As in the overview of photosynthesis, the light-dependent reactions utilise light energy to convert it into more usable chemical energy.
So naturally, it starts with light. This is the brief sequence of events:
1. The electrons present in the chlorophyll of the plant’s chloroplasts are brought to a higher energy level (they enjoy dancing more) by light energy. This takes place on the thylakoid membrane, and more specifically in a conglomerate of proteins/enzymes dedicated to this reaction, called photosystem II. It’s known as photoionisation.
2. To maintain a fresh supply of dancing electrons, light also splits (photolysis) the H2O into… electrons, protons and… wait for it. Wait for it. Wait… Oxygen! So that’s how the oxygen by-product is made.
3. What’s the deal with the dancing electrons? They’re picked up by electron carriers (nightclub bouncers) and thrown out, one by one. This releases energy every time a poor electron is pushed down another flight of stairs (thylakoids are multi-story clubs thank you very much) all the way to photosystem I. Ouch. I sure hope that energy is put to good use.
4. The sweat and blood and tears of electrons passing down the electron transport chain is used to pump the elite clientèle into the thylakoid. Who is this clientèle, I hear you ask. It is none other than the protons! You know, the ones snatched from the H2O.
They rush inside all at once as soon as the electrons are suitably thrown out – just couldn’t stand all that… negativity. They are stuffed inside the thylakoids like sardines on a hot day, to the point where the nightclub is filled with positivity and the outside (the stroma) is totally missing out.
5. The proton gradient formed as a result (lots of protons inside the thylakoid, few outside) enables their movement subsequently in the opposite direction, down their concentration gradient. Unfortunately for them, there are only a few exits back outside. These are gates – enzymes – called ATP synthase. They have the absolute cheek to charge every proton to get out energy currency. This energy makes ATP from ADP + Pi.
6. Meanwhile, what are the electrons doing at photosystem I? They’re electrons, what else are they going to do if not get excited – again – and end up in trouble – again. Light strikes them at PSI, even harder this time, and they roll-rollety-roll along to electron carrier NADP (nicotinamide adenine dinucleotide phosphate, of course you were dying to know) where they are coerced into making friends (?!) with a proton from the stroma and sticking together to form reduced NADP.
Phew. Did I call that a BRIEF sequence of events? Hahahaha sorry, my bad.
On the upside, you now get to see the gorgeous summary diagram of it all happening at once. You wouldn’t have wanted to see that first.