4.2.54 · Cell Membranes and Transport

Investigating Transport Processes in Plants — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 11 parts Lesson 12 of 20 1 quiz

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

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Lesson summary

4.2.5 Investigating Transport Processes in Plants — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Plan, carry out and evaluate practical investigations of osmosis, membrane permeability and water uptake using plant tissue.

Suitable tissue
No ethical issues
Independent variable
Control variables
Shape of the line
Zero-change point

Part 1 of 9Why plant tissue is used

Suitable tissue

Suitable tissue potato, beetroot, carrot and onion epidermis are cheap and easy to cut into standard sizes.

No ethical issues

No ethical issues plant tissue allows osmosis, permeability and diffusion to be investigated with simple apparatus.

Planning

Planning identify the independent, dependent and control variables, state a method, decide the number of replicates and say how results are processed.

Part 2 of 9Osmosis with potato cylinders

Prepare a range of sucrose solutions, for example 0.0 to 1.0 mol dm^-3, by serial dilution of a stock solution.

Cut potato cylinders with a cork borer and trim them to the same length, for example 40 mm, using a ruler and scalpel on a tile, taking them from the same potato.

Blot each cylinder dry, then measure and record its mass (or length) to the nearest 0.01 g.

Place cylinders in equal volumes of each solution in labelled tubes for the same time, for example 30 minutes, at the same temperature.

Remove the cylinders, blot them in the same way and measure mass or length again.

Calculate percentage change = (final − initial) ÷ initial × 100.

Independent variable

Independent variable sucrose concentration.

Dependent variable

Dependent variable percentage change in mass.

Control variables

Control variables volume of solution, time, temperature, surface area, source of potato and blotting technique.

Part 3 of 9Processing and graphing results

1Graph

Graph plot percentage change in mass (y-axis) against sucrose concentration (x-axis).

2Shape of the line

Shape of the line it decreases, because at low concentration water enters the tissue (mass gain) and at high concentration water leaves (mass loss).

3Zero-change point

Zero-change point where the line crosses zero there is no net water movement, so the solution has the same water potential as the tissue.
A calibration curve of water potential against sucrose concentration converts this to the water potential of the potato cells.

4Why percentage change

Why percentage change cylinders may start with slightly different masses.

5Reliability

Reliability calculate a mean of replicates, identify anomalies and exclude them with a reason, and use the range or error bars.

Part 4 of 9Interpreting the cylinders

Result	Meaning	Cell state
Mass increases	Solution had higher ψ than the tissue, so water entered by osmosis	Turgid, firm
No change	Solution and tissue have the same ψ	No net movement
Mass decreases	Solution had lower ψ than the tissue, so water left by osmosis	Flaccid, floppy, may be plasmolysed

Part 5 of 9Other plant transport investigations

Other plant transport investigations

Beetroot permeability

Beetroot permeability betalain pigment leaks out when membranes are damaged by high temperature or organic solvents.
Measure the colour of the surrounding water with a colorimeter after a standard treatment, where higher absorbance means a more permeable membrane.

Plasmolysis in onion epidermis

Plasmolysis in onion epidermis mount strips in solutions of different concentration and count the percentage of plasmolysed cells under the microscope.
The concentration at which 50% are plasmolysed is incipient plasmolysis, where cell water potential equals that of the solution.

Potometer

Potometer it measures water uptake by a cut shoot as a proxy for transpiration.
The shoot is cut under water and the apparatus is sealed to avoid air bubbles.
Movement of an air bubble over a measured time gives the rate of uptake.

Part 6 of 9Sources of error and improvements

Surface water

Surface water incomplete or uneven blotting adds mass, so blot each cylinder for the same time.

Variation in tissue

Variation in tissue cylinders are not identical, so use the same potato and a mean of several.

Evaporation

Evaporation long incubation allows water loss, so use covered tubes.

Timing and temperature

Timing and temperature use a digital timer and a water bath.

Precision

Precision use a balance of greater precision and more replicates.

Part 7 of 9Safety

Sharp tools

Sharp tools cut on a tile, away from the body, when using a scalpel or cork borer.

Chemicals

Chemicals wear eye protection with acids or solvents and wipe up spills.

Plant material

Plant material use only what is needed and dispose of it properly.

Risk assessment

Risk assessment state the hazard, the risk and the precaution.

Part 8 of 9More from the specification

Active Transport Mechanisms

Active transport moves ions or molecules across a selectively permeable membrane against their concentration gradient, requiring metabolic energy from ATP hydrolysis and specific transmembrane carrier proteins.

Channel vs. Carrier Facilitated Diffusion

Facilitated diffusion transports hydrophilic molecules down their concentration gradient through water-filled channel proteins or carrier proteins without requiring metabolic cellular energy.

Investigating Plant Tissue Osmosis

When plant tissue cylinders are placed in dilute or concentrated sucrose solutions, water molecules move across selectively permeable membranes by osmosis,
resulting in measurable changes in mass and cell volume.

Endocytosis and Bulk Vesicular Transport

Endocytosis is an active, energy-requiring bulk transport process where the plasma membrane invaginates around extracellular materials to form intracellular vesicles.

Active transport

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

Facilitated diffusion

Passive movement through transport proteins.

Channel protein

Hydrophilic pore for specific ions or water.

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.

Concentration gradient

The difference in concentration of a substance between two regions.

Carrier

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

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.

Part 9 of 9More on this topic

Risk assessment
- hot water, acid and a sharp blade are the main hazards.

Key terms

Term	Meaning
Independent variable	The variable that the investigator changes.
Dependent variable	The variable that is measured as the outcome.
Control variable	A variable kept constant to make a test fair.
Percentage change	(final value − initial value) ÷ initial value × 100.
Plasmolysis	Shrinking of the protoplast away from the cell wall when water leaves the cell.
Incipient plasmolysis	The point where about half of cells show plasmolysis.
Calibration curve	Graph used to convert a measurement into the quantity of interest.
Replicate	A repeat measurement used to calculate a mean.

Avoid these mistakes

Using change in mass rather than percentage change when cylinders start at different masses.

Forgetting to blot the cylinders before weighing — extra surface water changes the mass.

Saying the experiment is fair without naming the specific variables that were controlled.

Reading the water potential from the point of maximum mass gain instead of the zero-change point.

Last updated: 15 October 2026