A circulatory system is very essential in mammals but not in smaller organisms like Amoeba because

Step 1: Analyze the question The question asks why large mammals need a circulatory system while small organisms like Amoeba do not. The answer must relate to how materials (oxygen, nutrients, wastes) move around the body.

Step 2: Explain the concept Substances move by diffusion, which is effective only over very short distances. It depends on the surface area to volume ratio:

• Amoeba is single-celled and microscopic. It has a large surface area relative to its volume, and every part of the cell is very close to the external environment. Oxygen, nutrients, and wastes can simply diffuse in and out across the cell membrane, so no transport system is needed.
• Mammals are large and multicellular. Most of their cells are far from the body surface, and their surface area to volume ratio is small. Diffusion alone would be far too slow, so they need a circulatory system (heart, blood vessels, blood) to carry materials around quickly.

Step 3: Check each option

• A. Amoeba lives in freshwater: Where it lives does not explain why it needs no transport system. Incorrect.
• B. Diffusion is sufficient to transport materials in Amoeba: This matches the explanation above. Correct.
• C. Amoeba lacks blood containing haemoglobin: This is true, but it is a result of having no circulatory system, not the reason. Many organisms with blood and a circulatory system also lack haemoglobin in their blood. Incorrect.
• D. Amoeba exhibits anaerobic respiration: Amoeba respires aerobically, taking in dissolved oxygen by diffusion. Incorrect.

Step 4: Answer

Exam tip: Remember "small size = large surface area to volume ratio = diffusion is enough". Whenever a question asks why a small organism has no transport system, think of diffusion over short distances. Large organisms need mass transport because their surface area to volume ratio is too small.