1) Thylakoid membranes in chloroplasts contain light-harvesting complexes of chlorophyll and other pigments that absorb light energy. 2) The light energy is passed to photosystems which contain clusters of chlorophyll. This initiates photosynthesis as electrons are transferred. 3) There are two pathways - the noncyclic pathway produces both ATP and NADPH as electrons move between two photosystems, while the cyclic pathway only produces ATP as electrons return to one photosystem.

1. © Cengage Learning 2015
6.4 How Do The Light-Dependent
Reactions Work?
• Thylakoid membranes contain millions of light-harvesting
complexes
– Circular arrays of chlorophylls, various accessory pigments,
and proteins
• After light absorption, light-harvesting complexes pass
energy back and forth
1

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How Do The Light-Dependent Reactions
Work? (cont’d.)
• Photosystem: group of hundreds of chlorophylls,
accessory pigments, and other molecules
– Photosynthesis begins when energy from light-harvesting
complexes reaches a photosystem
2

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How Do The Light-Dependent Reactions
Work? (cont’d.)
3
photosystem light-harvesting complex
Figure 6.5 - A view of
some components of
the thylakoid
membrane as seen
from the stroma.
Molecules of
electron transfer
chains and ATP
synthases are also
present, but not
shown for clarity.

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How Do The Light-Dependent Reactions
Work? (cont’d.)
• Light-dependent reactions involve:
– Noncyclic pathways
– Cyclic pathways
4

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The Noncyclic Pathway
• Steps of noncyclic pathway:
1) Light energy ejects electrons from photosystem II
2) Photosystem II pulls electrons from water molecules and
breaks them apart into oxygen and hydrogen ions
• Photolysis: process by which light energy breaks down a molecule
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The Noncyclic Pathway (cont’d.)
• Steps of noncyclic pathway (cont’d.):
3) Electrons enter an electron transfer chain in the thylakoid
membrane
4) Hydrogen ion gradient forms across the thylakoid membrane
5) After electrons move through first electron transport chain,
light energy ejects electrons from photosystem I
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The Noncyclic Pathway (cont’d.)
• Steps of noncyclic pathway (cont’d.):
6) Ejected electrons move through a second electron transfer
chain; NADPH forms
7) Hydrogen ions in the thylakoid compartment are propelled
through ATP synthases
8) ATP synthases phosphorylate ADP; ATP is formed in the
stroma
• Electron transfer phosphorylation: process by which electron flow
through electron transfer drives ATP formation
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The Noncyclic Pathway (cont’d.)
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ATP
synthase
light energy light energy1
2
3
4
5
7
8
stroma
O2
thylakoid
compartment
H2O
photosystem II
electron
transfer
chain
photosystem I
electron
transfer
chain
6
Figure 6.7 - {Animated} Light-dependent reactions, noncyclic
pathway. ATP and oxygen gas are produced in this pathway.
Electrons that travel through two different electron transfer
chains end up in NADPH.

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The Noncyclic Pathway (cont’d.)
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“uncharged” particles with
and “charging component
The “charged” particles
resulting from harvesting light
energy

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The Cyclic Pathway
• Electrons ejected from photosystem I enter an electron
transfer chain, and then return to photosystem I
• Similarities with noncyclic pathway:
– Hydrogen ions are moved into the thylakoid compartment,
driving ATP formation
• Differences from noncyclic pathway:
– NADPH and oxygen gas are not produced
10

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The Cyclic Pathway (cont’d.)
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The cyclic pathway is the reaction that really pumps out the ATP. REMEMBER that ATP is the major energy currency of cells!