Section 2 d) Summary

There are three ways in which substances can move into and out of cells:

• Diffusion
• Osmosis
• Active transport

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. This can happen in any fluid, and at the end the substance will be evenly dispersed. It is a passive process, meaning it happens in the direction of the concentration gradient and therefore doesn't require any energy. 

Examples include gas exchange in the lungs, gas exchange in the leaf, and assimilation in the small intestine. 

Experiments:
Diffusion can be investigated by placing food colouring or potassium permangenate 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.

Factors affecting the rate of diffusion include

• Distance: Shorter distances mean faster diffusion
• Temperature: Higher temperatures mean more kinetic energy, and faster moving particles
• Surface area: Larger surface area means faster diffusion
• Size of particles: Smaller particles move more quickly
• Concentration gradient: Steeper concentration gradients mean faster diffusion
• Pressure (in gas): This is the same principle as concentration, there is more particles in a smaller volume of space 

Osmosis is the net movement of water particles across a semi-permeable membrane from an area of high water potential to an area of low water potential. 

Examples of this include plants using root hair cells to take in water from the soil, water moving around plant cells for evenly distributed turgidity, which supports the plant. 

Osmosis is relatively similar to diffusion, the difference being osmosis is in reference only to the movement of water particles across a semi-permeable membrane. 

Experiments:

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.

Active transport is the movement of particles against a concentration gradient, from an area of low concentration to an area of high concentration. It is an active process; it requires energy from respiration, unlike diffusion and osmosis. 

Examples include reabsorption of glucose in the nephron and root hair cells in a plant taking in mineral ions.