Phases of Photosynthesis:

There are two types of reactions in photosynthesis the light dependent reaction and, the dark reaction (light independent reaction).

Light Dependent Reaction Hill’s Reaction (Photochemical Phase):

This is the photochemical phase of photosynthesis. As the name suggests, this is a light dependent reaction, that is, light plays a key role in this reaction. This reaction occurs in the thylakoids of grana in chloroplasts. A series of chemical reactions occur in quick succession, initiated by light. The main steps of this reaction are as follows: Step 1: Excitation of Chlorophyll: i.

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The photosynthetic pigments (chlorophyll) absorbs light energy in the form of photons (smallest unit of light energy). ii. After being exposed to light energy, the chlorophyll molecule gets activated and emits electrons, which travel through electron transport chain in chloroplasts. Step 2: Formation of ATP from ADP (photophosphorylation): The above mentioned reactions in step 1is mediated by electron acceptors and Adenosine triphosphate (ATP) is synthesized from Adenosine diphosphate (ADP) and inorganic phosphate (Pi). This is known as photophosphorylation.

ATP is used during dark reaction. ADP + Pi + energy —–> ATP Step 3: Splitting of Water (Photolysis): The splitting of water also known as photolysis (photo means light and lysis means breaking) takes place in light reaction during which, water is broken down into highly reactive hydrogen (H+) ions and oxygen. 2H20 energy of 4 photons> 4H+ + 02 + 4e– The free oxygen is the oxygen gas given off during photosynthesis.