Rate of photosynthesis — AQA GCSE Biology
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Rate of photosynthesis explained
Photosynthesis rate depends on several interacting factors.
Read the full explanation
Light intensity supplies energy for the reaction, so raising it increases the rate until another factor becomes limiting. Carbon dioxide is a reactant, so increasing its concentration raises the rate until limited elsewhere. Temperature affects enzyme activity: the rate rises with temperature up to an optimum, then falls rapidly as enzymes denature. Chlorophyll amount determines how much light energy can be absorbed, so more chlorophyll supports a faster rate. A practical example is pondweed exposed to a lamp: moving the lamp closer increases light intensity and oxygen bubbles per minute, but eventually the rate levels off.
Students should be able to: • measure and calculate rates of photosynthesis • extract and interpret graphs of photosynthesis rate involving one limiting factor • plot and draw appropriate graphs selecting appropriate scale for axes • translate information between graphical and numeric form.
Photosynthesis rate is the speed at which a plant produces oxygen or uses carbon dioxide. A common method uses pondweed: count bubbles per minute, or collect gas in a syringe to record volume per minute. Rate is calculated by dividing gas volume by time, e.g., 12 cm³ ÷ 4 min = 3 cm³/min. Graphs show rate against light intensity, carbon dioxide concentration or temperature. For light and CO₂, the line rises steeply then levels off (plateaus) when another factor becomes limiting. However, for temperature, the rate peaks at an optimum then falls sharply as enzymes denature. When plotting, choose a scale using most of the grid, label axes with units, and plot accurately.
(HT only) These factors interact and any one of them may be the factor that limits photosynthesis.
In higher tier, you must understand that light intensity, carbon dioxide concentration, temperature and chlorophyll amount do not act in isolation. At any moment, the factor in shortest supply limits the rate, and changing other factors has little effect until that limiting factor is relieved. For example, if carbon dioxide is limiting, increasing light intensity will not raise the rate further; the graph plateaus. If carbon dioxide is then increased, the rate may rise again until another factor, such as temperature, becomes limiting. Explaining these interactions requires linking each factor to its biological role and using the limiting factor concept to account for plateaus and changes in gradient.
(HT only) Students should be able to explain graphs of photosynthesis rate involving two or three factors and decide which is the limiting factor.
A limiting factor is the factor in shortest supply that holds the rate below its potential maximum. On a graph of rate against light intensity, the curve rises steeply, then flattens into a plateau. The plateau shows that light is no longer limiting, so another factor, such as carbon dioxide concentration or temperature, now controls the rate. If the carbon dioxide concentration is raised, the plateau shifts to a higher rate, showing that carbon dioxide was limiting. With three factors, compare curves at the same value on the x-axis: the curve with the highest rate has the most favourable combination, and the factor that differs between curves is the one limiting the lower curve. To decide the limiting factor, identify the factor being changed, then ask which other factor could raise the rate further.
(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 because the light spreads out over a larger area. The inverse square law states that light intensity is inversely proportional to the square of the distance: intensity ∝ 1 ÷ distance². If the distance doubles, the intensity falls to one quarter of its previous value; if the distance trebles, the intensity falls to one ninth. In photosynthesis, this means that moving a lamp twice as far away can reduce the rate if light is the limiting factor, but only while light remains limiting. To use the law, square the distance, then take the reciprocal. For example, at 0.50 m the intensity is proportional to 1 ÷ 0.50² = 4 arbitrary units, and at 1.00 m it is proportional to 1 ÷ 1.00² = 1 arbitrary unit. The law predicts the shape of a rate against distance graph, which rises steeply as distance decreases.
(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.
Growers can raise the rate of photosynthesis in greenhouses by adding artificial light, heating, or enriching the air with carbon dioxide. Each enhancement costs money, so the aim is to increase growth and yield enough to cover those costs and still make a profit. The limiting factor idea guides decisions: if light is limiting, adding more carbon dioxide will not help until light is increased. Once one factor is no longer limiting, another factor takes over, so spending more on the first factor gives diminishing returns. For example, heating a greenhouse may speed growth in winter, but if light intensity is low, the extra heat may not raise the rate much. A grower compares the cost of each enhancement with the extra income from the improved yield, and stops adding a factor when the extra cost exceeds the extra income.
