Lipids (College Board AP® Biology): Study Guide

Structure & function in lipids

• Lipids are macromolecules which contain carbon, hydrogen and oxygen atoms

• They are typically nonpolar, hydrophobic molecules

• One type of lipid is fatty acids

  • Fatty acids contain a methyl group at one end of a hydrocarbon chain (known as the R group) and at the other is a carboxyl group

    • The shorthand chemical formula for a fatty acid is RCOOH

Saturation

• Fatty acids can vary in:

  • length of the hydrocarbon chain (R group)

  • saturation of the fatty acid chain (R group), which may be saturated or unsaturated

• Differences in saturation determine the structure and function of lipids

  • Saturated fatty acids contain only single bonds between the carbon atoms

    • These chains are straight

    • Being straight means that the fatty acids can pack together tightly, and so these fats are solid at room temperature

    • E.g. butter

  • Unsaturated fatty acids contain at least one carbon-carbon double bond

    • These chains have a bend wherever a double bond is present

    • The bends in the chain prevent the fatty acids from packing together tightly

    • The more double bonds in a fatty acid, the more unsaturated the lipid becomes and therefore the more liquid it is at room temperature.

    • E.g. olive oil

Functions of lipids

• Lipids are energy-dense and provide energy storage and support cell function

  • E.g. Thermal insulation from subcutaneous fat, such as whale blubber

• Steroids are a class of lipids; they are hormones that support physiological functions, including growth and development, energy metabolism, and homeostasis

• Another lipid is cholesterol which provides essential structural stability to animal cell membranes

• When fatty acids are combined with the monomer glycerol and a phosphate ion, they form phospholipids, which group together to form the lipid bilayers found in plasma and cell membranes