Biology GCSE what does it mean

Because cells themselves are minute [a few microns in diameter usually], yes your idea is correct. The importance of this fact is that O2, nutrients etc moving into the cell and waste products, CO2 out of cell can do so SIMPLY BY DIFFUSION; in larger organisms, this distance from the external environment to the "deeper" parts of the organisms is greater, so diffusion alone cannot suffice, hence the evolution over millions of years of specialized organs/systems to achieve process of exchange of substances between the inside of the animal/plant and the outside, yeah?

Hope this helps!
M

Because cells themselves are minute [a few microns in diameter usually], yes your idea is correct. The importance of this fact is that O2, nutrients etc moving into the cell and waste products, CO2 out of cell can do so SIMPLY BY DIFFUSION; in larger organisms, this distance from the external environment to the "deeper" parts of the organisms is greater, so diffusion alone cannot suffice, hence the evolution over millions of years of specialized organs/systems to achieve process of exchange of substances between the inside of the animal/plant and the outside, yeah?

Hope this helps!
M