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IGCSE Biology · 0610

IGCSE Biology: Plant Nutrition and Photosynthesis Questions

Photosynthesis questions reward precise equations and clear reasoning about limiting factors. These worked examples cover the word and symbol equations, graph interpretation and the leaf starch test exactly as Cambridge sets them.

What you need to know

  • Word equation: carbon dioxide plus water, in the presence of light and chlorophyll, produce glucose plus oxygen.
  • Balanced symbol equation: 6CO2 plus 6H2O gives C6H12O6 plus 6O2.
  • Chlorophyll in the chloroplasts absorbs light energy for the reaction.
  • Light intensity, carbon dioxide concentration and temperature can each act as a limiting factor.
Light intensity Rate of photosynthesis another factor limiting light limiting
As light intensity rises the rate increases, then plateaus when another factor becomes limiting.

Practice questions with answers

Question 12 marks

Write the word equation for photosynthesis.

Carbon dioxide plus water, in the presence of light and chlorophyll, produce glucose plus oxygen.

Mark schemeCorrect reactants: carbon dioxide and water (1). Correct products: glucose and oxygen (1). Light and chlorophyll shown above or with the arrow gains credit but is not required for both marks.

Common mistakeWriting oxygen as a reactant, or naming the product as starch rather than glucose.

Exam tipPut light and chlorophyll above the arrow, not in the list of reactants.

Question 23 marks

A graph shows the rate of photosynthesis rising as light intensity increases, then levelling off. Explain the shape of the graph.

At low light intensity, light is the limiting factor, so as light increases the rate increases. At high light intensity, light is no longer limiting. Another factor, such as carbon dioxide concentration or temperature, becomes the limiting factor, so the rate levels off.

Mark schemeLight limiting at low intensity so rate rises (1). Rate levels off (1). Another named factor, carbon dioxide or temperature, becomes limiting (1).

Common mistakeSaying the plant is tired or has enough light without naming the new limiting factor.

Exam tipName the second limiting factor explicitly to secure the final mark.

Question 34 marks

Describe how you would test a leaf to show that starch is present.

Boil the leaf in water to kill it and break down the cell membranes. Then boil it in ethanol using a water bath to remove the chlorophyll. Dip the leaf in hot water to soften it, then add iodine solution. A blue-black colour shows that starch is present.

Mark schemeBoil in water (1). Ethanol to remove chlorophyll, using a water bath, no naked flame (1). Add iodine solution (1). Blue-black shows starch (1).

Common mistakeHeating ethanol directly over a flame. Ethanol is flammable, so a water bath must be used.

Exam tipMention the water bath. Cambridge often awards a mark for the safe method with ethanol.

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Frequently asked questions

What is the balanced equation for photosynthesis?

6CO2 plus 6H2O produces C6H12O6 plus 6O2, in the presence of light and chlorophyll.

What are the limiting factors of photosynthesis?

Light intensity, carbon dioxide concentration and temperature. The one in shortest supply limits the overall rate.

Why is a leaf boiled in ethanol during the starch test?

Ethanol removes the green chlorophyll so the colour change with iodine can be seen clearly against the pale leaf.

What colour does iodine turn if starch is present?

Iodine solution turns from orange-brown to blue-black when starch is present.

More IGCSE Biology practice

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