A red onion cell in concentrated sucrose can keep its rectangular outline while its purple interior contracts. Which boundary should you measure? The wall gives you a reference. The first gap between that wall and the living contents is the observation you will score.

Recognize the three boundaries

TurgidEarly separationPlasmolysedWall and protoplastSmall visible gapLarger gap
Figure 1. The green wall retains its outline. The purple living contents withdraw as the vacuole loses water; the pale gap fills with external solution.

Starting outside, distinguish the porous cell wall, the plasma membrane around the protoplast, and the tonoplast around the vacuole. The purple pigment marks the sap. With a school microscope the membranes themselves are usually too thin to resolve individually. Infer their positions from the wall and pigment boundaries, and look for a clear space between the wall and the protoplast.

The term plasmolysis refers to shrinkage and separation of the protoplast. Water leaves when the external water potential is lower. The wall is permeable to the bathing solution, so the space outside the retracting protoplast contains liquid. An air bubble has a prominent circular outline and may cross several cells.

Prepare a comparison you can repeat

Add solutionCoverslipWick hereSlideOnion peel
Figure 2. Replace the bathing solution without lifting the coverslip. The absorbent strip draws liquid across the stationary peel.

1. Take thin, pigmented inner epidermis from comparable red onion scales. Use similarly sized, single-layer strips. Mount one in water, lower the coverslip and find an intact field under low power. Increase magnification to see the cell corners. Record the initial condition.

2. Prepare separately labelled sucrose baths, for example 0.20, 0.30, 0.40 and 0.50 mol/L, plus a water control. These concentrations illustrate a starting range. Use fresh strips for each concentration. Add test solution to one edge of the coverslip and wick from the opposite side. Replace enough liquid to displace the original bath.

3. Pilot the observation time, then use the same interval, temperature and scoring rule for all slides. Count fields in a predefined order. Include intact cells with a clearly visible wall and score a cell once if a distinct gap is visible at any corner or edge. Exclude torn cells and record exclusions.

4. Return one clearly plasmolysed strip to water and observe the same cells. Expansion toward the wall is deplasmolysis. A damaged or severely stressed cell may fail to recover. For independent repeats, use tissue from different bulbs and retain counts separately.

Calculate the midpoint from counts

Cells (%)050100.20.30.40.50Sucrose concentration (mol/L)
Figure 3. Illustrative counts, not class measurements. The dashed reference crosses the example response between 0.30 M and 0.40 M sucrose.

Example only: out of 20 eligible cells per bath, 2, 7, 12 and 18 cells show gaps at 0.20, 0.30, 0.40 and 0.50 M. The percentages are 10%, 35%, 60% and 90%. At 0.30 M the class is 15 percentage points short of 50%; the next interval gains 25 points. Linear interpolation gives 0.30 + (15/25) × 0.10 = 0.36 M.

The midpoint lies inside a measured interval. It is an estimate for this tissue, scoring rule and exposure time. At the midpoint, half the counted cells meet the criterion; an individual cell has its own onset. For a better estimate, test narrower intervals around 0.36 M and plot each independent bulb.

Relate the result to water potential

For a single cell at incipient plasmolysis, pressure potential relative to the surroundings is approximately zero. If near equilibrium, the cell water potential approximates the external solution potential. Under the ideal dilute-solution approximation, Ψs = −iCRT. For sucrose i ≈ 1. At 20 °C use 293 K and R = 0.008314 L MPa mol⁻¹ K⁻¹. The illustrative midpoint implies −1 × 0.36 × 0.008314 × 293 ≈ −0.88 MPa.

This is an approximate population estimate under model assumptions. External dilution, solute entry, inadequate equilibration and variation among cells all affect it. Water loss also changes cell sap concentration. Assigning this number to each untreated cell would overstate the result.

Troubleshoot an unexpected slide

If the control already shows detached contents, inspect a fresh peel before interpreting the sucrose series. If every concentration gives a similar response, check solution labels and replacement of the initial water. If only the edges of a field show shrinkage, compare a central field and check for drying. Record the pattern before changing the method.

