The electron acceptor that is reduced by Chl* is the first in a chain of electron carriers in the thylakoid membrane. Electrons are passed from one carrier to another in an energetically “downhill” series of reductions and oxidations. The final electron acceptor is NADP⁺, which gets reduced:

NADP⁺ + H⁺ + 2 e^–→NADPH                                               (10.7)

As in mitochondria, ATP is produced chemiosmotically in the thylakoid membrane during the process of photophosphorylation, which we will illustrate shortly. The overall process involves two *electron transport processes, noncylic and cylic. The noncyclic electron transport reactions that use the energy from light to generate ATP and NADPH are illustrated in Figure 10.7.

As seen in Figure 10.7, two coordinated photosystems, each with its own reaction center, collaborate to produce ATP and NADPH:

Let’s look in more detail at these photosystems, beginning with photosystem II.

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PHOTOSYSTEM II After an excited chlorophyll in the reaction center (Chl*) gives up its energetic electron to reduce a chemical acceptor molecule, the chlorophyll lacks an electron and is very unstable. It has a strong tendency to obtain an electron from another molecule to replace the one it lost—in chemical terms, it is a strong oxidizing agent. The replenishing electrons come from water, splitting the H—O—H bonds:

H₂O → ½ O₂ + 2 H⁺ + 2 e^–                                              (10.8)

2 e^– + 2 Chl⁺ → 2 Chl                                                  (10.9)

Overall: 2 Chl⁺ + H₂O → 2 Chl + 2 H⁺ + ½ O₂                              (10.10)

Notice that the source of O₂ in photosynthesis is H₂O (as demonstrated in Investigating Life: What Is the Chemistry of Photosynthesis, and How Will Increasing CO₂ in the Atmosphere Affect It?).

Back to the electron acceptor in the electron transport system: the energetic electrons are passed through a series of membrane-bound carriers to a final acceptor at a lower energy level. As in the mitochondrion, a proton gradient is generated and is used by ATP synthase to make ATP (see below).

PHOTOSYSTEM I In photosystem I, an excited electron from the Chl* at the reaction center reduces an acceptor. The oxidized chlorophyll (Chl⁺) now obtains an electron, but in this case the electron comes from the last carrier in the electron transport system. This links the two photosystems chemically. They are also linked spatially, with the two photosystems adjacent to one another in the thylakoid membrane. The energetic electrons from photosystem I pass through several molecules and end up reducing NADP⁺ to NADPH.

Next in the process of harvesting light energy to produce carbohydrates is the series of carbon-fixation reactions. These reactions require more ATP than NADPH. If the pathway we just described—the linear or noncyclic pathway—were the only set of light reactions operating, there might not be sufficient ATP for carbon fixation. Cyclic electron transport makes up for this imbalance. This pathway uses photosystem I and the electron transport system to produce ATP but not NADPH; it is cyclic because an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll (Figure 10.8).