Factors affecting photosynthesisIB MYP Biology: Revision notes
Section 1
Limiting factors
The rate of photosynthesis depends on several factors. A limiting factor is the factor that is in shortest supply, so it sets the rate. Increasing a limiting factor increases the rate, until another factor becomes limiting.
The four main factors:
- Light intensity: light provides the energy.
- Carbon dioxide concentration: a raw material. Air contains only about 0.04%, so it is often limiting.
- Temperature: affects the speed of the enzymes that control photosynthesis.
- Chlorophyll: needed to absorb light. Less chlorophyll (for example, yellow leaves lacking magnesium) means slower photosynthesis.
At night, light is limiting. On a cold bright day, temperature is often limiting.
Do not say a limiting factor 'stops' photosynthesis. It limits (sets) the rate.
Section 2
Temperature and enzymes
As the temperature rises, particles move faster and collide more often, so the rate increases. Photosynthesis is controlled by enzymes. Above the optimum temperature (roughly 40 °C for many plants) the enzymes begin to be denatured: their active sites change shape and cannot work, so the rate falls sharply.
So a temperature graph rises, reaches a peak, then falls steeply.
Section 3
Reading rate graphs
When you read a rate graph described in a question:
- A rising section means the factor on the x-axis is limiting.
- A flat section (plateau) means a different factor is limiting.
- A fall at high temperature is due to enzymes being denatured.
Example: the rate rises steadily to 8 units at 40 au and then stays at 8 units. Light is limiting up to 40 au; above that, carbon dioxide or temperature is limiting. If the rate is proportional to light, then at 20 au it is 4 units.
When the graph levels off, name the factor that is NOT on the x-axis.
Section 4
Investigating with pondweed
Aquatic plants such as pondweed release bubbles of oxygen when they photosynthesise. To investigate light intensity:
- Place a piece of pondweed in water with a lamp at a measured distance.
- Count the bubbles from the cut stem in one minute.
- Repeat at different distances and find a mean.
Variables: independent = distance from the lamp; dependent = bubbles per minute; control = same pondweed, same temperature (use a water bath or heat shield) and same carbon dioxide (add sodium hydrogencarbonate).
Counting bubbles is not very accurate because bubbles are different sizes. A gas syringe measures the volume of oxygen more accurately.
Must know
- Light, carbon dioxide, temperature and chlorophyll can limit photosynthesis.
- A limiting factor is the one in shortest supply.
- A plateau means a different factor is limiting.
- High temperatures denature enzymes, so the rate falls.
- Pondweed bubble counting measures rate, but is not accurate.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Factors affecting photosynthesis
- A student in Dubai grows basil in pots on a balcony. In summer the afternoon temperature on the balcony can rise above 45 °C, and in the early morning, just after sunrise, the light is dim. The student notices that the plants grow best in the spring.Explain why the rate of photosynthesis of the basil falls when the temperature rises above about 40 °C.2 marks
- A student measures the rate of photosynthesis of a plant at 25 °C in air containing 0.04% carbon dioxide. From 0 to 40 arbitrary units (au) of light intensity the rate rises in a straight line from zero, reaching a rate of 8 units at 40 au. From 40 au to 100 au the rate stays at 8 units. At 80 au she then increases the carbon dioxide concentration to 0.1%, and the rate rises to 14 units.Explain why the rate stayed at 8 units above 40 au, and why it then rose when the carbon dioxide concentration was increased.2 marks
- A student investigates how light intensity affects the rate of photosynthesis of pondweed. She places a piece of pondweed in a beaker of water and shines a lamp on it from different distances. She counts the bubbles of gas released from the cut stem in one minute. She records 48 bubbles per minute at 10 cm from the lamp, 22 at 20 cm, 10 at 30 cm and 5 at 40 cm.Describe how the student should carry out this investigation so that her results are valid.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).