Page 33 - Q & A STPM 2022 Biology

Biology STPM Chapter 7 Gaseous Exchange

(i) Explain why the dissociation curves have a sigmoid or ‘S’ shape.
(1 mark)
(ii) Which of the following curves is due to the presence of a high level of
carbon dioxide. (2 marks)
(iii) Describe the physiological advantage resulting from the altered
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dissociation curve in the presence of a high level of carbon dioxide.
(2 marks)
(iv) Give suggestions on how the effect of carbon monoxide on the
oxygen dissociation curve shown contributes to the toxic effect of the
gas. (2 marks)

Answer:
(a) (i) Airway

P
Semester
2

(ii) – Lungs are only able to inflate when the air pressure in the lungs is
higher than the atmospheric pressure.
– A puncture at P will cause the air pressure in the right lung to
become equal to the atmospheric pressure (it cannot drop below
atmospheric pressure). Hence the right lung becomes deflated
(collapse).
(b) (i) – Initially, the binding of oxygen to haemoglobin is slow.
– But the binding of the first oxygen molecule changes its shape,
exposing other haem groups to oxygen. Thus, making subsequent
binding of oxygen to haemoglobin easier.
(ii) Curve C
(iii) – When the cells are actively respiring, the level of CO is high
2
whereas the oxygen concentration is low. The curve is shifted to
the right (Bohr effect).
– The affinity of haemoglobin for oxygen decreases, enabling oxygen
to be easily released into the active cells.
– The haemoglobin molecule is less saturated for the same partial
pressure of oxygen.
(iv) – Haemoglobin has a higher affinity for carbon monoxide compared
to oxygen.
– Hence, in the presence of carbon monoxide, haemoglobin will be
prevented from combining with oxygen. This will deprive oxygen
from tissues and can be fatal.

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Chapter 7.indd 158 11/2/21 9:27 AM