light reactions

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6 Terms

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Photosystems

What are photosystems?

Inside the chloroplasts, there are clusters — like groups — of chlorophyll molecules and proteins stuck together.
These clusters are called photosystems.

The job of a photosystem is to capture light energy and start moving electrons with that energy.


Inside a photosystem, you have two main parts:

  • An antenna complex.

  • A reaction center.

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Antenna Complex

The antenna complex is like a web or net made of lots of chlorophyll molecules.
When sunlight hits the antenna complex, the energy jumps from molecule to molecule like a hot potato, until it reaches the reaction center.

Real-world analogy:
Imagine a crowd at a concert. When the music drops, the energy travels through the crowd, everyone starts dancing, and finally it reaches the front where the lead singer (reaction center) is.

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Reaction Center

The reaction center is the special chlorophyll molecule that actually donates an electron after getting excited.

Once the electron gets excited, it leaves the chlorophyll and goes into an electron transport chain (which we will explain super deeply soon).

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Photosystem I and Photosystem II

Plants have two different photosystems:

  • Photosystem I (PSI)

  • Photosystem II (PSII)

Each photosystem is named after when it was discovered, NOT the order they happen.
Photosystem II actually comes first in the light reactions.


Each reaction center in the photosystems has a special version of chlorophyll:

  • Photosystem I uses a chlorophyll called P700 (because it best absorbs light at 700 nm wavelength).

  • Photosystem II uses a chlorophyll called P680 (because it best absorbs light at 680 nm wavelength).

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Non-Cyclic Electron Flow

This is the main pathway during light reactions.

It’s called non-cyclic because electrons do not return to where they started.
They move in one direction, like a river flowing downhill.

This whole process is sometimes called the Z-scheme because if you draw the path of the electrons, it looks kind of like a slanted "Z".

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Cyclic Electron Flow

Sometimes the plant needs more ATP and less NADPH.

In that case, it uses a shortcut called cyclic electron flow.


In cyclic flow:

  • Electrons from Photosystem I are passed back to the b6-f complex instead of going to NADP+.

  • This builds the proton gradient again and makes more ATP.

  • No NADPH is made.

  • No oxygen is made.


🔥 WHY do plants do this?

Because the Calvin Cycle uses more ATP than NADPH, so sometimes you gotta make extra ATP.