describe the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of: haemoglobin, carbonic anhydrase, the formation of haemoglobinic acid, the formation of carbaminohaemoglobin

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Transport of Oxygen and Carbon Dioxide

Transport of Oxygen and Carbon Dioxide

Learning Objective

Describe the role of red blood cells (RBCs) in transporting oxygen (O₂) and carbon dioxide (CO₂) with reference to:

  • Haemoglobin (Hb)

  • Carbonic anhydrase

  • Formation of haemoglobinic acid (Hb‑H⁺)

  • Formation of carbaminohaemoglobin (Hb‑CO₂)

Key Concepts

1. Structure of Red Blood Cells

RBCs are biconcave, anucleate cells packed with haemoglobin molecules (\overline{270} million per cell). Their large surface‑to‑volume ratio facilitates rapid gas exchange.

2. Haemoglobin – The Primary Carrier

Each haemoglobin molecule contains four heme groups, each capable of binding one O₂ molecule. The binding is reversible and cooperative (sigmoidal O₂‑dissociation curve).

3. Carbonic Anhydrase – Catalysing CO₂ Conversion

Carbonic anhydrase, abundant in the cytoplasm of RBCs, accelerates the reaction:

\$\text{CO}2 + \text{H}2\text{O} \;\xrightleftharpoons[\text{CA}]{\text{catalysed}}\; \text{H}2\text{CO}3 \;\xrightleftharpoons{}\; \text{H}^+ + \text{HCO}_3^-\$

This reaction is essential for the efficient transport of CO₂ as bicarbonate ion (HCO₃⁻) in plasma.

4. Formation of Haemoglobinic Acid (Hb‑H⁺)

When O₂ binds to haemoglobin in the lungs, the protein undergoes a conformational change that reduces its affinity for H⁺, releasing H⁺ into the cytoplasm:

\$\text{Hb} + \text{O}2 \;\rightleftharpoons\; \text{HbO}2\$

\$\text{HbO}2 + \text{H}^+ \;\rightleftharpoons\; \text{HbH}^+ + \text{O}2\$

In the tissues, where CO₂ is high, the reverse occurs: H⁺ binds to deoxy‑Hb forming Hb‑H⁺, which promotes O₂ release (Bohr effect).

5. Formation of Carbaminohaemoglobin (Hb‑CO₂)

CO₂ can bind directly to the amino groups of the globin chains, forming carbaminohaemoglobin:

\$\text{HbNH}2 + \text{CO}2 \;\rightleftharpoons\; \text{HbNHCOO}^- + \text{H}^+\$

This accounts for \overline{5}‑10 % of CO₂ transport and also contributes to the Bohr effect by stabilising the deoxy‑Hb conformation.

Overall Transport Pathway

LocationO₂ TransportCO₂ Transport
Lungs (alveoli)O₂ diffuses into plasma → binds to Hb forming HbO₂ (≈ 98 % of O₂)CO₂ diffuses from blood → converted to HCO₃⁻ by carbonic anhydrase → HCO₃⁻ exits RBC via Band 3 protein → plasma carries HCO₃⁻ (≈ 70 %)
Systemic tissuesHbO₂ releases O₂ → Hb becomes deoxy‑Hb (facilitated by H⁺ and CO₂)CO₂ produced by metabolism enters RBC → reacts with H₂O (carbonic anhydrase) → H⁺ binds to deoxy‑Hb (forming HbH⁺) and CO₂ binds to Hb (forming Hb‑CO₂) → HCO₃⁻ formed and exchanged for Cl⁻ (Hamburger shift)
Return to lungsDeoxy‑Hb picks up O₂ againHCO₃⁻ re‑enters RBC, combines with H⁺ → H₂CO₃ → H₂O + CO₂ (catalysed by carbonic anhydrase) → CO₂ expelled in alveoli

Key Points to Remember

  1. Haemoglobin carries the majority of O₂ (≈ 98 %) and a small proportion of CO₂ (as Hb‑CO₂).

  2. Carbonic anhydrase speeds up the interconversion of CO₂ and HCO₃⁻, allowing rapid CO₂ transport.

  3. Formation of haemoglobinic acid (Hb‑H⁺) and carbaminohaemoglobin both lower Hb’s affinity for O₂, enhancing O₂ release in tissues (Bohr effect).

  4. The “chloride shift” (Hamburger shift) maintains electro‑neutrality as HCO₃⁻ moves in and Cl⁻ moves out of RBCs.

Suggested diagram: Schematic of O₂ and CO₂ transport pathways showing haemoglobin binding, carbonic anhydrase activity, formation of Hb‑H⁺ and Hb‑CO₂, and the chloride shift.