Practise Rate of photosynthesis. 19 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The factors that affect the rate of photosynthesis, the idea of a limiting factor, and the required practical on light intensity. Higher tier adds graphs with two or three factors, the inverse square law for light intensity, and whether it pays to add heat, light or carbon dioxide in a greenhouse. Expect graphs, calculations and practical questions, including 6-mark method answers.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the factors affecting photosynthesis rateTemperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
4
Explain what a limiting factor isThe factor in shortest supply, which stops the rate of photosynthesis increasing; raising it raises the rate.
4
Describe the light intensity required practicalPut pondweed at different distances from a lamp and count the oxygen bubbles, or measure the volume of oxygen, in a set time.
5
Explain the effect of temperatureThe rate rises up to an optimum temperature, then falls quickly because the enzymes that control photosynthesis denature.
5
Explain a one-factor rate graphWhere the line rises the factor on the x-axis is limiting; where it levels off another factor has become limiting.
7
Find the limiting factor on multi-factor graphs (HT)Compare curves: if raising the second factor raises the curve, that factor was limiting the lower curve.
7
Use the inverse square law (HT)Light intensity is proportional to 1 ÷ distance2, so doubling the distance gives a quarter of the light intensity.
8
Evaluate greenhouse costs against extra yield (HT)Use data to decide whether the value of the extra crop from adding heat, light or carbon dioxide is more than the extra cost.
Notes
Factors and limiting factors
Four factors affect the rate of photosynthesis: temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
A limiting factor is the one in shortest supply. It holds the rate back: increasing it increases the rate, but increasing the other factors does not.
Light intensity and carbon dioxide: the rate rises as the factor increases, then levels off when a different factor becomes limiting.
Temperature: photosynthesis is controlled by enzymes, so the rate rises up to an optimum and then falls steeply as the enzymes denature.
Chlorophyll absorbs the light, so leaves with less chlorophyll (for example, leaves damaged by tobacco mosaic virus) photosynthesise more slowly.
Required practical: light intensity and pondweed
Put pondweed in a boiling tube of dilute sodium hydrogencarbonate solution, which supplies carbon dioxide.
Place a lamp 10 cm away, measured with a metre rule, and leave the pondweed a few minutes to adjust.
Count the oxygen bubbles released in one minute, or collect the gas in an upturned measuring cylinder for a set time.
Repeat at 20, 30, 40 and 50 cm, three times at each distance, and calculate means.
Control the temperature (LED lamp, or a beaker of water as a heat shield), the carbon dioxide concentration and the piece of pondweed. Darken the room so the lamp is the only light.
Rate of photosynthesis = volume of oxygen (or number of bubbles) ÷ time.
Inverse square law and graphs with more than one factor (Higher tier only) grade 7+
Light intensity is inversely proportional to the square of the distance from the lamp: \(\text{light intensity} \propto \dfrac{1}{d^2}\).
Double the distance and the light intensity falls to a quarter; triple it and it falls to a ninth.
On a graph with two or three curves, where the curves rise together the x-axis factor is limiting.
Where a curve levels off, look at the curve above it: if a higher carbon dioxide concentration or temperature gives a higher rate, that factor was limiting the lower curve.
Greenhouse economics (Higher tier only) grade 8+
Growers can add heat, light and carbon dioxide to a greenhouse to increase the rate of photosynthesis, and so the yield.
A paraffin heater releases carbon dioxide as well as heat.
Adding more of a factor that is not limiting wastes money, because the rate does not increase.
It is only worth it if the value of the extra crop is greater than the extra cost: the aim is the maximum rate while still making a profit.
Cheatsheet
Factors: temperature, light intensity, carbon dioxide concentration, amount of chlorophyll
Limiting factor = the factor in shortest supply, which stops the rate increasing
Rate = volume of oxygen (or number of bubbles) ÷ time
Too hot: enzymes denature, so the rate falls
Sodium hydrogencarbonate solution supplies carbon dioxide in the pondweed practical
\(\text{light intensity} \propto \dfrac{1}{d^2}\): double the distance → ¼ of the light intensity grade 7+
Extra heat, light or CO2 is worth it only if the value of the extra crop is more than the extra cost grade 8+
How to answer each type of question
Describe and explain a rate graph with one factor
2 to 3 marks5
Describe the rising part, then the flat part, quoting values from the graph.
Explain the rising part: the factor on the x-axis is limiting.
Explain the flat part: name another factor that is now limiting.
Example. A graph shows that the rate of photosynthesis of a plant increases as light intensity increases from 0 to 6 arbitrary units. From 6 to 10 arbitrary units the rate stays constant. Explain the shape of the graph.
