Page 38 - Q & A STPM 2022 Biology

Biology STPM Chapter 7 Gaseous Exchange
(iii) – about 85% carbon dioxide is transported in the form of hydrogen
carbonate ions (HCO )
-
3
– carbon dioxide from the active tissue cells diffuses from the tissue
fluid through the blood capillaries into the blood plasma and then
into the red blood cells
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– In the red blood cells, carbon dioxide combines with water to form
carbonic acid.
– The reaction is catalysed by carbonic anhydrase.
Carbonic anhydrase
CO + H O H CO
3
2
2
2
Carbon Water Carbonic
dioxide acid
– The carbonic acid dissociates into hydrogen ion (H ) and a
+
hydrogen carbonate ion (HCO ), catalysed by carbonic anhydrase.
-
3
H CO H + HCO 3 – Semester
+
2
3
Carbonic Hydrogen Hydrogen
acid ion carbonate ion
– The hydrogen carbonate ions (HCO ) diffuse out of the red blood 2
–
3
cell into the blood plasma. The red blood cell membrane is more
permeable to anions than to cations such as Na and K . In order to
+
+
maintain electrical neutrality, the chloride ions (Cl ) diffuse from
–
the blood plasma into the red blood cell. This process is called a
chloride shift.
– The increase in concentration of H ions in the red blood cell
+
lowers the pH. This causes the oxyhaemoglobin to dissociate into
oxygen and haemoglobin. Oxygen is released from the red blood
cells into tissues for respiration.
– The hydrogen ions (H ) combine with haemoglobin to form
+
haemoglobinic acid (HHb). Hence, haemoglobin acts as a buffer.
– In lungs, these processes are reversed. Then carbon dioxide diffuses
out into the alveolus from the blood capillaries.

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Chapter 7.indd 163 11/2/21 9:27 AM