Name four factors that can limit photosynthesis.

Light intensity, carbon dioxide concentration, temperature and chlorophyll.

Why does the rate increase with light intensity at first?

Light provides the energy for photosynthesis.

What does a plateau on a rate graph mean?

A different factor is now limiting the rate.

Why does the rate fall above the optimum temperature?

Enzymes are denatured, so they can no longer work.

What is the carbon dioxide concentration of air?

About 0.04%, so it is often limiting.

What is the independent variable when moving a lamp towards pondweed?

The distance of the lamp from the pondweed.

How can the temperature be controlled in a pondweed investigation?

Place the beaker in a water bath or use a heat shield.

Why is counting bubbles inaccurate?

Bubbles vary in size; a gas syringe measures volume more accurately.

Why do yellow leaves photosynthesise slowly?

They have less chlorophyll, so absorb less light.

Why do greenhouse growers add carbon dioxide?

To raise the rate when carbon dioxide is the limiting factor.

Factors affecting photosynthesisIB MYP Biology: Flashcards

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What is a limiting factor?

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What is a limiting factor?: The factor in shortest supply, which sets the rate of photosynthesis.
Name four factors that can limit photosynthesis.: Light intensity, carbon dioxide concentration, temperature and chlorophyll.
Why does the rate increase with light intensity at first?: Light provides the energy for photosynthesis.
What does a plateau on a rate graph mean?: A different factor is now limiting the rate.
Why does the rate fall above the optimum temperature?: Enzymes are denatured, so they can no longer work.
What is the carbon dioxide concentration of air?: About 0.04%, so it is often limiting.
What gas bubbles from pondweed during photosynthesis?: Oxygen.
What is the independent variable when moving a lamp towards pondweed?: The distance of the lamp from the pondweed.
How can the temperature be controlled in a pondweed investigation?: Place the beaker in a water bath or use a heat shield.
Why is counting bubbles inaccurate?: Bubbles vary in size; a gas syringe measures volume more accurately.
What could be added to the water to increase carbon dioxide?: Sodium hydrogencarbonate.
Why do yellow leaves photosynthesise slowly?: They have less chlorophyll, so absorb less light.
Why do greenhouse growers add carbon dioxide?: To raise the rate when carbon dioxide is the limiting factor.

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

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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).