Osmosis — AQA GCSE Combined Science
Test yourself on Osmosis with AQA GCSE practice questions.
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Osmosis explained
Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution to a concentrated solution.
Read the full explanation
A dilute solution has a higher water potential, meaning more free water molecules per unit volume, while a concentrated solution has a lower water potential. The membrane allows small water molecules through but restricts larger solute molecules. Water therefore moves down its water potential gradient. In animal cells, water entering by osmosis can cause the cell to swell and possibly burst, while water leaving can cause shrinkage. In plant cells, water entering the vacuole increases turgor pressure, making the cell firm; water leaving causes plasmolysis. Osmosis is passive and requires no energy from respiration.
use simple compound measures of rate of water uptake
This statement requires you to use a compound measure, which is a quantity made from two other measurements, to express how quickly water enters plant tissue by osmosis. Rate of water uptake is calculated as change in mass or volume divided by time taken, giving units such as g/min, g/h, cm³/min or cm³/h. For example, if a potato cylinder gains 1.8 g in 15 minutes, the rate is 1.8 g ÷ 15 min = 0.12 g/min. You should be able to read data from a table or graph, calculate the rate, compare rates between different solution concentrations, and explain that the rate depends on the concentration gradient across the partially permeable membrane.
use percentages
A percentage expresses a quantity as a fraction of 100, so it allows fair comparison between samples of different starting sizes. In osmosis practical work, you may need to calculate what proportion of a plant tissue's original mass has been gained or lost, or express a change relative to the starting value. To use percentages, identify the whole or original value, divide the part by that whole, then multiply by 100. For example, if a potato cylinder starts at 2.0 g and ends at 2.4 g, the change is 0.4 g; as a percentage of the original mass this is (0.4 ÷ 2.0) × 100 = 20%. Always state whether the change is a gain or a loss and give the percentage with appropriate units or context.
calculate percentage gain and loss of mass of plant tissue.
In osmosis experiments, plant tissue such as potato or beetroot cylinders is weighed before and after being placed in solutions of different concentrations. The change in mass shows whether water has entered or left the cells. To calculate percentage gain or loss, first find the change in mass by subtracting the original mass from the final mass. A positive change is a gain; a negative change is a loss. Then divide the change by the original mass and multiply by 100. For example, if a cylinder starts at 5.0 g and finishes at 4.0 g, the change is −1.0 g, so the percentage loss is (1.0 ÷ 5.0) × 100 = 20%. Always report the direction of change clearly.
Students should be able to plot, draw and interpret appropriate graphs.
Graph skills underpin osmosis investigations. When plotting, place the independent variable (salt or sugar concentration) on the x-axis and the dependent variable (change in mass) on the y-axis, using a sensible scale that fills the grid. Plot points accurately with small crosses, then draw a line or curve of best fit rather than joining dots. Interpreting means describing the trend: as concentration increases, mass change falls from positive to negative, crossing zero at the isotonic point where external solution concentration matches cell contents. Read values from the graph, including the intercept, and link steepness to the rate of water movement by osmosis.
Required practical activity 2: investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.
In this required practical, cut equal-sized pieces of plant tissue such as potato or beetroot, blot them, and record initial masses. Place one piece in each of several salt or sugar solutions of different concentrations, plus a distilled water control, for a set time. Remove, blot dry and reweigh. Calculate change in mass for each piece. In dilute solutions, water enters cells by osmosis, so mass increases; in concentrated solutions, water leaves, so mass decreases. The concentration giving no change is approximately isotonic with the tissue. Control variables include tissue type, size, temperature and time. Plot change in mass against concentration and interpret the graph.
Your focus
- Define osmosis as the diffusion of water through a partially permeable membrane.
- Explain how water moves from a dilute solution to a concentrated solution.
- Describe the effects of osmosis on plant and animal cells.
Show all 18 objectives
- Calculate rate of water uptake from change in mass or volume and time, using correct compound units.
- Compare rates of water uptake between different concentrations and relate differences to concentration gradients.
- Interpret tabulated or graphical osmosis data to determine change in mass, time and rate.
