Question

why unicellular organisms respire through the process of diffusion

Answer 1

In unicellular organisms such as amoeba and paramecium exchange of gases takes place through plasma membrane by simple diffusion. We know that amoeba is a unicellular organism which lives in fresh water. This organism does not have a specialised organ to do the process of respiration. So in amoeba respiration takes place through its cell membrane also called plasma membrane. Amoeba gets oxygen gas dissolved in surrounding water through its plasma membrane by the process of diffusion. The oxygen gas diffused inside the body is used up by amoeba. In the body the oxygen gas absorbed is used to break down the complex food material into simple molecules. During these metabolic reactions in the body of amoeba the oxygen gas is converted into carbon dioxide gas. The carbon dioxide gas is also liberated in the surrounding water through the same process of diffusion.

Answer 2

Unicellular organisms breathe through direct diffusion via the cell membrane. As the cell membrane is semi-permiable it allows substances to move from a high concentration to a low concentration ie. oxygen diffuses in and carbondioxide diffuse out. Organisms such as amoeba and paramoecium breathe this way.