**Light and Dark Reactions in Photosynthesis**

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. It occurs in two main stages: the light reactions and the dark reactions.

**1. Light Reactions**
The light reactions occur in the thylakoid membrane of the chloroplasts and require light energy to take place. These reactions involve the conversion of light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

**2. Dark Reactions**
The dark reactions, also known as the Calvin cycle or the light-independent reactions, occur in the stroma of the chloroplasts and do not directly require light energy. These reactions utilize the ATP and NADPH produced during the light reactions to convert carbon dioxide (CO2) into glucose.

Now let's discuss the specific roles of light reactions in the dark reactions:

**a) Carboxylation of RUBP (Ribulose-1,5-bisphosphate)**
RUBP carboxylation is the first step of the Calvin cycle, and it involves the attachment of carbon dioxide to the five-carbon sugar molecule, RUBP. This reaction is catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). The carboxylation of RUBP requires ATP and NADPH, which are produced during the light reactions. ATP provides the necessary energy for the reaction, while NADPH provides the reducing power.

**b) Regeneration of RUBP**
After carboxylation, RUBP is converted into two molecules of 3-phosphoglycerate (PGA). To continue the Calvin cycle, RUBP needs to be regenerated for further carboxylation. The regeneration of RUBP requires ATP, which is generated during the light reactions. ATP provides the energy required to convert PGA into RUBP, ensuring the continuous functioning of the Calvin cycle.

**c) Reduction of PGA**
The reduction of PGA is the next step in the Calvin cycle. PGA is converted into glyceraldehyde-3-phosphate (G3P), which is a three-carbon molecule. This conversion requires ATP and NADPH, both of which are generated during the light reactions. ATP provides energy, while NADPH provides the necessary electrons and hydrogen ions for the reduction reaction.

**d) Formation of Hexose Sugar**
Finally, G3P molecules are used to form hexose sugars, such as glucose and fructose. Several rounds of the Calvin cycle are required to produce one molecule of hexose sugar. The energy and reducing power provided by the light reactions (ATP and NADPH) are essential for the synthesis of hexose sugars.

In summary, the light reactions of photosynthesis provide the necessary ATP and NADPH for the dark reactions to occur. These reactions are interconnected and work together to convert carbon dioxide into glucose, ultimately storing light energy in the form of chemical energy.

Dark reaction depend on product of light reaction that is ATP and NADPH and for carboxylatiin of RUBP and reduction of PGA and regeneration of RUBP
ATP and NADPH are required