Photosynthesis - Light Dependent and Light Independent Reaction

in StemSocial3 months ago

Hello everyone, it's good to be here today. Thanks to everyone who reads my post, and leaves a comment, and most especially those who reply to my comments on their posts, it means a lot to me as an individual. Today, we will be doing a little bit of plant science, looking at Photosynthesis. A lot of people who are in medical school, or have to do with animal science, find plant science a little boring, even when it is very interesting. Well, without wasting any time, let me quickly discuss photosynthesis.

Photosynthesis can occur in two phases, light-dependent and light-independent reactions. As the name implies, light-dependent photosynthetic reaction converts light energy into chemical energy where it absorbs photons and water (H2O) to release Oxygen (O2) and Energy (NADPH/ATP) in the Thylakoid membrane where it involves photoexcitation (photon excite electrons) where a photon brings an electron to a high energy state which is harnessed (the energy) by an electron transport chain and the electron goes back to a lower energy state. This state needs light to function and so occurs during the day. The light-independent reaction (Calvin cycle/C3 cycle/Dark reaction) uses the energy from the light phase (NADPH/ATP), and Carbon dioxide (CO2) and RuBP to produce G3Ps (carbon-based molecule) and Carbohydrate (Glucose) which the plant uses. This doesn't need light, so they can occur anytime.

The plant leaves consist of Chloroplast which has the green pigment in plants known as Chlophyl. It is made up of the Stroma and the Granum which are piles of Thylakoid disks. The light reaction usually occurs in the Thylakoid membrane of the granum and it is performed with the following complexes from Photosystem II, Oxygen-evolving complex, cytochrome B6F, Photosystem 1, Ferredoxin NADP Reductase, and ATP synthase. Let me quickly break this down. In the light phase, A photon is needed to excite an electron, which occurs at photosystem II and Photosystem I. In the Photosystems, there are chlorophylls in them, and the photon hits the chlorophyll molecules in photosystem II, energy goes from one chlorophyll molecule to another in the complex. Two excited electrons are produced in the process. The oxygen-evolving complex is converted to an unstable state where it splits water to help replenish the electron used in the photosystem II and releases oxygen atoms which combine to form O2 which is regarded as a waste product. Also the water functions during hydrogen release in the lumen to increase the proton gradient to increase the concentration of protons in the lumen compared to the Stroma. Plastoquinone QB which is a lipid-soluble molecule that plays a critical role in the electron transport chain of photosynthesis picks up the two excited electrons from the reaction center in photosystem II, and also picks up two protons from the Stroma, and carried them to the Cytochrome B6F where the protons are released to the Lumen after which Plastocyanin picks up the two electrons and takes them to the photosystem I where the electrons get two more photons which boost its energy. At this point, another mobile carrier known as Ferredoxin carries the electron to Ferredoxin NADP Reductase where the two electrons and the proton from the stroma convert NADP+ to NADPH. The proton which has been kept in the lumen is used by ATP synthase to power the production of energy molecules (ATP). For three protons, an inorganic phosphate combines to make ATP.

In the dark reaction which uses the Calvin cycle, the high-energy molecules which were created in the light reaction, to produce carbon-based molecules of carbohydrate (glucose) from Riboluse 1,5-biphosphate (RuBP) as well as CO2 the plant gets from the surrounding air through its stomata. The cycle starts with carbon fixation where CO2 is attached to RuBP with RuBisCo as a catalyst. This step creates an unstable 6-carbon intermediate which breaks down into 2 X 3-carbon chains known as phosphoglycerates (PGA) after which reactions that lead to the production of 2ATP and 2ADP + 2P occur in Catabolic Exothermic backward reactions which become endothermic reaction. The carbon cycle has to do this in 6 rounds happening at the same time where they produce one molecule of glucose with one for each carbon, thereby producing 12 G3Ps which can be used to produce glucose while the remaining carbons can be used to produce RuBPs.

It is important to know that for photosynthesis to occur, there needs to be water, carbon dioxide, and sunlight remember that dark reactions need the high energy molecule from the light reaction. It is important that in other to get water, the root takes the water to the xylem, and carbon dioxide and oxygen get into and out of the leaves through the stomata, while photons are absorbed through chlorophyll. With this, you can understand how the plant gets its required compounds for photosynthesis to occur.



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