Lipids

Lipids provide long-term storage of energy (9 kcal/gram) in humans. They also provide padding and insulation, store fat soluble vitamins (A, D, E, etc.), and form the backbone of hormones and cell membranes. You can find lipids in dairy products, meats, nuts, and greasy food. Structurally, lipids have long chains of hydrocarbons and few oxygens. In fact, lipids are considered to be saturated when the carbon chain is linked with as many hydrogens as possible. Lipids are considered unsaturated when there is a double bond in the chain. There are four common lipids:

• Fats are three fatty acids linked to an ester of glycerol. They can be saturated or unsaturated. Essential fatty acids include linoleic and linolenic acid.
  
  
• Waxes are similar to fats but are composed of much longer fatty acids. Molecular attractions between fatty acid chains accounts for their higher melting point.
  
  
• Phospholipids are similar to fats but one of the fatty acid groups is replaced by a phosphate group. The structure of this lipid gives it some interesting characteristics. The ester of glycerol is hydrophilic but the fatty acid chains are hydrophobic. Cell membranes utilize this contradiction to form a double-layered membrane with the hydrophilic heads on the outside and the hydrophobic tails on the inside.
  
  
• Steroids are the last common kind of lipids. Steroids have a four carbon ring structure and a high hydrogen to carbon ratio. Examples of steroids include cholesterol and hormones like testosterone and estrogen.