Three main factors affect the rate of diffusion:
Single-celled organisms, such as amoeba. have huge surface areas for their volume. In other words, they have high surface area to volume ratios. So, they can use diffusion to transport substances in and out of their cell.
Let’s imagine an organism is shaped like a cube. The length of each side is 1 metre on the first cube and 2 metres on the second cube.
The surface area to volume ratio of cube 1 is 6:1 and with cube 2, the ratio falls to 3:1. So, as the size of the organism increases, the surface area to volume ratio decreases.
This creates a problem with multicellular organisms, as their surface area is not large enough for their volume of cells. Many cells are too far from the surface to diffuse oxygen into them.
Multicellular organisms solve this problem in two main ways: