Lipids

Question 1

Give two reasons why triglycerides are used as energy storage molecules

Answer 1

Because they contain lots of chemical energy and they’re insoluble in water.

Question 2

Explain why the structure of phospholipids makes them able to form the bilayer of cell membranes

Answer 2

Phospholipid heads are hydrophilic and their tails are hydrophobic, so they form a double layer with their heads facing out towards the water on either side.
This makes the centre of the membrane bilayer hydrophobic, so water-soluble substances can’t easily pass through it.

Question 3

Describe how a triglyceride molecule is formed

3 marks

Answer 3

1. One glycerol and three fatty acids;
2. Condensation (reactions) and removal of three molecules of water;
3. Ester bond(s) (formed);
   Accept all marks in suitably labelled diagram OR in a balanced equation

Question 4

Scientists investigated the percentages of different types of lipid in plasma membranes from different types of cell. The table shows some of their results:

The scientists expressed their results as Percentage of lipid in plasma membrane by mass. Explain how they would find these values.

2 marks

Answer 4

1. Divide mass of each lipid by total mass of all lipids (in that type of cell);
2. Multiply answer by 100.

Question 5

Explain why phospholipids can form a bilayer but triglycerides cannot.

3 marks

Answer 5

1. Phospholipid both hydrophobic and hydrophilic
   OR
   Phospholipid polar
   OR
   Phosphate group is charged;
2. Triglycerides only hydrophobic
   OR
   Fatty acid/triglyceride is non-polar;
3. Hydrophilic/phosphate group attracts water (to either side of bilayer);

Question 6

The ratio of saturated to unsaturated fatty acids in a cell-surface membrane determines the extent of the membrane’s fluidity.

Scientists provided a cell culture of mouse phagocytes with liquid broth rich in unsaturated fatty acids.

The scientists observed:

• an increase in the proportion of phospholipids in the phagocytes containing unsaturated fatty acids

• more phagocytosis.

Suggest and explain why there was more phagocytosis.

3 marks

Answer 6

1. More unsaturated fatty acids increases fluidity (in cell-surface membrane);
2. (Making cell-surface) membrane more fluid/ flexible;
3. Easy to engulf;