lesson
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Every second, the sun blasts Earth with trillions of photons β tiny packets of light energy that travel 93 million miles across space.
Plants capture these fleeting electromagnetic waves and lock them into durable chemical bonds inside biological batteries.
How does a green leaf trap incoming solar energy instead of letting it bounce away as heat?
Harvesting Photons
Plant cells contain disc-shaped organelles called chloroplasts, which house light-absorbing pigment molecules like chlorophyll a and chlorophyll b.
These pigments absorb blue (430β450 nm) and red (640β660 nm) wavelengths while reflecting green light (500β550 nm), giving leaves their characteristic color.
πInteractive/animated chart comparing the absorption spectra of chlorophyll a (teal line) and chlorophyll b (green line) across visible wavelengths (400 nm violet to 700 nm red). A smooth rainbow background spans 400nm to 700nm. The curves show steep peaks in the blue (400-470nm) and red (640-680nm) regions and a deep valley in the green (500-560nm) region labeled 'Reflected Green Light'. Hovering or looping points highlight photon absorption efficiency at blue vs green vs red wavelengths.
In 1883, botanist Theodor Engelmann placed filamentous algae under a prism-separated light spectrum and observed that oxygen-seeking bacteria gathered densely around the red and blue zones, proving those wavelengths drive photosynthetic activity.
When a photon hits a chlorophyll molecule, what happens to that absorbed energy at the subatomic level?
Exciting Electrons and Splitting Water
Absorbing a photon boosts an electron in the chlorophyll ring from a low-energy ground state to an unstable excited state with higher potential energy.
In Photosystem II (PSII), a protein-pigment complex embedded in the thylakoid membrane, this high-energy electron is captured by a primary electron acceptor before it can drop back down.
πStep-by-step diagram of Photosystem II in the thylakoid membrane. 1) Yellow photon strikes P680 chlorophyll reaction center. 2) An electron (e-) jumps up an energy axis from ground state to excited state. 3) Water-splitting complex extracts electrons from 2 H2O molecules, producing 4 H+, 4 e-, and 1 molecule of O2 gas that diffuses outward. 4) The extracted electron replenishes the oxidized P680+ reaction center. Clean animated pulse showing photon entry -> electron pop -> water split.