Explain carbon-dioxide transport in plasma
Describe the role of plasma in carrying carbon dioxide, especially as hydrogencarbonate ions.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which form carries most CO₂ in plasma?
What you will learn
- State how carbon dioxide is transported in plasma
- Distinguish dissolved CO₂ from hydrogencarbonate transport
Build the explanation
Most carbon dioxide in blood is transported as hydrogencarbonate ions in plasma. The conversion begins mainly in red blood cells: carbon dioxide and water form carbonic acid under the action of carbonic anhydrase, then carbonic acid dissociates. Hydrogencarbonate leaves red blood cells during the chloride shift and is carried in plasma. A smaller fraction of carbon dioxide is dissolved in plasma, and some is carried bound to globin as carbaminohaemoglobin.
Apply it carefully
When blood reaches the lungs, hydrogencarbonate returns to red blood cells and is converted back to carbon dioxide. This dissolved gas diffuses down its partial-pressure gradient into alveoli. Do not say plasma transports most carbon dioxide as dissolved gas; the main plasma form is hydrogencarbonate.
Pause and explain
Where does most hydrogencarbonate formation start?
Worked example
What is the main form in which carbon dioxide is transported in plasma?
Show the worked solution
- Consider the product formed after CO₂ hydration in red cells.
- That ion moves into plasma.
Answer Hydrogencarbonate ions (HCO₃⁻).
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Describe the route by which carbon dioxide from a respiring tissue becomes transported in plasma.
Give me a hint
Include diffusion into a red cell and ion exchange.
Compare my reasoning
- CO₂ diffuses into red blood cells and is converted by carbonic anhydrase to H₂CO₃, then H⁺ and HCO₃⁻.
- HCO₃⁻ leaves during the chloride shift and travels in plasma.
Carbon dioxide diffuses from tissue into a red blood cell. Carbonic anhydrase catalyses formation of carbonic acid, which dissociates into H⁺ and HCO₃⁻. Hydrogencarbonate leaves as chloride enters and is then carried in plasma.
Look for these in your work
- Red-cell conversion described
- Carbonic anhydrase named
- Chloride shift and plasma transport included
A student says: 'Most CO₂ is dissolved as gas in the plasma.' Correct the statement.
Give me a hint
Distinguish a minor dissolved fraction from the main plasma form.
Compare my reasoning
- Most CO₂ is converted to hydrogencarbonate.
- HCO₃⁻ is carried in plasma; only a smaller fraction stays dissolved as CO₂.
Most carbon dioxide is carried in plasma as hydrogencarbonate ions formed mainly in red blood cells. A smaller amount is dissolved directly in plasma, and some is bound to haemoglobin as carbaminohaemoglobin.
Look for these in your work
- Main form corrected
- Minor dissolved form acknowledged
- Bound form distinguished
Explain how plasma-borne hydrogencarbonate contributes to CO₂ exhalation in the lungs.
Give me a hint
Follow the reverse shift and subsequent diffusion.
Compare my reasoning
- HCO₃⁻ returns to red cells as chloride leaves.
- It is converted to CO₂, which diffuses into alveoli down a partial-pressure gradient.
In lung capillaries, hydrogencarbonate returns to red blood cells in the reverse chloride shift. It is converted back to carbon dioxide, which diffuses down its partial-pressure gradient into alveoli and is exhaled.
Look for these in your work
- Reverse shift stated
- CO₂ regeneration explained
- Alveolar diffusion linked to gradient
Common mistakes
- Equating plasma transport with dissolved CO₂ only
- Placing carbonic anhydrase in plasma rather than red cells
- Forgetting that CO₂ is regenerated in lungs before diffusion
What can you explain now?
Which form carries most CO₂ in plasma?
Compare with the explanation
Hydrogencarbonate ions
Most carbon dioxide is carried in plasma as HCO₃⁻.
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Make it stick
What is the main plasma form of transported CO₂, and how is it regenerated in the lungs?
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