Introduction

Photosynthesis is the process by which plants prepare their food by using light energy synthesising organic compounds (sugar). Photosynthesis is the primary source of all food on earth. It also releases oxygen into the atmosphere. It is an anabolic process regulated by various enzymes. There are two types of reactions that occur during photosynthesis- light reaction and dark reaction. In light reactions, grana’s light energy is absorbed to synthesise ATP and NADPH. In a dark reaction, light is not necessary, and it occurs during the later part of photosynthesis, in which the carbon dioxide is reduced to sugar. The phase in which dark reaction occurs is considered the dark phase of photosynthesis, also known as the biosynthetic phase.

Light reaction and photochemical phase

• The first stage of the photosynthesis process is depicted by the light reaction.
• The light reaction involves converting solar energy to chemical energy in the form of ATP and NADPH. 
• The chlorophyll pigment absorbs the sunlight energy and converts it into chemical energy in the form of electron charge carrier molecules.
• Photosystem I and photosystem II, which are present inside the thylakoid membrane of the chloroplast, both utilise light energy. 
• The water molecule is split with the help of light energy, and then the electrons are extracted from photosystem II.
• The extracted electrons move from PSII to cytochrome and then to the PS I and then but electrons are reduced in the form of energy. 
• PS I again provide energy to the electrons.
• Then the electrons are reduced into NADP+ and NADPH.
• Thus, light reactions convert the energy from the light into ATP, which is a biologically useful form and also produces NADPH.

Photochemical and biosynthetic phases of photosynthesis

• The biosynthetic phase of photosynthesis is also known as the dark reaction and is referred to as the dark phase. 
• The biosynthetic phase does not require light, but its role is to deal with the products of light reactions. 
• It is light-independent and occurs in the later phase of photosynthesis.
• The main site of the dark reaction is the stroma of the chloroplast.
• In the biosynthetic phase of photosynthesis, the end product is glucose which is formed by using ATP and NADPH. 
• This phase requires the use of hydrogen ions.
• It is also completed in various steps and involves the conversion of simple substrates into more Complex products.
• Two main cycles are involved in the process of dark reaction- the Calvin cycle and C4 Pathway or Hatch and Slack Pathway.

The Calvin Cycle

All photosynthetic plants undergo the Calvin cycle. The three main stages of the Calvin cycle are as follows:

• Carboxylation: This process is also known as fixation. The carbon dioxide binds with RuBP, combines with an organic molecule, and forms two molecules of 3-PGA. This enzyme is found in the stroma, and slowly it forms molecules of RuBP.
• Reduction: Reduction is the second step in the Calvin cycle and involves a series of reactions that lead to glucose formation. The 3-PGA molecules are converted into the molecules of simple sugar- glucose. This stage uses the products of light reaction ATP and NADPH and converts the sunlight energy into chemical form. In this stage, the electrons are transferred from G3P to 3-PGA, hence referred to as reduction.

Factors that affect photosynthesis 

• Light: The rate of photosynthesis increases with the increased light intensity till the light saturation point.
• Carbon dioxide concentration: Till the compensation point is achieved, the rate of photosynthesis increases with the increase in the concentration of carbon dioxide.
• Temperature: At higher temperatures, the activity of enzymes is inhibited.
• Water: photosynthesis increases due to an increase in the amount of water, but after the saturation, there is no effect of water on photosynthesis.

Conclusion

Thus, photosynthesis is an important process that acts as a primary source for all food on the earth. There are two types of reactions that occur during photosynthesis- light reaction and dark reaction. The phase in which dark reaction occurs is considered the dark phase of photosynthesis and is also known as the biosynthetic phase. The light reaction is the first stage that involves converting solar energy to chemical energy in the form of ATP and NADPH. The biosynthetic phase of photosynthesis is also known as the dark reaction and is referred to as the dark phase. The biosynthetic phase does not require light, but its role is to deal with the products of light reactions. There are two main cycles that are involved in the process of dark reaction- the Calvin cycle and C4 Pathway or Hatch and Slack Pathway. The Calvin cycle includes three main stages- carboxylation, reduction and regeneration. At the end of the C4 pathway, CO2 released in the bundle sheath cells enters the Calvin cycle, where the enzyme RuBisCO forms sugar.