Page 37 - Q & A STPM 2022 Biology

Biology STPM Chapter 7 Gaseous Exchange
IV : The hydrogen carbonate ions (HCO ) in the red blood cells diffuse
-
3
out. This causes the red blood cell to become positively charged.
V : Chloride ions diffuse into the red blood cells to neutralise the positive
charge in the cells. This process is called the chloride shift.
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Section C Essay Questions

Question 1

(a) Explain the structure of haemoglobin and how it is adapted to carry
oxygen. (8 marks)
(b) Describe how carbon dioxide is transported by the blood. (7 marks)

Answer:
(a) – Haemoglobin is a conjugated protein
– with a quaternary structure.
Semester
2 – It consists of four polypeptide chains, two alpha and two beta chains
coiled to form a spherical structure.
– Each polypeptide chain contains a prosthetic group, haem.
– Within each haem group there is a ferrous iron atom
– Each haem group is able to combine with an oxygen molecule. Hence,
there are 4 haem groups in a haemoglobin molecule; therefore each
haemoglobin molecule can combine with 4 oxygen molecules.
high partial pressure of oxygen
Hb + 4O HbO 4
2
Haemoglobin Oxygen low partial pressure of oxygen Oxyhaemoglobin
– In the lung alveolar capillary, where the partial pressure of O is high,
2
when the first oxygen molecule binds to a haem group, it causes a change
in shape of the haemoglobin so that the affinity of the other haem groups
for oxygen increases. This allows rapid binding with oxygen in the lungs.
Haemoglobin shows cooperativity.
– In the tissue, when the partial pressure of O is low and partial pressure
2
of CO is high, oxygen dissociates readily from haemoglobin.
2
(b) Carbon dioxide is transported in the blood in three ways:
(i) – about 5% carbon dioxide dissolves in the blood as a physical
solution
(ii) – about 10% carbon dioxide combines with the amino acid group
(NH ) in the protein part of the haemoglobin molecule to form
2
carbaminohaemoglobin

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Chapter 7.indd 162 11/2/21 9:27 AM