Required practical activity 6: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
Pondweed releases oxygen bubbles when it photosynthesises. The rate can be measured by counting bubbles per minute or by collecting the gas in a capillary tube and measuring the volume. Place a lamp a measured distance from the pondweed, leave it to acclimatise, then count bubbles for one minute. Repeat at several distances, such as 10 cm, 20 cm, 30 cm and 40 cm, and repeat each distance to improve reliability. Control variables include temperature, carbon dioxide concentration, the length of pondweed and the time allowed for acclimatisation. As the lamp moves further away, light intensity falls, so the rate of bubbling usually decreases. The results can be plotted as a graph of rate against distance or against 1 ÷ distance². A control experiment can use a darkened tube to show that bubbles are linked to light.
AT skills covered by this practical activity: AT 1, 2, 3, 4 and 5.
This statement identifies the specific apparatus and techniques (AT) used when investigating photosynthesis. AT 1 involves using apparatus to make accurate measurements, such as a ruler for distance or a stopwatch for time. AT 2 covers the safe use of heating devices, like a water bath to control temperature. AT 3 refers to using apparatus to observe biological processes, such as collecting gas in a syringe. AT 4 involves the safe use of living organisms, like handling pondweed carefully to measure its physiological response to light. AT 5 focuses on measuring rates of reaction, such as calculating the volume of oxygen produced per minute. These specific techniques ensure the practical yields valid data.
This practical activity also provides opportunities to develop WS and MS. Details of all skills are given in Key opportunities for skills development.
This statement explains that the photosynthesis practical develops working scientifically (WS) and mathematical skills (MS), which are listed in the Key opportunities for skills development. WS includes planning investigations, using apparatus, recording data and evaluating methods. MS includes calculating means, rates and percentages, and interpreting graphs. For example, a student might calculate the mean number of oxygen bubbles per minute at each light intensity and plot a line graph. They might also use the inverse square relationship to predict how light intensity affects the rate. These skills are assessed through practical questions and mathematical questions in exams, so students should practise applying them to photosynthesis data.
Your focus
- Describe how changing light intensity, carbon dioxide concentration, temperature and chlorophyll amount affects the rate of photosynthesis.
- Explain each effect using the underlying biological process, including enzyme activity and limiting factors.
- Interpret simple rate graphs to identify when a factor becomes limiting.
Show all 27 objectives
- Measure oxygen production or carbon dioxide uptake over time and calculate a rate with correct units.
- Extract values and describe trends from a graph showing rate against one limiting factor.
- Plot a graph using a suitable scale and translate between graphical and numeric information.
- Explain how factors interact to limit the rate of photosynthesis.
- Identify the limiting factor at different points on a graph.
- Predict the effect of changing one factor when another is limiting.
- Interpret a rate graph and identify the plateau.
- State which factor is limiting at a given point on a graph.
- Compare two or three curves to justify a limiting factor.
- Apply the inverse square law to calculate relative light intensity.
- Explain how distance from a lamp affects the rate of photosynthesis.
- Interpret a rate against distance graph using the inverse square relationship.
- Explain how limiting factors affect greenhouse management.
- Compare the costs and benefits of enhancing greenhouse conditions.
- Justify a commercial decision using limiting factor reasoning.
- Carry out the pondweed practical safely and accurately.
- Record and process data on bubble rate or gas volume.
- Explain how control variables and repeats improve the investigation.
- Describe how to use appropriate apparatus to measure variables accurately during a photosynthesis investigation.
- Explain how to safely use living organisms and heating devices to measure physiological responses.
- Calculate the rate of photosynthesis by measuring the production of gas over time.
- Identify the WS and MS skills used in a photosynthesis practical.
- Apply mathematical skills to calculate rates and interpret graphs from photosynthesis data.
- Explain how working scientifically skills improve the validity of an investigation.
Rate of photosynthesis exam tips
Marking Points
- Light intensity: as light intensity increases, the rate of photosynthesis increases because more energy is supplied, until another factor becomes limiting.
- Carbon dioxide concentration: increasing carbon dioxide concentration increases the rate because it is a reactant used to make glucose, until another factor becomes limiting.
- Temperature: the rate increases with temperature up to an optimum because enzyme activity increases, then decreases at higher temperatures because enzymes denature.
- Chlorophyll amount: a greater amount of chlorophyll allows more light energy to be absorbed, so the rate of photosynthesis increases.
- Limiting factor idea: at any point the factor in shortest supply limits the rate, so changing other factors may have little effect until that factor is relieved.
- State that rate of photosynthesis can be measured by the volume of oxygen produced per unit time or the volume of carbon dioxide taken in per unit time.
- Calculate rate using rate = volume of gas ÷ time, with correct units such as cm³/min or cm³ s⁻¹.
