Osmosis

What is Osmosis?

If you leave a wilted carrot in a glass of water, it becomes crisp again within a few hours. This happens because of a fundamental biological mechanism called osmosis.
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
It is a passive process, meaning it requires NO energy from respiration.
Water molecules move randomly in both directions across the membrane, but the net movement is always towards the more concentrated solution until equilibrium is reached.

Osmosis in Animal and Plant Cells

A partially permeable membrane has tiny pores that allow small molecules like water to pass, but completely block larger solute molecules (like sugar or salt).
Animal cells (such as red blood cells) do NOT have a rigid cell wall.
If placed in pure water or a dilute solution (hypotonic), animal cells gain water, swell, and will burst (lyse). If placed in a highly concentrated solution (hypertonic), they lose water and shrivel (crenate).
Plant cells possess a strong cell wall that prevents bursting.
In a dilute solution, water enters the plant cell, causing the vacuole to swell and push the cytoplasm against the cell wall until the cell becomes firm and turgid.
In a concentrated solution, plant cells lose water, the vacuole shrinks, and the cell membrane pulls away from the cell wall, leaving the cell plasmolysed.
If a cell is placed in an isotonic solution, the internal and external concentrations are identical, resulting in zero net movement of water.

Required Practical 3: Investigating Osmosis

First, use a cork borer to cut several potato cylinders to ensure they all have the exact same diameter, standardising the surface area to volume ratio.
Next, trim the cylinders to the same length (e.g., 3 cm) using a scalpel and ruler, and ensure the potato skin is removed because it acts as a barrier to osmosis.
Then, record the initial mass of each cylinder using a highly precise top-pan balance.
Place the cylinders into a range of at least five different concentrations of sugar or salt solution, ensuring you include a distilled water (0.0 M) control because it contains NO dissolved substances.
After a set time, remove the cylinders and gently blot them dry by rolling them on a paper towel. This crucial step removes excess surface water which would otherwise cause an overestimate of the final mass.
Finally, reweigh the cylinders to find their final mass.
In this experiment, the independent variable is the solution concentration, the dependent variable is the percentage change in mass, and the control variables include temperature, time left in the solution, and volume of the solution.

Calculating the Rate of Water Uptake

You must be able to calculate simple compound measures for the rate of water uptake, such as grams per minute (g/min) or cubic centimetres per hour (cm³/hr).
You may also be asked to identify percentiles in transport data, determining the value below which a specific percentage of observations fall.

Equation:

\text{Rate} = \frac{\text{Change in Mass}}{\text{Time}}

Worked Example:

A plant tissue gains 0.24 g of mass over 30 minutes. Calculate the rate of water uptake in g/hour.

Step 1: Identify the values and the required units.

Change in mass = $0.24\text{ g}$
Time = $30\text{ minutes}$

Step 2: Calculate the rate in g/min first.

$\text{Rate} = \frac{0.24}{30} = 0.008\text{ g/min}$

Step 3: Convert the rate to g/hour by multiplying by 60 (since there are 60 minutes in an hour).

$0.008 \times 60 = 0.48\text{ g/hour}$

Calculating Percentage Change in Mass

Biologists calculate the percentage change in mass rather than the actual mass change because the potato cylinders may have had slightly different starting masses.
Using a percentage allows for a mathematically valid comparison between the samples.

Equation:

\text{Percentage Change} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100

Worked Example (Mass Gain):

A potato cylinder has an initial mass of 2.40 g and a final mass of 2.76 g. Calculate the percentage change.

Step 1: Substitute the values into the equation.

$\frac{2.76 - 2.40}{2.40} \times 100$

Step 2: Calculate the difference, then divide by the initial mass and multiply by 100.

$\frac{0.36}{2.40} \times 100 = +15\%$ (The positive sign indicates water moved INTO the plant tissue.)

Worked Example (Mass Loss):

A potato cylinder has an initial mass of 2.40 g and a final mass of 2.00 g. Calculate the percentage change.

Step 1: Substitute the values into the equation.

