AQA GCSE Biology (8461) — Cell Biology
Equation:
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.
Step 2: Calculate the rate in g/min first.
Step 3: Convert the rate to g/hour by multiplying by 60 (since there are 60 minutes in an hour).
Equation:
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.
Step 2: Calculate the difference, then divide by the initial mass and multiply by 100.
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.
Step 2: Calculate.
Students often divide by the final mass when calculating percentage change. You must always divide the change in mass by the INITIAL (starting) mass.
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'.
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.
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.
AQA marks often accept 'selectively permeable' or 'semi-permeable' as valid synonyms for partially permeable in osmosis definitions.
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.
Put your knowledge into practice — try past paper questions for Biology
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.
