Rate of photosynthesis — AQA GCSE Combined Science
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Rate of photosynthesis explained
The rate of photosynthesis is affected by limiting factors.
Read the full explanation
As light intensity, carbon dioxide concentration or chlorophyll amount increases, the rate rises until another factor becomes limiting, then the graph plateaus. Temperature affects enzyme-controlled reactions: the rate increases with temperature up to an optimum, then falls sharply as enzymes denature. Chlorophyll amount matters because it absorbs light; fewer chloroplasts or less chlorophyll means less light absorbed. A typical investigation uses pondweed and counts oxygen bubbles per minute at different distances from a lamp, keeping other variables constant. Explain each factor by linking it to the supply of energy or reactants, and identify the limiting factor from the shape of the graph.
measure and calculate rates of photosynthesis
This statement requires you to both collect data from a photosynthesis experiment and turn that data into a rate. A common method uses pondweed in water with a light source at a set distance. As the pondweed photosynthesises, oxygen bubbles are released; you count the bubbles in a fixed time, or collect the gas in a syringe and read the volume. The rate is then the change divided by the time, for example 30 bubbles ÷ 2 minutes = 15 bubbles per minute, or 4.8 cm³ ÷ 4 minutes = 1.2 cm³ per minute. To make the test valid, keep temperature and carbon dioxide concentration constant while changing light intensity, and allow the pondweed to settle before timing. Repeating readings and calculating a mean improves reliability. You should also be able to compare rates at different light intensities and explain the pattern using the idea that light is a limiting factor.
extract and interpret graphs of photosynthesis rate involving one limiting factor
Photosynthesis rate depends on light intensity, carbon dioxide concentration and temperature. When one factor is limiting, increasing it raises the rate until another factor becomes limiting, so the graph plateaus. To extract information, read the axis labels and units, then read a value from the curve at a stated condition. To interpret, describe the trend: as the variable on the x-axis increases, rate rises steeply at first, then levels off. The plateau shows the named factor is no longer limiting; another factor, such as temperature or carbon dioxide concentration, has become limiting. For example, on a rate against light intensity graph at 20 °C, the curve rises then flattens; adding more light cannot increase rate because carbon dioxide concentration or temperature now limits it. Compare curves at different conditions to identify which factor limits.
plot and draw appropriate graphs selecting appropriate scale for axes
Plotting photosynthesis data requires choosing axes, scales and a graph type that show the relationship clearly. Put the independent variable, such as light intensity or carbon dioxide concentration, on the x-axis and the dependent variable, such as rate of photosynthesis or volume of oxygen produced, on the y-axis. Choose a scale that uses most of the grid: each major square should represent a sensible amount, such as 1, 2, 5 or 10 units, not 3 or 7. Label each axis with the quantity and unit. Plot points accurately with small crosses, then draw a smooth line of best fit or join points with straight lines only if instructed. For example, if light intensity ranges from 0 to 50 arbitrary units and rate ranges from 0 to 20 mm³ min⁻¹, use 10 units per major square on x and 5 units per major square on y.
translate information between graphical and numeric form.
This skill means moving between a graph and the numbers it represents, in either direction. From a graph, read a value off an axis at a chosen point, or find the rate from the gradient of a straight-line section. From numbers, plot points accurately and join them appropriately. In photosynthesis, a graph of rate against light intensity typically rises then levels off; reading the plateau shows the maximum rate, while the initial gradient shows how quickly rate increases. To go from numeric to graphical, label axes with quantity and unit, choose a sensible scale, plot each pair, then draw a line or curve of best fit. To go from graphical to numeric, identify the required axis value, read across or up to the line, then down or across to the other axis, and state the unit.
(HT only) These factors interact and any one of them may be the factor that limits photosynthesis.
Higher tier only. The rate of photosynthesis is limited by light intensity, carbon dioxide concentration, temperature, or the amount of chlorophyll. These factors interact; any one can limit the rate if it is in shortest supply. For example, increasing light intensity increases the rate until it plateaus, meaning another factor (like carbon dioxide or temperature) is now limiting. Raising carbon dioxide can lift this plateau, provided other factors are adequate. Temperature affects enzyme activity, peaking at an optimum before denaturation reduces the rate. Chlorophyll levels, affected by disease or mineral deficiency, can also limit photosynthesis. Students must explain these interactions and deduce limiting factors from graphs.
(HT only) Students should be able to explain graphs of photosynthesis rate involving two or three factors and decide which is the limiting factor.