$\frac{2.00 - 2.40}{2.40} \times 100$

Step 2: Calculate.

$\frac{-0.40}{2.40} \times 100 = -16.67\%$ (The negative sign indicates water moved OUT of the plant tissue.)

Graphing and Interpreting Osmosis Data

When plotting osmosis data, the concentration of the solution goes on the x-axis, and the percentage change in mass goes on the y-axis.
Because cylinders can lose mass, your y-axis must include a negative scale extending below the x-axis.
A positive percentage change plotted above the x-axis shows that the external solution was more dilute than the plant tissue.
A negative percentage change plotted below the x-axis shows that the external solution was more concentrated than the plant tissue.
The isotonic point is exactly where your line of best fit crosses the x-axis (the x-intercept).
At this exact concentration, the percentage change in mass is zero, meaning the external solution matches the internal concentration of the plant cell sap.

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Exam Tips

Common Mistake
Students often divide by the final mass when calculating percentage change. You must always divide the change in mass by the INITIAL (starting) mass.
2
When defining osmosis, you will lose marks if you simply say 'water moves from a high to low concentration' — you must explicitly specify a 'dilute to concentrated solution' or 'high to low water concentration'.
3
In 6-mark methodology questions for Required Practical 3, examiners specifically look for you to state that the potato cylinder must be 'blotted dry' to remove excess surface water before the final weighing.
4
Always include the negative sign (-) when recording mass loss in your tables or calculations to clearly indicate the net movement of water out of the tissue.
5
AQA marks often accept 'selectively permeable' or 'semi-permeable' as valid synonyms for partially permeable in osmosis definitions.

Key Terms(22)

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution: A solution containing a high concentration of water molecules and a low concentration of dissolved solute.
Concentrated solution: A solution containing a low concentration of water molecules and a high concentration of dissolved solute.
Partially permeable membrane: A membrane that allows only certain small molecules (like water) to pass through it, while blocking larger solute molecules.
Passive process: A cellular process that happens naturally down a concentration gradient and requires no energy from respiration.
Net movement: The overall, cumulative direction of particle movement from an area of higher concentration to lower concentration.
Hypotonic: A solution with a lower solute concentration (more dilute) than the inside of the cell.
Lyse: The bursting of an animal cell when placed in a dilute solution due to the intake of too much water by osmosis.
Hypertonic: A solution with a higher solute concentration than the inside of the cell.
Crenate: The shrivelling of an animal cell when placed in a concentrated solution due to water loss by osmosis.
Turgid: A firm plant cell where the vacuole is swollen with water, pushing the cytoplasm hard against the cell wall.
Plasmolysed: A plant cell that has lost water, causing the vacuole to shrink and the cell membrane to pull away from the cell wall.
Isotonic: A solution that has the exact same solute concentration as the inside of a cell, resulting in no net movement of water.
Cork borer: A metal tool used to cut uniform cylinders of plant tissue to standardise the surface area.
Top-pan balance: A piece of highly precise digital equipment used to measure the mass of an object.
Distilled water: Pure water that contains no dissolved substances, often used as a control in osmosis experiments.
Independent variable: The variable that you actively alter or change in an experiment (e.g., the concentration of the sugar solution).
Dependent variable: The variable that you measure as the outcome of an experiment (e.g., the percentage change in mass).
Control variables: The variables that must be kept exactly the same to ensure a fair test (e.g., temperature and time).
Percentiles: A statistical value indicating the point below which a specific percentage of observations in a group of data fall.
Isotonic point: The specific internal concentration of a plant cell, found on a graph exactly where the line of best fit crosses the x-axis.
Line of best fit: A straight line or smooth curve drawn through the centre of data points on a scatter graph to show the general trend.

