Aerobic respiration produces carbon dioxide as a waste product. Carbon dioxide waste leaves protozoans by diffusion to the water surrounding the protozoan. Aerobic respiration is possible only if oxygen is present in protozoan cells. Oxygen enters protozoans by diffusion from the water surrounding the protozoan. Protozoans are a diverse group of organisms.
Protozoans have several special features.
Freshwater protozoans have contractile vacuoles. Protozoans living in freshwater have lower water concentrations in their cytoplasm than the water concentration in the almost pure water surrounding them. If the freshwater protozoans had no means of ridding themselves of excess water, they would swell up and burst. Fortunately they have contractile vacuoles which collect excess water and expel it from the protozoan. Marine protozoans and protozoans that are internal parasites lack contractile vacuoles because the water concentration in their cytoplasm is the same as that in their environment.