how is diffusion used by single celled organisms and multicellular organisms for gaseous exchange ?

Answer 1

In protozoans the diffusion of gases occurs through the plasma membrane of the cells body. In porifera exchange of gases occurs through the general body surface and internally as the water passes along the canals. In platyhelminthes it occurs though the integument. In annelids too integument serves in diffusion of gases.
In arthropods there are gills, tracheae, book lung and book gills.
In Mollusks there are gills and lungs in some snails.
Some Echinoderms have respiratory tree.
Fish have gills and accessory respiratory organs.
Frog has integument, buccal cavity, and the lungs.
Reptiles, birds and mammals have lungs where exchange of gases occurs. The law of diffusion is the same in all organisms,

Answer 2

Single-celled -- they exchange oxygen and carbon dioxide by diffusion through the plasma membrane - directy trading these gases with the environment.

Multicellular organisms vary:
-- Vascular plants have pores called stomata. Gases diffuse in or out of the stomata and then continue to diffuse in or out of particular cells.
-- Some animals experience diffusion of gases through a thin skin, through tracheae, through gills, or in the lungs.
-- Humans, in particular, must rely on gases diffusing into and out of the capillaries, into and out of red blood cells, and into and out of individual cells.

Answer 3

Single celled organisms are easy. They go all the way on the first date.