Diffusion or passive transport occurs in living organisms without any additional input of energy. The process simply occurs if the concentration gradient is such that there is a concentration difference of a particular substance outside and inside a cell membrane. The substance must have small enough molecules to fit through the pores in the membrane. For example, if there is a higher concentration of glucose in the bloodstream,relative to the level inside the body cells, glucose passes through the capillary walls and will diffuse into the cells, from high to low concentration. Another example would be osmosis, a special type of diffusion that occurs when the substance water passes through a cell membrane. If a cell is surrounded outside, by a high concentration of water relative to the inside of the cell, as when you water a plant, the water passes into the cell from high concentration outside, to low concentration inside. Eventually, when an equilibrium is established on either side of the membrane, diffusion will stop.
In animals and plants the process of diffusion is involved in the transfer of materials within the cells. Diffusion is central to life functioning since it is a feature of many processes that control and supply vital substances to the living body. Examples are respiration and excretion in animal bodies. Oxygen is passed from lungs to cells by a process of diffusion. The carbon dioxide excreted by the cells is diffused to the lungs. Diffusion is also central to the process of excretion. When there is excess water it is diffused to the kidneys, and when there is less water it is diffused back to the body.
Diffusion of materials in and out of cells in plants and animals.
And frogs use diffusion through their skin as a method of respiration.