This diagram may seem difficult but below is a step by step explanation of the entire light reaction process so don't be scared!
1. Water gets split apart by the z enzyme in the thylakoid lumen at the base of photosystem 2 (the first integral protein), this is shows using this equation:
H20 -> 1/2 O2 + 2H+ + 2e or 2H2O -> 4H+ + O2 + 2e for all the chemists out there. This reaction is called photolysis The H+ ions stay inside the thylakoid lumen, adding to the concentration gradient which will be talked about later, the oxygen leaves the cell and the electrons enter the integral protein, photosystem 2. 2. The 2 electrons from the previous step, get excited by photons from the sun that hit the top of photosystem 2. The electrons use their energy to float up and enter what we call the q-cycle. This means that they use the principle energy accepter (PEA) plastiquinone as a shuttle to get from photosystem 2 to the plastiquinome cytochrome complex (the second integral protein). |
3. When the 2 electrons get to the plastiquinome cytochrome complex, they use their energy and attractive force to pull H+ ions from the stroma into the thylakoid lumen, adding to the concentration gradient that will be used later. After the electrons do this they move down to ride the plastocyanin (PEA) to get to photosystem 1 (the third integral protein).
4. At photosystem 1, the electrons enter one by one and, once again, get excited by another photon of light hitting the top of photosystem 1. The electron then carries this energy to another primary electron accepter, ferredoxin. 5. Ferredoxin then carries the electron to the fourth integral protein, NADP+ reductase. Here they use their energy and attatch themself with a H+ ion to a NADP+ molecule to form the compound NADPH. This molecule gets used in the calvin cycle. This reaction also aids the concentration gradient of H+ ions because it uses them from the stroma side so there aren't many there and there are a lot on the thylakoid membrane side from the previous steps. |
6. All the protons to this point have accumulated on the thylakoid lumen side, this creates a chemical gradient of a surplus of protons on that side and a deficit of protons on the stroma side. It is this difference that drives chemiosmosis and ATP synthase. In an effort to seek an equilibrium H+ ions will rush through the fifth integral protein, ATP synthase to try and get to the other side. When there are four H+ ions inside the protein, on the stroma side, an ADP uses this to combine with an inorganic phosphate to make ATP. This process is called photophosphorylation. This ATP is used in other parts of the plant cell such as the calvin cycle to make glucose, you can find more about that in the next lesson!
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For a great animation on the location and function of light reactions click this link: http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf
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Video #1: An informative but kind of dull explanation of the light reactions
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Video #2: An entertaining and easy to comprehend video on photosynthesis
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