Lipids (College Board AP® Biology): Study Guide

Structure & function in lipids

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

• There are two groups of lipids:

  • Triglycerides

  • Phospholipids

Triglycerides

• Triglycerides are nonpolar, hydrophobic molecules

• The monomers are glycerol and fatty acids

  • Glycerol is an alcohol

  • 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

• A triglyceride is formed through a dehydration synthesis reaction

• Covalent bonds form between glycerol and the fatty acid chains

• For each bond formed a water molecule is released

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

  

Saturation

• Fatty acids can vary in two ways:

  • Length of the hydrocarbon chain (R group)

  • The fatty acid chain (R group) may be saturated or unsaturated

• Differences in saturation determine the structure and function of lipids

  • Saturated fatty acids

    • contain no double bonds between the carbon atoms

    • are solid at room temperature

    • originate from animals

  • Unsaturated fatty acids

    • contain at least one carbon-carbon double bond

    • liquid at room temperature (oil)

    • originate from plants

    • can be monounsaturated or  polyunsaturated

Functions of lipids

• Lipids are energy-dense

• They 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

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

• Other roles of lipids include:

  • Physical protection of soft organs, e.g., visceral fat around the heart

  • Thermal insulation from subcutaneous fat, e.g. whale blubber

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

  • Waterproofing secretions, e.g. birds' preening glands or waxy cuticles on leaf surfaces

  • Electrical insulation, e.g. the myelin sheath around certain nerve axons

  • Certain photosynthetic pigments, e.g. carotenoids

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

Phospholipids

• Phospholipids are a type of lipid; they are formed from the monomer glycerol and fatty acids

• Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule as one has been replaced by a phosphate ion (PO₄^3-)

• As phosphate is polar it is soluble in water (hydrophilic)

• The fatty acid "tails" are nonpolar and, therefore, insoluble in water (hydrophobic)

The bipolar nature of phospholipids

• Phospholipids contain both hydrophobic and hydrophilic parts so they can be regarded as bipolar

• As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

Functions of phospholipids

• Phospholipids are the main component of cell membranes

  • The presence of hydrophobic fatty acid tails acts as a barrier to water-soluble molecules

  • The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalize

    • This enables the cells to organize specific roles into organelles helping with efficiency

  • The composition of phospholipids contributes to the fluidity of the cell membrane

    • If there are mainly saturated fatty acid tails, the membrane will be less fluid

    • If there are mainly unsaturated fatty acid tails, the membrane will be more fluid

  • Phospholipids control membrane protein orientation

    • Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer