The light-dependent reactions occur only during the daylight .i.e in the presence of sunlight and are mainly observed in the plants, particularly in the biological process of photosynthesis. The light reactions usually take place in the structures called Grana.
Grana are membrane-bound sacs called as thylakoids which are present within the chloroplasts. These structures harvest the light called as photosystems; a chloroplast has around a various number of photosystems.
The photosystems have pigment molecules. In the plants, chlorophyll is one of the primary pigment which actively takes part in the process of light reactions like photosynthesis, but the accessory pigment like carotenoids are also present. These pigments allow the plant to absorb a wide range of spectrum than wavelengths of the light is absorbed by chlorophyll.
In the light-dependent reactions, the light energy is converted into NADPH and ATP.
Mechanism of Light Reactions
Photosynthesis is the first stage of light-dependent reactions in which the solar energy is converted into chemical energy in the form of ATP and NADPH. The by-products obtained are utilized in the light-independent reactions. The protein complexes and the pigment molecules help in the production of NADPH as well as ATP.
Process of Light Reactions
The process of light-dependent reactions is as given below-
- In light reactions, energy from the sunlight is absorbed by the pigment chlorophyll and is converted into chemical energy in the form of electron charge carrier molecules such as NADPH and ATP.
- Light energy is utilized in both the Photosystems 1 and 2, present inside thylakoid membranes of the chloroplasts.
- The carbohydrate molecules are obtained from the carbon dioxide by the use of chemical energy gathered during the reactions.
- The Light energy tends to split into water and later extracts the electrons from the photosystem 2; then the electrons move from the PSII to b6f( cytochrome) to the photosystem 1 (PS1) and reduce in the form of energy.
- The electrons are re-energized in the Photosystems 1 and the electrons of high energy reduce NADP+ into NADPH.
- In the process of non-cyclic photophosphorylation, the cytochrome uses the electron energy from Photosystem 2 to pump the ions of hydrogen from the lumen to stroma; later, this energy allows the ATP synthase to bind to the third phosphate group to the ADP molecule, which then forms the ATP.
- In the process of cyclic photophosphorylation, the cytochrome b6f uses electron energy from both the Photosystems 1 and 2 to create a number of ATP and stops the production of the NADPH, thus maintaining the right quantities of ATP and NADPH.
Thus, the light reactions harness the light energy to drive the transport of electrons and the pumping of the proton, to convert the energy from the light into the biological and useful form of the ATP and to biologically produce a usable source of reducing the power -NADPH.
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