Frequently asked questions

Does a pale onion work? It can plasmolyse, but a pigmented peel makes changes in vacuolar volume easier to follow.

Is a flaccid cell always visibly plasmolysed? Loss of turgor and a readily visible gap are different observations. A microscope may miss the earliest separation.

Why use sucrose rather than salt in the calculation? Sucrose is a nonelectrolyte. Salt dissociates, so an equivalent molarity does not give the same ideal particle concentration.

20 explained practice questions

1. Which event defines visible plasmolysis?

  1. Wall digestion
  2. Nuclear division
  3. Chloroplast division
  4. Protoplast detachment
Reveal answer and reasoning

Protoplast detachment. Loss of water can cause living contents to pull away from the wall.

2. Which boundary remains outermost during plasmolysis?

  1. Cell wall
  2. Tonoplast
  3. Nuclear envelope
  4. Vacuolar pigment
Reveal answer and reasoning

Cell wall. The cellulose-rich wall usually retains its visible position during the observation.

3. Which membrane encloses vacuolar sap?

  1. Cuticle
  2. Tonoplast
  3. Cell wall
  4. Outer nuclear membrane
Reveal answer and reasoning

Tonoplast. The tonoplast separates vacuolar sap from cytosol.

4. What usually gives red onion vacuoles their colour?

  1. Haemoglobin
  2. Melanin
  3. Anthocyanin
  4. Chlorophyll
Reveal answer and reasoning

Anthocyanin. Anthocyanins dissolve in vacuolar sap in pigmented epidermal cells.

5. What drives net water movement across a permeable membrane?

  1. Cell count difference
  2. Wall thickness alone
  3. Slide size
  4. Water potential difference
Reveal answer and reasoning

Water potential difference. Water moves down a water-potential gradient under the stated conditions.

6. How does adding sucrose affect the solution’s solute potential?

  1. It becomes more negative
  2. It becomes more positive
  3. It stays zero
  4. It becomes turgor pressure
Reveal answer and reasoning

It becomes more negative. Dissolved solute reduces water potential relative to pure water at the same pressure.

7. What is near zero at incipient plasmolysis in the simple model?

  1. Temperature
  2. Pressure potential
  3. Solute concentration
  4. Vacuole volume
Reveal answer and reasoning

Pressure potential. The first detachment marks near loss of the positive turgor contribution.

8. Why wick liquid under the coverslip?

  1. Fix cells permanently
  2. Stain DNA
  3. Exchange the bath while retaining the field
  4. Dissolve the wall
Reveal answer and reasoning

Exchange the bath while retaining the field. Capillary flow displaces the previous medium while the peel can remain in position.

9. Which preparation gives a clear scoring plane?

  1. Folded peel
  2. Whole bulb
  3. Stacked epidermal sheets
  4. Single epidermal layer
Reveal answer and reasoning

Single epidermal layer. Overlapping layers hide walls and create misleading boundaries.

10. Which is the relevant control for ordinary mounting and handling?

  1. Water-mounted matched peel
  2. Dry slide without tissue
  3. Boiled sucrose alone
  4. Unlabelled stock bottle
Reveal answer and reasoning

Water-mounted matched peel. A similarly handled water mount establishes whether handling itself produces gaps.

11. Which observation supports reversible plasmolysis?

  1. Nucleus disappears
  2. Protoplast expands toward its wall
  3. Wall dissolves
  4. Pigment leaves every cell
Reveal answer and reasoning

Protoplast expands toward its wall. Water uptake can restore volume if the cell remains viable. Reversibility is an observation to test.

12. What should be standardized across concentrations?

  1. Pencil colour
  2. Counting formula
  3. Exposure time
  4. Eyepiece label
Reveal answer and reasoning

Exposure time. Different waiting times can change the fraction of cells that have visibly responded.

