LIght and Dark reaction

Asked by sonali221994 | 21st Dec, 2009, 06:50: PM

Expert Answer:

Photosynthesis occurs in two stages. 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.

Light reaction is an energy trapping, photochemical reaction, taking place in the grana of chloroplast and resulting in the formation of ATP and NADPH2. During light reaction, chlorophyll absorbs energy, gets excited and releases a high energy electron which passes through a system of electron acceptors. During this electron transfer, energy is released in the form of ATP. Water is split in the process, releasing oxygen as a by-product of the reaction.

In light reaction, there are 2 pathways of electron transfer, which are cyclic and non-cyclic electron transfer. During non-cyclic electron transfer, photolysis of water takes place. Under the influence of light, water molecule splits into H+ and OH-. Photolysis results in the generation of free oxygen (O2). This process is responsible for generating the majority of breathable oxygen in earth's atmosphere


The dark reactions or 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. Here, ATP and NADPH2 is utilized.



Answered by  | 22nd Dec, 2009, 04:57: PM

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