Living cells experiment

Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to:

  1. cut equal-sized pieces of potato
  2. blot with tissue paper and weigh
  3. put pieces into different concentrations of sucrose solution for a few hours
  4. remove, blot with tissue paper and reweigh

The percentage change in mass can be calculated for each piece of potato:

percentage\:change\: in\:mass = \xrightarrow[start\:mass]{(end\:mass-\:start\:mass)}\times100

Example 1

A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end.

percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100 = 0.5 ÷ 2.5 × 100 = +20%

The plus sign shows that it has gained mass - it will have gained water by osmosis.

Example 2

A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end.

percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100 = –0.5 ÷ 2.5 × 100 = –20%

The minus sign shows that it has lost mass - it will have lost water by osmosis.

Example results

A graph of change in mass (vertical axis) against concentration of sucrose (horizontal axis) can be plotted.

Graph. Y axis change in mass per cent, -40 to 30. X axis concentration of sucrose in mol/dm3, 0 to 1.4. Plotted line begins at approx 28, y axis and 0, x axis. Ends at -30, y-axis and 1, x axis.

Where the line crosses the horizontal axis at 0% change in mass, the sucrose concentration is equal to the concentration of the contents of the potato cells. The sucrose concentration is isotonic with the cells' cytoplasm, so there is no net movement of water by osmosis.

Move on to Video
next