A. Light Reactions
Light strikes chlorophyll pigments in a photosystem.
This “excites” the electrons of the chlorophyll molecule
Photosystem
photon
Light-harvesting complexes
Reaction center
Primary electron acceptor
Thylakoid
membrane
Transfer of energy
Pigment molecules
Pair of chlorophyll a molecules
Photosystems• Consists of light-harvesting complexes:
(pigment molecules bound to proteins)
• These pigment-protein clusters surround a
reaction center complex which contains:
• A pair of chlorophyll a molecules
• A primary electron acceptor
• Pigments absorb photons and pass the energy from
molecule to molecule (yellow lines) until it reaches the
reaction center.
• See prior slide.
Two Photosystems
• The reaction center chlorophyll a of photosystem II
is called P680
– It absorbs red light @ 680 nm.
• The reaction center chlorophyll a of photosystem I
is called P700
– It absorbs far-red light @ 700 nm.
• They work together to generate ATP and NADPH
The Photosystem Connection
• Light energy  ATP & NADPH
• Electrons from the splitting of water pass from
photosystem II to photosystem I to NADP+
• How do the electrons get from Photosystem II to
Photosystem I?
– Electrons move down an ETC located between the
photosystems.
– This provides energy to make ATP
• How do electrons get from photosystem I to NADP+?
– Electrons move down a short ETC.
Light Reactions:
STEPS 1-3
1. Pigment molecule absorbs
light within light-harvesting
complex; energy is passed
along, reaches reaction
center of photosystem II,
and excites electron of
chlorophyll P680.
2. Electron is captured by
primary electron acceptor.
3. Water is split; oxygen is
released.
Light Reactions:
STEP 4
4. Each photo-excited
electron passes from
photosystem II to
photosystem I via
the ETC.
• Hydrogen gradient is
formed; ATP is
synthesized via
chemiosmosis.
Light Reactions:
STEPS 5-6
5. Light energy excites
electron of chlorophyll
P700 of photosystem I;
electron is captured by
primary electron
acceptor.
6. Excited electron is
passed to NADP+ via a
short ETC, resulting in
NADPH.
Electron flow in the light reactions of photosynthesis. The
energy from light drives electrons from water to NADPH
A mechanical analogy of the light reactions
Primary electron acceptor
Primary electron acceptor
Input of light
Input of light
Role Playing
To model the result of light striking a photosystem
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In Photosystem II, Chlorophyll 1, 2, and 3 (the reaction center chlorophyll) sit in the three chairs each with a
tennis-ball electron.
Primary Electron Acceptor & the Water Molecule stand near Chlorophyll 3
The water molecule should hold a tennis ball electron behind its back.
The Light Source shines the flashlight on chlorophyll 1
Chlorophyll 1 gets “excited”—standing up quickly, waving its electron and then “wakes up chlorophyll 2.
Chlorophyll 2 stands up excitedly and wakes up chlorophyll, while chlorophyll 1 sits back down,
“exhausted”.
Chlorophyll 2 sits down next.
Then as chlorophyll 3 gets up and waves its electron, the Primary Electron Acceptor grabs it.
Chlorophyll 3 then sits back down acting startled to have “lost” its electron.
The Water Molecule comes over, waves its electron and hands it to chlorophyll 3.
This causes the Water Molecule to split, releasing Oxygen.
The Electron Acceptor passes the electron down the electron transport chain.
This process pumps Hydrogen ions across the thylakoid membrane.
In Photosystem I, light excites the electrons the same way.
These electrons are transferred to NADP+ and are replaced by electrons coming from the electron transport
chain.
Hydrogen ions cross back over the thylakoid membrane and bond to NADP+ becoming NADPH
The hydrogen pump powers the production of ATP.
B. The Calvin Cycle: Converting CO2 to Sugars
• The Calvin Cycle functions like a sugar factory
within the stroma.
• Inputs:
– CO2 from the air
– Energy from ATP
– Electrons from NADPH
• Output:
• This cycle produces a sugar called G3P, which the
plant uses to make glucose
– G3P = glyceraldehyde-3-phosphate
Photosynthesis Reviewed and Extended