1. Movement in and out of cells
Learning Objectives
- to know that dissolved substances can move in and out of cells by diffusion.
- to know that particles move from a high concentration to a low concentration.
- to know that the greater the difference in concentration the faster the rate of diffusion

2. Movement
Dissolved substances can move in and out of cells by diffusion, osmosis and active transport.
Diffusion and osmosis are passive and require no energy.
Active transport is active and requires energy.

3. Diffusion in action: one gas
Teacher notes
This activity could be used as a starter exercise to work on diffusion.

4. Diffusion in action: two gases
Teacher notes
This activity could be used as a starter exercise to work on diffusion.

5. Changing concentrations
During diffusion molecules move from an area of high concentration to an area of low concentration.
They are said to move down a concentration gradient.
high
concentration
low
Diffusion is a passive process which means that no energy is needed.
Molecules diffuse until they are evenly spaced apart and equilibrium is reached.

6. What happens in the alveoli?
Teacher notes
This four-stage animation shows how oxygen and carbon dioxide are exchanged in the alveoli. Suitable prompts could include:
Start: What is special about the shape of the alveoli and their position to the capillary blood supply?
Stage 1: Why is the blood arriving at the lungs deoxygenated?
Stage 2: What colour is haemoglobin?
Stage 3: Which other substances are produced during respiration?
Stage 4: Approximately what percentage (approx). of exhaled air is carbon dioxide? (4%)

7. Diffusion across the alveoli
The gases move from a high concentration to a low concentration.
So oxygen moves from inside the alveoli to the blood where it binds to haemoglobin.
Carbon dioxide is present in high concentration so moves into the alveoli and is breathed out.
The greater the difference in concentration the faster the rate of diffusion.

8. Diffusion and the placenta
How does the placenta work?
blood to mother
low in O2/nutrients,
high in CO2/waste
umbilical cord
placental villi increase surface area for diffusion
umbilical artery
blood from mother
high in O2/nutrients,
low in CO2/waste
umbilical vein