Photosynthesis rate depends on light intensity, carbon dioxide concentration and temperature. When one factor is in short supply it limits the rate; increasing it raises the rate until another factor becomes limiting. On a graph of rate against light intensity, the line rises then plateaus. The plateau shows light is no longer limiting, so carbon dioxide or temperature now limits. If raising carbon dioxide lifts the plateau, carbon dioxide was limiting. With two or three factors plotted, compare the curves: the factor whose increase produces the greatest rise is limiting. At low temperature, enzyme activity limits, so curves flatten early. Decide the limiting factor by identifying which change increases the rate.
(HT only) Students should understand and use inverse proportion – the inverse square law and light intensity in the context of photosynthesis.
Light intensity decreases with distance from a lamp. The inverse square law states that intensity is inversely proportional to the square of the distance: intensity ∝ 1 ÷ distance². If distance doubles, intensity falls to one quarter; if distance trebles, intensity falls to one ninth. In photosynthesis, rate increases with light intensity until another factor limits. To use the law, measure distance from lamp to pondweed, calculate 1 ÷ d², and plot rate against this value; a straight line through the origin shows rate is proportional to light intensity. This explains why moving a lamp further away sharply reduces the rate.
(HT only) Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit.
In a greenhouse, growers raise light intensity, carbon dioxide concentration and temperature to increase photosynthesis. Each factor eventually becomes limiting, so adding more gives diminishing returns. The economics involve comparing the extra yield gained against the cost of providing the enhanced condition, such as heating, lighting or CO₂ enrichment. Maximum profit occurs when the marginal gain from further enhancement no longer exceeds its cost. For example, adding CO₂ boosts photosynthesis until another factor, like light or temperature, limits the rate. You must explain how limiting factors determine optimum conditions and why unlimited enhancement is not economically sensible.
Required practical activity 5: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
In this required practical, pondweed is placed in water with a light source at a measured distance. As the pondweed photosynthesises, oxygen bubbles are released and counted, or the volume of gas collected is measured over a set time. Light intensity is varied by changing the distance between the lamp and the pondweed; light intensity is inversely proportional to the square of the distance. The rate of photosynthesis is indicated by the number of bubbles per minute or the volume of oxygen per minute. Control variables include temperature, carbon dioxide concentration and the pondweed itself. A graph of rate against light intensity typically rises and then plateaus when another factor becomes limiting. Students should use appropriate apparatus, repeat readings and identify anomalies.
Your focus
- Explain how temperature, light intensity, carbon dioxide concentration and chlorophyll amount each affect the rate of photosynthesis.
- Interpret graphs of rate against a factor to identify the limiting factor.
- Describe a valid investigation into one factor affecting the rate of photosynthesis, controlling other variables.
Show all 30 objectives
- Measure oxygen production from pondweed over a known time period.
- Calculate a rate of photosynthesis using change divided by time and express it with correct units.
- Explain how changing light intensity affects the rate and identify when another factor becomes limiting.
- Read a rate value from a photosynthesis graph at a stated condition.
- Explain why the rate plateaus when one factor is limiting.
- Identify and name the factor that becomes limiting at the plateau.
- Select a suitable scale for axes using photosynthesis data.
- Plot points accurately and draw an appropriate line or curve.
- Label axes correctly with quantity and unit.
- Read values and gradients accurately from a rate graph and quote them with correct units.
- Plot tabulated photosynthesis data accurately on suitably scaled and labelled axes.
- Explain what a plateau and an initial steep section of a rate graph indicate about photosynthesis.
- Explain how light intensity, carbon dioxide concentration, temperature, and chlorophyll amount interact to limit photosynthesis.
- Deduce which factor is limiting the rate of photosynthesis from graphical data.
- Describe the effect of temperature on enzyme activity and how chlorophyll levels affect light absorption.
- Interpret rate graphs involving two or three factors and identify the limiting factor from the evidence.
- Explain how changing light intensity, carbon dioxide concentration or temperature affects the rate until another factor limits.
- Compare curves to justify which factor is limiting at a given point.
- Apply the inverse square law to calculate relative light intensity at different distances.
- Explain how light intensity changes with distance and how this affects the rate of photosynthesis.
- Interpret graphs of rate against 1 ÷ distance² to identify proportionality.
- Describe how light intensity, carbon dioxide concentration and temperature can be enhanced in a greenhouse and act as limiting factors.
