life11e_ch03

key concept3.4Lipids Are Defined by Their Solubility Rather Than by Chemical Structure

Lipids—colloquially called fats—are hydrocarbons that are insoluble in water because of their many nonpolar covalent bonds. As you saw in Key Concept 2.2, nonpolar hydrocarbon molecules are hydrophobic and preferentially aggregate together, away from water, which is polar. When nonpolar hydrocarbons are sufficiently close to one another, weak but additive van der Waals forces help hold them together. The huge macromolecular aggregations that can form are not polymers in a strict chemical sense, because the individual lipid molecules are not covalently bonded. With this understanding, it is still useful to consider aggregations of individual lipids as a different sort of polymer.

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Triglycerides are simple lipids composed of glycerol bonded to three fatty acids via ester linkages.
Because phospholipids are amphipathic, they are able to aggregate to form phospholipid bilayers, which are useful in creating membrane structures.
Carotenoids, steroids, certain vitamins, and waxes are classified as lipids and have a variety of functions, depending on their chemical structures.

There are several different types of lipids, and they play a number of roles in living organisms:

Fats and oils store energy.
Phospholipids play important structural roles in cell membranes.
Carotenoids and chlorophylls help plants capture light energy.
Steroids and modified fatty acids play regulatory roles as hormones and vitamins.
Fat in animal bodies serves as thermal insulation.
A lipid coating around nerves provides electrical insulation.
Oil or wax on the surfaces of skin, fur, feathers, and leaves repels water and prevents excessive evaporation of water from terrestrial animals and plants.