Noncyclic Photophosphorylation

Photophosphorylation is the process of making ATP from ADP and Pi (phosphorylation) using energy derived from light (photo). Noncyclic photophosphorylation begins with PS II and follows the steps:

1. Photosystem II. Electrons trapped by P680 in photosystem II are energized by light.

2. Primary electron acceptor. Two energized electrons are passed to a molecule called the primary electron acceptor. This electron acceptor is called “primary” because it is the first in a chain of electron acceptors.

3. Electron transport chain. Electrons pass through an electron transport chain. This chain consists of proteins that pass electrons from one carrier protein to the next. Some carrier proteins, like ferredoxin and cytochrome, include nonprotein parts containing iron.

4. Phosphorylation. As the two electrons move “down” the electron transport chain, they lose energy. The energy lost by the electrons as they pass along the electron transport chain is used to phosphorylate, on average, about 1.5 ATP molecules.

5. Photosystem I. The electron transport chain terminates with PS I (with P700). Here the electrons are again energized by sunlight and passed to a primary electron acceptor (different from the one associated with PS II).

6. NADPH. The two electrons pass through a short electron transport chain. At the end of the chain, the two electrons combine with NADP+ and H+ to form NADPH. NADPH is a coenzyme. Since the electrons have a considerable amount of energy left, NADPH is an energy-rich molecule.

7. Photolysis. The two electrons that originated in PS II are now incorporated into NADPH. The loss of these two electrons from PS II is replaced when H2O is split into two electrons,

2 H+ and 1.2 O2. The process is called photolysis and literally means decomposition (lysis) by light (photo). A manganese-containing protein complex catalyzes the reaction. The two electrons from H2O replace the lost electrons from PS II. One of the H+ provides the H in NADPH.

In summary, photophosphorylation takes the energy in light and the electrons in H2O to make the energy-rich molecules ATP and NADPH. Because the reactions require light, they are often called the light-dependent reactions or, simply, light reactions. The following equation informally summarizes the process:

H2O + ADP + Pi + NADP+ + light _ATP + NADPH + O2 + H+