How is diffusion used in animals? How is diffusion used in plants?

Diffusion is the act of a greater concentration of particles mixing with a lower concentration of particles, in the effort of achieving an equilibrium on both sides.  It occurs quite naturally, without the use of energy, except in the process known as active transport. 

In animals, glucose and oxygen are carried by the blood stream to every cell in the body, where the two diffuse into the cell, where it is used by the cells...

Diffusion is the act of a greater concentration of particles mixing with a lower concentration of particles, in the effort of achieving an equilibrium on both sides.  It occurs quite naturally, without the use of energy, except in the process known as active transport. 

In animals, glucose and oxygen are carried by the blood stream to every cell in the body, where the two diffuse into the cell, where it is used by the cells mitochondria.  A chemical process breaks apart the glucose, where it is combined with the oxygen to create energy for the cell in the form of ATP, carbon dioxide, and water.  The carbon dioxide and water are diffused out of the cell as waste products, which the blood carries to each chemical's respective exit destinations. 

Plants use light energy, usually from the sun, and combine that with carbon dioxide, which diffuses into the plant from the air and water, from which it is diffused into the root system of the plant.  The simple sugar glucose is formed, with oxygen released as a waste product, which diffuses out of the plant into the air.