OCR AS Biology

Revision Notes

3.2.11 The Role of Haemoglobin

The Role of Haemoglobin

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 – this is cooperative binding
  • 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

Transport of Carbon Dioxide, downloadable AS & A Level Biology revision notes Transport of carbon dioxide.

The oxygen dissociation curve

  • The oxygen dissociation curve describes the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin

The Oxygen Dissociation Curve, downloadable AS & A Level Biology revision notes

The oxygen dissociation curve. The S-shaped curve is indicative of cooperative binding.

  • It’s often easier to understand the curve by starting in the top right-hand corner of the graph. This area shows what happens in the lungs
  • The partial pressure of oxygen is high and so haemoglobin picks up oxygen rapidly, forming oxyhaemoglobin
  • The bottom left-hand portion of the graph shows what happens next
  • As oxygen reaches where it’s needed in respiring tissues, the partial pressure of oxygen in the tissues is comparatively low
  • As a result, oxygen diffuses out into the body cells down a concentration gradient to the tissues

Explaining the Oxygen Dissociation Curve

  • A small change in the partial pressure of oxygen can have a very large impact on the percentage saturation of haemoglobin
  • This is because haemoglobin has such a high affinity for oxygen
  • The partial pressure of oxygen in the lungs is high, so haemoglobin picks up oxygen rapidly
  • In respiring tissues, the partial pressure of oxygen is low, so oxygen is dropped off rapidly
  • This ensures that oxygen is picked up where there’s plenty of it and delivered to where it is needed in respiring tissues

The Bohr shift

  • Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr shift or Bohr effect
  • The Bohr effect explains how the ability of haemoglobin to bind to and release its oxygen changes
  • When the partial pressure of carbon dioxide is high, in respiring tissues, for example, haemoglobin’s affinity for oxygen is reduced
  • This is a helpful change because it means that haemoglobin gives up its oxygen much more readily
  • This occurs because CO2 lowers the pH of the blood (by forming carbonic acid), which causes haemoglobin to release its oxygen
  • Carbon dioxide levels in the lungs are comparatively very low, haemoglobin’s affinity for oxygen is increased, which makes it easier for oxygen to bind to haemoglobin

The Bohr Effect, downloadable AS & A Level Biology revision notes

The ‘S’ shaped curve in this oxygen dissociation curve shows the saturation of haemoglobin against the partial pressure of oxygen.

The Chloride shift

  • The chloride shift describes the movement of chloride ions into red blood cells
  • Within the cytoplasm of red blood cells, an enzyme called carbonic anhydrase catalyses the reaction:

CO2 + H2O ⇌ H2CO3 ⇌ HCO3 + H+

  • Where:
    • H2CO3 = carbonic acid
    • HCO3= hydrogen carbonate ion
    • H+ = hydrogen ion (proton)
  • Negatively-charged hydrogen carbonate 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, then red blood cells would become positively charged as a result of a buildup of hydrogen ions (formed from the dissociation of carbonic acid)

Exam Tip

To avoid mistakes in the exam, be sure to learn the differences between the Bohr shift and chloride shift. The Bohr shift occurs when a high partial pressure of carbon dioxide causes haemoglobin to release oxygen into respiring tissues.

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