4.2.94 · Cell Membranes and Transport

Estimating Water Potential in Plants — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 16 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

4.2.9 Estimating Water Potential in Plants — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how the water potential of plant tissue is estimated by the gravimetric and incipient plasmolysis methods, and interpret the results.

Indirect estimate
Measures used
Steepness of the line
Different tissues
Making solutions
Evaporation

Part 1 of 7The principle

Indirect estimate

Indirect estimate water potential of tissue cannot be measured directly with school apparatus,
- so tissue is placed in solutions of known water potential.

Key idea

Key idea in the solution where the tissue neither gains nor loses water there is no net movement, so tissue ψ equals solution ψ.

Value

Value the result is the overall water potential of the tissue, a negative number in kPa.

Measures used

Measures used change in mass, change in length, or the proportion of cells showing plasmolysis.
- (gravimetric method)

Part 2 of 7Gravimetric method with potato

Prepare a dilution series of sucrose, for example 0.0 to 1.0 mol dm^-3,

from a stock solution using a measuring cylinder or syringe, and record the volumes used.

Cut cylinders of equal length with a cork borer from the same tuber.

Blot and weigh each cylinder, then place one (or three replicates) in each solution for a fixed time, for example 60 minutes, at constant temperature.

Remove, blot dry in the same way and reweigh.

Calculate the percentage change in mass for each, and the mean.

Plot percentage change against concentration and read the x-intercept where the change is zero.

Use a calibration curve of water potential (kPa) against sucrose concentration to convert the intercept into the water potential of the tissue.

Part 3 of 7Incipient plasmolysis method

Peel strips of epidermis from coloured onion or rhubarb, where vacuole pigment makes cells easy to see.

Mount strips in a range of sucrose solutions for about 15 minutes.

Observe with a light microscope and count how many cells in a field of view are plasmolysed, with the membrane pulled away from the wall.

Plot percentage of plasmolysed cells against concentration.

Incipient plasmolysis

Incipient plasmolysis the concentration at which 50% of cells are plasmolysed.

Meaning

Meaning pressure potential of the cell is zero, so the cell water potential equals its solute potential and equals that of the solution.

Part 4 of 7Interpreting the data

Mass loss

Mass loss the solution has a lower water potential than the tissue.

Mass gain

Mass gain the solution has a higher water potential than the tissue.

Steepness of the line

Steepness of the line a steeper line shows the tissue is more sensitive to differences in water potential.

Different tissues

Different tissues differences show differences in cell sap concentration,
- and plants from salty or dry environments often have more negative cell water potential,
which lets them take up water from soil.

Part 5 of 7Limitations and improvements

Limitation	Improvement
Tissue differs between parts of a tuber	Use the same tuber and several replicates
Blotting is variable	Use equal blotting time
Incubation too short to reach equilibrium	Leave for longer
Solutes leak from cells and sucrose may enter damaged cells	Use intact tissue and consider other methods
Intercept lies between tested concentrations	Repeat with a narrower range around the intercept and use a balance accurate to 0.001 g

Part 6 of 7Accuracy of the dilution series

Making solutions

Making solutions any error shifts the intercept, so measure volumes carefully, mix well and label each tube.

Evaporation

Evaporation keep the tissue in covered tubes, since evaporation would change the concentration.

Graph

Graph draw a line of best fit rather than joining points, and mark the reading with a construction line.

Part 7 of 7More from the specification

Principles of Osmosis and Water Potential

Osmosis is the net movement of water molecules from a region of higher water potential (less negative) to lower water potential (more negative) across a selectively permeable membrane.

Estimating Water Potential in Plant Tissues

In a classic potato cylinder experiment, potato tissue is bathed in graded sucrose solutions;
- the sucrose concentration producing zero percentage change in mass corresponds to the tissue's water potential.

Passive Simple and Facilitated Diffusion

Simple diffusion allows small non-polar molecules to cross the bilayer directly,
while facilitated diffusion transports polar molecules or ions down their concentration gradient using channel or carrier proteins.

Surface Area to Volume Ratio in Diffusion

As cell or agar cube dimensions increase, the surface area to volume ratio decreases,
increasing the average diffusion distance and reducing the rate of substance exchange.

Dilute solution

Solution with a high water concentration and low solute concentration.

Animal cell

A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.

Indicator

A chemical that changes colour depending on pH.

Endocytosis

Bulk transport into a cell using a membrane vesicle.

Bilayer

A double layer of phospholipids with tails facing inwards.

Selectively permeable

Allowing only some substances through.

Active transport

Movement of particles against a concentration gradient using energy from respiration.

Facilitated diffusion

Passive movement through transport proteins.

Phospholipid bilayer

Two layers of phospholipids with hydrophobic tails inwards

Channel protein

Hydrophilic pore for specific ions or water.

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
- as a result of random movement.

Osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Permeable

Allowing substances to pass through.

Protein

A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Carrier

A female with one recessive sex-linked allele who is unaffected.

Key terms

Term	Meaning
Gravimetric method	A method that uses changes in mass.
Plasmolysis	Protoplast and membrane pulling away from the cell wall due to water loss.
Incipient plasmolysis	Stage when 50% of cells show plasmolysis.
Dilution series	A set of solutions of decreasing concentration.
Calibration curve	Graph that converts a measured value into the property of interest.
Cell sap	Solution of solutes inside the vacuole.

Avoid these mistakes

Reading the answer from where the line starts instead of where it crosses the axis.

Saying the cell has the same solute concentration as the solution — the claim is about water potential.

Omitting the percentage calculation when starting masses differ.

Forgetting that the final value must be negative and in kPa.

Last updated: 15 October 2026