Photosynthesis utilizes light energy to excite electrons in pigment molecules, ultimately converting sunlight into chemical energy stored in the bonds of sugars.
Here's a more detailed explanation:
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Light Absorption: Photosynthetic pigments, like chlorophyll, are arranged in light-harvesting complexes called photosystems within the thylakoid membranes of chloroplasts. These pigments absorb specific wavelengths of light from the sun.
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Electron Excitation: When a pigment molecule absorbs light, an electron within that molecule is excited to a higher energy level. This energized electron now holds potential chemical energy.
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Photosystems (PSI and PSII): There are two main photosystems involved: Photosystem II (PSII) and Photosystem I (PSI).
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Photosystem II (PSII): Light energy absorbed by PSII excites electrons. These electrons are then passed along an electron transport chain. PSII replenishes its electrons by splitting water molecules (photolysis), releasing oxygen as a byproduct. This process is a major source of the oxygen in our atmosphere.
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Electron Transport Chain: As electrons move down the electron transport chain, the energy they release is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient. This gradient is a form of stored potential energy.
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Photosystem I (PSI): Electrons exiting the electron transport chain from PSII enter PSI. Light energy absorbed by PSI re-energizes these electrons.
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NADPH Formation: The re-energized electrons from PSI are then used to reduce NADP+ to NADPH. NADPH is an electron carrier that will be used in the Calvin cycle.
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ATP Synthesis: The proton gradient created by the electron transport chain drives the synthesis of ATP through a process called chemiosmosis. Protons flow down their concentration gradient through an enzyme called ATP synthase, which uses the energy to convert ADP to ATP.
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The Calvin Cycle: The ATP and NADPH produced during the light-dependent reactions are then used in the Calvin cycle (light-independent reactions) to fix carbon dioxide and produce sugars.
In summary, light energy is captured by pigments, exciting electrons. These electrons drive the formation of ATP and NADPH, which are then used to power the synthesis of carbohydrates from carbon dioxide.