- Describe a valid method, for example using pondweed and counting bubbles per minute or collecting gas in a capillary tube or syringe.
- Interpret a graph of rate against one limiting factor by identifying the initial rise and the plateau for light or CO₂, or the peak and subsequent fall for temperature due to enzyme denaturation.
- Select an appropriate scale for axes so that plotted points occupy most of the grid and are easy to read.
- Translate between graphical and numeric form by reading a rate from a curve at a stated value or estimating a value from a plotted line.
- State that the factor in shortest supply limits the rate of photosynthesis at any given time.
- Explain that increasing a non-limiting factor has little or no effect on the rate until the limiting factor is relieved.
- Describe interactions, for example that raising carbon dioxide concentration can increase the rate until temperature or light becomes limiting.
- Use the limiting factor concept to explain plateaus and changes in the gradient of rate graphs.
- Define a limiting factor as the factor in shortest supply that restricts the rate of photosynthesis.
- Describe the shape of a rate graph: an initial steep rise, then a plateau where the rate stops increasing.
- Explain that on the plateau, the factor plotted on the x-axis is no longer limiting.
- Use differences between curves at the same x-value to identify which factor is limiting the lower curve.
- Apply the reasoning to two or three factors, including light intensity, carbon dioxide concentration and temperature.
- Recognise that temperature affects enzyme activity, so it can limit the rate even when light and carbon dioxide are plentiful.
- State that light intensity is inversely proportional to the square of the distance from a point source.
- Use the relationship intensity ∝ 1 ÷ distance² to compare intensities at two distances.
- Calculate the effect of doubling or trebling the distance on light intensity.
- Explain that the inverse square law applies while light intensity is the limiting factor.
- Interpret a graph of rate against distance in terms of the inverse square relationship.
- Recognise that the law assumes a point source and no other limiting factors.
- Identify ways greenhouse conditions can be enhanced, such as artificial lighting, heating and carbon dioxide enrichment.
- Explain that each enhancement has a cost that must be balanced against increased yield and income.
- Use limiting factor reasoning to show that adding a non-limiting factor does not increase the rate.
- Describe diminishing returns when a factor stops being limiting.
- Evaluate a commercial decision by comparing the cost of an enhancement with the value of the extra growth.
- Recognise that maximum profit may occur before the maximum possible rate of photosynthesis is reached.
- Measure the rate of photosynthesis by counting oxygen bubbles per minute or collecting the gas volume.
- Change light intensity by moving a lamp to different measured distances from the pondweed.
- Control temperature, carbon dioxide concentration, pondweed length and acclimatisation time.
- Repeat readings at each distance and calculate a mean to improve reliability.
- Plot a graph of rate against distance or against 1 ÷ distance² and describe the trend.
- Use a control, such as a darkened tube, to show that light is needed for the bubbles.
- AT 1: Use appropriate apparatus to accurately measure variables, such as a ruler for light distance and a stopwatch for time.
- AT 2: Safely use heating devices, such as a water bath, to control the temperature of the pondweed's environment.
- AT 3: Use appropriate apparatus, like a gas syringe or inverted measuring cylinder, to observe and measure oxygen production.
- AT 4: Safely use living organisms, such as Elodea or Cabomba pondweed, to measure physiological responses to environmental changes.
- AT 5: Measure the rate of reaction by recording the volume of gas produced over a specific time period.
- WS: plan an investigation into light intensity and photosynthesis, identifying variables and a control.
- WS: record observations accurately in a table with correct headings and units.
- MS: calculate a mean rate from repeat measurements, for example total bubbles divided by total time.
- MS: plot and interpret a graph of rate against light intensity, describing the pattern.
- MS: use the inverse square law to explain why rate decreases as distance from the lamp increases.
Examiner Tips
- 💡Link each factor to its biological role: light and chlorophyll for energy absorption, carbon dioxide as a reactant, temperature for enzyme activity.
- 💡Use the phrase 'until another factor becomes limiting' when describing a plateau on a graph.
- 💡When explaining temperature, refer to enzymes and denaturation rather than saying heat 'kills' the plant.
- 💡Always include units when stating a rate, for example cm³/min, and show the division by time in your working.
- 💡When describing a graph, quote figures from the axes to support the trend, such as rate rising from 2 cm³/min at 10 °C to 8 cm³/min at 25 °C.
- 💡For graph-plotting questions, check that the scale is uniform, axes are labelled with quantity and unit, and points are plotted within half a small square.
- 💡Use the phrase 'the factor in shortest supply' when defining the limiting factor.
