Single-celled organisms like amoeba have a huge surface area for their volume
Single-celled organisms can rely on diffusion to transport molecules in and out of their cell
Calculating surface area to volume ratio -
Ratio = surface area / volume
As organisms get larger, the surface area to volume ratio falls sharply
Problem for multicellular organisms
Surface area is not large enough for their volume
Cells in the center cannot get enough oxygen by diffusion
How animals solve the problem
Special structures for gas exchange with high surface area (e.g. lungs)
Transport system to carry gases around the body
How fish get oxygen
Oxygen-rich water passes into the mouth
Flows over gills where oxygen diffuses into the bloodstream
Deoxygenated blood passes into the gill filaments, oxygen diffuses in, oxygenated blood returns to the body
Adaptations of gill filaments
Massive surface area
Thin membrane for short diffusion pathway
Efficient blood supply to maintain concentration gradient
Cells on the surface can get enough oxygen simply by diffusion however not enough oxygen can diffuse into the cells in the centre of the organism. They are too far away from the surface.
