🌡 Temperature
Photosynthesis is controlled by enzymes. As the temperature rises the reaction speeds up, because the particles move faster and meet more often.
Above the optimum temperature the enzymes become denatured and the rate falls quickly.
Photosynthesis » Rate of Photosynthesis
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.4.1.2
- How temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll affect the rate of photosynthesis
- How to measure and calculate a rate of photosynthesis
- How to read, plot and draw graphs of photosynthesis rate
- How to move between a graph and a table of numbers
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Unlock This CourseWhat is the rate of photosynthesis?
Some plants make glucose quickly and some make it slowly. The same plant can make glucose faster on a bright summer day than on a dull winter one. How fast photosynthesis happens is called its rate.
Four things can change the rate: temperature, light intensity, the concentration of carbon dioxide, and the amount of chlorophyll in the plant. Each one is a limiting factor when it is in short supply, because it stops the rate going any higher.
Key terms:
- Rate of photosynthesis: how much photosynthesis happens in a set amount of time.
- Light intensity: how bright the light is.
- Limiting factor: something that is in short supply and so stops the rate of photosynthesis from going up.
- Plateau: the flat part of a graph where the line stops rising.
How the four factors change the rate
☀ Light intensity
Light supplies the energy for the reaction. In dim light the rate is low. As the light gets brighter, the rate goes up.
At very low light intensity photosynthesis nearly stops, because there is not enough energy.
🍃 Carbon dioxide concentration
Carbon dioxide is a reactant. Air contains only a small amount, about 0.04%, so it is often in short supply.
More carbon dioxide means more of the reactant, so the rate goes up.
🌿 Amount of chlorophyll
Chlorophyll absorbs the light. A leaf with less chlorophyll absorbs less light, so photosynthesis is slower.
Pale, yellow or patchy leaves have less chlorophyll than healthy green ones.
Common mistakes
Many students write that a higher temperature always means a faster rate. It does not. After the optimum, the rate drops because the enzymes are denatured. Another mistake is to say that the plant "burns" or "dies" from too much heat. The correct answer is that the enzymes are denatured.
Measuring and calculating the rate
Photosynthesis makes oxygen, so you can measure the rate by finding how much oxygen a plant gives off. A water plant such as pondweed gives off bubbles of oxygen that you can count, or collect and measure.
You could also measure how much carbon dioxide is used up, or how much glucose is made, but oxygen is the easiest to see.
The calculation
rate = amount of oxygen produced ÷ time taken
Worked example
A piece of pondweed gives off 18 bubbles in 3 minutes. What is the rate?
rate = 18 ÷ 3 = 6 bubbles per minute.
A second plant collects 24 cm3 of oxygen in 8 minutes. rate = 24 ÷ 8 = 3 cm3 per minute.
Always give the unit. It tells the reader what you measured and over what time, for example cm3/min or bubbles per minute. To compare two plants fairly, work out a rate per minute for each.
Reading graphs with one limiting factor
Scientists show the effect of one factor on a line graph. The factor you change goes on the x-axis. The rate of photosynthesis goes on the y-axis.
Light intensity results (rate in cm3 of oxygen per minute)
Light intensity (arbitrary units): 0, 1, 2, 3, 4, 5
Rate: 0, 2, 4, 6, 6, 6
Look at the shape of the line:
- The rising part (0 to 3 units): the rate goes up as light intensity goes up. Light is the limiting factor here, because adding more light makes the rate faster.
- The plateau (3 units and above): the rate stays the same at 6 cm3/min, even though the light gets brighter. Light is no longer what limits the rate. The lesson Limiting Factors and Greenhouses looks at what takes over.
The carbon dioxide graph has the same rise-then-flatten shape. The temperature graph is different:
📈 Rising
Rate increases as temperature increases, because the enzymes work faster.
🏆 Peak
The highest point is at the optimum temperature.
📉 Falling
The rate drops steeply as the enzymes are denatured.
When you answer a question, quote numbers from the graph. For example: "between 1 and 3 units the rate triples from 2 to 6 cm3/min". Do not just say it goes up.
Plotting your own graph
Here is a set of results for temperature:
Temperature results
Temperature (°C): 10, 20, 30, 40, 50, 60
Rate (cm3 of oxygen per minute): 2, 4, 8, 10, 3, 0
- Put the factor you changed (temperature) on the x-axis and the rate on the y-axis.
- Choose a scale so that the points fill more than half of the grid. Count in equal steps such as 2s or 10s.
- Label each axis with its name and unit.
- Plot each point carefully with a small cross.
- Join the points with a smooth curve or a line of best fit. Do not force the line through every point if there is a clear smooth trend.
With a graph drawn, you can translate between graph and numbers. You can read a value by going up from the x-axis to the line and then across to the y-axis. You can also turn values from a graph back into a table.
Worked example
From the table above, at what temperature is the rate highest?
The highest rate is 10 cm3/min at 40 °C.
What is the change in rate from 30 °C to 40 °C?
10 − 8 = 2 cm3/min increase.
Exam-style question
A student measured the oxygen collected from pondweed in 5 minutes at different temperatures.
Temperature (°C): 10, 20, 30, 40, 50
Volume of oxygen in 5 minutes (cm3): 5, 10, 20, 25, 5
(a) Calculate the rate of photosynthesis at 30 °C. Give the unit. [2 marks]
(b) Describe how the rate changes from 10 °C to 40 °C. [2 marks]
(c) Explain why the rate is lower at 50 °C than at 40 °C. [2 marks]
Model answer
(a) 20 ÷ 5 (1) = 4 cm3/min (1).
(b) The rate increases as the temperature increases (1), from 1 to 5 cm3/min (1).
(c) The temperature is above the optimum, so the enzymes are denatured (1) and the reaction is slower (1).
Exam tip
In part (a), the volume is for 5 minutes, not 1 minute. Divide by the time to get a rate per minute, and do not forget the unit.