Trace oxygen and carbon-dioxide transport
Explain the roles of haemoglobin, carbonic anhydrase, haemoglobinic acid and carbaminohaemoglobin in red-cell gas transport.
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
What forms when oxygen binds reversibly to haemoglobin?
What you will learn
- Describe oxygen transport by haemoglobin
- Describe red-cell carbon-dioxide transport and name the relevant products
Build the explanation
Oxygen binds reversibly to haemoglobin in red blood cells to form oxyhaemoglobin. In the lungs, higher oxygen partial pressure favours loading; in respiring tissues, lower oxygen partial pressure favours unloading. Carbon dioxide enters red blood cells. Carbonic anhydrase catalyses the reversible reaction between carbon dioxide and water to form carbonic acid, which dissociates into hydrogen ions and hydrogencarbonate ions. Some hydrogen ions bind to haemoglobin, forming haemoglobinic acid. Some carbon dioxide binds directly to haemoglobin to form carbaminohaemoglobin.
Apply it carefully
Keep the forms distinct: oxyhaemoglobin carries oxygen; hydrogencarbonate ions carry most carbon dioxide in blood; carbaminohaemoglobin carries some carbon dioxide bound to globin; haemoglobinic acid is haemoglobin bound to hydrogen ions, helping buffer pH. Carbonic anhydrase speeds the reversible reaction but is not consumed. Do not call haemoglobinic acid a carbon-dioxide compound.
Pause and explain
Which molecule is formed when carbon dioxide binds to globin?
Worked example
Which enzyme speeds conversion of carbon dioxide and water into carbonic acid in red blood cells?
Show the worked solution
- Identify the enzyme associated with rapid carbon-dioxide hydration.
- It catalyses the reversible reaction.
Answer Carbonic anhydrase.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the products when carbon dioxide and water react in a red blood cell, and state the enzyme involved.
Give me a hint
Write the intermediate before dissociation.
Compare my reasoning
- Carbon dioxide and water form carbonic acid.
- Carbonic anhydrase catalyses the reaction; carbonic acid dissociates into H⁺ and HCO₃⁻.
Carbonic anhydrase catalyses CO₂ + H₂O ⇌ H₂CO₃, and carbonic acid dissociates into H⁺ and HCO₃⁻.
Look for these in your work
- Carbonic acid identified
- Enzyme named
- Dissociation products correct
Distinguish carbaminohaemoglobin from haemoglobinic acid.
Give me a hint
Ask what haemoglobin binds in each case.
Compare my reasoning
- Carbaminohaemoglobin forms when CO₂ binds to globin.
- Haemoglobinic acid forms when H⁺ binds to haemoglobin.
Carbaminohaemoglobin is formed when carbon dioxide binds to globin; haemoglobinic acid forms when haemoglobin binds hydrogen ions. They are different products with different binding partners.
Look for these in your work
- CO₂ binding stated
- H⁺ binding stated
- Products not conflated
A respiring tissue has low oxygen partial pressure and releases carbon dioxide. Explain how red cells can unload oxygen and assist carbon-dioxide transport.
Give me a hint
Connect the lung/tissue oxygen gradient to reversible binding and CO₂ chemistry.
Compare my reasoning
- Lower oxygen partial pressure favours oxyhaemoglobin dissociation.
- CO₂ enters red cells; carbonic anhydrase supports HCO₃⁻ formation, while some CO₂ binds globin and H⁺ binds haemoglobin.
At lower oxygen partial pressure, oxyhaemoglobin dissociates and releases oxygen to the tissue. Carbon dioxide diffuses into red cells; carbonic anhydrase catalyses its conversion with water to carbonic acid and then H⁺ and HCO₃⁻. Some CO₂ binds globin as carbaminohaemoglobin, and H⁺ binds haemoglobin as haemoglobinic acid.
Look for these in your work
- Oxygen unloading linked to partial pressure
- Carbonic anhydrase route included
- Both haemoglobin products distinguished
Common mistakes
- Calling carbonic anhydrase a product rather than an enzyme
- Confusing haemoglobinic acid and carbaminohaemoglobin
- Saying all carbon dioxide is carried bound directly to haemoglobin
What can you explain now?
What forms when oxygen binds reversibly to haemoglobin?
Compare with the explanation
Oxyhaemoglobin
Oxygen loading forms oxyhaemoglobin in red blood cells.
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Make it stick
What does haemoglobin bind to form oxyhaemoglobin, carbaminohaemoglobin and haemoglobinic acid?
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