RP03 - Water potential of plant tissue

Reading osmosis from mass change

A chip's change in mass shows which way water moved, and that points to the water potential of the tissue.

Water crosses the partially permeable cell-surface membranes of potato cells by osmosis, according to the water potential difference between the cells and the solution.

So −800 kPa is lower than −400 kPa, and net water movement runs towards the lower value.

What each mass change shows

You can read a mass change as water movement because sucrose is a large, polar molecule that crosses the cell-surface membrane far more slowly than water.

The last row is your target. A solution with the same water potential as the tissue is isotonic with it. With no water potential gradient, water molecules still cross the membranes in both directions, but at equal rates, so there is no net movement.

Published data give the water potential of a sucrose solution, so finding the solution that causes no mass change gives an estimate for the tissue.

Dilution series and method

You make six solutions of known concentration, then measure what each one does to the mass of a potato chip.

Diluting from a stock solution

One workable method uses 1.0 $\mathrm{mol\,dm^{-3}}$ sucrose stock solution and distilled water to prepare six 20 $\mathrm{cm^3}$ solutions.

For a direct dilution, the amount of sucrose is conserved:

To make 20 $\mathrm{cm^3}$ of 0.3 $\mathrm{mol\,dm^{-3}}$, rearrange to get $V_{\text{stock}}=(0.3/1.0)\times20=6\,\mathrm{cm^3}$, then add 14 $\mathrm{cm^3}$ of water.

Serial dilution

Instead of making each solution separately from stock, a serial dilution transfers a measured volume into solvent, mixes, then uses that dilution to make the next.

For a twofold series, mix equal volumes of solution and water at each stage: 1.0, 0.5, 0.25 and 0.125 $\mathrm{mol\,dm^{-3}}$. For a tenfold step, use 1 part of solution to 9 parts of water, making 10 parts in total.

Mix before each transfer and pipette accurately, because an early error carries through every later dilution.

Measuring the change in mass

In the diagram, note the fixed steps in concentration and what stays the same: one chip per tube, one 30 °C water bath, the same measuring equipment.

[DIAGRAM: asset_name: RP 03 - Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue - Diagram 1; asset_slug: RP 03 - Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue - Diagram 1; recommended_method: retained_png; caption: Six labelled boiling tubes in a 30°C water bath, each containing a different sucrose concentration and one potato chip, with a balance and pipette shown alongside.; description: Six labelled boiling tubes in a 30°C water bath, each containing a different sucrose concentration and one potato chip, with a balance and pipette shown alongside.]

1. Label six boiling tubes and add the volumes of sucrose solution and water with a graduated pipette or syringe, because the volumes set the concentration. Mix each tube so the concentration is even throughout.
2. Stand the tubes in a 30 °C water bath and check with a thermometer, so each solution is at temperature before its chip goes in.
3. Cut six chips from the same tuber with a potato chip cutter or cork borer and trim them to the same length, so tissue and size are comparable.
4. Blot each chip without squeezing, then record its initial mass. Surface liquid would add mass, and squeezing would force water out.
5. Put one chip in each tube for a fixed exposure time, such as 20 minutes, that gives a measurable change without tissue deterioration.
6. Remove the chips, blot them in the same way and record their final masses.
7. Repeat with at least three independent chips at each concentration, calculate each percentage change and then the mean, so one unusual chip has less effect.

Each concentration becomes an x-value on your graph, so a wrong volume moves a point and can shift your final estimate.

Variables and controls

Only sucrose concentration should differ between tubes, so every other influence on mass has to be held steady.

These control variables would otherwise change the result:

Why percentage change, not raw change

Potato chips rarely start with exactly the same mass, so raw changes can mislead: a 5.00 g chip gaining 0.20 g and a 2.50 g chip gaining 0.10 g have both changed by +4%. Percentage change in mass divides by initial mass, which standardises the results for fair comparison.