The reactions of photosynthesis — lesson. Science CBSE, Class 10.

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The mechanism of photosynthesis takes places two sequential stages:

Mechanism of photosynthesis

1. Light dependent reactions:

Light reactions

Light-dependent processes occur in the thylakoid membrane and require a constant source of light energy.
The light-dependent processes overall goal is to transform light energy into chemical energy. The Calvin cycle will use this chemical energy to fuel the construction of sugar molecules.
These reactions start in a photosystem II, which is a collection of pigment molecules and proteins.
A photon of light travels to hits a chlorophyll molecule. An electron in chlorophyll "excited" as a result of the photon hit. The electron is given enough energy to liberate from one of the chlorophyll molecule's atoms. As a result, chlorophyll is said to "donate" one electron.
In order to replace this electron in the chlorophyll, the water molecule is split, releasing oxygen and hydrogen electrons (refer to the previous theory).
The energy absorbed by the chlorophyll is converted and stored as chemical energy in the form of ATP and NADPH (electron carrier molecule).
All the electrons emitted in the light-dependent reactions are carried to photosystem II by a set of electron carriers (will be discussed in higher classes). The ATP and NADPH synthesized are also passed to the photosystem II.
This whole process of electrons or energy transfers from one photosystem to another photosystem in the form of a Z symbol and is known as the Z-scheme.

The Z scheme of Photosystems

Image Credits: Photosynthesis - PS I & II

2. Light independent reactions:

Calvin cycle

These are the reactions that take place in the stroma and do not directly require light.
Instead, these reactions use ATP and NADPH, electrons formed in the light-dependent reaction to reduce carbon dioxide into glucose.
In this process, ATP is broken down to release energy, and the NADPH donates its electrons to convert carbon dioxide molecules into sugars.
This is also known as the Calvin cycle and includes a series of reactions discussed in your higher classes.
All the plants which undergo this $C_{3}$ cycle are known as $C_{3}$ plants.

Reference:

https://commons.wikimedia.org/wiki/File:Figure_08_01_06.jpg

https://physicscatalyst.com/biology/photosynthesis-in-higher-plants.php

https://upload.wikimedia.org/wikipedia/commons/2/2f/Hill_reaction_4.png

https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/5.4/

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12. The reactions of photosynthesis