PS: Simplified
Light Reactions
Photosynthesis is a process that occurs within plants typically photoautotrophs. This process allows for the input of H2O (water) and CO2 (carbon dioxide) along with photons of light to produce oxygen (O2) as a byproduct of light dependent reactants. The process of photosynthesis can be broken down into two processes. First is the light dependent reaction which occurs in the grana which are stacks of thylakoids. The second reaction occurs in the stroma which is the fluid within the stroma. All of these processes occur within the chloroplasts.
To learn photosynthesis on a molecular scale, you must first realize that this entire process occurs within. the chloroplast and the structure inside the organelle along with the function of each part. Using the image above, the stacks of slabs that look comparable to pancakes are called grana, a singular stack is granum and singular piece is thylakoid. Within the thylakoid light reaction dependent reactions occur. This reactions only occur with the presence of light photon hitting photosystem II which will be explained later. Within the stroma which is the fluid everywhere else inside the chlorplast, the light independent reactions occur, meaning they are not dependent on light shining through chlorophyll but rather, are dependent on the byproducts of the light dependent reactions
Above is an electron transport chain that occurs within the thylakoids, known as the thylakoid membrane. This is the light-dependent reaction meaning it requires light photons to occur. The net reactants used is the input of H2O which immediately gets broken, simultaneously as light photons strike photosystem II. Do not get confused with photosystem II (PSII) and photosystem I (PSI) being ordered unconventionality. Due to how photosystem II was discovered second after photosystem I which is later down the electron transport chain (ETC)
A light photon hits photosystem II the light capturing complex. You can also imagine it striking chlorophyll.
As a result the splitting of water will take place as H2O --> 2H+ + 2e- + 1/2 O2. (Key Note: Oxygen (O2) will be kept as a byproduct of the entire photosynthesis reaction, the two electrons and two hydrogens come into play)
Due to the photon of light hitting photosystem II simulataneouslly as the splitting of water, at P680 (P680 just means the length which is 680 nanometers where light is absorbed) the two electrons from earlier, go from the ground state to the excited state. This just means the electrons gain electrical charge, and are able to supply energy to the electron carriers which include plastoquinone (PQ), plastocyanin (PC), and ferredoxin (Fd).
Through the supplementation of energy as these electrons are moving through the ETC, at Cytochrome Bf-6 specifically (you can just mention ETC on a test), it is pumping out H+ from the stroma and the water splitting, into the thylakoid lumen. Basically creating a proton gradient to form along the lumen. The thylakoid lumen is in the inner space inside the thylakoid, whereas the stroma is outside fluid inside the chloroplast. This is important because the proton gradient from the proton pump proteins will allow for a easier, and favorable flow of H+ ions into ATP synthase which is produced from the light dependent reactions.
Furthermore, as electrons begin to slowly lose energy in their travel from electron carriers, they need more energy. This is where they get hit by another photon of light at P700 (700 nanometers absorbance) at photosystem I (PSI). Remember its just named this because it was found first, this makes the electrons excited again from the ground state to excited state (chemistry review). Thus supplying energy to move onto the next electron carrier previously mentioned, ferrodoxin (Fd for short). This is importance since it sends these electrons to the next step.
Transporting with the assistance of Fd and energy accumulated from PSI, these electrons move to special NADP+ reductase a special enzyme protein that oxidizes NADP+ into NADPH which will be used in the Calvin Cycle the light-independent reactions.
Along with this the proton gradient mentioned earlier from the input of H+ into the thylakoid lumen, this will cause the proteins to FAVORABLY flow into the ATP Synthase, another enzyme protein that creates ATP. This is critical for the light independent reactions coupled with the formation of NADPH because it supplies its phosphate group in the Calvin Cycle. ATP Synthase takes in H+ ions as energy, and starts spinning around combining ADP + Phosphate group. Just remember ADP stands for adenine diphosphate, the di suffix represents two. ATP stands for adenine triphosphate, the tri suffix implying three phosphate groups.
Pictures shown above show ADP and NADP+ oxidizing to become ATP and NADPH and their molecular structures.
We will go onto the dark reations portion of photosynthesis known as the Calvin Cycle in the next topic part of PS: Simplified.
