The shape of the lesson
By the end of the lesson, learners can
- The S-shaped curve: Drag the dot along the curve or move the pO 2 slider from the lungs band (green) down to the respiring tissue band (orange).
- Carbon dioxide, step by step: Step through the six chips in order.
- Sigmoid curve: cooperative binding of four O2 per haemoglobin.
- Lungs: high pO2, near full saturation. Tissues: steep part, big unloading.
- Bohr shift: more CO2, curve to the right, lower affinity, more O2 released.
- CO2: carbonic anhydrase, H+ to HHb, HCO3- out, Cl- in.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, 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. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: The S-shaped curve | Drag the dot along the curve or move the pO 2 slider from the lungs band (green) down to the respiring tissue band (orange). Project tab 5 of the lab (Transport in mammals lab, oxygen dissociation curve) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Carbon dioxide, step by step | Step through the six chips in order. Project tab 6 of the lab (Transport in mammals lab, carbon dioxide transport) and let a student drive it while the class predicts. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Carbon dioxide, step by step, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A curve shifted to the right means lower affinity: haemoglobin is less saturated at any given pO 2 and gives up oxygen more readily. | Do not say the Bohr shift helps haemoglobin pick up oxygen. |
| 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. |
Transport of oxygen and carbon dioxide: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
State three ways in which carbon dioxide is transported in the blood, and where each is carried.
Describe the role of red blood cells in the transport of carbon dioxide from respiring tissues, including the chloride shift.
Explain the importance of the chloride shift.
Explain the importance of the shape of the oxygen dissociation curve at the partial pressures of oxygen found in the lungs and in respiring tissues.
Describe the Bohr shift and explain its importance during exercise.
Blood leaves the lungs 97% saturated. In resting muscle (pO2 5 kPa) it is 75% saturated; in exercising muscle with high CO2 (pO2 2.5 kPa) it is 20% saturated. Fully saturated blood carries 20 cm3 O2 per 100 cm3. Calculate the oxygen unloaded per 100 cm3 of blood in each muscle.
Answer key and marking notes
- As hydrogencarbonate ions (most, about 85%), carried in solution in the plasma.
- As carbaminohaemoglobin (about 10%), CO2 bound to haemoglobin in red blood cells.
- Dissolved as CO2 in the plasma (about 5%).
- CO2 diffuses from respiring cells into the plasma and into red blood cells.
- Carbonic anhydrase catalyses the reaction of CO2 with water to form carbonic acid, which dissociates into H+ and HCO3-.
- H+ combines with haemoglobin to form haemoglobinic acid, buffering the pH and causing haemoglobin to release oxygen.
- HCO3- diffuses out of the red blood cell into the plasma.
- Cl- ions diffuse in to maintain electrical neutrality: the chloride shift. Some CO2 also binds haemoglobin to form carbaminohaemoglobin.
- As negatively charged HCO3- ions leave the red blood cell, Cl- ions enter to balance the charge.
- This keeps the red blood cell (and plasma) electrically neutral.
- It allows HCO3- to keep leaving, so HCO3- does not build up and carbonic anhydrase can keep converting CO2, letting large amounts of CO2 be carried in the plasma.
- In the lungs pO2 is high, where the curve is flat near the top, so haemoglobin becomes almost fully saturated.
- Small changes in pO2 in the lungs hardly reduce loading.
- In respiring tissues pO2 is low, on the steep part of the curve, so a small fall in pO2 causes a large fall in saturation.
- So a large amount of oxygen is unloaded where it is needed, and more in very active tissues with lower pO2.
- High carbon dioxide concentration (more H+, lower pH) lowers the affinity of haemoglobin for oxygen.
- The dissociation curve moves to the right, so at any pO2 the saturation is lower.
- During exercise muscles respire faster and make more CO2, so more oxygen is unloaded to the muscles that need it.
- Resting: (97 - 75)% of 20 = 0.22 × 20 = 4.4 cm3 per 100 cm3.
- Exercising: (97 - 20)% of 20 = 0.77 × 20 = 15.4 cm3 per 100 cm3.
- So exercising muscle receives 3.5 times as much oxygen, because of the lower pO2 and the Bohr shift.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the maximum number of oxygen molecules that one molecule of adult haemoglobin can carry?
- A4
- B1
- C2
- D8
Where in the blood is carbonic anhydrase found?
