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. Therefore, a specialised transport system is needed to carry oxygen from one place to all parts of the body.
Source: Chapter 5, Section 5.1 — What are Life Processes?
The examiner expects two linked points: (1) cells are not in contact with the environment in large/complex organisms, and (2) diffusion is too slow/limited to reach all cells — hence a transport system is required. Avoid over-explaining; two clear sentences earn full marks. The answer must come from Section 5.1 of the textbook.