Cambridge International A Level Biology Revision Guide

Chapter 2: Biological molecules
7 The diagram below shows a disaccharide called lactose. The carbon atoms are numbered. You are not expected to
have seen this structure before. Lactose is a reducing sugar found in milk. It is made from a reaction between the
two monosaccharides glucose and galactose.
H OH
6
CH 2 OH
3
C 2
C
H
H
OH H
4
C 1
C
H
OH
5
C
O
O
H
5
C
O
H
4
C
H
OH H
1
C
OH
3
C 2
C
6
CH 2 OH
X
H
OH
a Suggest two functions that lactose could have. [2]
b What is the name given to the reaction referred to above that results in the formation of lactose? [1]
c Identify the bond labelled X in the diagram. [1]
d Draw diagrams to show the structures of separate molecules of glucose and galactose. [2]
e Using the information in the diagram, is the alpha or beta form of glucose used to make lactose?
Explain your answer. [2]
f Like lactose, sucrose is a disaccharide. If you were given a solution of lactose and a solution of sucrose,
state briefly how you could distinguish between them. [2]
[Total: 10]
8 a The diagram below shows the structures of three amino acids.
OH
51
H
H
CH 3
H
CH 2
O
H
O
H
N C C
N C C
N C C
OH
H
OH
H
H
H
H
alanine glycine serine
O
OH
i Draw a diagram to show the structure of the tripeptide with the following sequence:
alanine–glycine–serine. [3]
ii What is the name given to the sequence of amino acids in a protein? [1]
iii What substance, apart from the tripeptide, would be formed when the three amino acids combine? [1]
iv Draw a ring around an atom or group of atoms making up an R group that could hydrogen bond with
a neighbouring R group. [1]
v Draw a ring around and label the peptide bond(s) you have drawn in the diagram. [1]
vi Draw a ring around a group of atoms which could hydrogen bond with a –CO – group in an
alpha helix (α-helix). Label this group A. [1]
b State three features that α-helices and beta sheets (β-sheets) have in common. [3]
c A protein can be described as a polymer. State the meaning of the term polymer. [2]
d X and Y represent two different amino acids.
i Write down the sequences of all the possible tripeptides that could be made with just
these two amino acids. [1]
ii From your answer to d i, what is the formula for calculating the number of different tripeptides
that can be formed from two different amino acids? [1]
[Total: 15]