66 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport)

Question 1

Determination of water potential by measuring changes in mass and length

The results table shows sample results for an experiment measuring the mass change of a potato cylinder.

It would be set out in the same way if cylinder length or the mass of a leaf fragment were measured instead.

Solute potential is read from the table.

1. Complete the table, using the appropriate number of decimal places:
   (a) Calculate the actual change in mass of each potato disc
   (b) Calculate the percentage change in mass of each potato disc
   (c) Calculate the mean mass change for the three potato discs in each concentration.
2. Use your calculated values to plot a graph of mean percentage mass change against solute potential. You can find the value for the solute potential of each sucrose concentration from the table on the left. Remember that water potential values are negative, so they are plotted going down the page, below the x-axis of the graph. Draw a line of best fit.
3. Read the intercept on the x-axis, i.e. the solute potential when there is no water movement.

Answer 1

Question 2

Determination of water potential by measuring changes in mass and length

Why is percentage change rather than actual change used?

Answer 2

Percentage change rather than actual change is used because the initial masses are different.

Question 3

Determination of water potential by measuring changes in mass and length

Why is a line of best fit more suitable for this experiment?

Answer 3

This experiment provides one of the rare occasions in biology in which a line of best fit is more suitable than Joining the data points with straight lines.

If they were joined with straight lines, only the two data
points either side of the x-intercept would contribute to the result.

A line of best fit ensures that all data points contribute.

Question 4

Determination of water potential by measuring changes in mass and length

Sources of error and remedies

Answer 4

Question 5

Determination of water potential by measuring changes in mass and length

Further work

Answer 5

Question 6

What does the time the potato discs spend in sucrose solutions affect or not affect?

Answer 6

The time the potato discs spend in the sucrose solutions does not affect where the line of best fit crosses the x-axis.

But mass changes will be biggest if discs are in solution long enough to reach equilibrium, and so the masses will be measured most accurately