A-LevelBiologyTransport in mammals9700/21 · Oct/Nov 2025 · Q212 Marks

Haemoglobin is a globular protein containing haem groups. Explain how the presence of haem groups allows the haemoglobin molecule to transport oxygen.

(a)Haemoglobin is a globular protein containing haem groups. Explain how the presence of haem groups allows the haemoglobin molecule to transport oxygen.[2]
(b)In certain situations, such as during intense exercise, a person may breathe very quickly and deeply. This is known as hyperventilation. This hyperventilation causes more carbon dioxide to be exhaled. As a result, there is a lower concentration of carbon dioxide in the blood that passes through the capillary network in respiring tissues. This increases the affinity of haemoglobin for oxygen so that there is a decrease in the release of oxygen from red blood cells. Explain why a decrease in the concentration of carbon dioxide in the blood passing through respiring tissues leads to a decrease in the release of oxygen from red blood cells.[3]
(c)(i)Fig. 2.1 is a diagram of a section through the heart. Describe and explain how the tunica media (middle layer of the wall) of blood vessel X adapts the blood vessel for its function.[3]
(c)(ii)Describe the role of the atrioventricular node (AVN) and the Purkyne tissue in the cardiac cycle.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

2(a) any two from: (each haem contains) an, iron ion / ferrous ion / Fe2+; A iron / iron atom
(each haem) binds to / AW, an oxygen molecule / two oxygen atoms ;
A forms a bond only if, iron ion / ferrous ion / Fe2+, stated
2(b) any three from: less carbon dioxide diffuses into red blood cells ;
less carbon dioxide reacts with water ;
less carbonic acid is formed;
less, dissociation of carbonic acid / formation of hydrogen ions and hydrogen carbonate ions ;
less haemoglobinic acid is formed ; A description e.g. H+ binds to haemoglobin
less oxygen (released from haem) diffuses out of red blood cells (to respiring tissues) ;
ref. to carbaminohaemoglobin ;
AVP; e.g. ref. to change in shape of haemoglobin when H+ binds
2(c)(i) idea that blood vessel X carries (oxygenated blood) at high pressure (to body tissues) ; A ref. to systemic circulation
any two from: difference
high proportion of elastic fibres (and low proportion of smooth muscle) ;
explanation (elastic fibres allow X to) expand / stretch, qualified ;
e.g. to accommodate / AW, blood from ventricular, systole / contraction
with, surges / pulses, of blood leaving heart
to prevent, bursting / rupture
recoil of elastic fibres, detail;
e.g.to allow surges / pulses, to continue
idea of allowing continued flow of blood to tissues
maintains, onward pressure / high pressure of blood
if no marks gained in this section allow one mark for: stretch and recoil with one detail or ref. to two correct details but stretch and recoil not stated
2(c)(ii) allow, impulses/wave of excitation, for electrical impulses R nerve impulses once and allow ecf
must attempt AVN and Purkyne tissue to gain max
any four from: both involved in, co-ordination / control, of, systole / contraction, of, heart / heart muscle / cardiac muscle / ventricles ;
AVN only pathway for impulses to travel from atria to ventricles ; AW
A impulses must pass through AVN for ventricular, contraction / systole
allow, a delay / 0.1 s, for impulse to pass from atria to ventricles ;
so, ventricles contract after atria contract / ventricular systole occurs after atrial systole ;
allows atria to (fully) empty before, ventricles (begin to) contract / ventricular systole or allows blood to fill ventricles before ventricular, contraction / systole ;
Purkyne tissue carries / transmits / AW, impulse(s), (down septum), to base of ventricles / through walls of the ventricles ;
for ventricular systole / AW ;
ventricular muscles contract / AW, together / at same time / simultaneously / from the base upwards / from apex of heart ;