Ch 2 · 13.2 Investigation of limiting factors · AS & A Level Biology 9700
Topic 2 of 2 in Photosynthesis — AS & A Level Biology 9700 — 68 lessons.
Introduction
The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature, but at any moment only one of these factors actually controls the rate. Understanding which factor is limiting, and why, lets you interpret experimental graphs, and the syllabus expects you to know how to investigate each factor in the lab. (Syllabus section 13.2.)
Key ideas — The concept of a limiting factor
A limiting factor is a variable that, when in shortest supply, restricts the rate of a process and prevents it from proceeding any faster, even if all other factors are at their optimum. This is sometimes called Blackman's principle of limiting factors.
🧑🏫 Teacher's explanation
Imagine you are running a factory assembly line. You have 100 workers (enzymes), plenty of raw materials (carbon dioxide), and plenty of electricity (light). However, you only have 1 conveyor belt (limiting factor). No matter how many more workers or materials you add, the factory cannot produce more goods because the single conveyor belt is the bottleneck.
📖 Definitions
🔍 Worked example — Easy
A student measures the rate of photosynthesis in an aquatic plant at different light intensities. At 10 lux, the rate is 5 bubbles/min. At 50 lux, the rate is 25 bubbles/min. At 100 lux, the rate remains 25 bubbles/min. Identify the limiting factor at 10 lux and 100 lux.
🔍 Worked example — Standard
Explain why the rate of photosynthesis levels off (plateaus) when carbon dioxide concentration is increased beyond a certain point, even if light intensity is increased simultaneously.
🔍 Worked example — Exam-style
In an experiment using a suspension of chloroplasts and DCPIP (a redox indicator), the time taken for the DCPIP to change color (from blue to colorless) is measured. If the time taken decreases as light intensity increases, what can you conclude about the rate of photosynthesis?
⚠️ Common errors
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🎯 Exam question
(a) The results are shown below:
Key ideas — Light intensity and carbon dioxide concentration
🧑🏫 Teacher's explanation
Think of photosynthesis like a factory assembly line. The light intensity is the speed of the conveyor belt bringing raw materials in, carbon dioxide is the amount of raw material available, and temperature is the speed at which the workers can assemble the products. If the conveyor belt is moving slowly, adding more raw material won't help; the belt is the 'limiting factor'.
📖 Definitions
🔍 Worked example — Easy
A student investigates the effect of light intensity on the rate of photosynthesis using an aquatic plant. They observe that when the light source is moved from 10 ext cm to 20 ext cm away, the number of oxygen bubbles produced per minute drops from 40 to 10. Calculate the percentage decrease in the rate of photosynthesis.
🔍 Worked example — Standard
In an experiment using a suspension of chloroplasts and DCPIP (a blue redox indicator), the time taken for the DCPIP to turn from blue to colourless was 5 minutes at 25^° C and 2 minutes at 35^° C. Calculate the factor by which the rate of photosynthesis increased.
🔍 Worked example — Exam-style
A graph shows the rate of photosynthesis against CO_2 concentration. The curve rises steeply from 0 to 0.05\% CO_2 and then becomes a horizontal line (plateau) from 0.05\% CO_2 to 0.10\% CO_2. Explain why the rate does not increase further when CO_2 concentration is increased from 0.05\% to 0.10\%.
⚠️ Common errors
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🎯 Exam question
(a) Explain why sodium hydrogen carbonate is added to the water. [2] (b) Describe how the rate of photosynthesis could be measured in this experiment. [2] [4 marks]
Key ideas — Temperature as a limiting factor
🧑🏫 Teacher's explanation
Think of photosynthesis like a factory assembly line. The enzymes (like Rubisco) are the machines, and the temperature is the speed of the conveyor belt. If the belt moves too slowly (low temperature), the machines don't get enough materials to work on. If the belt moves at a perfect speed (optimum temperature), production is maximized. However, if the belt moves too fast (high temperature), the machines actually break down and stop working entirely.
📖 Definitions
🔍 Worked example — Easy
A student measures the rate of photosynthesis in an aquatic plant at 10 ^circC, 25 ^circC, and 45 ^circC. Describe the expected trend in the rate of photosynthesis.
🔍 Worked example — Standard
Calculate the percentage change in the rate of photosynthesis when the temperature increases from 20 ^circC to 30 ^circC, given the rate at 20 ^circC is 5.0 units/min and at 30 ^circC is 8.5 units/min.
🔍 Worked example — Exam-style
Explain why increasing the temperature from 35 ^circC to 45 ^circC causes a rapid decline in the rate of photosynthesis in a plant where light and CO_2 are abundant.
⚠️ Common errors
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🎯 Exam question
(a) Describe the effect of temperature on the rate of photosynthesis based on the data. [2] (b) Explain why the rate decreases significantly at 40 ^circC. [2] [4 marks]
Key ideas — Interpreting limiting factor graphs
When interpreting such graphs:
🧑🏫 Teacher's explanation
Imagine you are running a factory assembly line. If you have 10 workers but only 1 conveyor belt, adding more workers won't speed up production; the conveyor belt is the 'limiting factor'. In photosynthesis, the plant has a set number of enzymes and a set amount of raw materials. If you increase the light, the rate goes up because you are providing more energy. However, once every available enzyme is working as fast as it can, or once you run out of CO_2 molecules to process, adding more light won't help. The graph will flatten out.
