Single-celled organisms get all the oxygen they need by diffusion across their surface.
Which statement explains why a large multicellular organism needs exchange surfaces and a transport system?
Its cells are much larger, so diffusion into each cell is too slow.
It has a smaller total surface area than a single-celled organism has.
Its outer surface is too thick for any useful substances to diffuse across.
It has a small surface area : volume ratio and long diffusion distances.
114 exam-style questions on OCR GCSE Biology 2.2 The challenges of size, covering 2.2.1 Exchange surfaces and surface area : volume ratio, 2.2.2 Substances transported in organisms, 2.2.3 The human circulatory system, 2.2.4 The heart and blood vessels, 2.2.5 Red blood cells and plasma, 2.2.6 Uptake of water and mineral ions by plants, 2.2.7 Transpiration and translocation, 2.2.8 Xylem and phloem, 2.2.9 Environmental factors and water uptake, and 2.2.10 Using a potometer. Each one has a worked solution and a mark scheme showing where the marks go.