Lipids

Question 1

Describe the difference between the structure of a triglyceride molecule and the structure of a phospholipid molecule

Answer 1

In phospholipid, one fatty acid replaced by a phosphate;

Question 2

Describe how you would test for the presence of a lipid in a sample of food

Answer 2

Add ethanol, then add water;

Reject ethanal/ethonal

Accept ‘Alcohol/named alcohol’

2. White (emulsion shows lipid);

also shake

sometimes may ask if a solid food say crush / grind

Question 3

The structure of a phospholipid molecule is different from that of a triglyceride.
Describe how a phospholipid is different

Answer 3

Phosphate / PO4;

“It” refers to phospholipid.

2. Instead of one of the fatty acids / and two fatty acids

Question 4

Describe how a saturated fatty acid differs in molecular structure from an unsaturated fatty acid

Answer 4

absence of a double bond;
in the (hydrocarbon) chain

Question 5

Describe how an ester bond is formed in a phospholipid molecule.

___________________________________________________________________

Answer 5

Condensation (reaction)

OR

Loss of water;

2. Between of glycerol and fatty acid

Question 6

Explain why triglycerides are not considered to be polymers

Answer 6

not made of monomers / many repeating units;

Question 7

Explain the arrangement of phospholipids in a cell-surface membrane.

Answer 7

Bilayer

OR

Water is present inside and outside a cell;

Accept annotated diagram for ‘bilayer’

Accept cytoplasm/tissue fluid for water

Accept for two marks, annotated diagram of bilayer with water labelled on each side

2. Hydrophobic (fatty acid) tails point away/are repelled from water

OR

Hydrophilic (phosphate) heads point to/are in/are attracted to water

Question 8

Mucus produced by epithelial cells in the human gas exchange system contains triglycerides and phospholipids.

Compare and contrast the structure and properties of triglycerides and phospholipids.

Answer 8

Both contain ester bonds (between glycerol and fatty acid);

All statements must be clearly comparative or linked by the candidate, not inferred from separate statements.

Accept mark points shown on adjacent annotated diagrams.

2. Both contain glycerol;
3. Fatty acids on both may be saturated or unsaturated;
4. Both are insoluble in water;
5. Both contain C, H and O but phospholipids also contain P;

Must relate to element.

6. Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;
7. Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region;

Accept ‘non-polar’ for hydrophobic and ‘polar’ for hydrophilic.

8. Phospholipids form monolayer (on surface)/micelle/bilayer (in water) but triglycerides don’t;