Learn About Diffusion

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.

Key Takeaways: Diffusion

  • Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration.
  • Passive diffusion is the movement of molecules across a membrane, such as a cell membrane. The movement does not require energy.
  • In facilitated diffusion, a molecule is transported across a membrane with the help of a carrier protein.
  • Osmosis is a type of passive diffusion in which water diffuses across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.
  • Respiration and photosynthesis are examples of naturally occurring diffusion processes.
  • Glucose movement into cells is an example of facilitated diffusion.
  • Water absorption in plant roots is an example of osmosis.

What Is Osmosis?

Osmosis is a special case of passive transport. Water diffuses across a semi-permeable membrane which allows some molecules to pass but not others.

Osmosis
Water passing through a semi-permeable membrane by osmosis into a region of higher sugar concentration.  ttsz/iStock/Getty Images Plus

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. In the example above, water moves from the left side of the semi-permeable membrane, where the sugar concentration is low, to the right side of the membrane, where the sugar molecule concentration is higher. If the molecule concentration were the same on both sides of the membrane, water would flow equally (isostonic) between both sides of the membrane.

Examples of Diffusion

Gas Exchange in the Lungs
Both oxygen and carbon dioxide diffuse into the blood and are transported around the body.  ttsz/iStock/Getty Images Plus

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.

Photosynthesis
Diffusion of gasses occurs in photosynthesis in plants.  snapgalleria/iStock/Getty Images Plus

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.

Facilitated Diffusion
This image shows facilitated diffusion of glucose across a cell membrane through a carrier protein.  ttsz/iStock/Getty Images Plus

In facilitated diffusion, larger molecules such as glucose, can not freely diffuse across cell membranes. These molecules must move down their concentration gradient with the help of transport proteins. Protein channels embedded in cell membranes have openings to the outside of a cell that allow certain molecules to fit inside. Only molecules with certain characteristics, such as a certain size and shape are allowed passage from outside of a cell to its intracellular space. Since this process does not require energy, facilitated diffusion is considered passive transport.

Osmosis Examples

Wilted Yellow Tulips
Water moving across plant cell membranes by osmosis helps to restore the plant to an erect position.  berkpixels/Getty Images

Examples of osmosis in the body include the reabsorption of water by nephron tubules in the kidneys and the reabsorption of fluid at tissue capillaries. In plants, osmosis is exhibited in 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.