Question

why does diffusion alone is sufficient to supply oxygen in both protists (protozoans) and flatforms​

Answer 1

Answer:

Direct Diffusion

For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. ... In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist and gases diffuse quickly via direct diffusion.

Answer 2

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Diffusion is unable to meet the oxygen requirements of multi-cellular organisms such as humans because diffusion is one of the slow processes. That is a lot of time is required for oxygen to circulate to all the cells of the body. ... Therefore, through diffusion, it becomes difficult for oxygen to reach all the cells.