7.3 Light Reactions

A. Two Pathways
1. Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway.
2. Both pathways produce ATP but only the noncyclic pathway also produces NADPH.
3. ATP production during photosynthesis is sometimes called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.

B. Noncyclic Electron Pathway (*SPLITS WATER, PRODUCES NADPH & ATP)

1. This pathway occurs in the thylakoid membranes and requires participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II).
2. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated.
3. Each photosystem has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid pigments).
4. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a.
5. Electrons in reaction-center chlorophyll a become excited; they escape to electron-acceptor molecule.
6. The noncyclic pathway begins with PSII; electrons move from H2O through PS II to PS I and then on to NADP+.
7. The PS II pigment complex absorbs solar energy; high-energy electrons (e-) leave the reaction-center chlorophyll a molecule.
8. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+ ions:
9. Oxygen is released as oxygen gas (O2).
10. The H+ ions temporarily stay within the thylakoid space and contribute to a H+ ion gradient.
11. As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs.
12. Low-energy electrons leaving the electron transport system enter PS I.
13. When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor.
14. The electron acceptor passes them on to NADP+.
15. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e- + H+  NADPH.
16. NADPH and ATP produced by noncyclic flow electrons in thylakoid membrane are used by enzymes in stroma during light-independent reactions.

C. Cyclic Electron Pathway
1. The cyclic electron pathway begins when the PS I antenna complex absorbs solar energy.
2. High-energy electrons leave PS I reaction-center chlorophyll a molecule.
3. Before they return, the electrons enter and travel down an electron transport system.

a. Electrons pass from a higher to a lower energy level.
b. Energy released is stored in form of a hydrogen (H+) gradient.
c. When hydrogen ions flow down their electrochemical gradient through ATP synthase complexes, ATP production occurs.
d. Because the electrons return to PSI rather than move on to NADP+, this is why it is called cyclic and also why no NADPH is produced.

D. ATP Production (chemiosmosis)
1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+ remain.
2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from the stroma into the thylakoid space.
3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce ATP from ADP + P by using an ATP synthase enzyme

**Now is a good time to look at the various animations of these processes. The trick is to VISUALIZE them*

Photosynthesis Quick Animation
Photosynthesis Quick Animation 2

Photosynthesis Movie 1
Photosynthesis Movie 2
Photsynthesis Movie 3
Photosynthesis McGraw-Hill Animations

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