Osmosis — AQA GCSE Biology
Test yourself on Osmosis with AQA GCSE practice questions.
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Osmosis explained
Cell membranes are partially permeable, so water can cross them by osmosis while most dissolved solutes cannot.
Read the full explanation
Osmosis is passive: it needs no energy from respiration and happens because water molecules move randomly, with a net movement from the side with a higher water concentration (dilute solution) to the side with a lower water concentration (concentrated solution). In a plant root hair cell, soil water is usually more dilute than the cytoplasm and vacuole sap, so water enters and travels to the xylem. In an animal cell placed in pure water, water enters, the cell may swell and burst; in a concentrated salt solution, water leaves and the cell shrinks. Plant cells change less because the cellulose cell wall resists expansion, making the cell turgid, or pulls away from the wall when water is lost, causing plasmolysis.
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
This statement gives the formal definition of osmosis. Water molecules move randomly in all directions, but a partially permeable membrane lets small water molecules pass while blocking larger solute molecules. If one side is a dilute solution and the other a concentrated solution, more water crosses from the dilute side to the concentrated side than in the reverse direction, producing a net movement of water. The dilute solution has the higher water concentration; the concentrated solution has the lower water concentration. Osmosis is therefore a special case of diffusion and is passive. For example, a potato cylinder in distilled water gains mass because water enters its cells, while a cylinder in strong sugar solution loses mass because water leaves.
Students should be able to: • use simple compound measures of rate of water uptake • use percentages • calculate percentage gain and loss of mass of plant tissue.
Osmosis practical work produces numerical data that must be processed. Rate of water uptake is a compound measure: divide the volume of water taken up by the time taken, for example 12 cm³ ÷ 4 minutes = 3 cm³ per minute. Percentage change in mass compares the change with the starting mass: percentage change = (change in mass ÷ initial mass) × 100. A potato cylinder starting at 5.0 g and finishing at 5.8 g gains 0.8 g, so the percentage gain is (0.8 ÷ 5.0) × 100 = 16%. A cylinder falling from 5.0 g to 4.5 g loses 0.5 g, so the percentage loss is (0.5 ÷ 5.0) × 100 = 10%. Percentage change allows fair comparison between samples of different starting masses.
Students should be able to plot, draw and interpret appropriate graphs.
Osmosis data are often displayed as graphs. When plotting, put the independent variable on the x-axis, such as concentration of sugar solution in mol/dm³, and the dependent variable on the y-axis, such as percentage change in mass. Choose a sensible scale, label both axes with quantity and unit, and plot points accurately. Draw either a line of best fit or a smooth curve as appropriate; do not join points with jagged straight segments unless the question asks for it. Interpreting the graph means describing the trend: as sugar concentration increases, percentage change in mass falls from positive to negative. The point where the line crosses the x-axis shows the concentration at which there is no change in mass, because water enters and leaves at equal rates.
Required practical activity 3: investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.
This required practical tests osmosis in plant tissue by measuring mass change after immersion in a range of salt or sugar concentrations. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. You prepare equal-sized tissue samples, blot them, record initial masses, immerse them in labelled solutions for a set time, then blot and reweigh. Calculate the percentage change in mass. In a dilute solution, water enters cells by osmosis, so mass increases; in a concentrated solution, water leaves, so mass decreases. Plotting the results helps identify the concentration where there is no net movement of water.
AT skills covered by this practical activity: AT 1, 3 and 5.
This practical develops three working scientifically skills. AT 1 is the use of appropriate apparatus to make measurements, here a balance for mass and a measuring cylinder for solution volume. AT 3 involves using appropriate apparatus and techniques to observe and measure biological changes, such as the change in mass of plant tissue due to osmosis. AT 5 is the measurement of rates of water uptake, which can be determined by calculating the change in mass over the specific time the tissue was submerged. You should also evaluate sources of error, for example incomplete blotting or evaporation, and suggest improvements such as repeats and consistent timing to ensure valid measurements of these biological processes.
Your focus
- Define osmosis in terms of water movement through a partially permeable membrane from a dilute to a concentrated solution.
- Predict the direction of water movement across a cell membrane from given solution concentrations.
