Photosynthesis is one of the most important processes in all of biology, and also one of the most complex. This is possibly the most complex thing you will have to learn in basic biology aside from Cellular Respiration. However, there are some simple rules that can be kept in mind to help to understand this chemical reaction.
One of the most important things to keep in mind is the obvious facts that you already know. You know that photosynthesis uses light to power it, (the photo- part) and that is creates sugars for the plant to use. (the -synthesis). You also know that plants take in CO2 to power this reaction and release oxygen as a waste product. That right there reveals much about the reaction itself.
The first step of the reaction takes place in the outer membrane of structures called thylakoids, which are found inside the chloroplasts of plant cells. Thylakoids are organized into stacks called granum and are bathed in a solution called the stroma. When light from outside the cell strikes a molecule of Chlorophyll a within the Photosystem II embedded in the membrane, it then excited one electron in the molecule and causes it to jump up into a higher energy level. Then, the electron is transferred along the Electron Transport Chain after first being transferred to the Primary Electron Accepter from the clorophyll molecule. Electrons continue along the chain until they lost most of their energy. The energy that they lost is used to move H+ ions into the thylakoid. As this is happeneing, another light particle excites a second electron in a separate chlorophyll in they Photosystem I. Those electrons more to another electron acceptor and the lost electrons are replaced by the electrons form the first chlorophyll molecule. From there, they move along another Electron Transport Chain and are bonded along with a proton and a molecule of NADP+ to form a mulecuor of NADPH. Also, a higher concentration of protons is built up inside the tylakoid membrane that will be used later in a process called Chemiosmosis.
Ok. That admittedly was a lot to take in. However, this can be broken down into basic steps.
- A photon (light particle) excites an electron in a molecule of Chlorophyll.
- The electrons flow into the Primary Electron Acceptor.
- From there, they proceed to move along the Electron Transport Chain.
- Light is absorbed into the Photosystem II at the same time as Photosystem I. The electron from Photosystem II restores the missing electron from Photosystem I. The electron from Photosystem I then is joined along with a proton and molecule of NADP+ to from a molecule of NADPH.
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