Previous Subtopic: DiffusionAdaptations of Exchange Surfaces in Plants and Animals Next Subtopic: Active TransportActive Transport

Back to Osmosis

Put your knowledge into practice — try past paper questions for Biology

Key Terms(22)

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution: A solution containing a high concentration of water molecules and a low concentration of dissolved solute.
Concentrated solution: A solution containing a low concentration of water molecules and a high concentration of dissolved solute.
Partially permeable membrane: A membrane that allows only certain small molecules (like water) to pass through it, while blocking larger solute molecules.
Passive process: A cellular process that happens naturally down a concentration gradient and requires no energy from respiration.
Net movement: The overall, cumulative direction of particle movement from an area of higher concentration to lower concentration.
Hypotonic: A solution with a lower solute concentration (more dilute) than the inside of the cell.
Lyse: The bursting of an animal cell when placed in a dilute solution due to the intake of too much water by osmosis.
Hypertonic: A solution with a higher solute concentration than the inside of the cell.
Crenate: The shrivelling of an animal cell when placed in a concentrated solution due to water loss by osmosis.
Turgid: A firm plant cell where the vacuole is swollen with water, pushing the cytoplasm hard against the cell wall.
Plasmolysed: A plant cell that has lost water, causing the vacuole to shrink and the cell membrane to pull away from the cell wall.
Isotonic: A solution that has the exact same solute concentration as the inside of a cell, resulting in no net movement of water.
Cork borer: A metal tool used to cut uniform cylinders of plant tissue to standardise the surface area.
Top-pan balance: A piece of highly precise digital equipment used to measure the mass of an object.
Distilled water: Pure water that contains no dissolved substances, often used as a control in osmosis experiments.
Independent variable: The variable that you actively alter or change in an experiment (e.g., the concentration of the sugar solution).
Dependent variable: The variable that you measure as the outcome of an experiment (e.g., the percentage change in mass).
Control variables: The variables that must be kept exactly the same to ensure a fair test (e.g., temperature and time).
Percentiles: A statistical value indicating the point below which a specific percentage of observations in a group of data fall.
Isotonic point: The specific internal concentration of a plant cell, found on a graph exactly where the line of best fit crosses the x-axis.
Line of best fit: A straight line or smooth curve drawn through the centre of data points on a scatter graph to show the general trend.

Osmosis

What is Osmosis?

If you leave a wilted carrot in a glass of water, it becomes crisp again within a few hours. This happens because of a fundamental biological mechanism called osmosis.
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
It is a passive process, meaning it requires NO energy from respiration.
Water molecules move randomly in both directions across the membrane, but the net movement is always towards the more concentrated solution until equilibrium is reached.

Osmosis in Animal and Plant Cells

A partially permeable membrane has tiny pores that allow small molecules like water to pass, but completely block larger solute molecules (like sugar or salt).
Animal cells (such as red blood cells) do NOT have a rigid cell wall.
If placed in pure water or a dilute solution (hypotonic), animal cells gain water, swell, and will burst (lyse). If placed in a highly concentrated solution (hypertonic), they lose water and shrivel (crenate).
Plant cells possess a strong cell wall that prevents bursting.
In a dilute solution, water enters the plant cell, causing the vacuole to swell and push the cytoplasm against the cell wall until the cell becomes firm and turgid.
In a concentrated solution, plant cells lose water, the vacuole shrinks, and the cell membrane pulls away from the cell wall, leaving the cell plasmolysed.
If a cell is placed in an isotonic solution, the internal and external concentrations are identical, resulting in zero net movement of water.

Required Practical 3: Investigating Osmosis

First, use a cork borer to cut several potato cylinders to ensure they all have the exact same diameter, standardising the surface area to volume ratio.
Next, trim the cylinders to the same length (e.g., 3 cm) using a scalpel and ruler, and ensure the potato skin is removed because it acts as a barrier to osmosis.
Then, record the initial mass of each cylinder using a highly precise top-pan balance.
Place the cylinders into a range of at least five different concentrations of sugar or salt solution, ensuring you include a distilled water (0.0 M) control because it contains NO dissolved substances.
After a set time, remove the cylinders and gently blot them dry by rolling them on a paper towel. This crucial step removes excess surface water which would otherwise cause an overestimate of the final mass.
Finally, reweigh the cylinders to find their final mass.
In this experiment, the independent variable is the solution concentration, the dependent variable is the percentage change in mass, and the control variables include temperature, time left in the solution, and volume of the solution.

