AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Bioenergetics › Photosynthesis
Practise Rate of photosynthesis. 8 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. Expect graphs, calculations and practical questions, including 6-mark method answers.
Key facts
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
Notes
Factors and limiting factors
diagram
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.
Left: the rate levels off when another factor limits it. Right: past the optimum, 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
diagram
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.
Move the lamp to change the distance, and count the bubbles each time.
How to answer each type of question
Describe and explain a rate graph with one factor
2 to 3 marksGrade 5
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. The graph shows how the rate of photosynthesis of a plant changes with light intensity.
Explain the shape of the graph.
Show the model answerHide 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).
Don’t lose marks
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.
In the practical, forgetting that a hot lamp warms the water, so temperature changes as well as light intensity.
More tips
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.
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'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the factors affecting photosynthesis rateTemperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
Grade 4
Explain what a limiting factor isThe factor in shortest supply, which stops the rate of photosynthesis increasing; raising it raises the rate.
Grade 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.
Grade 5
Explain the effect of temperatureThe rate rises up to an optimum temperature, then falls quickly because the enzymes that control photosynthesis denature.
Grade 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.
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