3.5 Role of Respiratory Pigments

Respiratory pigments are coloured molecules that act as oxygen carriers by binding reversibly to oxygen. They play a crucial role in the transport and storage of oxygen in the body. The two well-known respiratory pigments in humans are haemoglobin and myoglobin.

Definition

Respiratory Pigments: Respiratory pigments are coloured molecules which act as oxygen carriers by binding reversibly to oxygen. All known respiratory pigments contain a coloured non-protein portion e.g., haem in the haemoglobin.

Haemoglobin: Haemoglobin is a respiratory pigment found in red blood cells. It consists of four globin protein chains, each associated with haem, an iron-containing group. It transports oxygen from the lungs to the tissues.

Myoglobin: Myoglobin is a respiratory pigment found in skeletal muscles. It consists of one polypeptide chain associated with an iron-containing ring structure. It stores oxygen in muscles.

Main Idea

• Bind reversibly to oxygen
• Transport oxygen (haemoglobin)
• Store oxygen (myoglobin)
• Contain a coloured non-protein portion (haem)
• Contain iron for oxygen binding

Core Concept

• Respiratory pigments = coloured molecules; oxygen carriers
• Haemoglobin = in RBCs; transports oxygen
• Myoglobin = in muscles; stores oxygen
• Haem = iron-containing group
• Globin = protein part
• Reversible binding = can bind and release oxygen
• Affinity = strength of binding
• PO₂ = partial pressure of oxygen

Detailed Explanation

Haemoglobin

• Contains four globin protein chains.
• Each chain is associated with haem, an iron-containing group.
• Iron combines loosely with oxygen.
• Each haemoglobin molecule can bind four molecules of oxygen.

• Transports oxygen in the blood.
• At high oxygen concentrations, the pigment combines with oxygen.
• At low oxygen concentrations, the oxygen is quickly released.

Location: - Found in red blood cells (RBCs).

• Has less affinity with oxygen compared to myoglobin.
• Loses oxygen at PO₂ 60 mmHg.

Myoglobin

• Consists of one polypeptide chain.
• This chain is associated with an iron-containing ring structure.
• This iron can bind with one molecule of oxygen.

• Serves as an intermediate compound for the transfer of oxygen from haemoglobin to aerobic metabolic processes of the muscle cells.
• Releases oxygen when the partial pressure of oxygen is below 20 mmHg.
• In this way, it stores oxygen in resting muscle, only releasing it when supplies of oxyhaemoglobin have been exhausted.

• Found in skeletal muscles.
• Is the main reason why meat appears red.

• Has more affinity with oxygen compared to haemoglobin.
• Loses oxygen at PO₂ 20 mmHg.

Table of Differences Between Haemoglobin and Myoglobin

Intermediate Level

Comparison of Haemoglobin and Myoglobin

Oxygen Dissociation Curves

Role of Haem

Role of Globin

Formula / Mathematical Form

Oxygen Saturation: SaO₂ = O₂ bound to HbO₂ carrying capacity × 100

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Respiratory pigments are coloured molecules.
• 2. They act as oxygen carriers.
• 3. Haemoglobin and myoglobin are the two well-known respiratory pigments.
• 4. Haemoglobin contains four globin chains.
• 5. Haemoglobin contains four haem groups.
• 6. Haemoglobin can bind four oxygen molecules.
• 7. Myoglobin contains one polypeptide chain.
• 8. Myoglobin contains one haem group.
• 9. Myoglobin can bind one oxygen molecule.
• 10. Haemoglobin is found in RBCs.
• 11. Myoglobin is found in muscles.
• 12. Haemoglobin transports oxygen.
• 13. Myoglobin stores oxygen.
• 14. Haemoglobin has less affinity for oxygen.
• 15. Myoglobin has more affinity for oxygen.
• 16. Haemoglobin loses oxygen at PO₂ 60 mmHg.
• 17. Myoglobin loses oxygen at PO₂ 20 mmHg.
• 18. Haem is the iron-containing group.
• 19. Globin is the protein part.
• 20. Iron is essential for oxygen binding.

Remember

• - Respiratory pigments = coloured oxygen carriers
• - Haemoglobin = in RBCs; transports oxygen
• - Myoglobin = in muscles; stores oxygen
• - Haem = iron-containing group
• - Globin = protein part
• - Four = globin chains in haemoglobin
• - One = polypeptide chain in myoglobin
• - Four = oxygen molecules bound by haemoglobin
• - One = oxygen molecule bound by myoglobin
• - Less affinity = haemoglobin
• - More affinity = myoglobin
• - 60 mmHg = haemoglobin loses oxygen
• - 20 mmHg = myoglobin loses oxygen

Common Mistakes

• 1. Confusing haemoglobin and myoglobin — Haemoglobin transports; myoglobin stores.
• 2. Thinking haemoglobin has one polypeptide chain — Haemoglobin has four.
• 3. Thinking myoglobin has four polypeptide chains — Myoglobin has one.
• 4. Mixing up the locations — Haemoglobin in RBCs; myoglobin in muscles.
• 5. Confusing the affinity — Myoglobin has higher affinity; haemoglobin has lower.
• 6. Mixing up the PO₂ values — Haemoglobin loses oxygen at 60 mmHg; myoglobin at 20 mmHg.
• 7. Forgetting the role of haem — Haem contains iron and binds oxygen.
• 8. Thinking oxygen binds to globin — Oxygen binds to haem.
• 9. Believing myoglobin transports oxygen — Myoglobin stores oxygen.
• 10. Mixing up the number of oxygen molecules — Haemoglobin binds four; myoglobin binds one.

Quick Revision

Respiratory pigments are coloured molecules that act as oxygen carriers by binding reversibly to oxygen. All known respiratory pigments contain a coloured non-protein portion, e.g., haem in haemoglobin. The two well-known respiratory pigments are haemoglobin and myoglobin. Haemoglobin contains four globin protein chains, each associated with haem, an iron-containing group. Iron combines loosely with oxygen, and in this way, oxygen is carried in the blood. At high oxygen concentrations, the pigment combines with oxygen; at low oxygen concentrations, the oxygen is quickly released. Myoglobin consists of one polypeptide chain associated with an iron-containing ring structure. This iron can bind with one molecule of oxygen. It is found in skeletal muscles and serves as an intermediate compound for the transfer of oxygen from haemoglobin to aerobic metabolic processes of muscle cells. Myoglobin releases oxygen when the partial pressure of oxygen is below 20 mmHg. Haemoglobin has less affinity for oxygen and loses oxygen at PO₂ 60 mmHg, while myoglobin has more affinity and loses oxygen at PO₂ 20 mmHg.

60-Second Revision

• Respiratory pigments = coloured oxygen carriers
• Haemoglobin = in RBCs; transports oxygen
• Myoglobin = in muscles; stores oxygen
• Haem = iron-containing group
• Globin = protein part
• Four = globin chains in haemoglobin
• One = polypeptide chain in myoglobin
• Four = oxygen molecules bound by haemoglobin
• One = oxygen molecule bound by myoglobin
• Less affinity = haemoglobin
• More affinity = myoglobin
• Warning: Oxygen binds to haem, not globin

NEXT TOPIC

• Chapter:
• 3. Respiratory System of Man

• Topic:
• 3.6 RESPIRATORY DISORDERS

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