Calculating the Rate of Water Uptake

You must be able to calculate simple compound measures for the rate of water uptake, such as grams per minute (g/min) or cubic centimetres per hour (cm³/hr).
You may also be asked to identify percentiles in transport data, determining the value below which a specific percentage of observations fall.

Equation:

\text{Rate} = \frac{\text{Change in Mass}}{\text{Time}}

Worked Example:

A plant tissue gains 0.24 g of mass over 30 minutes. Calculate the rate of water uptake in g/hour.

Step 1: Identify the values and the required units.

Change in mass = $0.24\text{ g}$
Time = $30\text{ minutes}$

Step 2: Calculate the rate in g/min first.

$\text{Rate} = \frac{0.24}{30} = 0.008\text{ g/min}$

Step 3: Convert the rate to g/hour by multiplying by 60 (since there are 60 minutes in an hour).

$0.008 \times 60 = 0.48\text{ g/hour}$

Calculating Percentage Change in Mass

Biologists calculate the percentage change in mass rather than the actual mass change because the potato cylinders may have had slightly different starting masses.
Using a percentage allows for a mathematically valid comparison between the samples.

Equation:

\text{Percentage Change} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100

Worked Example (Mass Gain):

A potato cylinder has an initial mass of 2.40 g and a final mass of 2.76 g. Calculate the percentage change.

Step 1: Substitute the values into the equation.

$\frac{2.76 - 2.40}{2.40} \times 100$

Step 2: Calculate the difference, then divide by the initial mass and multiply by 100.

$\frac{0.36}{2.40} \times 100 = +15\%$ (The positive sign indicates water moved INTO the plant tissue.)

Worked Example (Mass Loss):

A potato cylinder has an initial mass of 2.40 g and a final mass of 2.00 g. Calculate the percentage change.

Step 1: Substitute the values into the equation.

$\frac{2.00 - 2.40}{2.40} \times 100$

Step 2: Calculate.

$\frac{-0.40}{2.40} \times 100 = -16.67\%$ (The negative sign indicates water moved OUT of the plant tissue.)

Graphing and Interpreting Osmosis Data

When plotting osmosis data, the concentration of the solution goes on the x-axis, and the percentage change in mass goes on the y-axis.
Because cylinders can lose mass, your y-axis must include a negative scale extending below the x-axis.
A positive percentage change plotted above the x-axis shows that the external solution was more dilute than the plant tissue.
A negative percentage change plotted below the x-axis shows that the external solution was more concentrated than the plant tissue.
The isotonic point is exactly where your line of best fit crosses the x-axis (the x-intercept).
At this exact concentration, the percentage change in mass is zero, meaning the external solution matches the internal concentration of the plant cell sap.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often divide by the final mass when calculating percentage change. You must always divide the change in mass by the INITIAL (starting) mass.
2
When defining osmosis, you will lose marks if you simply say 'water moves from a high to low concentration' — you must explicitly specify a 'dilute to concentrated solution' or 'high to low water concentration'.
3
In 6-mark methodology questions for Required Practical 3, examiners specifically look for you to state that the potato cylinder must be 'blotted dry' to remove excess surface water before the final weighing.
4
Always include the negative sign (-) when recording mass loss in your tables or calculations to clearly indicate the net movement of water out of the tissue.
5
AQA marks often accept 'selectively permeable' or 'semi-permeable' as valid synonyms for partially permeable in osmosis definitions.