- Explain why increasing a limiting factor only increases the rate of photosynthesis until another factor becomes limiting.
- Evaluate the economic trade-off between the cost of enhancing conditions and the income from increased photosynthesis or yield.
- Carry out a practical procedure to measure the rate of photosynthesis in pondweed at different light intensities.
- Record and process data appropriately, including repeats, means and a graph of rate against light intensity or distance.
- Explain the relationship between light intensity and the rate of photosynthesis, including the inverse square law and limiting factors.
Rate of photosynthesis exam tips
Marking Points
- Explains that increasing light intensity increases the rate because more light energy is absorbed by chlorophyll for the light-dependent stage.
- Explains that increasing carbon dioxide concentration increases the rate because CO₂ is a reactant needed to make glucose.
- Explains that temperature increases the rate up to an optimum because enzyme activity increases, but above the optimum enzymes denature and the rate falls.
- Explains that a greater amount of chlorophyll increases the rate because more light is absorbed in the chloroplasts.
- Identifies the limiting factor from a graph: the factor whose increase raises the rate before the plateau.
- Describes a valid method, such as pondweed and bubble counting, with control of other variables to test one factor at a time.
- Select a suitable dependent variable, such as number of oxygen bubbles or volume of oxygen gas collected.
- Use the equation rate = change ÷ time and substitute measured values correctly.
- Give the rate with a correct compound unit, for example bubbles min⁻¹ or cm³ min⁻¹.
- Control key variables such as temperature, carbon dioxide concentration and the distance of the light source.
- Interpret a set of rate values to identify a trend, such as rate increasing with light intensity until another factor becomes limiting.
- State the independent variable on the x-axis and the dependent variable on the y-axis, including units, before reading any value.
- Describe the initial trend as the limiting factor increases: rate increases because the factor is in short supply and is limiting.
- Identify the plateau: rate stops increasing because a different factor has become limiting.
- Name the factor that becomes limiting at the plateau, using the context given in the graph or question.
- When comparing two curves, link a higher curve to a condition that allows a faster rate, such as a higher temperature or carbon dioxide concentration.
- Read values accurately from the curve, interpolating between grid lines where needed, and quote the unit.
- Place the independent variable on the x-axis and the dependent variable on the y-axis.
- Choose a scale where each major square represents a simple value and the data fill most of the grid.
- Label both axes with the quantity and its unit, for example 'Rate of photosynthesis in mm³ min⁻¹'.
- Plot points accurately using small crosses or dots, then draw a smooth curve or line of best fit.
- Use a ruler for straight-line sections and keep the line consistent with the overall trend.
- Include a clear title or key where needed to identify the graph.
- Read a value from a graph by locating the required point on one axis, moving to the line or curve, then across to the other axis, and quoting the value with its unit.
- Determine a rate from the gradient of a straight-line section by dividing the change in the y-axis quantity by the change in the x-axis quantity, including units.
- Plot supplied numeric data accurately by choosing a scale that uses most of the grid, labelling both axes with quantity and unit, and plotting points to within half a small square.
- Interpret a plateau on a rate graph as the maximum rate under those conditions, and a steeper initial section as a faster increase in rate.
- Translate in reverse by taking values from a table and constructing a line or curve of best fit, recognising when a straight line or a smooth curve is appropriate.
- Use the graph to predict an unmeasured value by interpolation between plotted points, or by extending a straight-line trend where the pattern supports it.
- Identify light intensity, carbon dioxide concentration, temperature, and chlorophyll amount as interacting limiting factors.
- State that the factor in shortest supply relative to demand limits the overall rate of photosynthesis.
- Explain that increasing a limiting factor increases the rate until a different factor becomes limiting, causing a plateau on a graph.
- Deduce the limiting factor from a graph: a rising curve indicates the x-axis factor is limiting, while a plateau indicates another factor is limiting.
- Explain that temperature affects kinetic energy and enzyme activity, with rates falling above the optimum due to denaturation.
- Describe how reduced chlorophyll (e.g., due to magnesium deficiency) limits photosynthesis by absorbing less light.
- State that the limiting factor is the factor in shortest supply that restricts the rate of photosynthesis.
- Describe how a plateau on a rate graph shows that the plotted factor is no longer limiting.
- Explain that increasing carbon dioxide concentration can raise a light-intensity plateau, showing carbon dioxide was limiting.
- Use enzyme activity to explain why low temperature limits the rate even when light and carbon dioxide are plentiful.
