Oxygen Transport and the Dissociation Curve

NCERT grounding

NCERT Class XI Biology, Chapter 14, Section 14.4.1 (Transport of Oxygen) is the syllabus anchor. The text states the headline distribution that NEET examiners repeatedly target: about 97 per cent of O₂ is transported by RBCs in the blood as oxyhaemoglobin, while the remaining 3 per cent of O₂ is carried in a dissolved state through the plasma. Haemoglobin is described as a red, iron-containing pigment present in RBCs that binds O₂ reversibly; each molecule carries a maximum of four molecules of O₂. The curve plotting percentage saturation against pO₂ is named the oxygen dissociation curve and is explicitly called sigmoid. NIOS Lesson 14 reinforces the same numbers and writes the reaction as Hb + 4 O₂ ⇌ Hb(O₂)₄, emphasising reversibility.

In the alveoli, where there is high pO₂, low pCO₂, lesser H⁺ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin; in the tissues, where low pO₂, high pCO₂, high H⁺ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from oxyhaemoglobin.

NCERT Class XI · Section 14.4.1

Mechanism of oxygen transport

Oxygen has a low solubility in plasma — only about 0.3 mL of O₂ dissolves per 100 mL of blood at an alveolar pO₂ of 104 mm Hg. If transport depended on physical solution alone, cardiac output would need to be more than twenty times its present value to meet resting metabolic demand. Haemoglobin solves this problem by acting as a reversible chemical reservoir for oxygen, multiplying the carrying capacity of every 100 mL of blood from ~0.3 mL to roughly 20 mL.

Each adult haemoglobin (HbA) molecule is a tetramer of two α-globin and two β-globin chains, each wrapped around one haem prosthetic group. The haem is a porphyrin ring with a central ferrous iron (Fe²⁺) atom; it is this iron that reversibly binds molecular O₂. Four haem groups give four binding sites per molecule — the NCERT-stated upper limit of "four molecules of O₂" per haemoglobin.

Because the binding is reversible and non-oxidative (the iron stays as Fe²⁺; if it were truly oxidised to Fe³⁺, methaemoglobin would result, which cannot carry O₂), the same molecule can repeatedly load O₂ at the lungs and unload it at the tissues during its 120-day RBC lifespan.

The reversible reaction

The combined statement of the binding reaction is written, following NIOS, as:

Hb   +   4 O₂   ⇌   Hb(O₂)₄
(deoxyhaemoglobin)    (oxyhaemoglobin)

The forward direction dominates at the alveolus (loading); the reverse direction dominates at the tissue (unloading). NCERT lists four factors that drive this equilibrium — pO₂, pCO₂, H⁺ concentration and temperature — and explicitly notes that "binding of oxygen with haemoglobin is primarily related to partial pressure of O₂". The other three are modulators that interfere with the binding.

The sigmoid dissociation curve

When percentage saturation of haemoglobin with O₂ (y-axis) is plotted against pO₂ in mm Hg (x-axis), the result is not a straight line and not a simple hyperbola — it is an S-shape, called a sigmoid or sigmoidal curve. The lower portion (pO₂ 0–20 mm Hg) is shallow; the middle portion (20–60 mm Hg) is steep; and the upper portion (above 60 mm Hg) is a plateau approaching 100 % saturation.

The biological reason for the S-shape is cooperative binding (positive cooperativity). When the first O₂ molecule binds to one of the four haem sites, it triggers a small conformational change in the globin chains — Hb transitions from the low-affinity "T" (tense) state toward the high-affinity "R" (relaxed) state. The remaining three sites now bind O₂ far more easily than the first. The fourth O₂ binds about 300 times more readily than the first. NEET does not test the T/R nomenclature, but it does test the consequence: the curve is sigmoidal because each Hb has four interacting binding sites.