Key Terms(22)

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution: A solution containing a high concentration of water molecules and a low concentration of dissolved solute.
Concentrated solution: A solution containing a low concentration of water molecules and a high concentration of dissolved solute.
Partially permeable membrane: A membrane that allows only certain small molecules (like water) to pass through it, while blocking larger solute molecules.
Passive process: A cellular process that happens naturally down a concentration gradient and requires no energy from respiration.
Net movement: The overall, cumulative direction of particle movement from an area of higher concentration to lower concentration.
Hypotonic: A solution with a lower solute concentration (more dilute) than the inside of the cell.
Lyse: The bursting of an animal cell when placed in a dilute solution due to the intake of too much water by osmosis.
Hypertonic: A solution with a higher solute concentration than the inside of the cell.
Crenate: The shrivelling of an animal cell when placed in a concentrated solution due to water loss by osmosis.
Turgid: A firm plant cell where the vacuole is swollen with water, pushing the cytoplasm hard against the cell wall.
Plasmolysed: A plant cell that has lost water, causing the vacuole to shrink and the cell membrane to pull away from the cell wall.
Isotonic: A solution that has the exact same solute concentration as the inside of a cell, resulting in no net movement of water.
Cork borer: A metal tool used to cut uniform cylinders of plant tissue to standardise the surface area.
Top-pan balance: A piece of highly precise digital equipment used to measure the mass of an object.
Distilled water: Pure water that contains no dissolved substances, often used as a control in osmosis experiments.
Independent variable: The variable that you actively alter or change in an experiment (e.g., the concentration of the sugar solution).
Dependent variable: The variable that you measure as the outcome of an experiment (e.g., the percentage change in mass).
Control variables: The variables that must be kept exactly the same to ensure a fair test (e.g., temperature and time).
Percentiles: A statistical value indicating the point below which a specific percentage of observations in a group of data fall.
Isotonic point: The specific internal concentration of a plant cell, found on a graph exactly where the line of best fit crosses the x-axis.
Line of best fit: A straight line or smooth curve drawn through the centre of data points on a scatter graph to show the general trend.

Previous Subtopic: DiffusionAdaptations of Exchange Surfaces in Plants and Animals Next Subtopic: Active TransportActive Transport

Back to Osmosis

Put your knowledge into practice — try past paper questions for Biology

Key Terms(22)

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Dilute solution: A solution containing a high concentration of water molecules and a low concentration of dissolved solute.
Concentrated solution: A solution containing a low concentration of water molecules and a high concentration of dissolved solute.
Partially permeable membrane: A membrane that allows only certain small molecules (like water) to pass through it, while blocking larger solute molecules.
Passive process: A cellular process that happens naturally down a concentration gradient and requires no energy from respiration.
Net movement: The overall, cumulative direction of particle movement from an area of higher concentration to lower concentration.
Hypotonic: A solution with a lower solute concentration (more dilute) than the inside of the cell.
Lyse: The bursting of an animal cell when placed in a dilute solution due to the intake of too much water by osmosis.
Hypertonic: A solution with a higher solute concentration than the inside of the cell.
Crenate: The shrivelling of an animal cell when placed in a concentrated solution due to water loss by osmosis.
Turgid: A firm plant cell where the vacuole is swollen with water, pushing the cytoplasm hard against the cell wall.
Plasmolysed: A plant cell that has lost water, causing the vacuole to shrink and the cell membrane to pull away from the cell wall.
Isotonic: A solution that has the exact same solute concentration as the inside of a cell, resulting in no net movement of water.
Cork borer: A metal tool used to cut uniform cylinders of plant tissue to standardise the surface area.
Top-pan balance: A piece of highly precise digital equipment used to measure the mass of an object.
Distilled water: Pure water that contains no dissolved substances, often used as a control in osmosis experiments.
Independent variable: The variable that you actively alter or change in an experiment (e.g., the concentration of the sugar solution).
Dependent variable: The variable that you measure as the outcome of an experiment (e.g., the percentage change in mass).
Control variables: The variables that must be kept exactly the same to ensure a fair test (e.g., temperature and time).
Percentiles: A statistical value indicating the point below which a specific percentage of observations in a group of data fall.
Isotonic point: The specific internal concentration of a plant cell, found on a graph exactly where the line of best fit crosses the x-axis.
Line of best fit: A straight line or smooth curve drawn through the centre of data points on a scatter graph to show the general trend.