In noncyclic electron transport, light energy is used to oxidize water, forming O2, H+, and electrons. Follow the steps in Figure 8.9 as you read this section.
tached to each ribose (see Figure 7.4). Whereas NAD+ partic-
Photosystem I ip^N
Electron transport chain
Photosystem I ip^N
5| Photosystem I reduces an oxidizing agent, ferredoxin (Fd), which in turn reduces NADP+ to NADPH + H+.
Electrons from water replenish the electrons that chlorophyll molecules lose when they are excited by light. As the electrons are passed from water to chlorophyll, and ultimately to NADP+, they pass through a chain of electron carriers. These redox reactions are exergonic, and some of the free energy released is used ultimately to form ATP by a chemiosmotic mechanism.
two photosystems are required. Noncyclic electron transport requires the participation of two different photosystems. These photosystems are light-driven molecular units, each of which consists of many chlorophyll molecules and accessory pigments bound to proteins in separate energy-absorbing antenna systems.
► Photosystem I uses light energy to reduce NADP+ to NADPH + H+.
► Photosystem II uses light energy to oxidize water molecules, producing electrons, protons (H+), and O2.
The reaction center for photosystem I contains a chlorophyll a molecule called P700 because it can best absorb light of wavelength 700 nm. The reaction center for photosystem II contains a chlorophyll a molecule called P680 because it absorbs light maximally at 680 nm. Thus photosystem II requires photons that are somewhat more energetic (i.e., shorter wavelengths) than those required by photosystem I. To keep noncyclic electron transport going, both photosystems I and II must constantly be absorbing light, thereby boosting electrons to higher orbitals from which they may be captured by specific oxidizing agents.
P700*, which then leads to the reduction of an oxidizing agent called ferredoxin (Fd) and the production of P700+. Then P700+ returns to the ground state by accepting electrons passed through the redox chain from photosystem II.
With this accounting for the source of the electrons entering photosystem II, we can now consider the fate of the electrons from photosystem I. These electrons are used in the last step of noncyclic electron transport, in which two electrons and two protons are used to reduce a molecule of NADP+ to NADPH + H+. In summary:
► Noncyclic electron transport uses a molecule of water, four photons (two each absorbed by photosystems I and II), one molecule each of NADP+ and ADP, and one Pi.
► Noncyclic electron transport produces NADPH + H+ and ATP and half a molecule of oxygen (1/2 O2).
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