- Calculate a percentage change in mass using the original mass as the denominator.
- Distinguish between percentage gain and percentage loss when interpreting osmosis data.
- Apply percentage calculations to compare results from plant tissue experiments.
- Calculate percentage gain or loss of mass from original and final masses of plant tissue.
- Interpret positive and negative mass changes in terms of water movement into or out of cells.
- Apply the percentage change method accurately to experimental osmosis data.
- Plot a graph of change in mass against concentration with correctly labelled axes and a suitable scale.
- Draw an appropriate line or curve of best fit through plotted points.
- Interpret the graph to identify the isotonic point and describe how mass change varies with concentration.
- Carry out the practical safely to obtain valid measurements of mass change in plant tissue.
- Calculate and compare changes in mass across a range of salt or sugar concentrations.
- Explain the results in terms of water movement by osmosis and identify the approximate isotonic concentration.
Osmosis exam tips
Marking Points
- Osmosis is the diffusion of water through a partially permeable membrane.
- Water moves from a dilute solution to a concentrated solution, or down a water potential gradient.
- A dilute solution has a higher water potential than a concentrated solution.
- The partially permeable membrane allows water molecules through but not larger solute molecules.
- In plant cells, water entering by osmosis increases turgor pressure and makes the cell firm; water leaving causes plasmolysis.
- In animal cells, water entering by osmosis may cause swelling or bursting, and water leaving may cause shrinkage.
- Osmosis is a passive process and does not require energy from respiration.
- Select the correct formula: rate of water uptake = change in mass or volume ÷ time taken.
- Calculate change in mass by subtracting the initial mass from the final mass, for example 5.2 g − 4.0 g = 1.2 g.
- Divide the change by the time in the stated unit, for example 1.2 g ÷ 10 min = 0.12 g/min, and include the compound unit in the answer.
- Compare rates at different concentrations and link a faster rate to a steeper concentration gradient between the external solution and the cell contents.
- Read values accurately from a graph or table, including choosing two points and finding the change in the y-value over the change in the x-value.
- Recognise that a negative rate indicates water loss by osmosis when the external solution is more concentrated than the cell contents.
- Identify the original or starting value as the whole amount before calculating any percentage.
- Calculate the change by subtracting the original value from the final value, or the final value from the original value, depending on whether mass is gained or lost.
- Divide the change by the original value, not by the final value, to find the proportion of the original mass.
- Multiply the proportion by 100 to convert it to a percentage.
- State clearly whether the result represents a percentage gain or a percentage loss.
- Use consistent units for mass throughout the calculation, such as grams for both original and final masses.
- Record the original mass of the plant tissue before placing it in the solution.
- Record the final mass after the tissue has been removed and any surface water has been blotted.
- Calculate the change in mass by subtracting the original mass from the final mass.
- Divide the change in mass by the original mass to find the proportional change.
- Multiply the proportional change by 100 to express it as a percentage.
- State whether the result is a percentage gain or a percentage loss, using the sign or wording to show direction.
- Axes correctly labelled with quantity and unit, for example 'concentration of salt solution / mol dm⁻³' on the x-axis and 'change in mass / g' on the y-axis.
- Suitable linear scale chosen so plotted points occupy at least half the grid, with evenly spaced major divisions.
- Points plotted accurately as small crosses or dots, with a line or curve of best fit drawn smoothly rather than point-to-point.
- Trend described correctly: as concentration increases, change in mass decreases from positive through zero to negative.
- Zero change in mass identified as the point where the external solution is isotonic with the plant tissue, so no net water movement.
- Values read from the graph, such as the concentration at which mass change is zero, quoted with the correct unit.
- Use plant tissue pieces of the same size, shape and type, and blot them dry before weighing to remove surface water.
- Prepare a range of salt or sugar concentrations, including distilled water as a control, and measure each concentration accurately.
- Keep temperature, soaking time and volume of solution the same for every piece to make the test fair.
- Record initial and final masses accurately, then calculate change in mass for each piece.
