The Heart and Cardiac Cycle — AQA A-level Biology

SPEC 3.3.4.1 - MASS TRANSPORT IN ANIMALS

Haemoglobin & the Bohr Effect

How haemoglobin loads oxygen at the lungs and unloads it at respiring tissues — cooperative binding, dissociation curves and the Bohr effect.

Why We Need Haemoglobin

Oxygen is only slightly soluble in blood plasma. In humans, plasma alone can carry far too little O₂ to sustain aerobic respiration in all our tissues.

The Solution

Haemoglobin (Hb) — a specialised oxygen-carrying protein packed inside red blood cells (erythrocytes). It dramatically increases the oxygen-carrying capacity of blood and, crucially, loads O₂ at the lungs and unloads it at respiring tissues.

Synoptic Link

Haemoglobin is a direct example of quaternary protein structure (from Topic 3.1.4.2). Its function depends entirely on the specific 3D arrangement of its four polypeptide subunits — a classic "structure determines function" example for Paper 3 essays.

Haemoglobin Structure

HAEMOGLOBIN

A globular protein with quaternary structure consisting of four polypeptide chains, each containing a haem (prosthetic) group with a central Fe²⁺ ion that reversibly binds one O₂ molecule.

Structural Details

4 polypeptide subunits: adult haemoglobin (HbA) has 2 α-chains and 2 β-chains

4 haem groups — one per subunit; each contains Fe²⁺ at its centre

Each Fe²⁺ binds ONE O₂ → max capacity 4 O₂ per Hb molecule

When fully loaded: Hb + 4O₂ → oxyhaemoglobin, often written Hb(O₂)₄

Key Definitions

OXYHAEMOGLOBIN

Haemoglobin combined with oxygen. When fully saturated: Hb + 4O₂ ⇌ Hb(O₂)₄. The binding is reversible — the double arrow is essential.

PARTIAL PRESSURE (pO₂)

The pressure exerted by oxygen in a mixture of gases. It is a measure of oxygen concentration. High pO₂ at the lungs (~13–14 kPa); low pO₂ at respiring tissues (~2–4 kPa).

AFFINITY FOR OXYGEN

The tendency of haemoglobin to bind oxygen. HIGH affinity → binds O₂ readily (loads easily but unloads less readily). LOW affinity → releases O₂ readily (unloads easily).

Examiner Language

AQA mark schemes require the term "partial pressure" — writing "concentration of oxygen" will NOT score. Use pO₂ in kPa.

Loading, Transport & Unloading

Haemoglobin's job has three stages — each depends on the partial pressure of oxygen (pO₂) in the surrounding tissue:

1

Loading (association) at the lungs — alveoli have HIGH pO₂ (~13 kPa) → Hb has HIGH affinity for O₂ → Hb binds 4 × O₂ → becomes oxyhaemoglobin (~97–98% saturated).

2

Transport — oxyhaemoglobin is carried in red blood cells around the body via the circulatory system.

3

Unloading (dissociation) at respiring tissues — tissues have LOW pO₂ (~3 kPa, O₂ used up in respiration) → Hb has LOWER affinity for O₂ → O₂ dissociates from Hb → diffuses into cells.

Exam Tip

The command word describe expects BOTH loading (at lungs) AND unloading (at tissues). The 2024 Paper 2 examiner report flagged that students who only described the muscle lost easy marks — oxyhaemoglobin supplies all respiring cells/tissues.

Cooperative Binding

COOPERATIVE BINDING

The property of haemoglobin whereby binding of the first O₂ molecule causes a conformational (shape) change in the quaternary structure, making it easier for subsequent O₂ molecules to bind.

1

First O₂ binds slowly — the haem group is partially hidden inside the quaternary structure. This explains the shallow start of the S-curve.

2

Conformational change — binding distorts the tertiary/quaternary structure, exposing the remaining haem groups.

3

2nd & 3rd O₂ bind more easily — producing the steep section of the curve.

4

4th O₂ binds slowly again — statistical: only one haem group left, harder to find. Curve plateaus at ~100% saturation.

The Payoff

Cooperative binding produces the sigmoid (S-shaped) dissociation curve — which allows efficient loading at the lungs AND efficient unloading at tissues. A linear binding protein could never do both.

