SB1. Key Concepts in Biology - Diffusion and Osmosis (including methods to investigate osmosis)

Question 1

SB1i - What is the difference between diffusion and osmosis?

Answer 1

• Diffusion is when particles move from a high concentration to a low concentration.
• In osmosis this is the same principal except across a semi permeable membrane where a solvent moves from and area of high concentration to an area of low concentration

Question 2

SB1i CP - Describe the method you would use to investigate osmosis through potatoes

Answer 2

• Cut discs of raw potato, blot them dry, and measure their mass.
• Put each disc in a sugar (or salt) solution of different concentrations
• After 30 minutes, measure the mass of each disc again.
• Subtract the initial mass from the final mass of each disc.
• Divide this number by the initial mass and then multiply by 100 to give a percentage change in mass.
• The highest change in mass means there was the greatest difference between water concentration as osmosis works with the concentration gradient This can work with any semi-permeable membrane

Question 3

SB1i CP - Explain why potatoes will gain mass when put into a very dilute solution

Answer 3

• The concentration of water in the solution will be higher than the concentration of water in the potatoes, so water will move into the potato by osmosis which increases mass

Question 4

SB1i CP - Explain why potatoes will lose mass when put into a very concentrated solution

Answer 4

• The water concentration of the potatoes will be higher than that of the solution, so water will move into the solution due to osmosis. Therefore, the potatoes will lose mass.

Question 5

SB1i - What are the 3 factors affecting Diffusion?

Answer 5

• Temperature
• Concentration Gradient
• Surface Area of Cell Surface Membrane

Question 6

SB1h CP - How do you calculate rate of reaction in enzyme-controlled reactions?

Answer 6

1/time

Question 7

Describe diffusion

Answer 7

Particles in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher
concentration, they will therefore move from this region to where they are in a lower concentration.

It is important to remember that the particles:
- will move in both directions, but there will be a net movement from high to low concentration.
- will end up evenly spread throughout the liquid or gas but will continue to move.

Question 8

How does concentration gradient affect rate of diffusion?

Answer 8

The greater the difference in concentration, the quicker the rate of diffusion

Question 9

How does temperature affect rate of diffusion?

Answer 9

The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly

Question 10

How does the surface area of the cell membrane separating different regions

Answer 10

The greater the surface area, the faster the rate of diffusion

Question 11

What is the equation to calculate diffusion rate? (Ficks law)

Answer 11

Question 12

What is the calculation for percentage change?

Answer 12

Question 13

How do you work out the concentration of the potato in the osmosis practical?

Answer 13

*Put each disc in a sugar (or salt) solution of different concentrations
* After 30 minutes, measure the mass of each disc again.
* Subtract the initial mass from the final mass of each disc.
* Divide this number by the initial mass and then multiply by 100 to give a percentage change in mass.
Plot the results on a graph, change in mass against concentration and where the plotted line crosses the horizontal axis at 0 per cent change in mass, the sucrose concentration of the solution is equal to the concentration of the contents of the potato cells. Therefore, the concentration of the water in the outer solution is equal to the concentration of the water inside the cells.