Why does green light produce less ATP than red or blue light?

Chlorophyll reflects green light — it's not absorbed, so photosystems aren't excited.

What would happen if you block the electron transport chain with DCMU?

Electrons can't flow from PSII → PSI. Water splitting stops, no O₂ produced, no NADPH made, ATP production drops dramatically.

Pro 🔒~25 min

Photosynthesis: Light Reactions

Photosystems I & II, electron transport chain, and ATP synthesis

How it works

The light reactions of photosynthesis occur in the thylakoid membrane. Photosystem II (P680) absorbs photons and splits water, releasing O₂ and electrons. These electrons pass through the electron transport chain (plastoquinone → cytochrome b6f → plastocyanin), pumping H⁺ into the thylakoid lumen. Photosystem I (P700) re-energizes the electrons, which reduce NADP⁺ to NADPH via ferredoxin. The H⁺ gradient drives ATP synthase (chemiosmosis), producing ATP. Red light (680 nm) is most efficiently absorbed by chlorophyll a; action spectra show peaks at ~430 nm (blue) and ~680 nm (red).

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Step-by-step

Adjust light intensity and wavelength to observe their effects on electron flow rate, ATP production, and O₂ evolution.
The animation shows electron movement through the thylakoid membrane components.

Key formulas

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ Overall photosynthesis equation
$2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$ Water splitting (photolysis) at Photosystem II

Frequently asked questions

Why does green light produce less ATP than red or blue light?: Chlorophyll reflects green light — it's not absorbed, so photosystems aren't excited.
What would happen if you block the electron transport chain with DCMU?: Electrons can't flow from PSII → PSI. Water splitting stops, no O₂ produced, no NADPH made, ATP production drops dramatically.