Question

what is light reaction?​

Answer 1

HERE IS YOUR ANSWER

In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane proteincomplexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.

The four photosystems absorb light energy through pigments—primarily the chlorophylls, which are responsible for the green color of leaves. The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain(ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI, the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis various electron donors are used.

Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.

The net-reaction of all light-dependent reactions in oxygenic photosynthesis is:

2H

2O + 2NADP+

 + 3ADP + 3Pi → O

2 + 2NADPH + 3ATP

The two photosystems are protein complexes that absorb photons and are able to use this energy to create an electron transport chain. Photosystem I and II are very similar in structure and function. They use special proteins, called light-harvesting complexes, to absorb the photons with very high effectiveness. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and is unique because it transforms light energy into chemical forms

Answer 2

$\textbf{Light Reaction }$:

It occurs in the Grana where photon are absorbed so as to excite electrons of chlorophyll to a higher energy level. These activated electrons are harnessed to form ATP from ADP and reduce NADP to NADPH. Water is also splitted during this and oxygen is released.

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More is here :

$\textbf{Photosynthesis}$:

6CO2 + 12H2O --chlorophyll,sunlight--> C6H1206 + 6O2 + 6H2O

It is the process by which green plants and some other organisms uses sunlight to synthesise nutrients from carbon dioxide and water. The main products forms here is glucose and by products are carbon dioxide and oxygen and water where which chlorophylls are the sites of photosynthesis.

Events occurring during Photosynthesis are as follows :

1. Absorption of light energy by chlorophyll.

2. Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen.

3. Reduction of carbon dioxide to carbohydrates.

Stages of Photosynthesis are as follows :

1. $\textbf{Light Reaction }$:

It occurs in the Grana where photon are absorbed so as to excite electrons of chlorophyll to a higher energy level. These activated electrons are harnessed to form ATP from ADP and reduce NADP to NADPH. Water is also splitted during this and oxygen is released.

2. $\textbf{Dark Reactions }$:

Calvin cycle occurring in chlorophyll here carbon dioxide combined with 5-carbon sugar call ribulose biphosphate. This unstable compound breaks down into three carbon compounds (phosphoglyeric acid) this then forms glucose.