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Light-dependent reactions of photosynthesis at the thylakoid membrane

The initial stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into potential energy.

The light dependent reaction produces oxygen gas and converts ADP and NADP⁺ into the energy carriers ATP and NADPH.

Light dependent reactions occur on the thylakoid membrane inside a chloroplast (this is where the chloroplasts are embedded). Inside the thylakoid membrane is called the thylakoid space, and outside the thylakoid membrane is the stroma.

The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.

History

The first ideas about light being used in photosynthesis were proposed by Jan Ingenhousz in 1779¹who recognized it was sunlight falling on plants that was required, although Joseph Priestly had noted the production of oxygen without the association with light in 1772². Cornelius Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce CO₂³. Then in 1939 Robin Hill showed that isolated chloroplasts would make oxygen, but not fix CO₂ showing the light and dark reactions occurred in different places⁴. This led later to the discovery of photosystem 1 and 2.

Cyclic photophosphorylation

See also: photophosphorylation

In cyclic electron flow, the electron begins in a pigment complex called photosystem II, passes from the primary acceptor to ferredoxin, then to a complex of two cytochromes (similar to those found in mitochondria), and then to plastoquinone before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H⁺ ions across the membrane; this produces a concentration gradient which can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces O₂, as well as ATP. Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons, but they are sent back to photosystem I. NADPH is NOT produced in cyclic photophosphorylation. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation.

Noncyclic photophosphorylation

The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. Being a light reaction, Noncyclic photophosphorylation occurs on thylakoid membranes inside chloroplasts. First, a water molecule is broken down into 2H⁺ + 1/2O₂ + 2e^- by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H⁺ and 1/2O₂ are left out for further use. Then a photon is absorbed by chlorophyll pigments on surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction which eventually transfers energy to the core of photosystem II, exciting the two electrons which are transferred to the primary electron acceptor. The deficit of electrons is replenished by taking electrons from another molecule of water. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, providing the energy for hydrogen ions (H⁺) to be pumped into the thylakoid space. This creates a gradient, making H⁺ ions flow back into the stroma of the chloroplast, providing the energy for the regeneration of ATP.

The photosystem II complex replaced its lost electrons from an external source, however, the two other electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin- NADP reductase|NADP⁺ reductase, for short FNR, which uses them to catalyst the reaction (as shown):

NADP⁺ + 2H⁺ + 2e^- → NADPH + H⁺

This consumes the H⁺ ions produced by the splitting of water, leading to a net production of 1/2O₂, ATP, and NADPH+H⁺ with the consumption of solar photons and water.

The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.

Steps

It is important to note that both photosystems are almost simultaneously excited; thus, both photosystems begin functioning at almost the same time.

1. The excited electronis passed along until it reaches P680 chlorophyll.
2. The excited electron is passed to the primary electron acceptor. Photolysis in the thylakoid takes the electrons from water and replaces the P680 electrons that were passed to the primary electron acceptor. (O₂ is released into the air as a waste product)
3. The electrons are passed to photosystem I via the electron transport chain (ETC) and in the process used to pump protons across the thylakoid membrane into the lumen.
4. The stored energy in the proton gradient is used to produce ATP which is used later in the Calvin-Benson Cycle.
5. P700 chlorophyll then uses light to excite the electron to its second primary acceptor.
6. The electron is sent down another ETC and used to reduce NADP⁺ to NADPH.
7. The NADPH is then used later in the Calvin-Benson Cycle to remove PGA that is produced from RuBisCO reaction and releases enzyme for continuation of steady state reaction [1]

See also

• Light-independent reaction
• Reaction center

References

1. ^ Ingenhousz, J (1779). Experiments Upon Vegetables. London: Elmsly and Payne. 
2. ^ Priestley, J (1772). Observations on Different Kinds of Air. 62. London: Phil. Trans. Roy. Soc.. pp.147. 
3. ^ van Niel, C. B. (1931.). "On the morphology and physiology of the purple and green sulfur bacteria". Arch. Microbial 3: 1–114. doi:10.1007/BF00454965. 
4. ^ Hill, R. (May 1939). "Oxygen Produced by Isolated Chloroplasts". Proceedings of the Royal Society of London. Series B, Biological Sciences 127 (847): 192–210. 

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