A-Level Biology 9700 / Topic 8 / AS

Load in the lungs, unload where it is needed.

Haemoglobin picks up oxygen almost completely in the lungs and gives a lot of it away in respiring tissues, helped by the Bohr shift. Carbon dioxide travels mainly as hydrogencarbonate ions, made in red blood cells by carbonic anhydrase.

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The key idea

Each haemoglobin molecule has four polypeptide chains, each with a haem group that binds one O₂, so it carries up to four oxygen molecules. Binding is cooperative: once one O₂ binds, the molecule changes shape and the next bind more easily, giving the S-shaped (sigmoid) oxygen dissociation curve. In the lungs (high partial pressure of oxygen) haemoglobin is almost fully saturated; in respiring tissues (low pO₂), on the steep part of the curve, a small fall in pO₂ releases a lot of oxygen. Carbon dioxide diffuses into red blood cells, where carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃, which dissociates to H⁺ and HCO₃^-. H⁺ combines with haemoglobin to form haemoglobinic acid, releasing oxygen and buffering the blood. HCO₃^- diffuses into the plasma, balanced by Cl^- moving in: the chloride shift. Some CO₂ binds haemoglobin as carbaminohaemoglobin and a little dissolves in plasma. High CO₂ lowers haemoglobin's affinity for oxygen and moves the curve to the right: the Bohr shift.

Fig. 1: oxygen dissociation curve of adult haemoglobin and the Bohr shift

Section 01

The S-shaped curve.

Drag the dot along the curve or move the pO₂ slider from the lungs band (green) down to the respiring tissue band (orange). Read the saturation and how many of the four haem groups hold oxygen. Then tick High CO₂.

Low pO₂: the first O₂ binds with difficulty, so the curve starts shallow.
Cooperative binding: binding one O₂ changes the shape of the haemoglobin, making it easier for the second and third to bind, so the curve rises steeply over the middle range (in the lab, 50% saturation is at about 2.8 kPa).
High pO₂: the curve levels off as the last site fills and haemoglobin approaches 100% saturation.
In the lungs (about 12 kPa) haemoglobin is about 98% saturated, so it loads almost fully. In respiring tissue (about 2 to 5 kPa) the curve is steep, so a small drop in pO₂ unloads a large amount of oxygen. The more active the tissue, the lower its pO₂ and the more oxygen it receives.
Bohr shift: high carbon dioxide (more H⁺, lower pH) lowers haemoglobin's affinity for oxygen. The curve moves to the right, so at the same low pO₂ more oxygen is released in actively respiring tissues, while loading in the lungs, where CO₂ is low, is hardly affected.

Examiner trap

A curve shifted to the right means lower affinity: haemoglobin is less saturated at any given pO₂ and gives up oxygen more readily. Do not say the Bohr shift helps haemoglobin pick up oxygen.

Stage 1 · Learn

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In the oxygen dissociation tab, what is the saturation of haemoglobin at the starting pO₂ of 12 kPa?

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What happens to the curve in the lab when High CO₂ is ticked?

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In the CO₂ transport tab, which enzyme catalyses the reaction of carbon dioxide with water?

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In step 5 of the CO₂ tab, which way do the ions move?

Section 02

Carbon dioxide, step by step.

Step through the six chips in order. Follow each token: CO₂, H₂CO₃, H⁺, HCO₃^-, Cl^-, HHb and carbamino-Hb.

Form	Where	About
hydrogencarbonate ions, HCO₃^-	made in red blood cells, carried in the plasma	85%
carbaminohaemoglobin	CO₂ bound to amine groups of haemoglobin in red blood cells	10%
dissolved CO₂	in solution in the plasma	5%

Carbonic anhydrase in red blood cells catalyses CO₂ + H₂O → H₂CO₃ very rapidly; carbonic acid dissociates to H⁺ + HCO₃^-.
Haemoglobinic acid: H⁺ combines with haemoglobin (HHb). This buffers the blood pH and makes haemoglobin release its oxygen to the tissues.
Chloride shift: HCO₃^- diffuses out of the red blood cell into the plasma through a carrier protein; Cl^- ions move in the other way to keep the cell electrically neutral. This keeps HCO₃^- from building up in the cell, so carbonic anhydrase can keep converting CO₂.
Plasma carries most of the carbon dioxide, as HCO₃^- in solution, plus the dissolved CO₂.
In the lungs every reaction runs in reverse and CO₂ diffuses into the alveoli.

Examiner trap

The chloride shift moves chloride ions into the red blood cell as hydrogencarbonate ions move out. It is about balancing charge, not about carrying carbon dioxide.

Stage 2 · Exam

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Why is the oxygen dissociation curve of adult haemoglobin S-shaped?

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What is the advantage of the Bohr shift to a mammal?

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How is most carbon dioxide transported in the blood?

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What is the importance of the formation of haemoglobinic acid in red blood cells?

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Six original Cambridge-style questions

Haemoglobin and the oxygen dissociation curve, loading in the lungs and unloading in tissues, the Bohr shift, carbonic anhydrase, haemoglobinic acid, carbaminohaemoglobin, the chloride shift and plasma.