- 💡When explaining a plateau, name the limiting factor and say what would happen if it were increased.
- 💡Link each factor to its role, such as carbon dioxide as a reactant or temperature as affecting enzymes.
- 💡Quote values from the graph, such as the light intensity where the plateau begins, to support your explanation.
- 💡Compare curves at the same x-value rather than at different points along the axis.
- 💡Name the limiting factor explicitly and justify your choice with evidence from the graph.
- 💡Show the squared distance in your working before taking the reciprocal.
- 💡Use ratio reasoning: doubling distance gives one quarter of the intensity.
- 💡Link any calculated change in intensity to the rate only when light is limiting.
- 💡Name the limiting factor before suggesting a change to greenhouse conditions.
- 💡Use comparative language such as greater than or less than when discussing cost and income.
- 💡Link each enhancement to a specific effect on the rate of photosynthesis.
- 💡State clearly how the dependent variable is measured, for example bubbles per minute.
- 💡Identify at least two control variables and explain why each is kept constant.
- 💡Describe repeats and a mean when explaining how reliability is improved.
- 💡When describing this practical, explicitly name the apparatus used to measure the dependent and independent variables.
- 💡Remember that controlling temperature using a water bath demonstrates a specific practical technique (AT 2).
- 💡When asked about skills, link your answer to the specific photosynthesis practical rather than giving a general answer.
- 💡For calculations, write the equation, substitute values and give the answer with units.
- 💡When interpreting a graph, quote data points to support your description of the trend.
Common Mistakes
- Saying the rate keeps rising indefinitely with light intensity; the correction is that it plateaus when another factor becomes limiting.
- Stating that high temperature increases the rate without limit; the correction is that above the optimum enzymes denature and the rate falls.
- Confusing chlorophyll amount with light intensity; the correction is that chlorophyll is the pigment amount in the chloroplast, while light intensity is the external light supply.
- Confusing rate with total gas volume: rate must be divided by time, so a reading of 30 cm³ after 5 minutes gives 6 cm³/min, not 30 cm³/min.
- Plotting points without choosing a scale that fills the grid, which compresses the curve and makes interpretation difficult; choose a scale where each major grid square represents a sensible round number.
- Assuming all graphs plateau; while light and CO₂ graphs level off when another factor limits the rate, temperature graphs fall after the optimum because enzymes denature.
- Claiming that all factors affect the rate equally at all times; the correction is that only the factor in shortest supply limits the rate.
- Saying that increasing light intensity always increases the rate; the correction is that it only does so if light is the limiting factor.
- Stating that increasing temperature relieves a carbon dioxide limitation; the correction is that only increasing carbon dioxide can relieve a carbon dioxide limitation.
- Saying the rate is zero on the plateau; correct this by stating the rate is constant and limited by another factor.
- Assuming the factor on the x-axis always limits the rate; correct this by checking whether raising another factor would increase the rate.
- Confusing the steep section with the plateau; correct this by linking the steep section to the plotted factor being limiting and the plateau to a different factor taking over.
- Treating intensity as inversely proportional to distance rather than to distance squared; correct this by squaring the distance before taking the reciprocal.
- Forgetting to square the distance when comparing two positions; correct this by writing distance² in the calculation.
- Assuming the rate always follows the inverse square law; correct this by noting that other factors limit the rate at close distances or high intensities.
- Assuming the maximum rate always gives maximum profit; correct this by comparing costs with income.
- Adding more of a factor that is not limiting and expecting a higher rate; correct this by identifying the actual limiting factor first.
- Ignoring running costs such as electricity or fuel; correct this by including them in the comparison.
- Starting to count bubbles immediately after moving the lamp; correct this by allowing time for the pondweed to acclimatise.
- Changing more than one variable at a time; correct this by keeping all factors except light intensity constant.
- Recording a single reading at each distance; correct this by repeating and calculating a mean.
- Confusing AT codes with generic working scientifically skills; correction: AT codes refer to specific apparatus and techniques, such as AT 5 for measuring rates of reaction.
- Failing to specify the apparatus used for measurements; correction: explicitly state the equipment, such as using a thermometer to monitor temperature (AT 1).
- Ignoring the safe use of living organisms; correction: recognise that handling pondweed to measure its response to light is a specific technique (AT 4).
- Confusing WS and MS skills; correction: WS covers investigation design and evaluation, while MS covers calculations and graph work.
- Forgetting units when calculating rates; correction: always include units such as bubbles per minute or cm³ per minute.
- Drawing a bar chart for continuous data; correction: use a line graph when both variables are continuous.