Chapter 13: Diffusion and Osmosis

Movements of Substances

Membranes can be:

• Permeable
  Substances can freely pass through the membrane.

• Selectively Permeable
  Allows some but not all substances to pass through.

• Impermeable
  Substances cannot freely pass in or out of the membrane.

Diffusion

Diffusion is the movement of a substance from a region of high concentration to an area of low concentration. It is a passive process (does not require energy).

• Biological membranes are selectively permeable.
  - Allows: Water, Carbon Dioxide, Oxygen
  - Doesn’t Allow: Sugars, Proteins and Salts

• Diffusion takes place along the concentration gradient.

Factors Affecting Diffusion

• Concentration of substances

• Temperature

• Size of molecules

• Surface area
  - Size of area that substance can move through
  - More surface available, more diffusion can occur

Examples of Diffusion

• Gaseous exchange in the lungs

• Absorption of digested food from intestine into blood

• Plant hormones are produced at the tips of the roots and shoots, and move through the plant via diffusion

• Gaseous exchange in the leaf

Osmosis

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of high water concentration to an area of low water concentration. It is a passive process (does not require energy).

Solvent
Is the liquid that dissolves other substances. Water is the most common (universal solvent)

Solute
The substance that has been dissolved.

Solution
Mixture of solvent + solute.

Animal Cells and Osmosis

Animal cells only have a cell membrane seperating the cell and the external environment. If such a cell was immeresed in pure water the net flow of water would be into the cell. Should this continue, the cell membrane would be unable to withstand the pressure as the cell fills up with water and the cell would burst.

Amoeba Cell in Less Concentrated Solution

Amoeba is a single-celled organism that lives in fresh water. The fresh water is less concentrated than the cell contents of the amoeba. Amoeba cannot stop water entering the cell so it has a special structure called the contractile vacuole which collects water and pushes it out of the cell. This contractile vacuole carries out osmoregulation. This is an active process (uses energy).

Animal Cell in Less Concentrated Solution

If red blood cells were put into a similar situation they would burst as they don’t have a contractile vacuole. This is known as haemolysis.

Animal Cell in More Concentrated Solution

If red blood cells were put into a high salt solution water would leave the cells by osmosis. The cells shrivel, which is a process called crenation, and die.

Plant Cell in Less Concentrated Solution

• If the plant cell has a higher concentration of solute than its surroundings then the water will enter the cell by osmosis and the cell will expand as the water flows into the vacuole, e.g. roots.

• The cytoplasm is pushed outwards against the cell wall. The cell expands but does not burst.

Turgor pressure is pressure of the cytoplasm and the vacuole against the cell wall of the plant.

Plant Cell in More Concentrated Solution

If a plant cell is placed into a highly concentrated solution it will lose water by osmosis. The cell wall shrinks as the cell membrane pulls away from the cell wall leaving a gap. This process is known as plasmolysis.

Osmosis and Food Preservation

The death of micro-organisms due to water loss from osmosis is the mechanism by which some forms of food preservation work.

High Sugar Level

Any micro-organisms that land on jam will lose water by osmosis. The micro-organisms cannot survive without water and die. This extends the shelf life of the jam.

High Salt Level

Foods such as fish and meat are soaked in salt solutions such as brine to preserve them. Salt draws water from micro-organisms and kills them.