- Compare two or three curves to identify which changed factor gives the greatest increase in rate and is therefore limiting.
- State that light intensity is inversely proportional to the square of the distance from the light source.
- Use the relationship intensity ∝ 1 ÷ distance² to calculate relative intensity at different distances.
- Explain that doubling the distance reduces light intensity to one quarter of its original value.
- Describe how to plot rate of photosynthesis against 1 ÷ distance² to test whether light intensity limits the rate.
- Interpret a straight line through the origin on a rate against 1 ÷ distance² graph as evidence that rate is proportional to light intensity.
- Identifies light intensity, carbon dioxide concentration and temperature as factors that can be enhanced in a greenhouse to increase the rate of photosynthesis.
- Explains that the rate of photosynthesis increases as a limiting factor is raised until another factor becomes limiting, so further increases have little or no effect.
- Applies the idea of limiting factors to explain why a grower would not increase all conditions indefinitely, because the cost of enhancement may exceed the value of extra growth.
- Uses the concept of profit as the difference between income from increased yield and the cost of providing enhanced conditions, such as heating, lighting or CO₂ enrichment.
- Describes an optimum where the rate of photosynthesis is maximised for the lowest additional cost, rather than simply maximised at any cost.
- Sets up pondweed in water with a light source at a measured distance and allows time for the plant to acclimatise before counting bubbles.
- Varies light intensity by changing the distance between the lamp and the pondweed, using a ruler or metre rule to measure distance accurately.
- Counts oxygen bubbles released per unit time or collects and measures the volume of oxygen produced per unit time as a measure of the rate of photosynthesis.
- Controls variables such as temperature, carbon dioxide concentration and the size or type of pondweed to ensure a valid comparison.
- Repeats readings at each distance and calculates a mean, identifying and excluding anomalous results where appropriate.
- Plots a graph of rate of photosynthesis against light intensity or distance and describes the trend, including any plateau when another factor becomes limiting.
- Uses the inverse square relationship between light intensity and distance to explain why moving the lamp further away reduces the rate.
Examiner Tips
- 💡Name the limiting factor and justify it using the graph shape, for example a plateau shows another factor is limiting.
- 💡For temperature, always mention enzymes and denaturation above the optimum.
- 💡When describing an investigation, state the independent variable, how the dependent variable is measured, and at least two control variables.
- 💡Show the substitution line in calculations so the examiner can see how your answer was obtained.
- 💡Use the phrase 'rate of oxygen production' rather than 'amount of photosynthesis' when describing the dependent variable.
- 💡When describing a graph, quote figures from the axes and state the trend before explaining it in terms of limiting factors.
- 💡Annotate the graph: mark the steep section, the plateau and the point where the curve flattens.
- 💡Use the phrase 'becomes limiting' when explaining a plateau, and name the factor explicitly.
- 💡If asked to compare curves, quote a value from each curve at the same x-value to support your comparison.
- 💡Check the range of each variable before choosing a scale, then count squares to confirm the data fit.
- 💡Label axes with both quantity and unit; marks are often available for correct labels.
- 💡Plot points with a sharp pencil and small crosses so the line of best fit is easy to draw.
- 💡Annotate the graph with the values you read, showing the construction lines, so your method is visible to the examiner.
- 💡When calculating a gradient, choose two points on the straight section that are far apart and clearly on the line, not two plotted data points.
- 💡Check that your answer's unit matches the axis units, converting if the question uses different units such as minutes and seconds.
- 💡When analysing a graph with multiple curves, identify what changes between the plateaus to find the new limiting factor.
- 💡Remember to include chlorophyll amount when listing factors that can limit photosynthesis.
- 💡Label each curve clearly with the factor changed, such as light intensity at 0.04% and 0.4% carbon dioxide.
- 💡As this is Higher Tier only content, when asked to decide the limiting factor, state the evidence from the graph, for example the rate rises when carbon dioxide increases.
- 💡Write the relationship as intensity ∝ 1 ÷ d² and show the substitution clearly when calculating.
- 💡As this is Higher Tier only, when explaining a graph, state that a straight line through the origin supports proportionality between rate and light intensity.
- 💡As this is Higher Tier only, when a question asks about greenhouses, link each enhanced condition to a named limiting factor and then to a cost or benefit.
- 💡Use comparative language such as 'increases until', 'plateaus' and 'no longer cost-effective' to show understanding of diminishing returns.
