describe the Bohr shift and explain the importance of the Bohr shift

Transport of Oxygen and Carbon Dioxide (Cambridge AS & A Level Biology – Topics 8.2 & 8.3)

1. Learning‑outcome map (Topic 8.2)

2. Structure & Function of Haemoglobin (Hb)

• Quaternary structure: tetramer (α₂β₂); each subunit contains a heme group with an Fe²⁺ ion.
• O₂ binding: each Fe²⁺ can bind one O₂ molecule → up to four O₂ per Hb molecule.
• CO₂ binding:

  • Carbamate formation – CO₂ reacts with the terminal ‑NH₂ groups of the globin chains → Hb‑CO₂ (≈ 20–30 % of total CO₂).
  • Carbamino‑haemoglobin – CO₂ binds directly to the protein backbone (minor contribution).
  • Haemoglobinic acid – H⁺ bound to deoxy‑Hb (T‑state) acts as a buffer.

• H⁺ buffering: deoxy‑Hb provides binding sites for H⁺, helping to maintain blood pH.

3. Forms of Gas Transport in Blood

4. Oxygen‑Haemoglobin Dissociation Curve

• Sigmoidal shape – reflects cooperative binding; the first O₂ molecule increases affinity for the next.
• P₅₀ – pO₂ at 50 % Hb saturation (≈ 26 mm Hg in resting adult blood). Higher P₅₀ = lower affinity.
• Physiological relevance:

  • Right‑shift (higher P₅₀) → easier O₂ release in metabolically active tissues.
  • Left‑shift (lower P₅₀) → favours O₂ loading in the lungs.

5. The Bohr Effect (Bohr Shift)

5.1 Definition

A decrease in blood pH (increase in H⁺) or an increase in pCO₂ reduces haemoglobin’s affinity for O₂, causing the O₂‑Hb dissociation curve to shift to the right.

5.2 Mechanism (step‑by‑step)

1. Metabolically active tissues produce CO₂.
2. CO₂ diffuses into RBCs and is hydrated (catalysed by carbonic anhydrase):
   

   \$\mathrm{CO2 + H2O \;\xrightleftharpoons[CA]{}\; H2CO3 \;\rightleftharpoons\; H^+ + HCO_3^-}\$

3. Elevated H⁺ protonates specific amino‑acid residues (e.g., histidine) on the globin chains.
4. Protonation stabilises the “tense” (T) conformation of Hb, which has a lower O₂ affinity.
5. The T‑state also enhances CO₂ binding as carbamate and H⁺ binding as haemoglobinic acid (Haldane effect, see §5.4).
6. Result: O₂ is released more readily where it is needed.

5.3 Factors that modify the Bohr shift

5.4 Importance of the Bohr Effect

• Targeted O₂ delivery: Rising CO₂ and H⁺ in active muscles force O₂ release exactly where metabolism is highest.
• Facilitates CO₂ removal (Haldane effect): De‑oxygenated Hb binds more CO₂ as carbamate and more H⁺ as haemoglobinic acid, increasing the blood’s CO₂‑carrying capacity.
• Acid‑base regulation: In the lungs, low pCO₂ and high pH shift the curve left, promoting O₂ uptake and CO₂ release, helping to stabilise systemic pH.
• Exercise adaptation: Exercise raises temperature, pCO₂ and H⁺; the combined right‑shift ensures rapid O₂ unloading and efficient CO₂ transport.

6. The Haldane Effect

When Hb releases O₂ (as in peripheral tissues) its affinity for CO₂ increases, so more CO₂ is taken up as carbamate and more H⁺ is buffered as haemoglobinic acid. Conversely, in the lungs O₂ binding reduces Hb’s capacity for CO₂, facilitating CO₂ release.

7. The Chloride Shift (Hamburger‑Freundlich)

• To keep RBCs electrically neutral, the bicarbonate ion (HCO₃⁻) that leaves the cell is exchanged for a chloride ion (Cl⁻) that enters.
• In tissues: CO₂ → HCO₃⁻ (leaves RBC) + Cl⁻ (enters RBC).
• In lungs: HCO₃⁻ re‑enters RBC, Cl⁻ exits, and CO₂ is expelled.
• This exchange is essential for rapid CO₂ transport and for stabilising intracellular pH.

8. The Heart (Topic 8.3)

8.1 External & Internal Anatomy

• Four chambers: two atria (receive blood) and two ventricles (pump blood).
• Major vessels: superior/inferior vena cava, pulmonary arteries & veins, aorta.
• Wall thickness: left ventricle > right ventricle (systemic vs pulmonary pressure).

8.2 Cardiac Cycle (one complete heartbeat)

1. Atrial systole – atria contract, filling ventricles (ventricular diastole).
2. Isovolumetric ventricular systole – ventricles contract, all valves closed, pressure rises.
3. Ventricular ejection – semilunar valves open; blood expelled to pulmonary artery (right) and aorta (left).
4. Isovolumetric ventricular diastole – ventricles relax, semilunar valves close.
5. Ventricular filling – AV (tricuspid/mitral) valves open, blood flows from atria to ventricles.

8.3 Conduction System

• SA node (sino‑atrial) – primary pacemaker; initiates impulse.
• AV node (atrioventricular) – delays impulse to allow complete ventricular filling.
• Bundle of His → Purkinje fibres – rapid conduction through ventricular walls, ensuring coordinated contraction.

9. Practical Investigation: Demonstrating the Bohr Effect with a Pulse Oximeter

10. Summary Diagram (suggested)