Light Dependant Reactions

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Light Dependant Reactions

The first reactions of photosynthesis require light energy, and are called light dependant reactions.

The aim is to produce ATP from ADP and inorganic P and harvest hydrogen so that carbon dioxide can be reduced to form a carbohydrate in the second series of reactions. The production of ATP using light is called photophosphorylation.

It will be useful if you refer to the Respiration Learn-it, and refresh your memory about electron carriers and the Chemiosmotic theory, as both reappear here.

There are many electron carriers involved, but the one that you must know about for the exams is NADP (nicotinamide adenine dinucleotide phosphate).


When light is absorbed by PS I and PS II, electrons in the chlorophyll molecule are boosted to higher energy levels. They are emitted and passed on to electron carriers.

The loss of electrons from PS II causes the splitting of water. The water gives up its electrons to PS II to fill the gap.

This leaves oxygen, which is given off as waste, and hydrogen ions. (See later in this Learn-it for loss of electrons from PS I).

The electrons are passed through a series of electron carriers at successively lower energy levels. This means that energy is released and used to form ATP out of ADP and P.

This occurs in a similar way to ATP production in mitochondria. The energy is actually used to pump hydrogen ions from the stroma across the thylakoid membrane. This creates an electrochemical gradient.

As the hydrogen ions diffuse back down the concentration gradient they flow through proteins. Part of each protein is ATP synthetase.

The energy released as the hydrogen ions diffuse down the concentration gradient is used to synthesise ATP. The electron eventually reaches PS I and fills the space.

Non-Cyclic Photophosphorylation

The electrons from PS I may also pass onto an electron carrier and then combine with the hydrogen ions (from the water) to reduce NADP to NADPH.

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This reduced NADP is used in the next series of reactions.

Note: No ATP has been made during this process.

This process is called non-cyclic photophosphorylation.

Cyclic Photophosphorylation

However, if there is plenty of NADPH, something different happens.

The electron from PS I is passed to the electron carriers used in PS II. ATP is formed and the electrons return to PS I to fill the space. This makes PS II redundant as no electrons are needed from there to fill the space in PS I. Only PS I is active. This is called cyclic photophosphorylation.

To summarise, in cyclic photophosphorylation...

  • No PS II is involved.
  • No oxygen is evolved.
  • No NADPH is made.
  • ATP is still made.