Why the S-shape is biologically perfect

The shape is not an accident — it is the curve that does the most physiological work. In the lungs the alveolar pO₂ of 104 mm Hg sits well into the flat upper plateau. Saturation is therefore ~97–98 %, and even if alveolar pO₂ drops to 80 mm Hg (mild hypoventilation, modest altitude), saturation falls only marginally to ~95 %. The plateau makes arterial oxygen content remarkably insensitive to small breathing changes.

At the tissue, by contrast, pO₂ sits on the steep middle of the curve (40 mm Hg → ~75 % saturation; 20 mm Hg → ~32 % saturation). A small fall in tissue pO₂ — exactly what happens when a muscle starts working — causes a large fall in Hb saturation and therefore a large release of O₂. The steep middle makes tissue unloading remarkably sensitive to small demand changes.

Bohr effect and curve shifts

The position of the sigmoid is not fixed. It slides left or right depending on chemical conditions in the local environment. The Bohr effect — discovered by Christian Bohr in 1904 — names the observation that rising pCO₂ and rising H⁺ (falling pH) shift the curve to the right, lowering Hb's affinity for O₂ and increasing release at the tissues. The opposite changes (low pCO₂, low H⁺) shift it left at the lungs.

Four physiological modulators are tested by NEET:

The four right-shifters at the tissue can be remembered as the "hot, acidic, panting" muscle mnemonic: high temperature, high H⁺, high pCO₂, plus high 2,3-BPG over the long term. Together they unload more oxygen exactly where catabolism needs it. The four opposing left-shifters at the alveolus — cool, alkaline, low pCO₂, low 2,3-BPG — favour loading.

P₅₀ — a number for the curve's position

P₅₀ is defined as the partial pressure of oxygen at which haemoglobin is exactly 50 per cent saturated. For normal adult HbA at pH 7.4, 37 °C, with normal 2,3-BPG, P₅₀ is approximately 26–27 mm Hg. A right-shifted curve has a higher P₅₀ (Hb releases O₂ more easily — for example P₅₀ ≈ 30 mm Hg in a hot, acidic muscle). A left-shifted curve has a lower P₅₀ (Hb holds O₂ more tightly — for example foetal Hb has a P₅₀ of ~19 mm Hg). NEET stems sometimes describe the shift verbally ("greater affinity / lesser affinity") rather than naming P₅₀, but the underlying logic is the same.

Foetal Hb and myoglobin — two diagnostic comparisons

The position and shape of the curve change when the binding protein itself changes. Two comparisons are useful for NEET-level understanding: foetal versus adult haemoglobin, and haemoglobin versus myoglobin.

Foetal haemoglobin (HbF)

Foetal haemoglobin replaces the two adult β-chains with two γ-chains (α₂γ₂). The γ-chains bind 2,3-bisphosphoglycerate (2,3-BPG) much less effectively than β-chains. Because 2,3-BPG normally lowers Hb's affinity for O₂, foetal Hb — which is largely "unaffected" by 2,3-BPG — retains a higher intrinsic affinity for O₂. Its dissociation curve sits to the left of the adult curve (lower P₅₀, ~19 mm Hg). Across the placenta, where maternal pO₂ in the intervillous space is only ~30–35 mm Hg, foetal Hb can still load O₂ efficiently — extracting it from the maternal HbA that releases it on its own (right-of-foetal) curve.

Myoglobin (Mb)

Myoglobin is a single-subunit globin found in cardiac and skeletal muscle. It has only one haem and one O₂-binding site, so there is no cooperativity. Its dissociation curve is therefore a steep hyperbola, not a sigmoid, and sits far to the left of haemoglobin's curve (P₅₀ ~2.7 mm Hg). Myoglobin is essentially saturated at any normal tissue pO₂; it only gives up O₂ when local pO₂ falls very low — making it an oxygen storage / buffer molecule for muscle during intense exercise, not a transport molecule.

Worked examples

Common confusion & NEET traps

Three confusion clusters generate the bulk of negative marking on this subtopic. NEET often phrases the trap as a "wrong statement" or "incorrect option" prompt — the candidate must spot the inversion.