Cells need to take in oxygen (for aerobic respiration) and nutrients. They have to excrete waste products (from aerobic respiration) and urea.
Surface are to volume ratio dictates how easily exchange can take place. The smaller ratio the larger the diffusion path. In order to overcome this multicellular organisms use exchange organs.
Single cell organisms have a short diffusion path meaning substances can diffuse directly into and out of the cell.
Multicellular organisms have a slow diffusion rate for two reasons:
1. Some cells are deep within the body.
2. Large animals…