- Explain the effects of water gain or loss on plant cells, including turgidity and plasmolysis.
Show all 18 objectives
- State the definition of osmosis including water, a partially permeable membrane, and movement from a dilute to a concentrated solution.
- Distinguish osmosis from the diffusion of solutes.
- Use the definition to explain observed mass changes in plant tissue experiments.
- Calculate the rate of water uptake from volume and time data.
- Calculate percentage gain or loss in mass of plant tissue using initial mass.
- Interpret percentage changes in mass in terms of water movement by osmosis.
- Plot osmosis data with correctly labelled axes, suitable scales and accurate points.
- Draw an appropriate line of best fit or smooth curve through plotted data.
- Interpret a graph of percentage change in mass against concentration to identify trends and the no-change intercept.
- Carry out a practical to measure mass change in plant tissue across a range of salt or sugar concentrations.
- Calculate percentage changes in mass to identify water uptake and water loss.
- Interpret results in terms of osmosis, using the concepts of dilute and concentrated solutions.
- Use appropriate apparatus to measure mass and volume accurately in an osmosis investigation (AT 1).
- Apply techniques to observe and measure biological changes, such as mass changes in plant tissue (AT 3).
- Measure and calculate the rate of water uptake over time to evaluate osmosis (AT 5).
Osmosis exam tips
Marking Points
- States that cell membranes are partially permeable, allowing water through but restricting most solutes.
- Defines osmosis as the net diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
- Explains that osmosis is passive and results from the random movement of water molecules, not from active transport.
- Uses the idea of water concentration or water potential gradient to predict the direction of water movement.
- Applies the process to named cells, such as a root hair cell absorbing soil water or a red blood cell in distilled water.
- Describes plant cell outcomes, including turgidity when water enters and plasmolysis when water leaves.
- Identifies the substance that moves in osmosis as water, not dissolved solutes.
- States that the membrane involved is partially permeable, so water passes through but larger solute particles do not.
- Describes the direction of net water movement as from a dilute solution to a concentrated solution.
- Links dilute and concentrated solutions to higher and lower water concentrations respectively.
- Explains that osmosis is a form of diffusion and is passive, requiring no energy from respiration.
- Applies the definition to a practical context, such as mass changes in potato tissue placed in different sugar solutions.
- Calculates rate of water uptake by dividing the volume of water taken up by the time taken, with correct units such as cm³ per minute.
- Calculates percentage change in mass using the formula (change in mass ÷ initial mass) × 100.
- Distinguishes percentage gain from percentage loss by the sign or direction of the mass change.
- Uses initial mass, not final mass, as the denominator when calculating percentage change.
- Interprets positive percentage change as water entering the tissue by osmosis and negative percentage change as water leaving.
- Compares percentage changes between samples to identify the solution concentration at which mass remains unchanged.
- Plots the independent variable on the x-axis and the dependent variable on the y-axis with correct labels and units.
- Chooses an even scale that uses the available grid and plots points accurately.
- Draws an appropriate line of best fit or smooth curve rather than joining every point with straight lines.
- Describes the trend, such as percentage change in mass decreasing as solute concentration increases.
- Identifies the intercept where the graph crosses the x-axis as the concentration with no net change in mass.
- Uses the graph to predict values, such as the percentage change in mass at a stated concentration.
- State that osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
- Describe preparing equal-sized samples of plant tissue, blotting them to remove surface water, and recording initial mass.
- Explain that samples are immersed in a range of salt or sugar concentrations for a controlled time, then removed, blotted and reweighed.
- Calculate percentage change in mass for each sample to allow valid comparisons between pieces of different starting masses.
- Interpret a graph of mass change: positive values show water uptake, negative values show water loss, and zero indicates no net movement.
- Identify and control key variables such as temperature, immersion time and blotting method to make the investigation valid.
- AT 1: Selects and uses appropriate apparatus, such as a balance for mass and a measuring cylinder for solution volume, to collect valid measurements.
- AT 3: Uses techniques to measure biological changes, such as accurately determining the initial and final mass of plant tissue to find the change in mass.
- AT 5: Measures the rate of water uptake by calculating the change in mass of the tissue over the duration of the experiment.
