Photosynthesis is the process by which light energy is converted into chemical energy, which can then be used by living organisms for energy production. It is a lengthy and complicated procedure that is carried out in several stages over time.

The process of photosynthesis is divided into two phases:

• The photochemical phase, also known as the light-dependent process, is the first phase.
• The second phase of the dark reaction of photosynthesis is known as the biosynthetic phase, and it is also known as the light reaction. This is the phase of the process that is not dependent on light.

All of the steps involved in photosynthesis take place entirely within the chloroplast.

Light Reaction and Dark Reaction

As previously stated, photosynthesis takes place in two phases: the light reaction and the dark reaction.

Dark Reaction

• Carbon-fixing reaction is another name for the dark reaction. 
• It is a light-independent process that results in the formation of sugar molecules from carbon dioxide and water molecules.
• The dark reaction takes place in the stroma of the chloroplast, where the products of the light reaction are used to fuel the process.
• Carbon dioxide is captured by plants from the atmosphere through their stomata, which then transports the carbon dioxide to the Calvin cycle.
• As a result of light-induced adenosine triphosphate (ATP) and NADPH formation, the Calvin cycle drives the reaction and converts six molecules of carbon dioxide into one sugar molecule, which is glucose.

Dark Reaction Process 

ATP releases stored energy that is used to transform molecules of carbon dioxide into sugars and other nutrients during the second phase of photosynthesis, which does not takes place in the presence of light.

To photosynthesize, the dark reactions of photosynthesis do not involve the addition of light.

The energy generated from ATP drives the formation of organic molecules from carbon dioxide during the second stage of photosynthesis, which does not require light.

What is Light Reaction?

• The light reaction, also known as the photolysis reaction, is a chemical reaction that occurs in the presence of light. 
• In chloroplasts, it usually takes place in the grana of the chloroplast.
• Pigment molecules are found in the photosystems. 
• Chlorophyll is one of the primary pigments found in plants, and it plays an important role in the process of light reactions such as photosynthesis. 
• Carotenoids are included among the accessory pigments. The chlorophyll in the thylakoid membrane of the chloroplasts is responsible for absorbing the energy from the sunlight. 
• The energy is then transferred to ATP and NADPH, which are both produced by two-electron transport chains, and used for energy storage. During the process, water is used, and oxygen is released into the atmosphere.

Light Reaction

In the light reaction, a series of events such as light absorption, hydrolysis, the release of oxygen, and the formation of ATP and NADPH take place in response to the presence of light in the environment.

The photosynthesis light reaction can only begin when the organism is exposed to sufficient light energy.

The photosystem is the arrangement of pigments, including chlorophyll, within thylakoids that allows light to pass through them.

Photosystem I (PS-I) and Photosystem II (PS-II) are the two types of photosystems found in plants (PS-II)

Photosystem I absorbs light at a wavelength of 700 nm, whereas Photosystem II absorbs light at a wavelength of 680 nm. Photosystem I and Photosystem II are complementary photosystems.

The thylakoids of the chloroplast are where the light reaction takes place. Upon contact with the light, chlorophyll an is excited to a higher energy state, which is followed by a series of chemical reactions. PS I and PS II are responsible for converting this energy into energy molecules such as ATP and NADPH. Additionally, hydrolysis occurs, resulting in the release of oxygen.

Process of Light Reaction

ATP and NADPH are produced as a result of the first stage of the photosynthesis process, during which solar energy is converted into chemical energy in the form of ATP. The protein complexes and pigment molecules both contribute to the production of NADPH and ATP, respectively.

The following is a description of the light reaction process:

• When sunlight is absorbed by the pigment chlorophyll, it is converted into chemical energy in the form of electron charge carrier molecules such as NADPH and ATP.  Light energy is utilised by both Photosystems I and II, which are found within the thylakoid membranes of the chloroplast.
• The carbohydrate molecules are formed from carbon dioxide through the use of chemical energy that is generated during the reactions.
• Photosystem II (PSII) electrons move from the photosystem II to b6f (cytochrome) electrons move from the photosystem II to the photosystem I (PSI) electrons move from the photosystem I (PSI) electrons reduce in the form of energy
• A re-energization of the electrons takes place in Photosystem I, and the electrons with high energy are responsible for the reduction of NADP+ into NADPH.
• Non-cyclic photophosphorylation occurs when the cytochrome uses the electron energy from Photosystem II to pump hydrogen ions from the lumen to the stroma; later, this energy is used by the ATP synthase to attach to the third phosphate group of an ADP molecule, resulting in the formation of ATP.
•  When performing cyclic photophosphorylation, the cytochrome b6f utilises electron energy from both the Photosystems I and II to generate a large amount of ATP while simultaneously inhibiting the production of NADPH, thereby maintaining the proper amounts of ATP and NADPH in the cell.
• This is accomplished through the use of photons to drive electron transport and proton pumping, converting the energy from the light into the biologically useful form of ATP, and creating a usable source of reducing the power NADPH.

Conclusion:

Photosynthesis and respiration are two processes that work in tandem. Respiration is responsible for the consumption of the products of photosynthesis (sugars).

Photosynthesis is the process by which unusable carbon from the atmosphere is fixed. Carbon, in the form of CO2 gas, is converted into sugars, which are then available for use by all living things on the planet. Photosynthesis is the process by which sugars and oxygen are produced. Photosynthesis uses CO2, H2O, and light energy to produce energy.It is possible to divide photosynthesis into two complementary sets of reactions: those that are light-dependent (light reactions) and those that are light-independent (dark reactions).

The light reactions result in the production of ATP, NADPH, and O2.The light reactions consume H2O as a byproduct. Chemiosmosis is used to synthesise the ATP that is generated by the light reactions. Using a proton gradient created by an electron transport chain, this can be accomplished.The dark reactions are a cyclic set of reactions that begin with carbon fixation and end with the production of sugars.In contrast to the light reactions, the dark reactions consume the ATP and NADPH produced by the light reactions.