Understanding:
• Reduced NADP and ATP are produced in the light dependent reactions
The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source
- Photophosphorylation may be either a cyclic process or a non-cyclic process
Cyclic Photophosphorylation
- Cyclic photophosphorylation involves the use of only one photosystem (PS I) and does not involve the reduction of NADP+
- When light is absorbed by Photosystem I, the excited electron may enter into an electron transport chain to produce ATP
- Following this, the de-energised electron returns to the photosystem, restoring its electron supply (hence: cyclic)
- As the electron returns to the photosystem, NADP+ is not reduced and water is not needed to replenish the electron supply
Non-Cyclic Photophosphorylation
- Non-cyclic photophosphorylation involves two photosystems (PS I and PS II) and does involve the reduction of NADP+
- When light is absorbed by Photosystem II, the excited electrons enter into an electron transport chain to produce ATP
- Concurrently, photoactivation of Photosystem I results in the release of electrons which reduce NADP+ (forms NADPH)
- The photolysis of water releases electrons which replace those lost by Photosystem II (PS I electrons replaced by PS II)
Cyclic vs Non-Cyclic Photophosphorylation
- Cyclic photophosphorylation can be used to produce a steady supply of ATP in the presence of sunlight
- However, ATP is a highly reactive molecule and hence cannot be readily stored within the cell
- Non-cyclic photophosphorylation produces NADPH in addition to ATP (this requires the presence of water)
- Both NADPH and ATP are required to produce organic molecules via the light independent reactions
- Hence, only non-cyclic photophosphorylation allows for the synthesis of organic molecules and long term energy storage
The key differences between cyclic and non-cyclic photophosphorylation are summarised in the table below:
Comparison of Cyclic and Non-Cyclic Photophosphorylation
Overview of the Light Dependent Reactions (Cyclic and Non-Cyclic)