Required practical: osmosis in plant tissue — AQA GCSE Biology Revision Notes

What you'll learn

This required practical investigates how different concentrations of sugar or salt solutions affect plant tissue through osmosis. You'll learn the complete method, how to control variables, process results using percentage change calculations, and interpret graphs to determine water potential. This practical is frequently tested in Paper 1 and Paper 2 of AQA GCSE Biology.

Key terms and definitions

Osmosis — the diffusion of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a partially permeable membrane.

Water potential — a measure of the tendency of water molecules to move from one region to another; pure water has the highest water potential.

Isotonic — a solution that has the same water concentration (water potential) as the plant tissue, resulting in no net movement of water.

Percentage change in mass — the change in mass expressed as a percentage of the original mass, calculated as: (final mass - initial mass) ÷ initial mass × 100.

Turgor pressure — the pressure of water pushing against the cell wall in a plant cell, making it rigid.

Plasmolysis — the process where the cell membrane pulls away from the cell wall as water leaves the plant cell by osmosis.

Independent variable — the variable you deliberately change in an experiment (in this practical, the concentration of the sugar/salt solution).

Dependent variable — the variable you measure in an experiment (in this practical, the change in mass of the plant tissue).

Core concepts

The standard method

The required practical follows a clear procedure that you must be able to describe and evaluate:

Preparation:

Use a cork borer to cut cylinders of potato (or other plant tissue) of the same diameter
Pat the cylinders dry with paper towels to remove surface moisture
Use a ruler to cut cylinders to the same length (typically 30mm)
Measure and record the mass of each cylinder using a balance (to at least 0.01g accuracy)

Setting up the experiment:

Prepare a range of sugar (sucrose) solutions with different concentrations, typically 0.0 M (distilled water), 0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1.0 M
Place one potato cylinder into each solution in a test tube or boiling tube
Ensure the solution completely covers the potato cylinder
Leave for a set time period (typically 20-30 minutes, though overnight gives better results)
Label each test tube clearly with the concentration

Taking measurements:

Remove each cylinder from its solution
Gently blot dry with paper towels to remove excess surface solution
Reweigh each cylinder and record the final mass
Calculate the change in mass and percentage change in mass

Variables and control

Understanding and controlling variables is essential for valid results:

Independent variable:

The concentration of sugar/salt solution
Must be varied systematically (e.g., 0.0 M, 0.2 M, 0.4 M etc.)
At least five different concentrations should be used

Dependent variable:

Change in mass of the potato cylinder
Measured by weighing before and after
Use the same balance throughout for consistency

Control variables (must be kept constant):

Volume of solution — use the same volume in each test tube (e.g., 10 cm³) to ensure cylinders are fully covered
Length/size of potato cylinders — use the same cork borer and cut to same length so surface area:volume ratio is constant
Type of plant tissue — use the same potato (or all from the same batch)
Temperature — keep all test tubes at room temperature in the same location
Time — leave all cylinders in solution for exactly the same duration
Source of potato tissue — cut all cylinders from the same region of the potato to ensure similar cell characteristics

Calculating percentage change in mass

Raw mass changes cannot be compared directly because cylinders may have slightly different starting masses. Percentage change in mass allows fair comparison:

Formula: Percentage change in mass = (final mass - initial mass) ÷ initial mass × 100

Interpreting results:

Positive percentage change — the cylinder gained mass because water moved into the cells by osmosis; the solution had higher water potential than the potato tissue
Negative percentage change — the cylinder lost mass because water moved out of the cells by osmosis; the solution had lower water potential than the potato tissue
Zero or near-zero percentage change — no net movement of water; the solution was isotonic with the potato tissue

Presenting and analysing results

Table of results: A typical results table should include:

Concentration of solution (M)
Initial mass (g)
Final mass (g)
Change in mass (g)
Percentage change in mass (%)

Graphing results:

Plot concentration (M) on the x-axis (independent variable)
Plot percentage change in mass (%) on the y-axis (dependent variable)
Draw a line of best fit (can be a smooth curve)
The graph typically shows a negative correlation — as concentration increases, percentage change in mass decreases

Finding the isotonic point:

The point where the line of best fit crosses the x-axis (where percentage change = 0%)
This concentration is isotonic with the potato tissue
It represents the concentration where the potato tissue and solution have the same water potential
Read the concentration value from the x-axis at this point

The biology behind the practical

In dilute solutions (low solute concentration):

Water concentration outside cells is higher than inside
Water moves into cells by osmosis
Cells become turgid (swollen and firm)
Mass of tissue increases
Turgor pressure increases, pushing against cell walls

In concentrated solutions (high solute concentration):

Water concentration outside cells is lower than inside
Water moves out of cells by osmosis
Cells become flaccid (limp)
Mass of tissue decreases
In very concentrated solutions, plasmolysis may occur

At isotonic concentration:

Water concentration is equal inside and outside cells
Water moves in and out at the same rate (dynamic equilibrium)
No net change in mass
Cells are neither turgid nor flaccid

Improving the practical

Understanding how to improve the method demonstrates higher-level thinking:

Accuracy improvements:

