Here we go again: step 1, light dependent reaction.

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As the name implies, in this stage of photosynthesis, light is needed. Duh.

Alrighty, well this process takes place in the chloroplasts of cells. You don't have chloroplasts, just a little by the way. Unless you're a plant... In which case, I guess I was under a rock when we figured out how to communicate with plants. Welcome fellow living things.

So this process takes places in the grana of the chloroplasts, which are stacks of thylakoids. These thylakoids have electron transport chains embedded into their membranes. The system gets kicked off in photosystem II of the electron transport chain. Yes. Photosystem II. Not one. It's not a typo. Photosystem II has two chlorophyll a molecules, which are a type of pigment. They also have a primary electron acceptor. Here's where the sun comes into play. A photon of light from the sun is captured by one of the pigment molecules inside Photosystem II, it is then passed along (like a hot potato) from one pigment molecule to the next, until it reaches one of the chlorophyll a molecules. The photon then proceeds to excite the electron of chlorophyll a to a higher energy level.

Backstep. The chlorophyll molecule's electron comes from a process called photolysis. Whereby water is split (using light), this results in two hydrogen ions and one oxygen atom. The two electrons on the hydrogen ions are handed over to two chlorophyll a molecules in photosystem II. The oxygen atom waits until this process happens one more time, bonds with the resulting oxygen, and leaves as a waste product.

Back to our negative friend the electron. This excited electron is captured by what is called the primary acceptor. From there it moves down the electron transport chain, first to a plastoquinone (PQ), then a cytochrome complex, and then another PQ. As the electron moves down the chain it loses energy, this energy is then used to undergo chemiosmosis. i.e. hydrogen ions are pumped into the thylakoid space, building up a high concentration of hydrogen ions in the thylakoid. These hydrogen ions then want to get out of the thylakoid to the lower concentration, they can only do this through the ATP synthase channel. Where they pass through and the energy released by them going through there is used to phosphorylate (photophosphorylate) ADP into ATP.

The electron then enters photosystem I, and is once again excited. It is once again captured by a primary electron acceptor. This electron is once again passed along another electron transport chain, and there ferrodoxin is used as the energy carrier. The energy created by passing along this electron is now used to reduce NADP+ to NADPH. An enzyme called NADP reductase catalyzes the movement of the electron from ferrodoxin to NADP.

Essentially, the whole point of this yet another lengthy process was to end up with ATP molecules and NADPH to be used in the light independent reaction.