Light Reaction – Class 11 | Chapter – 13 | Biology Short Notes Series PDF

Light Reaction: Light reaction is the first stage of photosynthesis process in which solar energy is converted into chemical energy in the form of ATP and NADPH. The protein complexes and the pigment molecules help in the production of NADPH and ATP.

Process of Light Reaction

  • In light reactions, energy from the sunlight is absorbed by the pigment chlorophyll and is converted into chemical energy in the form of electron charge carrier molecules such as NADPH and ATP.
  • Light energy is utilized in both the Photosystems I and II, present inside thylakoid membranes of the chloroplasts.
  • The carbohydrate molecules are obtained from the carbon dioxide from the use of chemical energy gathered during the reactions.
  • The Light energy tends to split into the water and later extracts the electrons from the photosystem II; then the electrons move from the PSII to b6f (cytochrome) to the photosystem I (PSI) and reduce in the form of energy.
  • The electrons are re-energized in the Photosystems I and the electrons of high energy reduce NADP+ into NADPH.
  • In the process of non-cyclic photophosphorylation, the cytochrome uses the electron energy from Photosystem II to pump the ions of hydrogen from the lumen to stroma; later, this energy allows the ATP synthase to bind to the third phosphate group to the ADP molecule, which then forms the ATP.
  • In the process of cyclic photophosphorylation, the cytochrome b6f uses electron energy from both the Photosystems I and II to create a number of ATP and stops the production of the NADPH, thus maintaining the right quantities of ATP and NADPH.

What Happens in Light Reaction?

Light from the Sun is captured during photosynthesis via a series of reactions, involving the chemical chlorophyll. This results in the synthesis of two high-energy chemical compounds: ATP and NADPH, the latter of which has chemical energy that can be easily transferred to other compounds. This series of reactions necessitates the use of water,  from which oxygen is released during the process. In the following phase of photosynthesis, the dark reactions, ATP and NADPH, are used to produce glucose from Carbon dioxide. 

  • The light reaction uses water, light (brought in from outside the cell), NADP+, phosphorus, and ADP (all created by the Calvin cycle) to produce ATP (both used in the Calvin cycle), and oxygen (a waste product).

  • The light excites the electrons, and the excited electrons’ energy is then used to join ADP and phosphate together to form ATP.

  • NADP+ then joins with the excited electrons.

  • Water is split in the process, and oxygen is generated as waste and is discharged.

  • In light reactions, sunlight energy is used to oxidise water (an electron donor) to oxygen, and then to pass these electrons to NADP+, resulting in NADPH. The conversion of ADP to ATP requires some light energy. The NADPH and ATP created, are then used to run the Calvin cycle, which produces sugar.

Light Reaction Equation

Water is oxidised, electrons from water are transported to NADP+, and ATP is generated during the light processes. Equations can be used to summarise the three elements of the light reaction.

Water oxidation is a process that occurs when water is exposed to oxygen

2H2O O2+4e+ 4H + 2H2O → O2 + 4e+ 4H+

NADP reduction

2NADP+ + 4e+ 4H+ 2NADPH + 2H + 2NADP+ + 4e+ 4H+ → 2NADPH + 2H+

Synthesis of ATP


The general equation for light reactions is obtained by combining these equations as given below:

2H2O + 2NADP+ + nADP + nPi + hv O2 + 2NADPH + 2H+ + nATP

Biology Quiz & Notes Physics Quiz & Notes Chemistry Quiz & Notes

Follow on Facebook

By Team Learning Mantras