Question

The action spectrum of photosynthesis shows a high light absorption for violet/blue and orange/red and low absorption for green/yellow. If we were to set up an experiment where narrow bands of unicoloured light were shown on an aquatic plant such as Hydrilla, and we counted the number of bubbles of oxygen produced as an indicator of photosynthetic rate, how would the graph appear?
A) Just like the action spectrum with peaks at violet/blue and orange/red and a trough at yellow/green
B) Exactly inverted from the action spectrum with a peak for yellow/green
C) Perfectly flat because many wavelengths are used by chlorophyll
D) A descending line because ultra violet has more energy than infrared

Answer 1

Hint: The action spectrum of photosynthesis shows a high light absorption for violet/blue and orange/red whereas low absorption for green/yellow. Visible light consists of electromagnetic waves, which have specific wavelengths and frequencies. Action spectrum of photosynthesis shows wavelengths used for light dependent reactions. It depicts which wavelength of light is most effectively used in a specific chemical reaction.

Complete answer:
Through the photosynthetic process plants take up carbon dioxide and release oxygen. The whole process is primarily dependent on light, oxygen, temperature and also the water supply from the surrounding environment. The efficiency of photosynthesis with monochromatic light, which is basically uni-colored light as a function of the wavelength of light, is known as the action spectrum of photosynthesis. The absorption of light by the different pigments is termed as an absorption spectrum, which is different from pigment to pigment. For photochemical reactions which involve a single pigment, the action spectrum has the same general shape as the absorption spectrum of the pigment.

Therefore we can say that the correct option is A which states that ‘Just like the action spectrum with peaks at violet/blue and orange/red and a trough at yellow/green’.

Note:
We must note that different colours have different wavelengths. A mix of violet blue has a greater content of blue light than red which is resultant in the peak. The action spectrum of photosynthesis is similar to that of the absorption spectrum of chlorophylls. The action spectrum shows the overall rate of photosynthesis at each wavelength of the light.