1. Diffusion

2. Diffusion and Osmosis - Objectives
Use concentration gradients to explain the direction of diffusion.
Explain the difference between diffusion and osmosis
Starter  Why does the smell of a stink bomb take a long time to reach everybody?

3. What is diffusion?
Diffusion is a process in which material spreads throughout a liquid or gas.
It is the overall movement of a substance from a region of high to low concentration.
The rate of diffusion depends upon the surface area of a cell membrane, the different in concentrations and the distance the particles have to travel.

4. Diffusion in Plants
Carbon dioxide and oxygen move in and out of the plants through the leaves.
Diffusion happens when there is a low concentration gradient (high to low).
Diffusion looks as if it has stopped when the particles are evenly spread out.

5. The Leaf Leaves are adapted so that diffusion happens quickly.
Leaves are thin to reduce the distance that gases have to diffuse through.
They are broad with a spongy layer inside to provide a large surface area.
Materials go into and out of cells by diffusion through the cell membrane.

6. Day and Night At night photosynthesis stops.
During the day CO2 is used up in photosynthesis.
The concentration inside the leaves is low. The concentration outside the leaves is high.
So CO2 diffuses into the plant (high to low).
O2 is high concentration in the leaves and low concentration outside the leaves. So O2 diffuses out of the plant (high to low).
At night photosynthesis stops.
Respiration is the dominant process.
The process of diffusion is reversed.
O2 diffuses into the leaves and CO2 diffuses out of the leaves.

7. What is osmosis?
Osmosis is the diffusion of water from a high water concentration to a low water concentration through a partially-permeable membrane.
The diffusion of water from a diluted solution to a concentration solution through a membrane that allows the passage of water molecules but not that solute.
Osmosis is a special kind of diffusion involving water molecules.

8. Partially-Permeable Membrane
Plant cells have a cellulose wall that provides support and lets different sizes of molecules through it. It does not affect the movement of substances in and out of the cell.
The cell membrane is a partially-permeable membrane that allows small particles such as water molecules through it, but not larger molecules like salt or sugar.

9. Turgid Plant Cells
Plant cells have a strong hard rigid cell wall on the outside of the cell membrane. This stops the cell bursting when it absorbs water by osmosis.
As the pressure from the water increases in the cell, it becomes more rigid. This is useful because plants to do not have an exoskeleton.
Instead the leaves and shoots can be supported by the turgor pressure, the pressure of water inside their cells.

10. Flaccid Plant Cells
If plant cells lose too much water by osmosis, they become less rigid.
Eventually the cell membrane shrinks away from the cell wall causing the plant to wilt. The cell is now flaccid.
Loss of water makes the cell limp and shrinks the cell membrane away from the cell wall.

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