IGCSE Biology 0610: Potato Osmosis Practical and Percentage Mass Change

A student investigates osmosis using potato cylinders. She cuts two identical cylinders of potato tissue, blots each dry with paper towel, and weighs each one. She places cylinder 1 in a beaker of distilled water (0.0 mol/dm3 sucrose solution) and cylinder 2 in a beaker of concentrated sucrose solution (0.8 mol/dm3). After 24 hours, she removes each cylinder, blots it dry again, and reweighs it.

Cylinder 1: initial mass 5.00 g, final mass 5.75 g. Cylinder 2: initial mass 5.00 g, final mass 4.30 g.

(a) Calculate the percentage change in mass of cylinder 1. Show your working. [2]

(b) Calculate the percentage change in mass of cylinder 2. Show your working, and give your answer as a negative value to indicate a decrease. [2]

(d) State the term used to describe potato tissue that has gained water and is pressing firmly against its cell walls, and the term used to describe potato tissue that has lost water and is no longer pressing firmly against its cell walls. [2]

Part (a): Percentage change in mass of cylinder 1

Percentage change in mass is calculated using:

$\text{percentage change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$

For cylinder 1:

$\frac{5.75 - 5.00}{5.00} \times 100 = \frac{0.75}{5.00} \times 100 = 15.0\%$

So cylinder 1’s mass changed by +15.0%.

Part (b): Percentage change in mass of cylinder 2

For cylinder 2:

$\frac{4.30 - 5.00}{5.00} \times 100 = \frac{-0.70}{5.00} \times 100 = -14.0\%$

So cylinder 2’s mass changed by -14.0%.

Part (c): Explaining the results using water potential

Distilled water contains no dissolved solutes at all, so it is more dilute than the cell sap inside the potato cells and therefore has a higher water potential than the cell sap. Water moves by osmosis from a region of higher water potential to a region of lower water potential, through the partially permeable cell membranes, so water moved into the cells of cylinder 1, increasing its mass.

The 0.8 mol/dm3 sucrose solution is more concentrated than the cell sap inside the potato cells, so it has a lower water potential than the cell sap. Here, water moved by osmosis out of the cells of cylinder 2, from the higher water potential inside the cells to the lower water potential of the surrounding solution, decreasing its mass.

Part (d): Naming the tissue states

The potato cells in cylinder 1 took in water, increasing the pressure of the cell contents against their cell walls; tissue in this state is described as turgid. The potato cells in cylinder 2 lost water, so they no longer press firmly against their cell walls, and the tissue feels soft; tissue in this state is described as flaccid. (Full plasmolysis, where the cell membrane visibly pulls away from the cell wall, is only confirmed by examining individual cells under a microscope, not simply by the firmness of the whole piece of tissue.)

Final answers

(a) +15.0%
(b) -14.0%
(c) Distilled water has a higher water potential than the potato cell sap, so water enters cylinder 1 by osmosis; the sucrose solution has a lower water potential than the cell sap, so water leaves cylinder 2 by osmosis.
(d) Cylinder 1’s tissue is turgid; cylinder 2’s tissue is flaccid.