1. When food is cooking in the kitchen you can smell it in other rooms in the house. Why is this?
Molecules in a gas or a liquid are constantly moving about freely, bumping into one another and bouncing this way and that. This takes place randomly and results in the molecules being spread out evenly. This tendency for molecules to become distributed evenly throughout the space they occupy is called diffusion.
Molecules are leaving the food as a gas and moving around at high speed, eventually reaching all parts of the house. Molecules are diffusing through a gas, ie gas. However, diffusion also travels through liquids, and this is its main importance in biology.

2. Diffusion
Diffusion - the net movement of particles from a high concentration to a low concentration until they are evenly spread out.
Diffusion is passive – it takes place along a concentration gradient from high to low concentration and uses no energy.
Students write this definition down.
Your cells need to take in substances such as oxygen and glucose. They also need to get rid of waste products and chemicals that are needed elsewhere in your body. Dissolved substances move in and out of your cells across the cell membrane. They do this in three ways: diffusion, osmosis and active transport. Today we are going to think about diffusion.
Diffusion takes place because of the random movement of the particles of a gas or a substance in water. All the particles are moving and bumping into each other and this moves them all around.
Ask them whether they know what concentrated means?

3. Factors that affect the rate of diffusion
Concentration gradient - the greater the difference in concentration, the faster the rate of diffusion.
Temperature – an increase in temperature means the particles in a gas or solution move more quickly.
Thickness of the membrane
Students write this down. The difference between two areas of concentration is called the diffusion gradient. What else may affect the rate of diffusion? Temperature.

4. Diffusion in living organisms
Many important substances can move across your cell membranes by diffusion. Water is one. Simple sugars and amino acids from the breakdown of proteins in your gut. The oxygen you need for respiration passes from the air into your lungs and into your cells by diffusion. Individual cells may be adapted to make diffusion easier and more rapid - increase in the surface area of the cell membrane over which diffusion occurs – more room for diffusion to take place.

7. Osmosis Objectives What is osmosis?
How is osmosis different to diffusion?
Why is osmosis so important?

8. Define solute and solvent.

9. The internal concentration of your cells needs to stay the same all the time for the reactions of life to take place.

10. Osmosis
Osmosis – the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water.

11. Osmosis
Differences in the concentrations of solutions inside and outside a cell cause water to move into and out of a cell by osmosis.

12. When the concentration of your body fluid is the same as in your red blood cells, equal amounts of water enter and leave the cell by osmosis and the cell keeps its shape. If the concentration of the solution around the red blood cells is higher than the concentration of substances inside the cell, water will leave the cell by osmosis. This makes it shrivel and shrink so that it can no longer carry oxygen around the body. If the concentration of your body fluids is lower than in your red blood cells, water enters the cells swell up, lose their shape and eventually burst.

13. When the concentration of your body fluid is the same as in your red blood cells, equal amounts of water enter and leave the cell by osmosis and the cell keeps its shape. If the concentration of the solution around the red blood cells is higher than the concentration of substances inside the cell, water will leave the cell by osmosis. This makes it shrivel and shrink so that it can no longer carry oxygen around the body. If the concentration of your body fluids is lower than in your red blood cells, water enters the cells swell up, lose their shape and eventually burst.