The pigment carrying oxygen in the blood is

Analyze the question The question asks for the pigment that carries oxygen in the blood. Two things matter here: it must be a pigment (a coloured molecule), and it must be the one that carries oxygen.

Explain the concept

• Haemoglobin is the red, iron-containing protein pigment found inside red blood cells. It binds oxygen in the lungs and carries it to the tissues.
• Oxyhaemoglobin is the compound formed when haemoglobin combines with oxygen. It is the product of that combination, not the carrier pigment itself.
• Plasma is the liquid part of blood. It is not a pigment and mainly transports dissolved substances such as nutrients, hormones and CO₂.
• Erythrocytes are the red blood cells. They are cells, not a pigment. They contain haemoglobin.

Step-by-step solution

1. Eliminate A (plasma): it is a fluid, not a pigment.
2. Eliminate D (erythrocytes): they are cells that contain the pigment, not the pigment itself.
3. Compare B and C: • Haemoglobin + oxygen ⇌ oxyhaemoglobin • The pigment that does the carrying is haemoglobin. Oxyhaemoglobin is what you get once oxygen is already attached.
4. So the pigment carrying oxygen is haemoglobin.

Answer: B. haemoglobin

Exam tip Remember the reaction: Hb + O₂ ⇌ HbO₂. Haemoglobin is the carrier, and oxyhaemoglobin is the carrier loaded with oxygen. If a question asks for the pigment that transports oxygen, choose haemoglobin. If it asks for the compound formed in the lungs, choose oxyhaemoglobin.