Human blood is red in colour due to the presence of which specific respiratory pigment?

Introduction / Context:

Blood appears bright red or dark red depending on its oxygen content, and this colour is due to a specific pigment present in red blood cells. Understanding which pigment is responsible is a basic part of human physiology and is often tested in general knowledge and entrance examinations. The question checks whether the learner can correctly name the respiratory pigment in humans and distinguish it from pigments found in plants or in other animals.

Given Data / Assumptions:

• The question asks about the red colour of human blood.
• We must identify the pigment responsible for this colour.
• Options include cytochrome, chlorophyll, hemocyanin, haemoglobin, and melanin.
• We assume standard knowledge of blood composition and pigments.

Concept / Approach:

Haemoglobin is an iron containing protein present in red blood cells. It binds oxygen in the lungs and carries it to tissues. When haemoglobin is oxygenated, it gives blood a bright red colour; when deoxygenated, the colour becomes darker. This pigment is responsible for the characteristic red colour of human blood. Other pigments listed in the options belong to different systems: chlorophyll is the green pigment in plant chloroplasts, hemocyanin is a blue copper containing pigment in some invertebrates, cytochromes are electron carriers in cells, and melanin is a pigment in skin, hair, and eyes, not in blood.

Step-by-Step Solution:

Step 1: Recall that red blood cells contain a special protein that binds oxygen for transport. Step 2: Remember the name of this protein, which is haemoglobin. Step 3: Connect haemoglobin with the red colour of blood due to its iron and oxygen binding properties. Step 4: Compare haemoglobin with other listed pigments and verify that none of them are the main pigment in human blood. Step 5: Choose Haemoglobin as the correct option.

Verification / Alternative check:

Textbooks of biology and physiology consistently describe haemoglobin as the red coloured respiratory pigment. Laboratory tests such as haemoglobin estimation directly measure the concentration of this pigment to assess anaemia. In contrast, melanin is discussed in chapters on skin and eye pigmentation, chlorophyll appears in plant photosynthesis, and hemocyanin is mentioned in discussions of invertebrate blood. These independent references confirm that haemoglobin is the pigment that gives human blood its red colour.

Why Other Options Are Wrong:

Option A, Cytochrome, refers to electron transport proteins in mitochondria and does not determine the colour of blood. Option B, Chlorophyll, is the green pigment in plants and algae and has no role in human blood. Option C, Hemocyanin, is a copper based blue pigment found in some molluscs and arthropods, not in humans. Option E, Melanin, is a pigment in skin, hair, and eyes and influences skin tone and protection from ultraviolet radiation but not blood colour.

Common Pitfalls:

Some students may confuse hemocyanin with haemoglobin because the names sound similar. Others may think melanin is involved in all body colours and mistakenly extend this to blood. Another error is to pick chlorophyll if they are thinking of plant related questions and not reading the question carefully. To avoid such confusion, learners should connect haemoglobin specifically with red blood cells and respiratory function, and remember that melanin is tied to skin, while chlorophyll is tied to plants.

Final Answer:

The red colour of human blood is due to the pigment Haemoglobin.