AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
In multicellular organisms like humans, all cells are not in direct contact with the surrounding environment. As body size increases and body design becomes more complex, simple diffusion cannot supply oxygen to all the cells adequately. Therefore, a specialised transport system is needed to carry oxygen from the point of intake to every cell in the body.
Source: Chapter 5, Section 5.1 — What are Life Processes?
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The examiner expects two linked points: (1) cells are not in direct contact with the environment in large/complex organisms, and (2) diffusion alone is too slow/insufficient to reach all cells. Mentioning the need for a specialised transport system rounds off the answer well. Avoid writing about the heart or circulatory system in detail — that exceeds the scope of a 2-mark answer here.