Show the model answer
From 0 to 6 units, light intensity is the limiting factor (1), so increasing it increases the rate of photosynthesis (1). Above 6 units another factor, such as carbon dioxide concentration or temperature, is limiting, so the rate stops increasing (1).
6-mark: describe the required practical
6 marks6
Say what you change and how: the distance of the lamp, measured with a metre rule.
Say what you measure and how: bubbles counted, or volume of oxygen collected, in a set time.
Give at least two control variables and how you would control them.
Include repeats and a mean, and how you would turn the results into a rate.
Example. Describe a method to investigate how light intensity affects the rate of photosynthesis of pondweed. [6 marks]
Show the model answer
Marked by levels. A Level 3 answer (5 to 6 marks) is a clear, logical method that includes most of these points: put a piece of pondweed in a boiling tube of sodium hydrogencarbonate solution; place a lamp 10 cm away, measured with a metre rule; leave for 5 minutes to adjust; count the bubbles released in 1 minute or collect the oxygen in an upturned measuring cylinder; repeat with the lamp at 20, 30, 40 and 50 cm; do three repeats at each distance and calculate a mean; keep the temperature constant with an LED lamp or a heat shield; use the same pondweed and the same concentration of sodium hydrogencarbonate; darken the room.
Calculate using the inverse square law (Higher tier only)
2 to 3 marks7
Find how many times further away the lamp is: new distance ÷ old distance.
Square that number to find how many times smaller the light intensity is.
If the rate is proportional to light intensity, divide the rate by the same number.
Example. A lamp is moved from 12 cm to 36 cm away from some pondweed. Light intensity is inversely proportional to the square of the distance. (a) How many times smaller is the light intensity at 36 cm? (b) At 12 cm the rate was 45 bubbles per minute. Assume light intensity is the only limiting factor and the rate is directly proportional to it. Predict the rate at 36 cm.
Show the model answer
(a) 36 ÷ 12 = 3 (1); 32 = 9, so 9 times smaller (1) (b) 45 ÷ 9 = 5 bubbles per minute (1)
Identify the limiting factor from data with two factors (Higher tier only)
2 marks each7
Name one factor as your answer.
Give a reason from the data: does increasing the x-axis factor raise the rate, and does changing the other factor make a difference?
Example. Rates of photosynthesis of a plant at 20 °C, in arbitrary units: Light intensity: 2, 4, 6, 8 Rate at 0.04% carbon dioxide: 10, 18, 20, 20 Rate at 0.10% carbon dioxide: 10, 20, 28, 30 (a) Which factor limits the rate at a light intensity of 2? Give a reason. (b) Which factor limits the rate at 0.04% carbon dioxide and a light intensity of 8? Give a reason.
Show the model answer
(a) Light intensity (1); the rate is the same at both carbon dioxide concentrations, so carbon dioxide is not limiting (1) (b) Carbon dioxide concentration (1); increasing light intensity from 6 to 8 does not increase the rate, but at 0.10% carbon dioxide the rate is higher (1)
Evaluate greenhouse data (Higher tier only)
3 to 4 marks8
For each option, convert the extra yield into money: extra yield × price.
Subtract the extra cost to find the extra profit or loss.
Give a clear conclusion that uses your numbers, and link it to limiting factors if you can.
Example. A grower sells lettuces for £2.00 per kg. Compared with an unheated greenhouse with normal air: heating increases the yield by 40 kg per week and costs £50 per week; adding carbon dioxide increases the yield by 15 kg per week and costs £35 per week. Use the data to evaluate which, if either, the grower should use. [4 marks]
Show the model answer
Heating: 40 × 2.00 = £80 of extra lettuces (1), so £80 − £50 = £30 extra profit per week (1). Carbon dioxide: 15 × 2.00 = £30, so £30 − £35 = a loss of £5 per week (1). The grower should heat the greenhouse but not add carbon dioxide; carbon dioxide was probably not the main limiting factor (1).
Shortcuts and memory tricks
Limiting factor = the weakest link: improve it and the rate rises; improve anything else and nothing changes.
Graph rule: rising line → the x-axis factor is limiting; flat line → something else is limiting.
Sense check: moving the lamp further away must give a lower light intensity and a lower (or equal) rate.
The temperature curve has the same shape as an enzyme activity curve (rise, peak at the optimum, steep fall) because enzymes control photosynthesis.
Where marks are lost
Writing 'more light, so the plant grows more' without saying that light is the limiting factor and the rate of photosynthesis increases.
