Protozoa such as amoeba depend on diffusion as a means of gaesous exchange. They have a large surface area to volume ratio for efficient diffusion.
Gaseous exchange occurs across the cell membrane by diffusion.Oxygen diffuses in and carbon (IV) oxide diffuses out.
Oxygen is used in the cell for respiration making its concentration lower than that in the surrounding water.Hence oxygen continually enters the cell along a concentration gradient.
Carbon (IV) oxide concentration inside the cell is higher than that in the surrounding water thus it continually diffuses out of the cell along a concentration gradient.