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Cell Biology & Energy (BIO 101) · Module 6 of 8

Photosynthesis: The Light Reactions

How chlorophyll captures light energy and converts it into ATP, NADPH, and oxygen.

In this module
  1. Chlorophyll & Light Absorption
  2. Photosystem II & the Splitting of Water
  3. The Z-Scheme & Photophosphorylation
  4. Products of the Light Reactions
  5. Check yourself

Chlorophyll & Light Absorption

  • Chlorophyll a and b, along with accessory pigments like carotenoids, absorb light primarily in the blue and red wavelengths, reflecting green light — which is why plants appear green.
  • Pigments are organized into photosystems (I and II) embedded in the thylakoid membrane, each with a reaction center that passes captured light energy to electrons.
Worked example

Why do most plants appear green to our eyes?

Solution. Chlorophyll absorbs mostly blue and red light for photosynthesis but reflects green light, which is why the leaves appear green to us.

Photosystem II & the Splitting of Water

  • Photosystem II absorbs light, exciting electrons that are passed to an electron transport chain; the electrons it loses are replaced by splitting water molecules, releasing O2 as a byproduct.
Worked example

Where does the oxygen released during photosynthesis actually come from?

Solution. It comes from the splitting (photolysis) of water molecules in Photosystem II, which replaces the electrons that Photosystem II loses after absorbing light.

The Z-Scheme & Photophosphorylation

  • Electrons flow from Photosystem II through an electron transport chain to Photosystem I, and the energy released pumps protons into the thylakoid space, creating a gradient that drives ATP synthase — photophosphorylation, similar to oxidative phosphorylation.
  • Photosystem I re-energizes the electrons with more light and ultimately passes them to NADP⁺, reducing it to NADPH.
The Z-scheme
electron energyP680lightETC → ATPP700lightNADPHH₂O → O₂
Light excites electrons in Photosystem II (P680), which are replaced by splitting water. They flow downhill through the ETC to Photosystem I (P700), are excited again by light, and finally reduce NADP⁺ to NADPH.
Worked example

How is ATP production during the light reactions similar to ATP production during cellular respiration?

Solution. Both use an electron transport chain to pump protons across a membrane, creating a gradient that drives ATP synthase via chemiosmosis — the same fundamental mechanism, just in a chloroplast's thylakoid membrane instead of a mitochondrion's inner membrane.

Products of the Light Reactions

  • The light reactions produce ATP, NADPH, and O2 (as a byproduct); ATP and NADPH are then used to power the Calvin cycle.
Worked example

What two energy-carrying molecules from the light reactions are used to power the Calvin cycle?

Solution. ATP and NADPH.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. Why do plants appear green?

2. What happens at Photosystem II to replace lost electrons?

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