- Explain results using osmosis: water moves into cells in dilute solutions and out of cells in concentrated solutions.
- Identify the concentration at which change in mass is zero as approximately isotonic to the plant tissue.
Examiner Tips
- 💡Always state the direction of water movement using the terms dilute and concentrated, or higher and lower water potential.
- 💡When describing cells, link water movement to observable effects such as turgidity, plasmolysis, swelling or shrinkage.
- 💡Use the phrase partially permeable membrane to show that you understand the role of the membrane in osmosis.
- 💡Write the formula, substitute the numbers with units, then give the answer with the correct compound unit.
- 💡If a question asks you to compare rates, calculate both rates first and then state which is greater and by how much.
- 💡Link any calculated rate back to osmosis by referring to the concentration gradient and the partially permeable membrane.
- 💡Write down the original mass and final mass before doing any arithmetic so the values are clear.
- 💡Show the calculation step by step, including the division and the multiplication by 100, to gain method credit where available.
- 💡Check that your percentage is sensible: a small change in mass should give a small percentage, while a doubling of mass gives a 100% gain.
- 💡Lay out your calculation clearly: change in mass, then change divided by original mass, then multiply by 100.
- 💡Include the percentage sign and state gain or loss in your final answer, for example '20% loss'.
- 💡Use the same balance and units for both masses to avoid unnecessary conversion errors.
- 💡Check the command word: 'plot' needs accurate points, 'draw' needs a best-fit line, and 'interpret' needs a described trend with data quoted.
- 💡Quote at least one pair of values from the graph to support your description, for example the concentration where change in mass is zero.
- 💡If asked to suggest why a point is anomalous, refer to the trend and state that the point lies away from the line of best fit.
- 💡State the independent variable (concentration), dependent variable (change in mass) and at least two control variables when describing the method.
- 💡Calculate percentage change in mass if asked, using (final mass − initial mass) ÷ initial mass × 100, and include the unit %.
- 💡When explaining results, link each direction of mass change to net water movement by osmosis across the cell membrane.
Common Mistakes
- Saying that osmosis is the movement of water from a high concentration of water to a low concentration of water without mentioning the membrane. Correction: osmosis must involve a partially permeable membrane.
- Confusing osmosis with diffusion of solute particles. Correction: in osmosis, it is water molecules that move through the partially permeable membrane.
- Thinking that osmosis requires energy. Correction: osmosis is passive and relies on the water potential gradient.
- Using final mass instead of change in mass in the calculation. Correction: first work out change in mass = final mass − initial mass, then divide by time.
- Giving a rate with no unit or with a single unit such as g. Correction: a rate is a compound measure, so write g/min, g/h, cm³/min or cm³/h.
- Assuming the rate is constant throughout the experiment. Correction: the concentration gradient changes as water moves, so the rate may decrease over time; use the overall change and total time unless told otherwise.
- Dividing the change by the final mass instead of the original mass; correct this by always using the starting mass as the denominator.
- Forgetting to multiply by 100 and leaving the answer as a decimal or fraction; correct this by completing the conversion to a percentage.
- Omitting the direction of change and simply giving a percentage; correct this by stating whether the tissue gained or lost mass.
- Using the final mass as the denominator instead of the original mass; correct this by always dividing by the starting mass.
- Ignoring the sign of the change and reporting a loss as a gain; correct this by checking whether the final mass is greater or smaller than the original.
- Failing to blot excess water from the tissue before weighing, which makes the final mass too high; correct this by gently drying the surface before recording the final mass.
- Plotting concentration on the y-axis: the independent variable always goes on the x-axis, so concentration belongs on the x-axis.
- Joining points dot-to-dot: draw a single line or curve of best fit that shows the overall trend.
- Forgetting units or using inconsistent units on an axis: label each axis with quantity and unit, for example 'change in mass / g'.
- Using different-sized tissue pieces: keep size and shape constant so mass changes can be compared fairly.
- Not blotting the tissue before weighing: surface water adds mass, so blot each piece before both weighings.
- Leaving pieces in solution for different times: fix the soaking time for all pieces so the comparison is valid.