🩸 Respiratory Pigments – Human Respiratory System
Respiratory pigments are special proteins in blood or tissues involved in transporting oxygen throughout the body. They also have other functions, such as:
- Oxygen storage
- Carbon dioxide transport
- Transport of substances other than respiratory gases
The two main respiratory pigments are haemoglobin and myoglobin.
1. Haemoglobin (Hb)
- Found in red blood cells (RBCs).
- Structure: Composed of four globin polypeptide chains – two α chains and two β chains.
- Amino acids: α chain has 141, β chain has 146 amino acids.
- Each chain has a haem group – an iron ion held in a porphyrin ring.
- Iron ion binds with four nitrogen atoms of the polypeptide chain.
- Oxygen binding: Each haem group binds one O₂ molecule, so one haemoglobin can carry four O₂ molecules.
- Chain interactions: Polypeptide chains are held together by salt bridges, hydrogen bonds, and hydrophobic interactions.
2. Myoglobin (Mb)
- Found in skeletal and cardiac muscles.
- Gives red or dark gray color to muscles.
- Structure: Monomer with one polypeptide chain (153 amino acids) and a single haem group.
- Oxygen binding: Can bind one O₂ molecule.
- Affinity: Myoglobin has higher affinity for oxygen than haemoglobin.
- Function: Acts as oxygen store in muscles, releasing O₂ when PO₂ drops below 20 mm Hg, supplying oxygen during high muscle activity.
3. Differences Between Haemoglobin and Myoglobin
- Haemoglobin consists of four polypeptide chains; myoglobin has one chain.
- Haemoglobin has four haem groups, myoglobin has one haem group.
- Haemoglobin is in blood (RBCs); myoglobin is in muscles.
- Haemoglobin carries four O₂ molecules; myoglobin carries one O₂ molecule.
- Haemoglobin transports oxygen, myoglobin stores oxygen.
- Haemoglobin has less affinity for oxygen; myoglobin has more affinity.
- Haemoglobin loses oxygen at PO₂ 60 mm Hg, myoglobin loses oxygen at PO₂ 20 mm Hg.
MCQs – Respiratory Pigments
Q1. Haemoglobin is primarily found in:
a) Plasma
b) Red blood cells ✅
c) Muscle cells
d) White blood cells
Answer: b) Red blood cells
Explanation: Haemoglobin is present in RBCs and is responsible for transporting oxygen from lungs to tissues.
Q2. Myoglobin is found in:
a) Liver
b) Kidneys
c) Skeletal and cardiac muscles ✅
d) Blood plasma
Answer: c) Skeletal and cardiac muscles
Explanation: Myoglobin stores oxygen in muscle cells and releases it when needed, especially during low PO₂.
Q3. How many oxygen molecules can one haemoglobin molecule carry?
a) 1
b) 2
c) 3
d) 4 ✅
Answer: d) 4
Explanation: Haemoglobin has four haem groups; each binds one oxygen molecule.
Q4. How many oxygen molecules can one myoglobin molecule carry?
a) 1 ✅
b) 2
c) 4
d) None
Answer: a) 1
Explanation: Myoglobin is a monomer with one haem group and binds a single oxygen molecule.
Q5. Which pigment has higher affinity for oxygen?
a) Haemoglobin
b) Myoglobin ✅
c) Both same
d) None
Answer: b) Myoglobin
Explanation: Myoglobin binds oxygen more strongly, allowing it to store oxygen in muscles.
Q6. At what PO₂ does haemoglobin start releasing oxygen to tissues?
a) 20 mm Hg
b) 60 mm Hg ✅
c) 100 mm Hg
d) 0 mm Hg
Answer: b) 60 mm Hg
Explanation: Haemoglobin releases oxygen at a higher partial pressure than myoglobin, which releases it at 20 mm Hg.
Q7. Haemoglobin is composed of:
a) Two polypeptide chains
b) Three polypeptide chains
c) Four polypeptide chains ✅
d) One polypeptide chain
Answer: c) Four polypeptide chains
Explanation: Haemoglobin has two α and two β chains, each with a haem group.
Q8. Myoglobin consists of:
a) Four haem groups
b) Two polypeptide chains
c) One polypeptide chain and one haem group ✅
d) No haem group
Answer: c) One polypeptide chain and one haem group
Explanation: Myoglobin is a monomeric protein with one haem group for oxygen storage.
Q9. The main function of haemoglobin is:
a) Store oxygen in muscles
b) Transport oxygen in blood ✅
c) Digest carbohydrates
d) Produce carbon dioxide
Answer: b) Transport oxygen in blood
Explanation: Haemoglobin carries oxygen from lungs to tissues and also helps in CO₂ transport.
Q10. The main function of myoglobin is:
a) Transport oxygen in blood
b) Store oxygen in muscles ✅
c) Transport carbon dioxide
d) Produce haemoglobin
Answer: b) Store oxygen in muscles
Explanation: Myoglobin stores oxygen in muscles and releases it when PO₂ drops below 20 mm Hg.
Q11. Which pigment gives red color to blood?
a) Myoglobin
b) Haemoglobin ✅
c) Chlorophyll
d) Carotenoids
Answer: b) Haemoglobin
Explanation: Oxyhaemoglobin is bright red, giving arterial blood its red color.
Q12. Which pigment gives red or dark gray color to muscles?
a) Haemoglobin
b) Myoglobin ✅
c) Bilirubin
d) Melanin
Answer: b) Myoglobin
Explanation: Myoglobin stores oxygen in muscles and contributes to their color.
Q13. Haemoglobin releases oxygen more readily when:
a) Blood pH decreases ✅
b) Blood pH increases
c) Temperature decreases
d) Oxygen partial pressure increases
Answer: a) Blood pH decreases
Explanation: Lower pH reduces haemoglobin’s affinity for oxygen (Bohr effect).
