Biology 2.2.6-7 Lipids

• Contain large amounts of carbon and hydrogen, and smaller amounts of oxygen
• Not polar, therefore insoluble in water
• Soluble in alcohol
• The most important types of lipids are:
  • triglycerides
  • phospholipids
  • steroids
• Not polymers, but do have different components bonded together
• Examples of macromolecules - large organic molecules

Triglycerides are made up of a glygerol and three fatty acids

Glycerol

• Three carbon atoms
• Is an alcohol, therefore has free -OH groups (three of them - tri)

       Glycerol:                Fatty acid:

Fatty acids

• Hydrocarbon tail between 2 and 20 carbons long
• Carboxyl group -COOH at one end, which ionises into H+ and -COO-
• Acidic due to the free H+ ions
• One double bond makes it monounsaturated, while many makes it polyunsaturated
• Double bonds make a kink, pushing the molecules apart and making them more fluid 

• During a condensation reaction an ester bond is formed and a water molecule is eliminated
• During a hydrolysis reaction an ester bond is broken when a water molecule is used up
• The reactions are between the -OH groups of the glycerol and the -COOH of the fatty acids
• In some cases the type of fatty acid on different -OH groups may be different
• If only one fatty acid has attached to a glycerol it is a monoglyceride molecule

• Energy source
  • broken down in respiration to release energy and generate ATP
  • ester bonds are hydrolysed, then glycerol and fatty acids break down into CO2 and water
  • respiration of a lipid produces more water than respiration of a sugar
• Energy store
  • insoluble so don't affect water potential of a cell
  • in mammals, fat is stored under the skin in adipose cells
  • high energy due to large numbers of hydrogen atoms
  • 1g of fat releases twice as much energy as 1g of glucose
• Insulation
  • adipose tissue acts as a heat insulator
  • lipids in the mylin sheath acts as an electrical insulator
  • animals preparing for hibernation store extra fat
• Buoyancy
  • fat is less dense than water, therefore aquatic mammals use it to stay afloat
• Protection
  • fat around delicate organs acts as a shock insulator
  • the peptidoglycan cell walls of some bacteria are covered in a lipid-rich outer coat

• Similar structure to triglycerides
• One fatty acid is replaced by a phosphate group
• There is an ester bond between an -OH group on the glycerol and an OH group on the phosphoric acid molecule (H3PO4)
• Most of the fatty acids found in phospholipids have an even number of carbon atoms (often 16 or 18)
• Commonly one of these chains is saturated and one is unsaturated

• In water the phosphate group has a negative charge, making it polar
• The fatty acid tails are non-polar
• This means the head is hydrophilic while the tail is hydrophobic
• This means phospholipids are amphipatic

• In a phospholipid bilayer individual phospholipids can move around their layer
• The membrane is selectively permeable, only small, non-polar molecules (e.g. O2, CO2) can pass through

• A steroid alcohol (sterol) - not made of glycerol or fatty acids
• Consists of four carbon-based rings / isoprene units
• Small, hydrophobic molecule - can sit in the phospholipid bilayer
• Regulates fluidity of the membrane
• In animals it is mostly made in the liver
• In plants, stigmasterol has a double bond between carbons 22 and 23
• Cholesterol makes up testosterone, oestrogen and vitamin D - these can pass through the membrane
• Steroids are abundant in plants
• Some steroids, when ingested, can be turned into animal hormones