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Light Dependent Reactions of Photosynthesis
(Please excuse my awful drawings, I try)
This excites electrons in a chlorophyll to a higher energy level. The photosystem is oxidized (Remember OILRIG: Oxidation is loss, reduction is gain…of electrons).The charge separation between the electrons and PSII also drives the photolysis of water over at the Oxygen Evolving Complex, but as water is a very stable molecule it takes 4 photons to split it! The word equation for this reaction looks like this…
2(H2O) ——> O2 + 4H+ + 4e-
What happens to the products of photolysis?
Well, Oxygen at this stage is a biproduct as we don’t need it and so it simply out of the chloroplast, out of the cell (if not used for respiration) and eventually out of the plant completely and into the air. The protons (H+) however, build up in the Thylakoid Lumen and contributes to a proton gradient. Whereas the electrons are used to reduce the chlorophyll and replace the excited electrons so that the photosystem is ready for another photon to oxidize it again and so the cycles continues.
An electron acceptor is reduced as it accepts the excited electron from Photosystem II
The Electron Acceptor is re-oxidized as it enters The Electron Transport Chain (a chain of proteins in the Thylakoid Membrane …shock horror) and a series of REDOX reactions occurs. As the electrons move down the ETC energy is released which is used to pump protons from the stroma and into the lumen (protons from the photolysis of water remain in the lumen) creating a high concentration of protons in the Thylakoid Lumen.
The high concentration of protons in the Thylakoid Lumen causes the H+ to move down a concentration gradient through the Thylakoid Membrane via ATPsynthase. This provides the energy for the photophosphorylation of ADP into ATP.
The yield of ATP is not as high as Oxidative Phosphorylation during Respiration.
Note: Pi = Inorganic Phosphate.
Anyway! Back to the electrons
Once the electrons have completed the series of REDOX reactions they replace an electron (that has been excited to a higher energy level by a photon) in Photosystem I.
The rest is alot similar to what went on in Photosystem II
The excited electron then reduces another Electron Acceptor
The electron acceptor is then oxidized as the electrons are passed onto another Electron Transport Chain
This time, instead of being passed onto another Photosystem, the electrons recombine with protons in the stroma to reduce NADP to form NADPH - the reducing power for the Light Independent Reactions
The whole point of the Light Dependent Reactions is to produce NADPH which are the reducing agents for The Calvin Cycle
Photosystems in Photosynthesis
Before we kick off this whole Photosynthesis thing we just need to clarify what exactly this mess is
This bad boy isn’t as complicated as he looks, he’s a Photosystem (Photsystem II to be exact) and basically it’s just a collection or pigments and proteins arranged in a certain way. The green stringy things you see in this diagram are chlorophyll.
In the Light Dependent reactions of Photosynthesis there are two of these photosystemy things involved - Photosystem II and Photosystem I (imaginative names), both of which are located in the Thylakoid Membrane of a chloroplast
Photosystem II also contains what is known as the Oxygen Evolving Complex which does just what it says on the tin - produces Oxygen, Protons & and Electrons from the photolysis of water molecules.