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The following is a breakdown of the energetics of the photosynthesis process from Photosynthesis by Hall and Rao: [6]. Starting with the solar spectrum falling on a leaf, 47% lost due to photons outside the 400–700 nm active range (chlorophyll uses photons between 400 and 700 nm, extracting the energy of one 700 nm photon from each one)
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the energy-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants , algae , and cyanobacteria .
There are two pathways of electron transfer. In cyclic electron transfer, electrons are removed from an excited chlorophyll molecule, passed through an electron transport chain to a proton pump, and then returned to the chlorophyll. The mobile electron carriers are, as usual, a lipid-soluble quinone and a water-soluble cytochrome.
These include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying ...
Electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain within the thylakoid membrane. Energy from PSI drives this process [ citation needed ] and is harnessed (the whole process is termed chemiosmosis ) to pump protons across the membrane, into the thylakoid lumen space from the chloroplast stroma.
The electrons then pass through Cyt b 6 and Cyt f to plastocyanin, using energy from photosystem I to pump hydrogen ions (H +) into the thylakoid space. This creates a H + gradient, making H + ions flow back into the stroma of the chloroplast, providing the energy for the (re)generation of ATP.
Photosystem II obtains electrons by oxidizing water in a process called photolysis. Molecular oxygen is a byproduct of this process, and it is this reaction that supplies the atmosphere with oxygen. The fact that the oxygen from green plants originated from water was first deduced by the Canadian-born American biochemist Martin David Kamen.
Chloroplasts also need to balance the ratios of photosystem I and II for the electron transfer chain. The redox state of the electron carrier plastoquinone in the thylakoid membrane directly affects the transcription of chloroplast genes encoding proteins of the reaction centers of the photosystems, thus counteracting imbalances in the electron ...