or reactions are called light reactions and it has been found to take place in grana, while the rest are called light independent or dark reactions and they occur in the stroma.

Mechanism of Photosynthesis

1. Light dependent reaction (Photochemical phase)

In this reaction light plays a key role. A series of chemical reactions occur in a very quick succession initiated by light and therefore the phase is technically called the photochemical phase or light dependent reaction.

The light reaction takes place in chlorophyll containing thylakoids called grana of chloroplasts.

Several steps occur in the light dependent reaction.

Step-I : The chlorophyll on exposure to light energy becomes activated by absorbing photons. (Photon is the smallest unit of light energy)

Step-II: The energy is used in splitting the water molecule into two component ions named hydrogen (H+), hydroxyl ion (OH- ).

H₂O  H⁺  + OH^- The reaction is known as photolysis, which means splitting by light (photo means light, lysis means breaking).

This was discovered by Robert Hill. Hence it is also called Hill’s reaction.

Step-III: The highly reactive H⁺ , OH^-- ions of water undergo quick change as described below.

OH^- ions through a series of steps produce water (H₂0) and Oxygen (O₂ ). Water may be used by the plant inside, but O₂ is usually released into the atmosphere.

H⁺ ions undergo series of changes in dark reaction. H⁺ ions produced in photolysis are immediately picked up by special compound NADP to form NADPH (reduced Nicotinamide Adenosine Dinucleotide Phosphate).

ADP and iP trap energy to form ATP (Adenosine Tri phosphate).

ATP and NADPH are formed at the end of the light reaction. These are (ATP, NADPH) called Assimilatory power.

2. Light independent reaction (Biosynthetic phase)

This reaction does not require the presence of light and extension of the phases after day time may occur in some plants (time gap between the two being less than even one thousandth of a second) and some times even in the dark.

This is also called dark reaction. But the term dark reaction or light independent reaction does not mean that they occur when it is dark at night.

It only means that these reactions are not depend on light. In the dark phase the hydrogen of the NADPH is used to combine it with CO2 by utilizing ATP energy and to produce glucose (C6 H12O6 ). This synthesis occurs in a