Osmosis

Question 1

What is osmosis?

Answer 1

Net movement of water molecules from an area of higher to lower water potential, across a partially permeable membrane

Question 2

Water potential

Answer 2

• The potential of water molecules to diffuse out of or into a solution
• Pure water has a water potential of zero
• Adding solutes to pure water lowers its water potential
• The more negative the water potential, the stronger the concentration of solutes in the solution

Question 3

What will happen if a cell is placed in an isotonic solution?

Answer 3

Stays the same - no net movement of water molecules (osmosis) as the cell has the same water potential as the solution

Question 4

What will happen if a cell is placed in a hypertonic solution?

Answer 4

Shrivel - net movement of water out of the cell as the solution has a lower water potential than the cell

Question 5

What will happen if a cell is placed in a hypotonic solution?

Answer 5

Swell - net movement of water molecules into the cell as the solution has a higher water potential than the cell

Question 6

How does water potential gradient affect rate of osmosis?

Answer 6

The higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time

Question 7

How does thickness of exchange surface affect rate of osmosis?

Answer 7

The thinner the exchange surface, the faster the rate of osmosis

Question 8

How does surface area affect rate of osmosis?

Answer 8

The larger the surface area, the faster the rate of osmosis

Question 9

What is a serial dilution?

Answer 9

A set of solutions that decrease in concentration by the same factor each time

Question 10

How can you make serial dilutions?

Answer 10

• Add 10cm3 solution to 1 test tube and 5cm3 distilled water to 4 other test tubes
• Using a pipette, add 5cm3 of the solution from the first test tube to the distilled water in the second test tube
• Repeat for the other test tubes, using the solution from the previous test tube

Question 11

How do you measure how much mass the potato cells gain or lose in each solution?

Answer 11

• Use a cork borer to cut potatoes into identically sized chips, about 1cm in diameter
• Divide the chips into groups of three and measure the mass of each group using a balance
• Place one group into each sucrose solution and leave in for 20 mins
• Remove the chips and pat dry with a paper towel
• Weigh each group again and record results
• Calculate the percentage change in mass for each group

Question 12

Calibration curve

Answer 12

Plot percentage change in mass on y-axis and concentration of sucrose solution on x-axis. The point at which your calibration curve crosses the x-axis is the point at which the solution is isotonic