- 💡State clearly how the dependent variable is measured, for example bubbles per minute or cm³ of oxygen per minute.
- 💡When describing the graph, refer to the initial rise and the plateau, and name the factor that may become limiting at the plateau.
- 💡Use the inverse square relationship to explain why the rate falls as the lamp is moved further away.
- 💡Mention repeats and means when asked how to improve the investigation, and identify control variables explicitly.
Common Mistakes
- Saying the rate keeps increasing indefinitely with light intensity: correct this by stating that another factor becomes limiting and the graph plateaus.
- Stating that high temperature increases the rate without limit: correct this by explaining enzyme denaturation above the optimum.
- Confusing chlorophyll amount with light intensity: correct this by saying chlorophyll is the pigment that absorbs light, while light intensity is the energy supply.
- Forgetting to convert seconds to minutes when the rate is expected per minute; correction: divide by 60 or time the experiment in minutes from the start.
- Using the distance from the lamp as if it were light intensity; correction: light intensity is inversely proportional to the square of the distance, so halving the distance greatly increases intensity.
- Averaging rates without checking for anomalous results; correction: identify and exclude anomalies before calculating a mean, and state why the anomaly is ignored.
- Saying the rate 'stops' at the plateau instead of saying it levels off or remains constant; correct by stating the rate is constant because another factor is now limiting.
- Naming the x-axis factor as limiting at the plateau; correct by naming a different factor, such as carbon dioxide concentration or temperature.
- Ignoring units when reading values; correct by copying the unit from the y-axis, for example arbitrary units or mm³ of oxygen per minute.
- Using an awkward scale such as 3 or 7 units per square; correct by using 1, 2, 5 or 10 units per square.
- Swapping the axes so the dependent variable is on the x-axis; correct by putting the independent variable on the x-axis.
- Joining points with a jagged dot-to-dot line when a smooth curve is appropriate; correct by drawing a smooth line of best fit through the trend.
- Reading the axes the wrong way round, for example treating the x-axis as rate; correction: check each axis label and unit before reading any value.
- Joining plotted points with straight segments when the relationship is a smooth curve; correction: draw a single smooth curve of best fit unless the data clearly form a straight line.
- Forgetting units or giving the gradient without units; correction: divide the y-axis unit by the x-axis unit and state the resulting unit with the answer.
- Forgetting that chlorophyll amount can be a limiting factor. Correction: Chlorophyll is essential for absorbing light, so low levels limit the rate.
- Claiming the factor with the highest value is limiting. Correction: The limiting factor is the one in shortest supply.
- Stating that increasing light always increases the rate. Correction: It only increases the rate until another factor limits it.
- Thinking the plateau means photosynthesis has stopped; correct by stating the rate is constant because another factor is now limiting.
- Assuming light is always limiting at high light intensity; correct by checking whether another factor, such as carbon dioxide or temperature, limits the plateau.
- Ignoring temperature when comparing curves; correct by noting that low temperature reduces enzyme activity and can limit the rate before light or carbon dioxide.
- Using intensity ∝ 1 ÷ distance instead of 1 ÷ distance²; correct by squaring the distance before dividing.
- Thinking doubling distance halves intensity; correct by stating intensity becomes one quarter.
- Plotting rate against distance and expecting a straight line; correct by plotting against 1 ÷ distance² to linearise the relationship.
- Stating that increasing any factor always increases the rate of photosynthesis indefinitely; correct by noting that once another factor becomes limiting, further increases have little or no effect on the rate.
- Confusing profit with yield, assuming maximum yield always gives maximum profit; correct by stating profit also depends on the cost of heating, lighting or adding CO₂, so maximum profit may occur at a lower yield.
- Ignoring the cost of enhanced conditions and treating greenhouse management as purely biological; correct by comparing the cost of enhancement with the extra income from improved growth.
- Error: starting to count bubbles immediately after setting up the apparatus. Correction: allow the pondweed to acclimatise for a few minutes so that the rate is steady before taking measurements.
- Error: changing more than one variable at a time, such as moving the lamp and altering the temperature. Correction: change only the distance between lamp and pondweed while keeping temperature, CO₂ concentration and pondweed size constant.
- Error: recording a single bubble count as the result. Correction: take repeat readings at each distance and calculate a mean to improve reliability.
- Error: assuming light intensity is directly proportional to distance. Correction: light intensity is inversely proportional to the square of the distance, so doubling the distance reduces intensity to one quarter.