The Oxyhaemoglobin Dissociation Curve

Reading the S-curve

Steep middle → small drop in pO₂ causes a large drop in saturation → lots of O₂ unloaded to tissues

Plateau at top → even small drops in lung pO₂ don't affect loading → safety margin

Shallow start → consequence of cooperative binding (first O₂ binds slowly)

The Bohr Effect

BOHR EFFECT

The effect of increased CO₂ concentration on the dissociation of oxyhaemoglobin. At higher pCO₂, haemoglobin's affinity for O₂ decreases → the dissociation curve shifts to the right → more O₂ is unloaded at any given pO₂.

1

Respiring tissues produce CO₂ as a waste product of aerobic respiration.

2

The higher partial pressure of CO₂ in the blood at respiring tissues lowers haemoglobin's affinity for O₂.

3

Dissociation curve shifts rightward → at any given pO₂, Hb is LESS saturated with O₂.

4

More O₂ dissociates from oxyhaemoglobin → more O₂ is available for respiring cells/tissues.

Adaptive Logic

The more active a tissue → the more CO₂ it produces → the greater the Bohr shift → the more O₂ it receives. Oxygen delivery is matched to demand.

Note on Depth

Some textbooks explain the mechanism via pH (CO₂ + H₂O → carbonic acid → H⁺, which binds Hb and alters its shape). This is extra detail beyond what AQA requires. The 2024 examiner report explicitly noted students who focused on pH detail missed simpler marks elsewhere. Stick to: increased CO₂ → reduced affinity → curve shifts right → more O₂ released.

Visualising the Bohr Shift

At pO₂ = 3 kPa (respiring tissue)

Normal curve: Hb ~25% saturated. Bohr-shifted curve: Hb drops to ~5% saturated. The difference = extra O₂ released exactly where active tissues need it.

Examiner Alerts — From Real Mark Schemes

The 2024 AS Paper 2 Q09.1 asked students to describe loading, transport and unloading of O₂. The examiner report revealed these recurring errors:

Error 1 — "Concentration" not "Partial Pressure"

Referring to "concentration of O₂" or "concentration of CO₂" scored no credit. AQA mark schemes require "partial pressure". Always use pO₂ and pCO₂ in kPa.

Error 2 — Forgetting Red Blood Cells

Students "frequently... did not mention red blood cells at all in the entire answer." MP4 (mentioning RBCs transport oxyhaemoglobin) was the lowest-scoring mark point. Always write "haemoglobin in red blood cells" — not just "blood."

Error 3 — Only Mentioning Muscles

"Weaker answers tended to refer only to muscles." Oxygen is delivered to ALL respiring cells and tissues, not just muscle.

Error 4 — "Affinity" Without "Loading/Unloading"

"Weaker answers focused on haemoglobin's affinity for O₂ changing and failed to gain marks by not describing loading and unloading." Always state explicitly: loads at the lungs, unloads at respiring tissues.

Why the S-Shape Matters

A common AQA exam question asks: "Explain the significance of the shape of the dissociation curve." Answer it in three parts:

1

Top plateau (at high pO₂, e.g. lungs ~13 kPa): Hb is ~98% saturated. Even if alveolar pO₂ drops slightly, loading is barely affected → reliable loading.

2

Steep middle section (pO₂ ~2–6 kPa): a small drop in pO₂ produces a large fall in saturation → efficient unloading of O₂ in exactly the pO₂ range found at respiring tissues.

3

Shallow start (very low pO₂): the first O₂ binds slowly because the haem groups are harder to access before any shape change has occurred → explains why cooperative binding is needed.

Exam Answer Formula

"The curve is steep across the pO₂ range found at respiring tissues, so a small reduction in pO₂ causes a large reduction in saturation. This means much more oxygen is unloaded — exactly where it is needed."

Different Haemoglobins, Different Jobs

Haemoglobins are a family of chemically similar proteins. Different organisms (and different life stages) have haemoglobins with different amino acid sequences → different 3D shapes → different affinities for O₂. Each is adapted to its environment.

Rule of Thumb

Curve shifted LEFT = HIGHER affinity → loads O₂ at LOWER pO₂. Useful where O₂ is scarce (high altitude, low-O₂ environments, across placenta).

Curve shifted RIGHT = LOWER affinity → unloads O₂ more readily. Useful for very active organisms needing rapid O₂ delivery.

Why Different Shapes?

Different primary structure (amino acid sequence) → different R-group interactions (H-bonds, ionic, disulfide bridges) → different tertiary/quaternary structure → different affinity for O₂. A structure-function example you can use in any Paper 3 essay.

