Diffusion and Osmosis

Diffusion is the movement of molecules from an area of high concentration to an area of lower concentration.

Gases diffuse in and out of cells

Osmosis is the movement of water from an area of high concentration to an area of lower concentration across a semi-permeable membrane.

Osmosis is a special form of diffusion.

Semi-permeable (or selectively permeable) membranes are cell membranes that act as a barrier. Many molecules cannot pass across them, but some molecules can. Therefore they control what leaves and enters the cell.

Diffusion.

Osmosis.

Animal cells and osmosis.

• Animal cells have a cell membrane that separates the contents of the cell from its external environment.
• If this cell is immersed in pure water, water will flow into the cell.
• If this flow of water continues the cell will be unable to withstand the pressure and the cell will eventually burst.

• If the cell is immersed in a solution that is highly concentrated, water will flow out of the cell.
• If this continued the cell will eventually shrivel up.

Amoeba.

• Amoeba is an organism that lives in fresh water.
• However unlike normal animal cells it has devised a way to prevent itself from bursting.
• It has a special structure called a contractile vacuole.
• As the water enters the cell, water is diverted into this vacuole.
• When the vacuole is full the water is pushed out of the cell.

Plant cells and osmosis.

• Plant cells have a cell wall surrounding their cell membrane.
• The cell wall is fully permeable.
• If a plant cell is immersed in pure water, the water will flow in by osmosis.
• The cell vacuole will expand and push the cytoplasm against the cell wall.
• The cell does not burst because of the cell wall.
• Such a cell is said to be turgid.
• Turgor pressure is the force of the cell contents against the cell wall in plant cells.

• If a plant cell is placed in a solution that is highly concentrated, it will lose water to its surroundings.
• As it loses water, the cell shrivels up and the cell membrane moves away from the cell wall.
• This leaves a gap between the cell wall and cytoplasm. This process is called plasmolysis.

Active transport.

• Active transport is the process where chemicals are taken into a cell against the diffusion gradient (i.e. from an area of low concentration to an area of high concentration)
• This process requires energy.