CIE A Level Biology (9700) 2019-2021

Revision Notes

8.1.6 Red Blood Cells, Haemoglobin & Oxygen

Red Blood Cells, Haemoglobin & Oxygen

Transport of oxygen

  • The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells
  • Each molecule of haemoglobin contains four haem groups, each able to bond with one molecule of oxygen
    • This means that each molecule of haemoglobin can carry four oxygen molecules (eight oxygen atoms in total)
  • When oxygen binds to haemoglobin, oxyhaemoglobin is formed:

4O2 + Hb (Haemoglobin)  → HbO8 (Oxyhaemoglobin)

  • Oxygen can also dissolve in the water of blood plasma; at normal body temperatures about 0.025 cm3 of oxygen can dissolve in water
  • 1 dm3 of blood contains 150 g of haemoglobin, which can carry up to 19.5 dm3 oxygen,
  • The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind
  • The reverse of this process happens when oxygen dissociates in the tissues
    • The dissociation of the last oxygen molecule is the hardest

Carbon dioxide transport

  • Waste carbon dioxide diffuses from tissues and into the blood following aerobic respiration
  • There are three main ways in which carbon dioxide is transported around the body
  • A very small percentage of carbon dioxide (~ 10 %) dissolves in blood plasma, forming H2CO3
  • A much larger percentage (~ 70 %) of carbon dioxide dissolves in the cytoplasm of red blood cells
  • 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 carbon anhydrase hence H2CO3 forms much more slowly in plasma than in the cytoplasm of red blood cells
  • Carbonic acid dissociates readily into H+ and HCO3- ions :

CO2 + H2O ⇌ H2CO3 ⇌ HCO3 + H+

  • The increase in H+ concentration results in a decrease in blood pH, which alters the structure of haemoglobin, encouraging the dissociation of oxyhaemoglobin to release oxygen
    • This is beneficial – when levels of carbon dioxide are higher, rates of aerobic respiration are greater and therefore the need for oxygen is higher
  • Hydrogen ions (protons) can combine with haemoglobin, forming haemoglobinic acid
  • Carbon dioxide can also bind to amino acids and therefore haemoglobin, forming carbaminohaemoglobin – this accounts for ~ 20 % of carbon dioxide transport in the blood

Worked example

There is around 150 g of haemoglobin in 1dm3 of blood.
In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.3cm3 of oxygen.

Calculate how much oxygen can be carried in 1dm3 of blood.

To answer this question, we must look at the information already given to us in the question. You do not need prior knowledge to gain full marks here.

In 1 dm3 of blood, there is 150 g of haemoglobin.

1 g of haemoglobin can carry 1.3 cm3 of oxygen.

Therefore 150 g of haemoglobin can carry 195 cm3 of oxygen.

195 cm3 is the final answer.

Exam Tip

In mathematical questions, remember that 1000 cm3 equals 1 dm3

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