Section 2 d) Specification

2.12 understand definitions of diffusion, osmosis and active transport

Diffusion: The net movement of particles from an area of high concentration to an area of low concentration in a fluid. Goes with the concentration gradient (passive), so doesn't require energy.

Osmosis: The net movement of water particles through a semi-permeable membrane from an area of high water potential to an area of low water potential, (an area with higher concentration of water particles to an area of low concentration of water particles) Goes with the concentration gradient (passive), so doesn't require energy.

Active Transport: The movement of particles from an area of low concentration to an area of high concentration, absorption against a concentration gradient. Requires energy from respiration.

2.13 understand that movement of substances into and out of cells can be by diffusion, osmosis and active transport

Diffusion: gas exchange in the alveoli

Osmosis: Water moves around plant cells, making them turgid for support

Active transport: Root hair cells absorb mineral ions from the soil

2.14 understand the importance in plants of turgid cells as a means of support

Plants don't have bones or an exoskeleton, so their means of support is turgid cells, which, with their cellulose cell walls, are strong and support the plant's shape. Without turgidity, the plant wilts and dies.

2.15 understand the factors that affect the rate of movement of substances into and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient

Diffusion and osmosis are affected by the following factors:

• Temperature. The particles move more quickly as they have more kinetic energy with increased temperature, meaning they move along the concentration gradient more quickly. 
• Concentration gradient. A greater difference in concentration means a steeper concentration gradient. (Fick's Law)
• Distance. Further distances mean slower diffusion, for obvious reasons. It takes longer for particles to travel a further distance. 
• Particle size. Larger particles in diffusion are heavier, and therefore move more slowly.
• Surface area to volume ratio. If the surface area is proportionally larger, there is more surface for passive transport to occur through. 

2.16 describe experiments to investigate diffusion and osmosis using living and non-living systems.

Osmosis can be investigated using Visking tubing, sugar solution and water. (Model cell)
1. Place sugar solution in one sealed Visking tube, and water in another. Weigh each.
2. Place the sugar tube in a beaker full of water, and the water tube in a beaker full of sugar solution.
3. Remove the tubes after 30 minutes and weigh again. Note the changes.
You can see that the sugar tube has increased in mass, because osmosis has caused water to enter the tube. The water tube has lost mass, as osmosis has caused water to leave the Visking tubing.

Osmosis can also be investigated using potato cylinders.
1. Using a cork borer, create a potato cylinder and cut into equal lengths of 3cm.
2. Weigh each cylinder, then place each into different concentrations of sugar solution, making sure to keep track of which is which.
3. After 30 minutes, remove and dry the potato cylinders, then reweigh and note the change in mass for each of them. ( percentage change in mass = (final mass - initial mass) x 100 / initial mass )
These results can be graphed to find the water potential of the potato, as this is the point at which the mass should not change.

Diffusion can be investigated by placing food colouring into water. Then by timing how long it takes for the water to become entirely one colour at different temperatures will tell us how temperature affects diffusion.