Learn About Diffusion

This graphic shows the diffusion of water and other molecules from left to right across a semi-permeable membrane. Larger molecules are not able to cross the barrier. Freemesm / Wikimedia Commons / CC BY-SA 3.0

What Is Diffusion?

Diffusion is the tendency of molecules to spread out in order to occupy an available space. Gasses and molecules in a liquid have a tendency to diffuse from a more concentrated environment to a less concentrated environment. Passive transport is the diffusion of substances across a membrane. This is a spontaneous process and cellular energy is not expended. Molecules will move from where a substance is more concentrated to where it is less concentrated. The rate of diffusion for different substances is affected by membrane permeability. For instance, water diffuses freely across cell membranes but other molecules can not. They must be helped across the cell membrane through a process called facilitated diffusion.

Osmosis is a special case of passive transport. Water diffuses across a semi-permeable membrane, which allows some molecules to pass, but not others. In osmosis, the direction of water flow is determined by solute concentration. Water diffuses from a hypotonic (low solute concentration) solution to a hypertonic (high solute concentration) solution.

Examples of Diffusion

A number of naturally occurring processes rely on the diffusion of molecules. Respiration involves the diffusion of gasses (oxygen and carbon dioxide) into and out of the blood. In the lungs, carbon dioxide diffuses from the blood into the air at lung alveoli. Red blood cells then bind the oxygen that diffuses from the air into the blood. Oxygen and other nutrients in the blood are transported to tissues where gasses and nutrients are exchanged. Carbon dioxide and wastes diffuse from tissue cells into the blood, while oxygen, glucose and other nutrients in the blood diffuse into body tissues. This diffusion process occurs at capillary beds.

Diffusion also occurs in plant cells. The process of photosynthesis that occurs in plant leaves depends on the diffusion of gasses. In photosynthesis, energy from sunlight, water, and carbon dioxide are used to produce glucose, oxygen, and water. Carbon dioxide diffuses from the air through tiny pores in plant leaves called stomata. Oxygen produced by photosynthesis diffuses from the plant through the stomata into the atmosphere.

Examples of osmosis include the reabsorption of water by nephron tubules in the kidneys, the reabsorption of fluid at tissue capillaries, and water absorption by plant roots. Osmosis is important to plant stability. Wilted plants are the result of a lack of water in plant vacuoles. Vacuoles help keep plant structures rigid by absorbing water and exerting pressure on plant cell walls. Water moving across plant cell membranes by osmosis helps to restore the plant to an erect position.