example: Phospholipid bilayer of the cellular membrane.