Biology - Diffusion and Osmosis

Substance move in and out of the cell membranes by diffusion. Diffusion is the movement of substances from a region of high concentration to a region of low concentration.

Particles move about in lots of different directions. This is called random movement. Diffusion is the net (overall) movement of particles from an area of high concentration to an area of low concentration.

The rate of diffusion is increased when:

• There's a grater surface area of the cell membrane.
• There's a grater difference between concentrations (a steeper concentration gradient).
• The particles have a shorter distance to travel.

Carbon dioxide (CO2) and oxygen (O2) move in and out of plants through their leaves.

During the day:

• Carbon dioxide is used up in photosynthesis. The concentration inside the leaves id lower than the concerntration outside the leaves.
• Carbon dioxide diffuses into plants through the stomata (tiny pores) on the bottom of their leaves.
• Oxygen, a product of photosynthesis, diffuses from the plant into our atmosphere.

At night, photosynthesis stops. Oxygen diffuses into leaf cells and carbon dioxide diffuses out of leaf cells.

The stomata on the underside of the leaves are specially adapted to:

• Open - to help increase the rate of diffusion of carbon dioxide and oxygen.
• Close - to prevent excess water loss in drought conditions.

Osmosis is the diffusion of water from high concentration of water (dilute solution) to low concentration of water (concerntration solution) through a partially-permeable membrane (a membrane that allows the passage of water molecules buy not solute molecules). Osmosis is a special type of diffusion involving water molecules.

Plant cells are surrounded by a membrane which allows water to move in and out of the cells. Water and solute molecules move freely through the provide support - it doesn't affect the movement of substances in or out of the cell.

In osmosis, the water molecules move randomly, colliding with each other and passing through the membrane in both directions. But, the net movement of molecules is from the area of low water concentration. This gradually dilutes the solution.

You can predict the direction of water movement if you know what the concentration of the water is. Remember, solute molecules can't pass through the membrane; only the water is always molecules can.

Movement of water is always from high to low water concentration.

Water also diffuses in and out of animal cells through the cell membrane by osmosis. But, animal cells don't have a cell wall , so too much water entering a cell could cause the cell to the burst.

Example - Red Blood Cell                                                  

1. When red blood cells are in solutions with the same concentration as their     cytoplasm, they retain their shape.

2. When in a weaker solution, they absorb water, swell up and may burst.

3. When in a more concentration solution, they lose water and shrivel up.         

Animal cells, unlike plant cells, don't have an inelastic cell wall.

Blood cells in a pure water solution will gain water by osmosis. Without a cell wall to prevent water from entering the cell, they absorb more and more water until they burst. This is called lysis.

Blood cells in a concentration solution (very little water) will lose water by osmosis. Without a cell wall to prevent water loss, they can shrivel up and become crenated (have rough edges).

Plant cells have inelastic cell walls which, together with the water inside the cells, are essential for the support of young non-woody plants. The cell wall:

• Prevents cells from bursting due to excess water.
• Contributes to rigidity.

The pressure of the water pushing against the cell wall is called tugor pressure.

A lack of water can cause plants to droop (wilt). as the amount of water inside the cell reduces, the cells become less and less rigid due to turgor pressure.

As water moves into plant cells by osmosis, the pressure inside the cell increases. The inelastic cell wall withstand the pressure ans the cell becomes very turgid (rigid). When all the cells are fully turgid, the plant is firm and upright. But, if water is in short supply, cells will start to lose water by osmosis. They lose turgor pressure and become flaccid (not rigid), and the plant begins to wilt.

When cells lose alot of water, the inside of the cell contracts. This is called plasmolysis.