Haemoglobin and Oxygen Transport Revision Notes | A Level AQA Biology

1. Why Haemoglobin Matters

Every cell in your body needs oxygen, yet oxygen barely dissolves in plasma. Without a dedicated carrier molecule, your tissues would be starved within seconds of activity. Haemoglobin solves this problem — and it does so with remarkable precision, loading oxygen where it is plentiful and releasing it exactly where it is needed most.

This lesson unpacks how haemoglobin's structure makes that precision possible, how the oxyhaemoglobin dissociation curve captures its behaviour, and why different organisms carry subtly different versions of the molecule. These are all directly assessed topics in AQA A Level Biology.

2. The Structure of Haemoglobin

Haemoglobin is a globular protein with quaternary structure — meaning it is made of more than one polypeptide subunit held together by interactions between those chains. In adult human haemoglobin, there are four subunits: two alpha chains and two beta chains. Each subunit folds around a haem group, which contains an iron(II) ion ($\text{Fe}^{2+}$) at its centre. That iron ion is the actual binding site for one oxygen molecule.

Because there are four subunits, one haemoglobin molecule can carry up to four oxygen molecules simultaneously. This four-seat capacity is central to its efficiency as a transport molecule.

Haemoglobins are described as a group of chemically similar molecules because the same basic principle — multiple subunits, each with a haem group — appears across a huge range of organisms, from fish to birds to humans. The amino acid sequences and the number of subunits can differ greatly between species, especially among invertebrates where molecules may contain many more chains than the four found in human haemoglobin. These differences alter the molecule's affinity for oxygen, but the presence of haem groups remains a unifying feature. This is why the plural 'haemoglobins' is used: the family shares a common structural theme while varying in its precise properties.

The 3D model above shows human haemoglobin (PDB: 4HHB). Notice the four subunits and the four haem groups nestled within them. The overall globular shape, together with hydrophilic R-groups on the outer surface of the protein, allows the molecule to remain soluble in the cytoplasm of red blood cells, as these groups interact with water molecules.

3. Loading, Transport, and Unloading of Oxygen

Haemoglobin's job has three distinct phases. At the lungs (or gills), oxygen loads onto haemoglobin — haemoglobin becomes saturated with oxygen and forms oxyhaemoglobin. The red blood cells then transport oxyhaemoglobin through the circulatory system. At respiring tissues, where oxygen concentration is low, oxyhaemoglobin dissociates — it releases oxygen, which diffuses into cells.

The key variable controlling loading and unloading is the partial pressure of oxygen ($p\text{O}_2$), measured in kilopascals (kPa). High $p\text{O}_2$ (as in the lungs, around 13 kPa) drives loading; low $p\text{O}_2$ (as in active muscle, around 2–4 kPa) drives unloading. The relationship between $p\text{O}_2$ and haemoglobin saturation is captured by the oxyhaemoglobin dissociation curve.

3a. The Oxyhaemoglobin Dissociation Curve

Picture a graph with partial pressure of oxygen on the x-axis (0–14 kPa) and percentage saturation of haemoglobin on the y-axis (0–100%). The resulting curve is not a straight line — it is sigmoid (S-shaped). That distinctive shape is not an accident; it is a direct consequence of how haemoglobin binds oxygen.

At low $p\text{O}_2$ values (0–3 kPa), the curve rises slowly — haemoglobin is reluctant to bind the first oxygen molecule. Then, between roughly 3 and 8 kPa, the curve rises steeply. At high $p\text{O}_2$ (above 10 kPa), the curve flattens as haemoglobin approaches full saturation. This pattern is explained by the cooperative nature of oxygen binding.

3b. Cooperative Binding — Why the Curve is S-Shaped

When the first oxygen molecule binds to one of haemoglobin's four subunits, it causes a conformational change — the shape of the whole haemoglobin molecule alters slightly. This change makes the remaining subunits more receptive to oxygen binding. The second oxygen binds more readily than the first, the third more readily than the second, and so on. This is cooperative binding.

Cooperative binding explains the S-shape directly. The initial slow rise reflects the difficulty of binding that first oxygen. The steep middle section reflects the accelerating ease of binding subsequent oxygens once the conformational change has occurred. The plateau at the top reflects saturation — all four haem groups are occupied.

This behaviour is physiologically brilliant. At the high $p\text{O}_2$ of the lungs, haemoglobin loads rapidly to near-full saturation. At the low $p\text{O}_2$ of active tissues, the cooperative effect works in reverse: once one oxygen dissociates, the conformational change makes the remaining oxygens easier to release too. The result is efficient unloading precisely where oxygen is most needed.

4. The Bohr Effect — Carbon Dioxide and Unloading

Now that you understand the basic dissociation curve, here is where it becomes even more powerful. Actively respiring tissues do not only have low $p\text{O}_2$ — they also produce carbon dioxide ($\text{CO}_2$). That $\text{CO}_2$ has a direct effect on haemoglobin's behaviour, and it is called the Bohr effect.

