Facilitated Diffusion

A second mechanism of passive transport is facilitated diffusion. Proteins in the cell membrane permit only certain molecules to pass across the membrane. The proteins allow molecules to move from a region of lower concentration of molecules to a region of higher concentration.

Image shows facilitated diffusion. Carrier proteins are being used to move a molecule through a semi permeable membrane
Adapted from Facilitated Diffusion. Source: Daegu University

This activity might not be viewable on your mobile device.Video segment. Assistance may be required. Watch this animation to learn more about facilitated diffusion.

Osmosis

Image shows movement of water molecules across a semipermeable membrane over time. As time increases water molecules move inside the cell until there is an equal number inside and out.

A third form of passive transport is osmosis. Osmosis is the movement of water from a region of higher concentration to one of lower concentration. Osmosis is a type of diffusion involving only water molecules.

 

The environment that a cell is exposed to can have a dramatic effect on the cell due to the fact that cell membranes are completely permeable to water. When one compound is dissolved in another, the whole mixture is called a solution. The substance capable of dissolving another substance is called a solvent. The substance that is dissolved in the solvent is called the solute. If you dissolve a teaspoon of salt in a glass of water, the water is the solvent and the salt is the solute. Solutions may be described as isotonic, hypertonic, or hypotonic.

Click on each picture to learn more.

Isotonic
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Image shows a isotonic solution and water moving into and out of a plant and animal cell. The cells are normal size.

Isotonic means "same strength." In an isotonic solution the solutes concentration is the same inside the cell as outside the cell.Close Pop Up

Hypertonic

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Image shows a isotonic solution and water moving into and out of a plant and animal cell. The cells are normal size.

A hypertonic solution is a solution having a greater solute concentration outside the cell than inside the cell. When a cell is placed in a hypertonic solution, the water will move out of the cell by osmosis. If water molecules continue to diffuse out of the cell, it will cause the cell to shrink.Close Pop Up

Hypotonic

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Image shows a isotonic solution and water moving into and out of a plant and animal cell. The cells are normal size.

A hypotonic solution is a solution having a lesser solute concentration outside the cell than inside. When a cell is placed in a hypotonic solution, the water will move into the cell by osmosis. If water molecules continue to diffuse into the cell, it will cause the cell to swell, up to the point that cytolysis (rupture) may occur. In plant cells, the cell will not always rupture. When placed in a hypotonic solution, the cell will have osmotic pressure and proceed with its normal functions.Close Pop Up

Video segment. Assistance may be required. Watch the following video to learn more about how osmosis works.

Source: How Osmosis Works, TOXCafe, YouTube

Video segment. Assistance may be required. Watch the following video to see osmosis in a real-life situation.

Source: Osmosis in the Kitchen, khymos, YouTube

Did the water flow into or out of the lettuce?

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Check Your Answer

The water flowed into the lettuce.Close Pop Up