| Date | May 2015 | Marks available | 2 | Reference code | 15M.2.HL.TZ1.2 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 2 | Adapted from | N/A |
Question
The diagram below shows the structure of lactase
A study of 600 adolescents in Sweden showed that milk consumption has a positive effect on height which shows continuous variation. However, milk contains lactose which some people can digest but some cannot.
State the pattern of inheritance that contributes to continuous variation.
Explain the production of lactose-free milk.
Identify the protein structures indicated by I and II.
I: ...............................................................
II: ...............................................................
Describe how structure I is held together.
This protein is described as a globular protein. Distinguish between globular and fibrous proteins.
Markscheme
polygenic / more than one gene
Accept polygenetic. Mark only first answer if more than one answer given.
lactase added to milk / lactase immobilised;
lactose hydrolysed/broken down into glucose and galactose;
for people who are lactose intolerant/lack lactase;
increases sweetness/solubility/smooth texture (in processed foods);
I is alpha helix and II is beta pleated sheet
Reject (α) double helix but accept α/A/a and β/B/b instead of alpha and beta.
hydrogen bonds;
Reject hydrogen and covalent bonds unqualified and hydrogen bonds between bases.
(hydrogen bonds) between N–H and C=O (on different amino acids);
Reject between amine and carboxyl groups.
(hydrogen bonds) between adjacent turns of the helix/every fourth amino
acid;
Accept above points in an annotated diagram.
A table is not required but for each feature the difference between globular and fibrous proteins must be made clear.
Examiners report
About half of candidates knew that polygenic inheritance contributes to continuous variation.
This question was generally well answered with stronger candidates able to score full marks. A few confused lactase with lactose and the products of lactose hydrolysis were not always known.
About a quarter of candidates knew the names of the two secondary structures.
Few candidates stated that hydrogen bonds stabilise secondary structures and even fewer earned a second mark for giving a detail of the hydrogen bonding.
N/A
