Where does the water go?

Learning goals

Students investigate how external sucrose concentration relates to net tissue mass change. They take initial and final balance readings, apply a consistent blotting procedure, use repeated fresh samples, and explain the pattern in terms of osmosis.

The interface introduces one task at a time. Students move the specimen through the procedure themselves. Percentage calculations, means, and graph construction are supplied; a manual percentage calculation is optional.

Prerequisites

Cell membranes, solutions and concentration, mass in grams, and a basic understanding of osmosis. Explain that net movement does not imply one-way molecular motion. Plant tissue behavior depends on water potential, including solute and pressure effects; external concentration alone is not a complete tissue model.

Procedure and controls

Predict how concentrated sucrose solution may affect tissue mass.
Place the balance, three prepared baths, sample tray, forceps, blotting paper, and timer.
Tare the empty balance. Move the highlighted fresh cylinder onto the pan to record initial mass.
Move the cylinder into the assigned concentration. Run the 30-minute modeled soak.
Transfer the sample to the paper, blot gently, and move it back to the balance.
Record final mass and percentage change. Repeat with fresh samples and fresh 100 mL solution aliquots: three cylinders each at 0.0, 0.3, and 0.6 M.
Compare means and ranges. Select the least-change solution and explain mass loss with net water movement.

Controlled conditions: similar-sized cylinders from one model tissue, consistent preparation, 100 mL starting solution per sample, 20°C, 30 minutes, the same balance and gentle blotting method. The app replenishes solution between samples. Forceps are part of the specimen-transfer action; students do not need to perform a separate forceps-selection step for every move.

Why blot? Surface liquid adds apparent mass without having entered the tissue. Why percentage change? Samples have slightly different initial masses. Why repeat? To assess consistency and avoid basing a claim on one sample. Samples are tested in a fixed concentration order in this introduction; randomizing order and using more concentrations are useful extensions.

Expected pattern in this teaching model

Sucrose bath	Expected mean mass-change pattern	Interpretation
0.0 M	Near +12%, with small sample/readout variation	Evidence consistent with net water uptake.
0.3 M	Near 0%, with small gains or losses	Least net change under the modeled conditions.
0.6 M	Near −12%, with small sample/readout variation	Evidence consistent with net water loss.

These numerical responses are prescribed synthetic data, not measured potato data or a universal biological prediction. Real tissue, exposure time, temperature, solute permeability, cutting damage, and pressure effects can change the result. Do not teach 0.3 M as an intrinsic isotonic concentration of all potatoes.

Near-zero mass change is consistent with little net water movement in this model. It does not prove identical internal and external solute concentrations, stopped molecular motion, or full biological equilibrium.

Calculation example

If initial mass is 5.00 g and final mass is 5.60 g, percentage change = (5.60 − 5.00) ÷ 5.00 × 100 = +12.0%. If final mass is 4.40 g, the change is −12.0%. Group means average the individual percentage changes. Graph bars show observed min–max ranges, not confidence intervals.

Assessment · 12 points

Criterion	Points
Identifies changed concentration and relevant controlled conditions.	0–3
Uses initial/final mass, consistent blotting, and repeated evidence correctly.	0–3
Supports a claim with a group mean and explains net water movement.	0–3
Explains a measurement or model limitation and an improved procedure.	0–3

3 = accurate and supported; 2 = mostly accurate but incomplete; 1 = limited or partly incorrect; 0 = absent. The app checks the least-change choice, a water-movement choice, and whether writing is present. A teacher evaluates the explanation’s scientific quality.

Discussion and extensions

Would a wet surface make final mass look too high or too low?
Why should cylinders come from comparable tissue and have similar dimensions?
What additional concentrations would help locate the region of least mass change?
What else would be needed before generalizing to another potato or another tissue?
How would a real experiment distinguish tissue water gain from uptake of a permeating solute?

Explicit response and measurement model

For the nominal 30-minute exposure, expected percentage mass change is 40 × (0.3 − external sucrose concentration in M), with independent sample variation of up to ±0.8 percentage points. Initial true sample mass varies within 5.00 ±0.04 g. Each weighing adds noise within ±0.005 g and rounds to 0.01 g. Blotting removes surface liquid automatically when students perform the step; its amount is not quantified.

This is not a mechanistic model of water potential, turgor, membrane permeability, solute entry, metabolism, or time to equilibrium. The shortened time-lapse and schematic specimen-size change are visual aids. All results are calculated from this stated response, not acquired from real samples.

Optional calculation feedback accepts ±0.15 percentage points from the calculation based on the displayed initial and final masses. Students should report to one decimal place. The report includes raw masses, percent changes, group means/ranges, controls, written explanation, and model limits.

Background reference: OpenStax, passive transport and osmosis

The guided scenario, response model, procedure, and assessment criteria are original instructional content.