Use more concentrations (e.g., 0.1 M intervals) to plot a more detailed graph
Use a more precise balance (to 0.001g rather than 0.01g)
Take repeat measurements at each concentration and calculate means
Use a micrometer to measure diameter of cylinders more precisely
Leave cylinders in solutions for longer to allow equilibrium to be reached

Validity improvements:

Control all variables carefully
Use fresh potato tissue (stored potato may have degraded cells)
Ensure all cylinders come from the same region of the potato
Blot cylinders with the same pressure/technique each time
Keep temperature constant using a water bath

Reliability improvements:

Carry out repeats (at least three) at each concentration
Calculate mean percentage change for each concentration
Identify and exclude anomalous results
Use multiple potatoes and calculate an overall mean

Worked examples

Example 1: Calculating percentage change in mass

Question: A student places a potato cylinder with a mass of 2.50 g into a 0.4 M sucrose solution. After 30 minutes, the cylinder has a mass of 2.20 g. Calculate the percentage change in mass. [2 marks]

Answer: Change in mass = 2.20 - 2.50 = -0.30 g [1 mark]

Percentage change in mass = (-0.30 ÷ 2.50) × 100 = -12% [1 mark]

Mark scheme guidance: Award 1 mark for correct calculation of change in mass (showing subtraction), 1 mark for correct percentage calculation with formula shown or correct final answer.

Example 2: Interpreting results and explaining osmosis

Question: A student investigated osmosis in potato cylinders using different concentrations of sugar solution. The graph shows the student's results.

[Assume a graph showing positive percentage change at 0.0 M declining to negative values, crossing x-axis at 0.3 M]

(a) Describe the relationship shown by the graph. [2 marks]

(b) Explain why the potato cylinder in distilled water (0.0 M) gained mass. [3 marks]

Answers:

(a) As the concentration of sugar solution increases, the percentage change in mass decreases [1 mark]. The relationship is negative/inverse correlation [1 mark].

(b) The distilled water has a higher water concentration/water potential than the potato cells [1 mark]. Water moves into the potato cells by osmosis [1 mark]. Osmosis is the diffusion of water from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane [1 mark].

Mark scheme guidance: In part (b), students must mention water moving by osmosis and reference concentration gradient/water potential. The definition of osmosis can gain a third mark.

Example 3: Evaluating method and suggesting improvements

Question: A student carried out the osmosis practical once at each concentration. Suggest two ways the student could improve the reliability of their results. [2 marks]

Answer: Any two from:

Carry out repeats/do the experiment three times at each concentration [1 mark]
Calculate a mean for each concentration [1 mark]
Identify and remove anomalous results [1 mark]

Mark scheme guidance: Must suggest specific improvements related to reliability (repeats, means, anomalies). General statements like "be more careful" do not score.

Common mistakes and how to avoid them

Confusing osmosis with diffusion — Always define osmosis specifically as the movement of water molecules through a partially permeable membrane, not just any substance moving. Mention water explicitly in your answer.
Getting percentage change formula wrong — Remember it's (change ÷ original) × 100, not (change ÷ final) × 100. The original/initial mass is always the denominator. Show your working clearly.
Saying water moves "from high to low concentration" — This is imprecise. Say water moves from a region of higher water concentration to a region of lower water concentration (or from higher to lower water potential). Be specific about what is moving.
Misidentifying the isotonic point — The isotonic point is where percentage change = 0 (where the line crosses the x-axis), not where the line crosses the y-axis. Read the concentration from the x-axis at this point.
Not controlling variables — In evaluation questions, identify specific control variables (volume, temperature, time, length of cylinder) rather than vague statements like "keep everything the same."
Forgetting to blot cylinders dry — Both before initial weighing and after removing from solution. Surface water will affect mass measurements and reduce accuracy. Explain this in method evaluation questions.

Exam technique for "Required practical: osmosis in plant tissue"

Command word "Calculate" — You must show your working clearly. Write out the formula, substitute values, and show each step. Marks are often awarded for correct method even if final answer is wrong. Always include units in your final answer.
Command word "Explain" — Link cause and effect using scientific reasoning. For osmosis questions, state the water concentration gradient, the direction of water movement, the process (osmosis), and the effect on mass/cells. Aim for 3-4 developed points for 3-4 marks.
Variables questions — Be specific. Name the actual variable (e.g., "concentration of sugar solution" not just "concentration") and state how you would control it (e.g., "use a measuring cylinder to measure 10 cm³ of solution in each tube").
Graph interpretation — Always quote data from the graph when describing patterns. Use the exact values shown. When finding the isotonic point, draw a clear line on the graph to show where percentage change = 0, then read across to the x-axis.

Quick revision summary

This required practical investigates osmosis by placing potato cylinders in different concentrations of sugar solution. Calculate percentage change in mass using (change ÷ initial mass) × 100. Positive values show water moved into cells (dilute solution), negative values show water moved out (concentrated solution). The isotonic point, where percentage change equals zero, shows the solution concentration matching the potato's water potential. Control variables include volume, temperature, time, and cylinder size. Plot concentration against percentage change to analyse results. Repeats and means improve reliability.