1.3 Lipids

Question 1

Describe how triglycerides are formed.

Answer 1

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid. A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond. The R-group of a fatty acid may be saturated or unsaturated.

Question 2

Contrast the structure of a phospholipid with the structure of a triglyceride.

Answer 2

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.

Question 3

Describe how you would test a liquid sample for the presence of lipid and how you would recognise a positive result.

Answer 3

1. (Mix / shake sample) with ethanol, THEN water;
2. White / milky (emulsion);
   Ignore cloudy
   Reject precipitate

Question 4

Lipids have many uses, including…

Answer 4

the bilayer of membranes
certain hormones (steroids)
respiratory substrates (high ratio of C-H bonds)

and also…
insulation
shock absorbing protection
waterproofing
buoyancy
source of water in desert animals (high ratio of hydrogen to oxygen means oxidation of triglycerides releases water)

Question 5

How are the properties of triglycerides related to their structures?

Answer 5

Large and non-polar, so insoluble (so storing them has no affect on water potential).
The less saturation there is in the fatty acids, the straighter they are, the more densely packed the triglycerides can be. The more densely packed the triglycerides are, the higher the melting point. This is the difference between fats (more saturated) and oils (polyunsaturated).

Question 6

How are the properties of phospholipids related to their structures?

Answer 6

Spontaneously form bilayers in water because the hydrophilic phosphate heads face out towards the water whilst the hydrophobic tails point inwards, shielded from the water.