RP3: Required practical activity 3: Osmosis in plant tissue
Not started yet — this one needs some love.
Aim: investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.
Independent variable: concentration of sugar (or salt) solution (mol/dm³).
Dependent variable: percentage change in mass of the plant tissue cylinder (%).
Control variables:
same tissue type (e.g. potato)
same initial length and diameter of each cylinder (cut with the same cork borer)
same volume of solution in each tube (e.g. 10 cm³)
same time left in solution
same temperature
same blotting technique before each weighing
Method
Use a cork borer to cut five cylinders of potato of the same diameter. Trim with a sharp knife on a white tile so all cylinders are the same length (approximately 3 cm). Remove any skin.
Measure and record the initial mass of each cylinder on a balance. Record in a table.
Measure 10 cm³ of each solution (e.g. 1.0 M, 0.75 M, 0.5 M, 0.25 M sucrose, and distilled water) into five separate labelled boiling tubes.
Add one potato cylinder to each boiling tube. Record which cylinder is in which tube.
Leave the tubes for at least 30 minutes at room temperature (overnight gives more reliable results).
Remove each cylinder and blot dry with paper towel using the same number of blots each time. Immediately re-weigh each cylinder.
Record the final mass. Calculate the percentage change in mass for each:
% change in mass = (final mass − initial mass) ÷ initial mass × 100
A positive value means the cylinder gained mass (water entered by osmosis — solution was more dilute than the cell contents).
A negative value means the cylinder lost mass (water left by osmosis — solution was more concentrated than the cell contents).
Plot a graph of % change in mass (y-axis) against concentration (x-axis). Draw a line of best fit. The point where the line crosses 0% is the isotonic concentration — the concentration of solutes inside the potato cells.
Safety
Cork borer and sharp knife — risk of cuts when preparing cylinders; always cut away from the body on a white tile; the teacher may prepare cylinders in advance to avoid this risk.
Electrical balance near water — keep the balance away from spilled solutions; dry hands before using electrical equipment.
Common exam mistakes
Use percentage change in mass, not just change in mass, because potato cylinders may not have the same starting mass.
Osmosis is the movement of water across a partially permeable membrane. Do not say salt, sugar or the whole solution moves by osmosis.
Explanations need the relative concentrations of the potato cells and solution; vague phrases such as "the potato dries out" or "water balances out" are common misconceptions.
Blot cylinders in the same way before weighing; surface liquid adds mass and makes the results unfair.