Photosynthesis is the fundamental method by which plants, algae, and certain bacteria convert light energy into chemical energy. This conversion occurs in specialized cellular structures through light-dependent reactions that capture energy from sunlight. These reactions initiate a flow of excited electrons, which temporarily store the light’s energy. This energized flow ultimately produces the high-energy molecules that power the subsequent stage of building sugars.
The First Step: Capturing Light Energy
Energy transfer begins in Photosystem II (PSII), a complex protein structure embedded in the thylakoid membranes of chloroplasts. When the chlorophyll pair P680 absorbs a photon of light, an electron becomes energized and is boosted to a higher orbit. This highly energetic electron is immediately passed to a primary electron acceptor, leaving P680 positively charged and unstable.
The electron lost by P680 is replaced by the splitting of a water molecule (photolysis). This process releases oxygen and protons (\(H^+\)) that accumulate within the thylakoid space. The excited electron then begins its journey down the first electron transport chain (ETC), where its energy is gradually released to pump protons into the thylakoid space. This proton build-up powers the enzyme ATP synthase, resulting in the creation of adenosine triphosphate (ATP).
Recharging the Electrons: Photosystem I
As the electron travels through the first electron transport chain, it loses significant energy and falls to a lower level. It eventually arrives at Photosystem I (PSI), where it is accepted by the chlorophyll pair P700. The electron at P700 is now at a medium energy state, having expended most of its power to create the proton gradient for ATP synthesis.
To complete its final task, the electron needs another injection of power. This second light excitation occurs when P700 absorbs another photon, re-energizing the electron to an even higher state than the first excitation in PSII. This boost is necessary because the final acceptor molecule requires an electron with strong reducing power.
The Ultimate Destination: Creating NADPH
After receiving its second energy boost from P700, the highly energized electron leaves PSI and enters the final leg of the electron transport chain. The electron is first transferred to ferredoxin (Fd), a small, iron-containing protein. Ferredoxin then acts as a shuttle, carrying the electron to the final enzyme, ferredoxin-NADP\(^+\) reductase (FNR).
FNR sits on the outer surface of the thylakoid membrane, interacting with molecules in the stroma. The enzyme catalyzes the terminal reaction by transferring the high-energy electron from ferredoxin, along with a proton (\(H^+\)) from the stroma, to the carrier molecule \(NADP^+\). This reduction converts \(NADP^+\) into its fully reduced, high-energy form, NADPH.
Why This Matters: Fueling the Next Stage
The cascade of light absorption, water splitting, and electron transport ultimately produces two primary energy-carrying molecules: ATP and NADPH. ATP, generated by the proton gradient, provides the direct chemical energy necessary for cellular work. NADPH provides the reducing power needed for biosynthetic reactions.
Both ATP and NADPH are immediately transported to the stroma, the fluid-filled space within the chloroplast. Here, they drive the Calvin Cycle, where carbon dioxide is converted into three-carbon sugar molecules. This completes the process of converting light energy into stable, long-term chemical energy stored in carbohydrates.