Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.4.5Exchange and Transport

The Bohr Effect — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 31 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

3.4.5 The Bohr Effect — AQA A Level Biology 7402 notes: Explain how raised carbon dioxide concentration shifts the dissociation curve to the right and why this benefits respiring tissues.Carbon dioxide produced in respiration is transported to the lungs in three ways: about 5% di…

  • carbonic anhydrase
  • partial pressure of CO2
  • haemoglobinic acid
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Section 1 of 9Carbon dioxide transport

Carbon dioxide produced in respiration is transported to the lungs in three ways: about 5% dissolved in plasma, about 10–20% bound to haemoglobin as carbaminohaemoglobin, and about 75–85% as hydrogencarbonate ions (HCO3−) in the plasma. Hydrogencarbonate is formed in red blood cells.

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−

The first reaction is catalysed by the enzyme carbonic anhydrase. The hydrogencarbonate ions diffuse out into the plasma and chloride ions move in to balance the charge (the chloride shift).

Notice also that the term partial pressure of CO2 (pCO2) is used in questions rather than concentration.

Carbon dioxide transport
In actively respiring muscle pCO2 may be 6 kPa or more, compared with about 5 kPa in the alveoli and a lower value in the air that is breathed in.
The Bohr effect is a rapid, reversible change that takes place within a single circulation of the blood,
  • so it is a physiological response requiring no change in gene expression.

Section 2 of 9The Bohr effect explained

The hydrogen ions released are taken up by haemoglobin, forming haemoglobinic acid (HHb). Haemoglobin acts as a buffer, preventing a large drop in pH. When H+ ions bind to haemoglobin, they change the molecule's shape, and reduce its affinity for oxygen, so oxyhaemoglobin releases its oxygen.

The Bohr effect is therefore the decrease in the affinity of haemoglobin for oxygen when the carbon dioxide concentration rises. On the graph the dissociation curve moves to the right: at any given pO2 the percentage saturation is lower, so more oxygen is released.

Section 3 of 9Why it is useful

Why it is useful
In actively respiring tissues, such as exercising muscle, CO2 production is high,
  • so the curve shifts right and more oxygen is released exactly where it is needed.
In the lungs CO2 diffuses out of the blood into the alveoli, so the curve shifts back left and haemoglobin loads oxygen efficiently.
The effect matches the supply of oxygen to the demand without needing any nervous or hormonal control.
LocationpCO2Affinity for O2Result
LungsLowHighLoading
Respiring tissueHighLowerUnloading

Section 4 of 9Interpreting shifted curves

Suppose, at pO2 = 4 kPa, the saturation is 70% with 3% CO2 and 50% with 6% CO2. The additional oxygen released is 20 percentage points. If the maximum capacity is 20 cm3 per 100 cm3 of blood, then the extra oxygen is 0.20 × 20 = 4 cm3.

A curve showing a shift to the right has a higher P50. When you draw a shifted curve, keep the S shape, start and finish at the same points, and clearly label the curves. Show the arrows indicating the effect at a given pO2.

Section 5 of 9Related ideas

Exercise also raises temperature and produces lactic acid, which lower pH and shift the curve further right. After the Bohr effect releases oxygen, increased ventilation rate removes CO2 to restore the gradients. Chemoreceptors detect the lower pH in blood and signal the medulla to increase breathing (3.2.4). This topic combines gas exchange, protein structure and respiration.

Section 6 of 9Putting the transport of CO2 and O2 together

It is useful to follow one cycle of gas exchange in a capillary of a muscle.

Putting the transport of CO2 and O2 together
Oxygen diffuses from the red blood cell, after unloading from oxyhaemoglobin, through the plasma, capillary wall and tissue fluid into the muscle cell.
At the same time, CO2 diffuses from the muscle cell into the plasma and into the red blood cell,
  • where carbonic anhydrase speeds up its conversion to carbonic acid.
The H+ ions bind to haemoglobin, which releases more oxygen (the Bohr effect), and the hydrogencarbonate ions move into the plasma.

In the lung capillary, the process runs in reverse.

1
The low pCO2 in the alveoli draws CO2 out.
2
Hydrogencarbonate re-enters the red cell and combines with H+ released from haemoglobin,
  • forming carbonic acid,
  • which carbonic anhydrase converts back to CO2 and water.
3
Haemoglobin, relieved of H+, binds oxygen with a higher affinity.
4
The cycle therefore couples the unloading of oxygen with the uptake of CO2, and the reverse in the lung.

Note that lactic acid from anaerobic respiration also lowers the pH and shifts the curve to the right, matching oxygen release to the level of tissue activity.

Section 7 of 9Exam-style questions

  1. Explain the Bohr effect and why it is an advantage during exercise. [4]4 marks

  2. Describe how most carbon dioxide is transported in the blood. [3]3 marks

  3. Explain the role of haemoglobin as a buffer. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Bohr effectReduced affinity of haemoglobin for oxygen at high CO2 concentration.
Carbonic anhydraseEnzyme that catalyses CO2 + H2O ↔ H2CO3.
Hydrogencarbonate ionHCO3−, the main form in which CO2 is transported.
Chloride shiftMovement of Cl− into red blood cells to balance charge.
CarbaminohaemoglobinHaemoglobin with CO2 attached.
BufferSubstance that resists changes in pH.

Section 9 of 9Exam technique

1
Saying that the curve moves to the left in the presence of CO2.
2
Stating that CO2 directly displaces oxygen from the haem group.
3
Forgetting carbonic anhydrase in the formation of hydrogencarbonate.
4
Mixing up affinity and saturationa shift to the right means lower affinity.
5
Not mentioning that H+ from carbonic acid causes the effect.

Last updated: