IGCSE Biology Paper-2: Specimen Questions with Answers 140 - 140 of 203

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Question 140



The diagram shows an experiment investigating the effect of light intensity on an aquatic plant.

The Effect of Light Intensity on an Aquatic Plant

Photosynthesis occurred in both test-tube P and test-tube Q. Both test tubes were kept at the same temperature. The number of bubbles produced in test-tube P was 5 bubbles per minute.

What is the most likely number of bubbles produced in one minute in test-tube Q?


Choice (4)













  • The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy, reaching to the earth about is used in photosynthesis and about is used in other metabolic activities.
  • Light varies in intensity, quality (Wavelength) and duration. The effect of light on photosynthesis can be studied under these three headings.
  • Light intensity: The total light perceived by a plant depends on its general form (viz. , height, size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80 % is absorbed, is reflected and 10 % is transmitted.
  • In general, rate of photosynthesis is more in intense light than diffused light. (Up to light is utilized in sugarcane, i.e.. , most efficient converter) .
  • Another photosynthetic superstar of field growing plants is Oenothera claviformis (winter evening primrose) , which utilizes about light.
  • However, this light intensity varies from plant to plant, e. g. , more in heliophytes (sun loving plants) and less in sciophyte (shade loving plants) . For a complete plant, rate of photosynthesis increases with increase in light intensity, except very high light intensity where ‘Solarisation’ phenomenon occurs, i.e.. , photo-oxidation of different cellular components including chlorophyll occurs.
  • Light quality: Photosynthetic pigments absorb visible part of the radiation i.e.. , to For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light. The green light has minimum effect. On the other hand, red algae show maximum photosynthesis in green light and brown algae in blue light.

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