$\text{CO}_2$ dissolves in the blood plasma and red blood cell cytoplasm, reacting with water to form carbonic acid ($\text{H}_2\text{CO}_3$), which dissociates to release hydrogen ions ($\text{H}^+$). The rise in $\text{H}^+$ concentration means a fall in pH. Those hydrogen ions bind to haemoglobin and alter its tertiary and quaternary structure, reducing its affinity for oxygen. The result: haemoglobin releases oxygen more readily at any given $p\text{O}_2$.

On the dissociation curve, the Bohr effect appears as a shift to the right. At the same partial pressure of oxygen, haemoglobin is less saturated when $\text{CO}_2$ concentration is high. This means more oxygen is unloaded at respiring tissues — exactly where it is needed. At the lungs, $\text{CO}_2$ is exhaled, pH rises, haemoglobin's affinity for oxygen is restored, and loading proceeds efficiently.

Notice in the chart above that the high-$\text{CO}_2$ curve sits to the right of and below the low-$\text{CO}_2$ curve. At a tissue $p\text{O}_2$ of around 4 kPa, the difference in saturation between the two curves represents the extra oxygen released due to the Bohr effect. This is the physiological advantage: the harder a tissue is working (more $\text{CO}_2$ produced), the more oxygen haemoglobin delivers to it.

Worked example: Reading the Dissociation Curve: Calculating Oxygen Released

Using the dissociation curve data, at a partial pressure of 2 kPa (respiring tissue), haemoglobin is 14% saturated under low CO2 conditions and 6% saturated under high CO2 conditions. At a partial pressure of 13 kPa (lungs), haemoglobin is 98.5% saturated under both conditions. Calculate the percentage of oxygen unloaded at the tissue under each condition, and determine the extra oxygen released due to the Bohr effect.

1. Saturation at lungs ($p\text{O}_2$ = 13 kPa) = 98.5% (both conditions) — This is the loading point — haemoglobin is nearly fully saturated when leaving the lungs.
2. Saturation at tissue ($p\text{O}_2$ = 2 kPa, low $\text{CO}_2$) = 14% — This is how saturated haemoglobin remains after passing through tissue under normal CO2 conditions.
3. Oxygen unloaded = 98.5% − 14% = 84.5% — The difference between loading saturation and unloading saturation gives the proportion of oxygen released.
4. Saturation at tissue ($p\text{O}_2$ = 2 kPa, high $\text{CO}_2$) = 6% — The Bohr effect has reduced haemoglobin's affinity, so it retains less oxygen at the same partial pressure.
5. Oxygen unloaded = 98.5% − 6% = 92.5% — More oxygen is released at the tissue when CO2 concentration is high.
6. Extra oxygen released = 92.5% − 84.5% = 8.0 percentage points — This 8 percentage point difference represents the additional oxygen delivered to the tissue as a direct result of the Bohr effect. In an active, respiring tissue producing more CO2, this difference is even larger.

Answer: Under low CO2: 84.5% of haemoglobin's oxygen is unloaded. Under high CO2: 92.5% is unloaded. The Bohr effect results in an additional 8.0 percentage points of oxygen being released at the tissue.

5. Haemoglobins Across Different Organisms

Different organisms face very different oxygen availability challenges, and natural selection has produced haemoglobins with different affinities for oxygen to match. The position of the dissociation curve on the x-axis tells you everything: a curve shifted to the left indicates high affinity (haemoglobin loads oxygen easily but releases it less readily), while a curve shifted to the right indicates lower affinity (haemoglobin loads less easily but releases oxygen more readily).

Consider three contrasting examples. A lugworm (Arenicola marina) lives in waterlogged sand where oxygen levels are very low. Its haemoglobin has a very high affinity for oxygen — its dissociation curve is far to the left — allowing it to load oxygen even at the tiny partial pressures available in its environment. A llama lives at high altitude where atmospheric $p\text{O}_2$ is reduced. Llama haemoglobin also has a higher affinity than human haemoglobin, enabling effective loading from thin air. A foetal haemoglobin (haemoglobin F) in humans has a higher affinity than adult haemoglobin (haemoglobin A), so that the foetus can extract oxygen from the mother's blood across the placenta.

The underlying principle is consistent: haemoglobin affinity is tuned by evolution to match the partial pressure of oxygen at the loading site. Where loading $p\text{O}_2$ is low (altitude, low-oxygen sediment, placenta), high-affinity haemoglobin is selected for. Where rapid unloading to active tissues is the priority, lower affinity or a pronounced Bohr effect serves the organism better.

Questions on haemoglobin and the dissociation curve appear regularly in AQA A Level papers, and they reward precise language. The following guidance covers the most common ways students lose marks and how to avoid them.

6. Exam Technique and Common Mistakes

Across this lesson, the central thread has been the relationship between structure and function: haemoglobin's quaternary structure enables cooperative binding, cooperative binding produces the sigmoid dissociation curve, the Bohr effect fine-tunes unloading at active tissues, and variation in the amino acid sequences of different haemoglobins allows the same basic design to serve organisms across vastly different environments. Mastering these connections — and expressing them with precise terminology — is what distinguishes a top-band answer from a mid-band one.