📖 Definitions
🔍 Worked example — Easy
A student plots a graph of the rate of photosynthesis against light intensity. The rate increases linearly from 0 to $50 \text{ $\mumol m^-2 exts^-1 and then remains constant at $20 \text{ $\mumol m^-3 exts^-1. Identify the limiting factor when light intensity is $30 \text{ $\mumol m^-2 exts^-1 and when it is $80 \text{ $\mumol m^-2 exts^-1.
🔍 Worked example — Standard
Two curves are plotted on a graph of 'Rate of Photosynthesis' vs 'Light Intensity'. Curve A reaches a plateau at $15 \text{ $\mumol m^-3 exts^-1. Curve B reaches a plateau at $25 \text{ $\mumol m^-3 exts^-1. Explain why Curve B has a higher plateau.
🔍 Worked example — Exam-style
Calculate the percentage increase in the rate of photosynthesis when light intensity increases from $10 \text{ $\mumol m^-2 exts^-1 to $40 \text{ $\mumol m^-2 exts^-1, given the rate at 10 is $5 \text{ $\mumol m^-3 exts^-1 and the rate at 40 is $15 \text{ $\mumol m^-3 exts^-1.
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🎯 Exam question
Key ideas — Investigating with a chloroplast suspension and a redox indicator
In the light-dependent stage, excited electrons normally reduce NADP. A redox indicator such as DCPIP (blue → colourless) or methylene blue (blue → colourless) can take those electrons instead, so the speed of its colour change measures the rate of the light-dependent reactions (the Hill reaction).
🧑🏫 Teacher's explanation
Imagine you are running a factory assembly line. The speed of the factory depends on how many workers (chloroplasts) are present, how many raw materials (CO2/light) are arriving, and how fast the machines (enzymes) can work. If one part of the line is too slow, it becomes the 'limiting factor'—no matter how many workers you add, the factory can't go faster until that specific bottleneck is fixed.
📖 Definitions
🔍 Worked example — Easy
A student uses a colorimeter to measure the absorbance of a DCPIP/chloroplast suspension. The absorbance decreases from 0.85 to 0.25 in 120 seconds. Calculate the rate of decolorization.
🔍 Worked example — Standard
A chloroplast suspension is placed at a distance of 20 cm from a light source. If the distance is increased to 40 cm, by what factor does the light intensity change?
🔍 Worked example — Exam-style
In an experiment investigating light wavelength, the rate of DCPIP decolorization was measured using different filters. The rates were: Red (0.05 s^-1), Blue (0.048 s^-1), and Green (0.005 s^-1). Interpret the data regarding chlorophyll absorption.
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🎯 Exam question
Key ideas — Investigating with whole plants
An aquatic plant such as Elodea or Cabomba gives off bubbles of oxygen from a cut stem. Count the bubbles per minute, or better, collect the gas in a capillary tube (a photosynthometer) and measure its volume.
🧑🏫 Teacher's explanation
Imagine you are running a factory. The speed of your production depends on how many workers you have (temperature), how much raw material you receive (CO₂), and how much electricity is available (light intensity). If you have plenty of workers and materials, but the power goes out, the factory slows down. This 'bottleneck' is what we call a limiting factor. In photosynthesis, the rate of the reaction is limited by whichever of these three factors is in the shortest supply.
📖 Definitions
🔍 Worked example — Easy
A student uses Elodea to measure the effect of light intensity. At a distance of 10 cm, the plant produces 20 bubbles/min. At 20 cm, it produces 5 bubbles/min. Calculate the percentage decrease in the rate of oxygen production.
🔍 Worked example — Standard
In an experiment to test the effect of temperature, a student finds that the rate of photosynthesis increases from 2.0 cm^3 min^-1 at 20circC to 4.5 cm^3 min^-1 at 30circC. Explain why the rate might suddenly drop if the temperature is increased to 50circC.
🔍 Worked example — Exam-style
A student investigates the effect of CO_2 concentration. They use 0.1\%, 0.2\%, and 0.5\%text sodium hydrogencarbonate. The rates of oxygen production are 1.2, 2.4, and 2.5 cm^3 min^-1 respectively. Interpret these results regarding limiting factors.
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🎯 Exam question
Worked example — Model answer
Question. A grower recorded the rate of photosynthesis of a tomato crop at increasing light intensities, at two different carbon dioxide concentrations. At low light intensity the rate was the same for both carbon dioxide concentrations, but at high light intensity the rate at 0.09% CO₂ was greater than at 0.04% CO₂. (a) Explain why the two curves were identical at low light intensity. [2] (b) Explain why the rate at high light intensity was greater at 0.09% CO₂. [3]
🔍 Worked example — Easy
A student observes that increasing the concentration of CO_2 from 0.01\% to 0.05\% does not increase the rate of photosynthesis in a plant kept in very dim light. Identify the limiting factor and explain why the rate did not change.
🔍 Worked example — Standard
Calculate the percentage increase in the rate of photosynthesis when the light intensity is doubled, assuming the rate increases from 1.2 μ mol CO_2 g^-1 leaf^-1 min^-1 to 2.4 μ mol CO_2 g^-1 leaf^-1 min^-1 and light was the limiting factor.
🔍 Worked example — Exam-style
An experiment uses a suspension of chloroplasts and DCPIP (a redox indicator). If the rate of decolorization of DCPIP is used as a proxy for the rate of photosynthesis, explain why the rate of decolorization increases when light intensity increases, provided CO_2 is in excess.
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