Explain light phase and dark phase

Asked by  | 20th Apr, 2009, 10:10: PM

Expert Answer:

The process of photosynthesis takes place in 2 stages:

i) Light phase or reaction      and 

ii) Dark  phase or reaction  


Light reaction is the first stage of photosynthesis. It is an energy trapping and photochemical reaction which takes place in the grana of chloroplasts and in presence of light. It results in the formation of ATP and NADPH2. During this reaction, chlorophyll-a absorbs light energy and gets excited, thereby releasing an energy rich electron, leaving it ionised. This electron passes through a series of electron acceptors and ATP is formed in the process. There are 2 pathways of electron transfer called cyclic electron transfer and non-cyclic electron transfer.


Dark phase or light-independent reactions is a synthetic process in which carbon dioxide is reduced to glucose. This process takes place in stroma and utilizes ATP and NADPH2. These reactions take the products of the light-dependent reactions and perform further chemical processes on them.

Though the light-independent reactions are also referred to as the dark reactions, that term may be misleading as they do not actually require darkness to proceed. The term "light-independent" is used to emphasize that the reactions occur regardless of the amount of light present as long as the proper substrate compounds are available. Even this term can be criticized, however, as the availability of substrates in plants depends on the light reactions, so the reactions cannot be said to be entirely "light-independent."


Answered by  | 21st Apr, 2009, 10:12: AM

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