Explaining a flat part of a graph with 'the plant is at its maximum'. Name another factor that is now limiting, such as carbon dioxide concentration or temperature.
Saying high temperatures 'kill' the enzymes. Say the enzymes are denatured.
Higher tier: halving the light intensity when the distance doubles (inverse proportion) instead of dividing it by four (inverse square).
In the practical, forgetting that a hot lamp warms the water, so temperature changes as well as light intensity.
Higher tier: comparing extra kilograms directly with extra pounds. Turn the extra yield into money first.
Exam technique
When asked which factor is limiting, name one factor and back it up with evidence from the graph or table.
In 6-mark method answers, cover what you change, what you measure, at least two control variables and repeats. Write the steps in order.
Use the full terms: 'light intensity', not just 'light', and 'carbon dioxide concentration', not 'more air'.
Higher tier: show every step of an inverse square calculation (distance ratio, square it, then divide) so you can gain method marks.
Higher tier: in greenhouse questions, work out profit = value of extra crop − extra cost for every option before you give your conclusion.
Required practical:Photosynthesis (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Pondweed releases bubbles of gas when it photosynthesises. Name the gas.
Oxygen
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
The rate of photosynthesis in a plant can be limited by several factors.
(a) Give three factors that affect the rate of photosynthesis.[3]
(b) What is meant by a limiting factor?[1]
(c) Pondweed releases bubbles of gas when it photosynthesises. Name the gas.[1]
(d) Suggest how a student could measure the rate of photosynthesis of a piece of pondweed.[1]
Show the answer and mark scheme
(a)
temperature
light intensity
carbon dioxide concentration
amount of chlorophyll
(b)
the factor in shortest supply, which stops the rate of photosynthesis increasing
(c)Answer: Oxygen
oxygen
(d)
count the number of bubbles (of oxygen) produced per minute / measure the volume of gas produced per minute
Question 2Medium5 marks
Instead of counting bubbles, a student used a data logger with an oxygen probe placed in the water next to some pondweed, to measure the concentration of dissolved oxygen every 10 seconds.
(a) Suggest two advantages of using a data logger with an oxygen probe, rather than counting bubbles, to measure the rate of photosynthesis.[2]
(b) Explain why some of the oxygen produced by the pondweed might not be detected by the oxygen probe.[2]
(c) Suggest one reason the student should let the apparatus settle for a short time after changing the light intensity, before taking readings.[1]
Show the answer and mark scheme
(a)
it gives a continuous, more precise numerical measurement of the oxygen produced, rather than relying on counting bubbles of varying size
it can take readings automatically and very frequently, which would be difficult to do accurately by counting bubbles by eye
results are stored and processed by a computer, reducing recording errors
(b)
some oxygen may escape from the water as bubbles rather than dissolving
some of the oxygen produced may be used by the plant itself for respiration
(c)
to allow the rate of photosynthesis, and the dissolved oxygen concentration, to reach a new steady rate at the new light intensity
Question 3Hard7 marks
The light intensity from a lamp is inversely proportional to the square of the distance from the lamp: \(\text{light intensity} \propto \dfrac{1}{d^2}\)
(a) A lamp is moved from 10 cm to 20 cm away from some pondweed. Calculate the light intensity at 20 cm as a fraction of the light intensity at 10 cm.[1]
(b) The relative light intensity at a distance d can be calculated using: \(\text{relative light intensity} = \dfrac{1}{d^2}\) Calculate the relative light intensity at 25 cm. Give your answer in standard form.[2]
(c) With the lamp 10 cm from the pondweed, the rate of photosynthesis was 60 bubbles per minute. Assume the rate of photosynthesis is directly proportional to light intensity. Predict the rate at 30 cm.[2]
(d) The student actually counted 12 bubbles per minute at 30 cm. Suggest two reasons why this was higher than predicted.[2]
Show the answer and mark scheme
(a)Answer: ¼ (0.25)
¼ / 0.25
(b)Answer: 1.6 × 10−3 arbitrary units
1 ÷ 252 = 1 ÷ 625
1.6 × 10−3
(c)Answer: 6.7 bubbles per minute
light intensity at 30 cm is 1/9 of that at 10 cm / 60 × (10 ÷ 30)2
6.7 (bubbles per minute)
(d)
at 10 cm another factor (e.g. carbon dioxide / temperature) was limiting, so the rate at 10 cm was lower than it would be if only light were limiting
light from other sources (e.g. room lights / a window) also reached the pondweed
the rate of photosynthesis is not directly proportional to light intensity at high light intensity