3.2.11 The Role Of Haemoglobin

Question 1

Transport of Oxygen

Answer 1

The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells
- red blood cells are also known as erythrocytes
Each molecule of haemoglobin contains 4 haem groups, each able to bond with 1 molecule of oxygen
- this means that each molecule of haemoglobin can carry 4 oxygen molecules, or 8 oxygen atoms in total
When oxygen binds to haemoglobin, oxyhaemoglobin is formed
Oxygen + Haemoglobin (rr) Oxyhaemoglobin
4O2 + Hb (rr) Hb4O2
The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making its easier for each successive oxygen molecule to bind
- this is cooperative binding
The reverse of this process happens when oxygen dissociates in the tissue

Question 2

Carbon Dioxide Transport

Answer 2

Waste carbon dioxide produced during respiration diffuses from the tissues into the blood
There are 3 main ways in which carbon dioxide is transported around the body
- a very small percentage of carbon dioxide dissolves directly in the blood plasma and its transported in solution
- carbon dioxide can bind to haemoglobin forming carbaminohaemoglobin
- a much larger percentage of carbon dioxide is transported in the form of hydrogen ions (HCO3-)

Question 3

Formation of Hydrogen Carbonate Ions

Answer 3

Carbon dioxide diffuses from the plasma into red blood cells
Inside red blood cells, carbon dioxide combines with water to form H2CO3
CO2 + H2O (rr) H2CO3
- red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
- without carbonic anhydrase, this reaction proceeds very slowly
- the plasma contains very little carbonic anhydrase hence H2CO3 forms more slowly in plasma than in the cytoplasm of red blood cells
Carbonic acid dissociates readily into H+ and HCO3- ions
H2CO3 (rr) HCO3- + H+
Hydrogen ions can combine within haemoglobin, forming haemoglobin if acid and preventing the H+ ions from lowering the pH of the red blood cells
- haemoglobin is said to act as a buffer in this situation
The hydrogen carbonate ions diffuse out of the red blood cell into the blood plasma where they are transported in solution

Question 4

The Chloride Shift

Answer 4

The chloride shift is the movement of chloride ions into red blood bells that occur when hydrogen carbonate ions are formed
Hydrogen carbonate ions are formed by the following process:
- carbon dioxide diffuses into red blood cells
- the enzyme carbonic anhydrase catalyses the combining of the carbon dioxide and eater to form carbonic acid (H2CO3)
- CO2 + H2O (rr) H2CO3
- carbonic acid dissociates to form hydrogen carbon ions and hydrogen ions
- H2CO3 (rr) HCO3- + H+
Negatively charged hydrogencarbonate ions formed from the dissociation of carbonic acid are transported out of red blood cells via a transport protein in the membrane
To prevent an electrical imbalance, negatively charged chloride ions are transported into the red blood cells via the same transport protein
- if this did not occur, the red blood cells would become positively charged as a result of buildup of the hydrogen ions formed from the dissociation of carbonic acid