Three Classic Examples

OrganismEnvironmentCurve PositionAdaptive Benefit

Llama (high altitude)Low atmospheric pO₂LEFT of humanHigher affinity → can still load O₂ at low altitude pO₂

Foetal Hb (HbF)Placenta — lower pO₂ than mother's bloodLEFT of maternal (HbA)Higher affinity → foetal Hb "pulls" O₂ FROM mother's Hb across the placenta

Lugworm (Arenicola)Mud burrows; water low in O₂ at low tideFar LEFTVery high affinity → loads O₂ when surrounding water has very low pO₂

Active mammals (e.g. mice)High metabolic rateRIGHT of larger mammalsLower affinity → unloads O₂ rapidly to meet high respiratory demand

Exam Strategy

For any "compare these two dissociation curves" question: (1) state which is left/right, (2) state which has higher/lower affinity, (3) link to the environment or lifestyle of that organism. Never stop at "has higher affinity" — always say WHY it matters.

Structuring the Extended Answer

How to structure a 6-mark "explain the Bohr effect and its importance" question using the DEEP framework:

DEEP: Bohr Effect Extended Answer

DDefine: The Bohr effect is the change in oxyhaemoglobin dissociation caused by increased carbon dioxide concentration; Hb's affinity for O₂ decreases and the curve shifts to the right.

EExplain mechanism: At respiring tissues, cells produce CO₂ via aerobic respiration, so pCO₂ is higher. This reduces haemoglobin's affinity for O₂, so oxyhaemoglobin dissociates and releases O₂ at that location.

EEvidence/Example: At a tissue pO₂ of ~3 kPa, normal Hb is ~25% saturated; under Bohr conditions saturation falls further at the same pO₂ → more O₂ unloaded.

PPractical/Quantitative: Adaptive significance — the more actively a tissue respires, the more CO₂ it produces, so the more O₂ Hb releases to it. Oxygen supply is matched to metabolic demand.

Practice Question 1

Describe how oxygen is loaded, transported and unloaded in the blood. [5 marks]

AQA AS Paper 2, June 2024 style question.

MP1

At the lungs, pO₂ is high → haemoglobin (in red blood cells) has a high affinity for O₂ → O₂ associates with / binds to Hb;

MP2

Cooperative binding — the binding of the first O₂ causes a shape change making further O₂ bind more easily → Hb becomes saturated (forming oxyhaemoglobin);

MP3

Oxyhaemoglobin is transported around the body in red blood cells;

MP4

At respiring cells/tissues, pO₂ is low (and pCO₂ is high);

MP5

Haemoglobin's affinity for O₂ decreases → oxyhaemoglobin dissociates / releases O₂ → O₂ diffuses into respiring cells.

Practice Question 2

A llama lives at 4000 m altitude. Its oxyhaemoglobin dissociation curve lies to the left of that of a human. Explain the advantage of this adaptation. [3 marks]

MP1

At high altitude, the partial pressure of oxygen (pO₂) in the atmosphere / alveoli is lower than at sea level;

MP2

Llama haemoglobin has a higher affinity for oxygen than human haemoglobin;

MP3

So at a lower pO₂, llama Hb still becomes highly saturated with O₂ / can still load O₂ efficiently at the lungs — enabling sufficient oxygen supply for aerobic respiration.

Summary

The Essentials

Haemoglobin: quaternary protein, 4 polypeptides, 4 haem groups with Fe²⁺, binds up to 4 O₂.

Cooperative binding: first O₂ changes Hb shape → subsequent O₂ bind more easily → S-shaped curve.

Loading: high pO₂ at lungs → high affinity → ~98% saturated.

Unloading: low pO₂ + high CO₂ at tissues → lower affinity → O₂ released.

Bohr effect: ↑CO₂ at tissues → Hb's affinity for O₂ decreases → curve shifts RIGHT → more O₂ unloaded.

Different Hbs: LEFT curve = higher affinity (llama, foetal, lugworm); RIGHT = lower affinity (active small mammals).

Must-Use Term

"partial pressure of oxygen (pO₂)" — never "concentration".

Must-Describe

BOTH loading AND unloading in RBCs — omitting either loses marks.

Top Exam Tip

For any Bohr question, the mark-scoring chain is: ↑CO₂ → Hb's affinity for O₂ decreases → curve shifts right → more O₂ unloaded at respiring tissues. Use the phrase "partial pressure" (pO₂) — never "concentration of oxygen". Always mention red blood cells and both loading AND unloading.