How do single-celled organisms gain substances such as oxygen?

How do multicellular organisms acquire substances such as oxygen?

Describe what happens to the surface area-to-volume ratio as the size of an organism increases.

How does this help to explain why single-celled organisms can only be a maximum size?

single celled organisms gain substances like oxygen through diffusion because they have a big surface area to volume ratio

multicellular organisms acquire substances like oxygen from different systems and diffusion, for example plants acquire oxygen through the stoma, and mammals through their lungs.

the surface area to volume ratio decreases as the size of the organism increases.

because they will die if they can't intake enough oxygen when they become too big in size.

simple diffusion

lung

As the cells goes bigger, the volume grow faster, ratio and rate of diffusion getting smaller

Its because of the decreasing surface area-to-volume ratio, which hampers efficient material exchange.