what are light and dark reactions????????///

Asked by akshitajain | 8th Mar, 2009, 12:44: PM

Expert Answer:


Photosynthesis is a two stage process. The first process is the Light Dependent Process (Light reaction), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or Dark reaction) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.


In the Light Dependent Processes, light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process.


The light-independent reactions, are chemical reactions that convert carbon dioxide and other compounds into glucose. They occur in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes. These reactions take the products of the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin Cycle: Carbon Fixation, Reduction reactions, and ribulose 1,5-diphosphate (RuDP) regeneration. 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 photosynthesis, so the reactions cannot be said to be entirely light-independent.

Answered by  | 9th Mar, 2009, 10:57: AM

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