Practical-Psi: Investigating Water Potential of Potato Tissue
- Scense Me
- Apr 13
- 2 min read
Updated: Apr 14
🎯 Aim
To determine the water potential of potato tissue by immersing potato cylinders in sucrose solutions of different concentrations and measuring the percentage change in mass.
🧰 Materials
Fresh potato
Cork borer (to cut uniform circular cylinders)
Scalpel and ruler (to trim equal lengths)
Balance (±0.01 g)
Blotting paper/tissue
6 labelled beakers or test tubes
50 mL each of sucrose solutions: 0.0 M, 0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M
Timer
Graph paper or excel spreadsheet (even better, instruct AI to construct your curve!)
🧫 Method
Use a cork borer to cut 6 potato cylinders of equal diameter. Trim each to the same length (e.g., 3 cm).
Blot dry gently and weigh each cylinder. Record the initial mass.
Place each cylinder into a separate container with 50 mL of sucrose solution.
Leave for 1 hour at room temperature.
Remove, blot dry, and weigh again. Record the final mass.
Calculate:
Change in mass = Final mass – Initial mass
% Change in mass =
Plot % change in mass (Y‑axis) against sucrose concentration (X‑axis).
Draw a smooth curve or line of best fit.
Identify the concentration where % change = 0. This is the isotonic point.
Sucrose Concentration (M) Initial Mass (g) Final Mass (g) Change in Mass (g) % Change in Mass Osmotic Potential (Ψ, MPa)
0.0
0.1
0.2
0.3
0.4
0.5
🧮 How to Fill the Osmotic Potential Column
Students use the formula:
Ψ=-iCRT
i=1.0(sucrose does not ionize)
C=molar concentration of sucrose (M)
R=0.0831" " L⋅bar⋅mol^(-1)⋅K^(-1)
T=295" K" (room temperature ~22°C)
Then convert bar → MPa (1 bar = 0.1 MPa).
📌 Example Calculation (for 0.3 M sucrose)
Ψ=-(1.0)(0.3)(0.0831)(295)
Ψ=-7.36" bar"=-0.736" MPa"
So in the Osmotic Potential column, students would enter -0.736 MPa for 0.3 M.
📈 Graph & Interpretation
Example Data Points for the Curve
Sucrose Conc. (M) % Change in Mass Osmotic Potential (MPa)
0.0 +7.5% 0.00
0.1 +4.0% -0.245
0.2 -0.8% -0.491
0.3 -3.4% -0.736
0.4 -5.2% -0.982
0.5 -7.7% -1.227
The curve crosses the X axis between 0.1–0.2 M, which corresponds to about -0.486 MPa.
This is the water potential of the potato tissue.
How the Graph Should Look
Y axis: % change in mass (gain or loss relative to original weight).
Lower X axis: Sucrose concentration in M (Molarity), e.g., 0.0 → 0.3 M.
Upper X axis: Osmotic potential (Ψ) in MPa, calculated using:
Ψ=-iCRT
where i=1(sucrose), R=0.0831" " L⋅bar⋅mol^(-1)⋅K^(-1), T=295K. Convert bar → MPa (1 bar = 0.1 MPa).
Interpretation for Students
At 0.0 M, the potato gains mass because water enters (solution is hypotonic).
At 0.5 M, the potato loses mass because water exits (solution is hypertonic).
The equilibrium point (~0.17 M) is where the potato neither gains nor loses mass.
This corresponds to a water potential of about –0.486 MPa, which is the osmotic potential of the potato tissue.
The Lab Manual in PDF:
Sample report as follows:
Related Images
Comments for Group Veronica: 15/20
The table of contents heading reads "TABLE OF CONTENT" rather than "TABLE OF CONTENTS." (line 20) You need to add the missing "S" to read "TABLE OF CONTENTS." The table body also uses "CONTENT" and "PAGE" as column headers rather than "CONTENTS" and "NO. PAGE / PAGE NUMBER," which is inconsistent with standard formatting; you should correct the column header to "CONTENTS" as well.
The section headings in the table of contents omit section numbers, and the heading "MATERIALS AND METHOD" is singular rather than plural. (line 21, table of contents) The table of contents lists "INTRODUCTION," "OBJECTIVES," "MATERIALS AND METHOD," and so on without their numbered prefixes (1.0, 2.0, 3.0, etc.), even though the actual section headings in…
Summary of Marks:
Comments for Group Imut People: 13/20
The table of contents uses the singular heading "CONTENT" instead of the standard plural "TABLE OF CONTENTS". Scientific lab reports follow the convention of a full "TABLE OF CONTENTS" heading; you need to correct this on the contents page so that it matches the expected format.
The section heading "3.0 OBJECTIVE" is singular and should read "3.0 OBJECTIVES" (line 65). The plural form is standard in scientific report writing, as multiple objectives are listed beneath it. You should change this heading to "OBJECTIVES" to be consistent with academic convention.
The abstract contains an incorrect apostrophe in "it's mass" (line 26). The possessive form does not take an apostrophe here; the correct phrasing is simply "its mass decreased."…
Comments for GroupMatcha Latte: 12/20 --> (18/20)
Resubmission: ❌ Issues Still Not Fixed
Line 23 — Table of Contents heading still singular: Still reads "TABLE OF CONTENT" instead of "TABLE OF CONTENTS."
Table of Contents — page numbers still blank: All page number cells remain empty. You need to fill in the correct page number for every section.
Lines 169–170 — "density" still used instead of "concentration": The opening paragraph of Discussion Section 1 still reads "positive for a density range of 0.0 M to 0.2 M" and "negative for a density range of 0.3 M to 0.5 M." You need to replace "density" with "concentration" throughout.
Line 175 — unrendered LaTeX string still present: The Hypotonic State bullet still contains the raw…
Comments for Group Power Rangers: 11/20
1. The report is missing a Table of Contents. Every formal lab report submitted for AGR3301 should include a Table of Contents immediately after the cover page, listing all sections and their page numbers. You need to add this before submission.
2. The section heading "2.0 OBJECTIVE" should be pluralised. At line 72, the heading reads "2.0 OBJECTIVE" rather than "2.0 OBJECTIVES". Similarly, line 80 reads "3.0 MATERIAL AND METHOD" rather than the standard "3.0 MATERIALS AND METHODS", and line 138 reads "4.0 RESULT AND CALCULATION" rather than "4.0 RESULTS AND CALCULATIONS". You need to correct all three to their standard plural forms throughout the report.
3. The objectives are formatted using bullet points rather than a numbered…