Lipids (College Board AP® Biology)

Molecular Structure of Lipids

Lipids

• Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates, lipids contain a lower proportion of oxygen

• Nonpolar and hydrophobic (insoluble in water)

• There are 2 groups of lipid that you need to know:

  • Triglycerides (the main component of fats and oils)

  • Phospholipids

Lipids as storage compounds

• They contain 2× more energy per gram than most carbohydrates

• Less body mass is required to store a given amount of energy

• This allows lipids to be energy-dense, maximizing the energy content per gram versus carbohydrates

• Lipids are insoluble so do not affect osmosis; therefore, they do not risk upsetting the water balance of the organism

• When lipids are respired, a lot of water is produced compared to the respiration of carbohydrates

  • This is called metabolic water and can be used as a dietary water source when drinking water is unavailable

    • A camel's hump is not a water sac, it is a lipid rich storage organ that yields metabolic water for the camel in its dry desert habitat

    • A bird's egg also makes use of lipid rich yolk to provide energy and metabolic water to the growing chick

• All these features make lipids ideal for long term energy storage

Other roles of lipids

• As well as energy storage molecules, lipids have a number of other roles

  • Physical protection of soft organs, eg, visceral fat around the heart

  • Thermal insulation from subcutaneous fat, eg, whale blubber

  • Subcutaneous fat as a buoyancy aid, eg, in seals (fat is less dense than water so assists flotation)

  • Waterproofing secretions, eg, birds' preening glands or waxy cuticles on leaf surfaces

  • Electrical insulation, eg, the myelin sheath around certain nerve axons

  • Certain photosynthetic pigments, eg, carotenoids

  • Glycolipids, typically as cell-surface recognition molecules/receptors

Triglycerides

• Are nonpolar, hydrophobic molecules

• The monomers are glycerol and fatty acids

• Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom)

• Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group

  • The shorthand chemical formula for a fatty acid is RCOOH

• Fatty acids can vary in two ways:

  • Length of the hydrocarbon chain (R group)

  • The fatty acid chain (R group) may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions, eg, coconut and palm oil)

• Unsaturated fatty acids can be monounsaturated or  polyunsaturated:

  • If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolized by enzymes

  • If H atoms are on opposite sides of the double bond they are trans-fatty acids and cannot form enzyme-substrate complexes and, therefore, are not metabolized

  • They are linked with coronary heart disease

Diagram of Different Lipid Structures

Examples of different types of fatty acids with the functional groups and presence of double bonds in unsaturated fatty acids highlighted

• Triglycerides are formed by esterification

• An ester bond forms when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of the fatty acid:

  • An H from glycerol combines with an OH from the fatty acid to make water

  • The formation of an ester bond is a dehydration synthesis reaction

  • For each ester bond formed a water molecule is released

  • Three fatty acids join to 1 glycerol molecule to form a triglyceride

  • Therefore, for 1 triglyceride to form, 3 water molecules are released

Formation of a Triglyceride Diagram

Formation of a triglyceride from a glycerol molecule and three fatty acid molecules