13. Which count belongs in the denominator?

  1. Only detached cells
  2. All slides prepared
  3. Only red cells with large gaps
  4. All eligible intact cells scored
Reveal answer and reasoning

All eligible intact cells scored. The denominator includes both positive and negative cells satisfying the eligibility rule.

14. What percentage is 7 detached cells out of 20?

  1. 35%
  2. 7%
  3. 65%
  4. 70%
Reveal answer and reasoning

35%. Divide the positive count by the eligible total and multiply by 100.

15. What is a practical tissue-level midpoint?

  1. Half the original cell volume
  2. Concentration giving 50% visibly plasmolysed cells
  3. Half the wall width
  4. Half the number of slides
Reveal answer and reasoning

Concentration giving 50% visibly plasmolysed cells. This population statistic differs from first detachment in a single cell.

16. What is the best design for biological replication?

  1. One photograph copied ten times
  2. Several adjacent pixels
  3. Several independent bulbs at each concentration
  4. One cell counted repeatedly
Reveal answer and reasoning

Several independent bulbs at each concentration. Fields from the same strip are subsamples. Independent bulbs capture additional biological variation.

17. Which assumption supports the sucrose calculation?

  1. Water is mobile
  2. A wall is present
  3. Temperature can be measured
  4. Solute effectively excluded during observation
Reveal answer and reasoning

Solute effectively excluded during observation. Uptake changes the effective gradient, weakening the intended interpretation.

18. What do you do when every bath gives less than 50%?

  1. Extend the tested concentration range
  2. Report the strongest bath as the midpoint
  3. Set the midpoint to zero
  4. Discard cells that stayed turgid
Reveal answer and reasoning

Extend the tested concentration range. Observed values must bracket the midpoint for interpolation rather than extrapolation.

19. What does visible purple outside a torn peel suggest?

  1. Extra chloroplasts
  2. Membrane damage
  3. Normal sustained turgor
  4. Cellulose synthesis
Reveal answer and reasoning

Membrane damage. Leakage makes the strip unreliable for testing a selectively permeable living system.

20. Which is evidence rather than interpretation?

  1. All sucrose stayed outside
  2. The cells were genetically identical
  3. A gap appeared at two corners
  4. The cell had exactly zero pressure
Reveal answer and reasoning

A gap appeared at two corners. The visible gap is directly observable. Pressure, transport and ancestry require additional evidence.

Test yourself with 20 new prompts

Choose one answer per question, then check your score.

1. A gap appears between wall and living contents. What event has occurred?
2. Which outline should be traced as a fixed reference before changing the solution?
3. A molecule leaving the vacuole first crosses which membrane?
4. A purple compartment shrinks as water leaves. Which pigment helps reveal it?
5. Two solutions are separated by a water-permeable membrane. Which difference determines net water direction?
6. At equal pressure, which change follows increasing dissolved sucrose?
7. The protoplast just ceases to press on the wall. Which quantity is approximately zero relative to ambient pressure?
8. Which aim is met by adding a drop at one edge and paper at the opposite edge?
9. Two preparations differ only in thickness. Which would you choose for counting?
10. What should accompany sucrose-treated peels when water is the solvent?
11. After adding water, which outcome supports recovery of the same cell?
12. One slide is scored at 2 minutes and another at 15. Which factor has been confounded?
13. A field contains intact and visibly torn cells. Use which total in a predefined intact-cell analysis?
14. An observer reports 14 detached cells among 40 eligible cells. What proportion is that as a percentage?
15. Which endpoint compares samples whose individual cells detach at slightly different concentrations?
16. Which sampling plan captures variation among bulbs?
17. If sucrose enters cells rapidly, which assumption has failed?
18. A repeat reaches only 35% even in the strongest bath. What is the useful next step?
19. A damaged strip leaks coloured sap before treatment. What is the most plausible concern?
20. Which sentence belongs in the observation column of a practical notebook?

Related practicals and sources

Osmosis in plant cells · Microscopy and cell size · Plant Physiology I course

Hillsborough College, Lab 6: Osmosis · Millersville University, plant biology laboratory manual