Passive transport

Passive transport requires no additional energy. It could be compared to a ball rolling down a hill.

• Diffusion
• Facilitated diffusion
• Osmosis

Diffusion

Concentration refers to the amount of a substance in a defined space. Another definition is that concentration only occurs when there is a mixture. So it refers to the amount of dissolved substance in a defined volume of another substance.

Diffusion is a passive process which means it does not require energy. Small particles diffuse faster than large ones. Eg substances like oxygen, carbon dioxide and glucose move in and out of cells by diffusion.

The greater the difference in concentration, the faster the rate of diffusion.

Facilitated diffusion

Like diffusion, facilitated diffusion is a passive process. Facilitated diffusion is when large molecules that cannot cross the plasma membrane are “helped” across by carrier proteins.

Osmosis

In osmosis, water particles move through a layer that only certain molecules can pass through (semi-permeable membrane) from an area of high water concentration to an area of lower water concentration.

When there is no longer any movement of water, osmotic equilibrium is reached. Cells can usually maintain the correct composition of fluids by osmoregulation.

Tonicity and osmosis

Tonicity is the concentration of a solution as compared to another solution. Concentration describes the amount of solutes dissolved by a solution. The way cells react depends on the surrounding solution.

In blood cells and bacteria, the cell bursts when it is placed in a solution that is more dilute than the cell contents. This occurs because water enters freely by osmosis. In blood cells this is known as haemolysis. The effects of different surrounding concentrations on blood cells are illustrated below.

• Hypertonic: A very concentrated solution where water moves out of cell. The cell implodes as the water has moved out of the blood cell.
• Isotonic: A surrounding solution with the same concentration as the cell contents therefore the cell remains undamaged.
• Hypotonic: A very dilute surrounding solution so water moves into the cell and the blood cell swells and can explode.