The different properties of triglycerides and phospholipids related to…

The idea

Both molecules are built on glycerol, so compare them by what is attached. A triglyceride has three hydrocarbon tails and no polar group, so the whole molecule is non-polar, hydrophobic and insoluble in water. That makes it an excellent energy store: the high proportion of carbon-hydrogen bonds releases about twice as much energy per gram as carbohydrate when oxidised, it can be packed into droplets without lowering the water potential of the cell, and its oxidation also releases metabolic water. Stored under the skin, it insulates and protects organs as well. A phospholipid has only two tails and a hydrophilic phosphate-containing head, so it is amphipathic rather than simply hydrophobic. In water, phospholipids arrange themselves into a bilayer with heads facing the aqueous environment and tails shielded inside, forming the basis of cell-surface and organelle membranes.

What earns the marks

• The triglyceride having three fatty acid tails and no polar group, so it is hydrophobic and insoluble in water.
• The triglyceride releasing about twice as much energy per gram as carbohydrate, because of the high proportion of carbon-hydrogen bonds.
• Insolubility meaning no effect on water potential, so stored fat does not cause water to enter the cell by osmosis.
• The phospholipid being amphipathic, with a hydrophilic phosphate-containing head and hydrophobic tails.
• The phospholipid forming a bilayer and so acting as the basis of membranes rather than as a store.
• Explaining that the hydrophobic tails face inwards and the hydrophilic heads face outwards in a bilayer in an aqueous environment.

Common mistakes and how to put them right

• Stating that triglycerides form the cell-surface membrane bilayer.

  Put it right: Triglycerides are storage lipids, while phospholipids form bilayers.

• Offering phospholipids as the main energy store of the cell.

  Put it right: Triglycerides are the main energy store.

• Saying triglycerides are insoluble 'because they are large'.

  Put it right: They are insoluble because they are non-polar and hydrophobic.

• Describing a phospholipid as entirely hydrophobic and then trying to explain bilayer formation.

  Put it right: Phospholipids are amphipathic, with a hydrophilic head and hydrophobic tails.

• Claiming lipids give twice as much energy as carbohydrate because they contain more oxygen.

  Put it right: They contain proportionally less oxygen and more hydrogen, and it is the high proportion of carbon-hydrogen bonds that releases more energy on oxidation.

Exam tips

• Structure the answer as a contrast in each sentence: three tails versus two tails and a phosphate-containing group, storage versus membranes.
• Metabolic water and thermal insulation are creditable extra properties of triglycerides; keep them ready for suggest questions about desert or arctic animals, but present them as possible advantages rather than proven causes of fat storage.
• When explaining insolubility, use the words non-polar and hydrophobic, since 'does not dissolve' alone is rarely enough.