H2o

A single photon of visible light does not have enough energy to break the bonds in water; four photons are required in this photolytic cleavage reaction.

The four electrons abstracted from water do not pass directly to P680 + , which can accept only one electron at a time. Instead, a remarkable molecular device, the oxygen-evolving complex (also called the watersplitting complex), passes four electrons one at a

NADPH + H+ NADP+

Photophosphorylation

Oxidative phosphorylation

FIGURE 19-55 Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria. Cyanobacteria use cytochrome b6f, cytochrome c6, and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (top to bottom) from water to NADP+. (b) In oxidative phosphorylation, electrons flow from NADH to O2. Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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