Light Reaction: Light reaction is the first stage of photosynthesis process in which solar energy is converted into chemical energy in the form of ATP and NADPH. The protein complexes and the pigment molecules help in the production of NADPH and ATP.
Light from the Sun is captured during photosynthesis via a series of reactions, involving the chemical chlorophyll. This results in the synthesis of two high-energy chemical compounds: ATP and NADPH, the latter of which has chemical energy that can be easily transferred to other compounds. This series of reactions necessitates the use of water, from which oxygen is released during the process. In the following phase of photosynthesis, the dark reactions, ATP and NADPH, are used to produce glucose from Carbon dioxide.
The light reaction uses water, light (brought in from outside the cell), NADP+, phosphorus, and ADP (all created by the Calvin cycle) to produce ATP (both used in the Calvin cycle), and oxygen (a waste product).
The light excites the electrons, and the excited electrons’ energy is then used to join ADP and phosphate together to form ATP.
NADP+ then joins with the excited electrons.
Water is split in the process, and oxygen is generated as waste and is discharged.
In light reactions, sunlight energy is used to oxidise water (an electron donor) to oxygen, and then to pass these electrons to NADP+, resulting in NADPH. The conversion of ADP to ATP requires some light energy. The NADPH and ATP created, are then used to run the Calvin cycle, which produces sugar.
Water is oxidised, electrons from water are transported to NADP+, and ATP is generated during the light processes. Equations can be used to summarise the three elements of the light reaction.
Water oxidation is a process that occurs when water is exposed to oxygen
2H2O → O2+4e− + 4H + 2H2O → O2 + 4e− + 4H+
NADP reduction
2NADP+ + 4e− + 4H+ → 2NADPH + 2H + 2NADP+ + 4e− + 4H+ → 2NADPH + 2H+
Synthesis of ATP
ATP → ADP + PiATP → ADP + Pi
The general equation for light reactions is obtained by combining these equations as given below:
2H2O + 2NADP+ + nADP + nPi + hv → O2 + 2NADPH + 2H+ + nATP
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