- Ainside red blood cells
- Bdissolved in the plasma
- Cin the walls of the capillaries
- Dinside lymphocytes
Use the oxygen dissociation curve. What is the percentage saturation of haemoglobin at a partial pressure of oxygen of 2 kPa?
- A25%
- B50%
- C10%
- D75%
Which substance forms when carbon dioxide combines directly with haemoglobin?
- Acarbaminohaemoglobin
- Bhaemoglobinic acid
- Coxyhaemoglobin
- Dcarbonic acid
Why does carbon monoxide poisoning reduce the oxygen carried by the blood?
- Acarbon monoxide binds to haem groups more strongly than oxygen does
- Bcarbon monoxide inhibits carbonic anhydrase in red blood cells
- Ccarbon monoxide shifts the dissociation curve to the right
- Dcarbon monoxide destroys red blood cells in the capillaries
Use the oxygen dissociation curve. Blood leaves the lungs at 12 kPa and reaches a muscle at 2 kPa. Fully saturated blood carries 20 cm3 of oxygen per 100 cm3. How much oxygen does each 100 cm3 of blood release to the muscle?
- A14.6 cm3
- B5.0 cm3
- C19.6 cm3
- D15.0 cm3
A drug blocks the protein that lets chloride ions and hydrogencarbonate ions exchange across the red blood cell membrane. Which effect is most likely?
- Ahydrogencarbonate builds up in red blood cells, so less carbon dioxide is carried as HCO3- in plasma
- Bcarbonic anhydrase is denatured, so no carbonic acid forms
- Cmore carbon dioxide is carried as hydrogencarbonate ions in the plasma
- Dhaemoglobin can no longer bind oxygen in the lungs
Llamas live at high altitude, where the partial pressure of oxygen in the air is low. Their haemoglobin has a dissociation curve to the left of human haemoglobin. Why is this an advantage?
- Atheir haemoglobin becomes highly saturated even at the low pO2 in their lungs
- Btheir haemoglobin releases oxygen more readily in the lungs
- Ctheir haemoglobin carries more carbon dioxide than human haemoglobin
- Dtheir haemoglobin shows a larger Bohr shift in the tissues
Quiz key
Haemoglobin has four polypeptide chains, each with one haem group containing an iron ion that binds one O2 molecule, so it carries four. One would be a single chain such as myoglobin; two counts only the alpha chains; eight counts oxygen atoms rather than molecules.
Carbonic anhydrase is in the cytoplasm of red blood cells, which is why HCO3- is made there and then diffuses out (the chloride shift). Plasma has little of the enzyme, so the reaction is slow there. It is not a capillary wall or lymphocyte enzyme in this process.
At 2 kPa the curve is level with the 25% gridline. 50% is reached at about 3 kPa, 75% at about 4 kPa, and 10% is below the curve at that pressure. Reading a curve means going up from the x value to the line, then across.
Carbon dioxide binds to the amine groups of haemoglobin, forming carbaminohaemoglobin. Haemoglobinic acid forms when H+ ions combine with haemoglobin. Oxyhaemoglobin carries oxygen. Carbonic acid is made from CO2 and water, not haemoglobin.
Carbon monoxide binds to the iron in haem almost irreversibly, much more strongly than oxygen, so fewer sites are free to carry oxygen. It does not act on carbonic anhydrase or destroy red cells, and a right shift would help unloading rather than block carriage.
At 12 kPa saturation is about 98%; at 2 kPa it is 25%. The fall is 73%, and 0.73 × 20 = 14.6 cm3. 5.0 cm3 is the oxygen still carried at 2 kPa (25%). 19.6 cm3 is the oxygen carried in the lungs. 15.0 cm3 assumes the blood leaves the lungs 100% saturated.
Without the exchange, HCO3- cannot leave the cell, so it accumulates and slows the carbonic anhydrase reaction (product build-up), and much less carbon dioxide reaches the plasma as HCO3-. The drug blocks a transport protein, not the enzyme, and it does not stop haemoglobin binding oxygen. Plasma transport falls, not rises.
A left-shifted curve means higher affinity, so the haemoglobin loads well even when pO2 in the lungs is low. Releasing oxygen in the lungs would be a disadvantage. The position of the oxygen curve does not show carbon dioxide carriage, and a larger Bohr shift is a separate property, not what a left shift means.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.