Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Answers to end-of-chapter questions
6 dissolve easily in water;
sweet;
general formula (CH 2 O) n / contain the
elements carbon, hydrogen and oxygen /
hydrogen and oxygen are present in ratio of
2 : 1;[3]
7 a lactose could be a source of energy;
it could be digested to, monosaccharides /
glucose and galactose, which could then be
used as building blocks for larger molecules;
[2]
b condensation;[1]
c glycosidic bond;[1]
d
OH on C atom 1 above
ring (β-galactose)
6 CH 2 OH
5
O
OH
OH
H
4 1
DH H
H
H
3 2
H OH
galactose
(drawn according to convention)
H
3
H
2
H
OH H
4 1
OH
H
5
O
OH
6 CH 2 OH
6 CH 2 OH
5
O
H
H
H
4 1
OH H
OH
2 OH
3
H OH
α-glucose
OH
galactose
(molecule rotated 180° in order
to line up with α-glucose)
OH on C atom 1
below ring
glucose correctly drawn;
galactose correctly drawn;[2]
Carbon atoms need not be numbered. Note
that galactose will probably be drawn
‘upside down’ as in the disaccharide – the
conventional way of drawing it is also shown
in the diagram above. The form used to make
the disaccharide is the beta form of galactose,
but students will not need to know this, other
than for interest.
e alpha glucose / α-glucose;
the –OH group on carbon atom 1 is below the
ring;[2]
f carry out a Benedict’s test on both solutions;
lactose would give a brick-red / brown
precipitate, sucrose would not;
accept positive result for lactose, negative
result for sucrose[2]
[Total: 10]
8 a i
H
H
N
CH 3 O H
C
H
vi alternative answers
A
H
H
O
v peptide bonds
A
H
OH
CH 2
C N C C N C C
C of COOH joined to N of NH 2 for both peptide
bonds;
peptide bonds shown as C=O joined to –NH
(i.e. water has been eliminated);
all three amino acids joined and in correct
sequence; accept even if errors in bonding[3]
ii primary structure;[1]
iii water;[1]
iv ring drawn around –OH or whole R group
(–CH 2 OH) of serine;[1]
v rings drawn around two peptide bonds
and bonds labelled appropriately;[1]
vi ring drawn around –NH group one side of a
peptide bond and group labelled A;[1]
b held in place by hydrogen bonding;
secondary structures;
all the –NH and –C=O groups of, peptide
bonds / polypeptide backbone, are involved;
[3]
c molecule made from repeating subunits;
subunits similar or identical to each other;
giant molecule / macromolecule; [max. 2]
d i XXX, XXY, XYY, XYX, YYY, YYX, YXX, YXY;[1]
ii 2 3; [1]
[Total: 15]
H
iv
O
OH
Cambridge International AS and A Level Biology © Cambridge University Press 2014