- Plots a graph of mean change in mass against concentration, using suitable scales and labels, and describes the trend.
- Interprets the graph to identify the concentration at which there is no net change in mass and relates this to the water concentration inside the cells.
- Evaluates the method by identifying sources of error, such as surface water, and suggests improvements.
Examiner Tips
- 💡Define osmosis using all three required ideas: water, partially permeable membrane, and movement from dilute to concentrated solution.
- 💡When predicting direction, compare water concentrations rather than solute concentrations, then state which side gains water.
- 💡Link each answer to the specific cell named in the question, such as a root hair cell or a plant leaf cell, instead of describing osmosis only in general terms.
- 💡Learn the definition as a complete sentence containing water, partially permeable membrane, and dilute-to-concentrated direction.
- 💡In application questions, name the dilute and concentrated sides before stating the direction of water movement.
- 💡Use the phrase net movement of water to show that water crosses both ways but the overall movement is in one direction.
- 💡Write the formula, substitute the values, then give the answer with the correct unit or percentage sign.
- 💡Show whether a mass change is a gain or a loss, and keep the sign consistent with the direction of water movement.
- 💡For rate questions, check that the time unit in the answer matches the time unit given in the question.
- 💡Label each axis with both the quantity and its unit before plotting any points.
- 💡Use a sharp pencil and small crosses for plotted points so the examiner can see exactly where each point lies.
- 💡When describing a graph, quote data from it, such as the value at a stated concentration or the x-axis intercept.
- 💡When describing the method, include how you control variables such as temperature and time, because these affect water movement.
- 💡When interpreting results, refer to the concentration of the solutions rather than water potential, and link mass increase to water entering cells by osmosis.
- 💡When asked about AT 1, name the apparatus (e.g., balance) and explain why it is suitable for the measurement being taken.
- 💡For AT 3, describe the techniques used to measure biological changes, such as carefully blotting the potato cylinders before weighing.
- 💡For AT 5, explain how to calculate the rate of water uptake by dividing the change in mass by the time the tissue was submerged.
Common Mistakes
- Saying that osmosis moves solutes rather than water; correct this by stating that osmosis is the movement of water molecules only.
- Describing osmosis as requiring energy from respiration; correct this by stating that osmosis is passive and driven by random molecular motion down a water concentration gradient.
- Claiming that water moves from a concentrated solution to a dilute solution; correct this by stating that water moves from the dilute solution to the concentrated solution.
- Defining osmosis as the movement of water from a concentrated solution to a dilute solution; correct this by stating that water moves from the dilute solution to the concentrated solution.
- Stating that solutes diffuse through the partially permeable membrane during osmosis; correct this by stating that only water molecules move through the membrane in osmosis.
- Describing the membrane as fully permeable or impermeable; correct this by stating that it is partially permeable, allowing water through but not larger solute molecules.
- Dividing by final mass instead of initial mass when finding percentage change; correct this by always dividing the change in mass by the starting mass.
- Forgetting to multiply by 100 and reporting a decimal as a percentage; correct this by completing the calculation with × 100 and adding the percentage sign.
- Calculating rate of water uptake as volume multiplied by time; correct this by dividing the volume taken up by the time taken.
- Swapping the axes so the dependent variable is on the x-axis; correct this by placing the variable you measured as a result on the y-axis.
- Joining plotted points with separate straight lines when a line of best fit or smooth curve is expected; correct this by drawing a single trend line or curve.
- Omitting units from axis labels; correct this by writing the quantity and its unit, such as percentage change in mass (%) or concentration (mol/dm³).
- Confusing osmosis with diffusion of solute particles; correct this by stating that osmosis specifically involves water movement from a dilute to a concentrated solution.
- Forgetting to blot samples before weighing; correct this by blotting gently and consistently to remove surface water that would otherwise affect mass readings.
- Using A-level terminology like water potential; correct this by using GCSE terms such as dilute and concentrated solutions.
- Recording masses without units; correct this by including grams (g) in column headings and throughout calculations.
- Plotting individual results without calculating a mean; correct this by repeating measurements and using the mean to reduce random error.
- Ignoring anomalous results; correct this by identifying outliers and considering whether to repeat or exclude them with justification.