Q14. Myoglobin releases oxygen when PO₂ falls below:
a) 60 mm Hg
b) 20 mm Hg ✅
c) 100 mm Hg
d) 40 mm Hg
Answer: b) 20 mm Hg
Explanation: Myoglobin provides oxygen to muscles during high activity or low oxygen levels.
Q15. Which pigment is a tetramer?
a) Myoglobin
b) Haemoglobin ✅
c) Both
d) None
Answer: b) Haemoglobin
Explanation: Haemoglobin has four polypeptide chains (tetramer) while myoglobin is a monomer.
Short Questions – Respiratory Pigments
• What are respiratory pigments?
Respiratory pigments are special proteins in blood or tissues that transport oxygen, store oxygen, and help in CO₂ transport.
• Name the two main respiratory pigments in humans.
Haemoglobin and myoglobin.
• Where is haemoglobin found?
In red blood cells (RBCs).
• How many polypeptide chains are present in haemoglobin?
Four (two α chains and two β chains).
• How many haem groups are present in haemoglobin?
Four haem groups.
• How many oxygen molecules can haemoglobin carry?
Four O₂ molecules.
• Where is myoglobin found?
In skeletal and cardiac muscles.
• How many polypeptide chains does myoglobin have?
One polypeptide chain.
• How many oxygen molecules can myoglobin carry?
One O₂ molecule.
• Which has higher oxygen affinity, haemoglobin or myoglobin?
Myoglobin has higher oxygen affinity.
• At what PO₂ does haemoglobin release oxygen?
At 60 mm Hg.
• At what PO₂ does myoglobin release oxygen?
At 20 mm Hg.
• What is the main function of haemoglobin?
To transport oxygen in the blood.
• What is the main function of myoglobin?
To store oxygen in muscles and release it when needed.
• Why does blood appear red and muscles dark red/gray?
Blood is red due to oxyhaemoglobin, muscles are dark red/gray due to myoglobin.
• Explain the Bohr effect in haemoglobin.
Lower pH reduces haemoglobin’s oxygen affinity, causing it to release oxygen more readily.
Long Questions – Respiratory Pigments
• Explain the structure and function of haemoglobin in humans.
Haemoglobin is a protein found in red blood cells (RBCs) responsible for transporting oxygen from lungs to tissues and aiding in carbon dioxide transport. It is composed of four globin polypeptide chains – two α chains and two β chains. Each chain has 141–146 amino acids and is folded to contain a pocket for the haem group, which has an iron ion held in a porphyrin ring.
The iron ion binds with oxygen molecules; each haem group binds one O₂ molecule, so a single haemoglobin molecule can carry four oxygen molecules. The polypeptide chains are held together by salt bridges, hydrogen bonds, and hydrophobic interactions.
Functions of haemoglobin include:
- Transporting oxygen from lungs to tissues
- Assisting in CO₂ transport from tissues to lungs
- Maintaining pH balance of blood (buffer function)
• Explain the structure and function of myoglobin in humans.
Myoglobin is a monomeric protein found in skeletal and cardiac muscles, giving muscles a red or dark gray color. It is composed of one polypeptide chain with 153 amino acids and a single haem group, capable of binding one oxygen molecule.
Myoglobin has a higher affinity for oxygen than haemoglobin, which allows it to store oxygen in muscles and release it during periods of low oxygen partial pressure (below 20 mm Hg), such as during intense exercise.
Functions of myoglobin include:
- Storing oxygen in muscle tissues
- Providing oxygen to muscles when PO₂ is low
- Supporting muscle activity during high energy demand
• Compare haemoglobin and myoglobin in terms of structure, oxygen binding, and function.
| Feature | Haemoglobin | Myoglobin |
|---|---|---|
| Polypeptide chains | Four (α₂β₂) | One |
| Haem groups | Four | One |
| Location | RBCs | Skeletal & cardiac muscles |
| Oxygen molecules bound | Four | One |
| Function | Oxygen transport in blood | Oxygen storage in muscles |
| Affinity for oxygen | Lower | Higher |
| Oxygen release PO₂ | 60 mm Hg | 20 mm Hg |
Explanation: Haemoglobin is designed for efficient oxygen transport and delivery to tissues, while myoglobin is specialized for oxygen storage in muscles.
• Describe the role of respiratory pigments in oxygen and carbon dioxide transport.
Respiratory pigments such as haemoglobin and myoglobin are crucial for oxygen transport, storage, and release. Haemoglobin in RBCs picks up oxygen in the lungs, forming oxyhaemoglobin, which releases oxygen at tissue level depending on PO₂ and pH (Bohr effect).
Myoglobin stores oxygen in muscles and releases it during periods of low oxygen demand. Respiratory pigments also assist in carbon dioxide transport:
- CO₂ binds to haemoglobin forming carbaminohaemoglobin
- Most CO₂ is carried as bicarbonate ions (HCO₃⁻) in plasma, maintaining acid-base balance
Together, haemoglobin and myoglobin ensure that oxygen supply meets tissue demand even under high physical activity.
• Explain why oxygen affinity differs between haemoglobin and myoglobin.
Haemoglobin exhibits lower oxygen affinity due to its tetrameric structure and cooperative binding; it releases oxygen more readily in tissues. Myoglobin, being a monomer, has higher affinity and only releases oxygen at very low PO₂. This difference ensures:
- Haemoglobin transports oxygen efficiently
- Myoglobin stores oxygen in muscles for sudden demand
Respiratory Pigments
Haemoglobin & Myoglobin | 8 Questions | 20s per Question
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