Photosynthesis — Question 6

Light reaction

The light reaction occurs in two main steps:

Step 1 — Activation of chlorophyll
The chlorophyll on exposure to light energy becomes activated by absorbing photons.

Step 2 — Splitting of Water
The absorbed energy is used in splitting the water molecule (H₂O) into its two components (Hydrogen and Oxygen) and releasing electrons.

$2 H_2O \overset{\text{energy of 4 photons}}{\longrightarrow} 4H^+ + 4e^- + O_2$

This reaction is known as photolysis of water.

End result of the products of photolysis
The hydrogen ions (H⁺) are picked up by a compound NADP (Nicotinamide adenine dinucleotide phosphate) to form NADPH.

$NADP^+ + e^- + H ^+ \overset{\text{enzyme}}{\longrightarrow} NADPH$

The oxygen (O) component is given out as molecular oxygen (O₂).

$2O \longrightarrow O_2$

The electrons (e^-) are used in converting ADP (adenosine diphosphate) into energy rich compound ATP (adenosine triphosphate) by adding one phosphate group P_i (inorganic phosphate).

$ADP + P_i \text{ (inorganic phosphate)} \longrightarrow ATP$

This process is called photophosphorylation.

Dark reaction

The reactions in this phase do not require light energy and occur simultaneously with the light reaction. The time gap between the light and dark reaction is less than one thousandth of a second. In the dark reaction, ATP and NADPH molecules (produced during light reaction) are used to produce glucose (C₆H₁₂O₆) from carbon dioxide. Fixation and reduction of carbon dioxide occurs in the stroma of the chloroplast through a series of reactions. The glucose produced is either immediately used up by the cells or stored in the form of starch.

Below is the summary of events in Light reaction and Light independent reactions of photosynthesis: