Light Dependent Reaction
Before we begin…
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P680 and P700 are identical chlorophyll a molecules, but they are associated with
different proteins which slightly alters their electron configurations. This
accounts for the difference in their light absorbing properties.
Photosystem I and Photosystem II were named in the order that they were
discovered. HOWEVER, Photosystem II comes first in noncyclic reactions.
Photosystem I is the only photosystem used in cyclic reactions.
What is the difference between cyclic and noncyclic reactions?
 Cyclic electron flow does not reduce NADP+ to NADPH; ATP is the only
product.
 Noncyclic electron flow produces ATP and NADPH in roughly equal quantities.
However, the Calvin cycle (light independent reaction) uses more ATP than
NADPH.
 A build up of NADPH coupled with a slow-down of the Calvin cycle could be
responsible for the shift to cyclic electron flow from noncyclic electron flow in
an attempt to increase the amount of ATP without creating more NADPH.
Light Dependent Reaction: Noncyclic electron flow
Non-cyclic electron flow is the most common pathway in light dependent reactions.
1) First, a photon is absorbed by a pigment in photosystem II (P680) which causes an
electron in the antenna pigment molecule to jump from its ground state energy level to a
higher energy level.
What is a photosystem?
A photosystem is composed of a reaction center surrounded by light-harvesting
complexes.
 Each light-harvesting complex consists of pigment molecules (chlorophyll a,
chlorophyll b, carotenoids) bound to particular proteins.
 The reaction center is a protein complex that includes two special chlorophyll a
molecules and a primary electron acceptor.
 Antenna: A large number of pigment molecules (100-5000) that harvest light
and transfer the electrons to the reaction center; the purpose is to maintain a
high rate of electron transfer in the reaction center, even at lower light intensities.
2) The electron then falls down to the low energy state, releasing its energy. However,
this energy is not lost as it is picked up by an adjacent pigment molecule where it is used
to raise an electron to a higher energy state, etc. etc., until this energy reaches the reaction
center (like a bucket brigade or "the wave" at a football game).
3) At the reaction center, a special chlorophyll a molecule receives energy from an
adjacent pigment molecule which excites one of its electrons to a higher energy level.
Instead of falling back to its ground state and releasing energy, the excited electron is
transferred to the primary electron acceptor during a redox reaction. The chlorophyll a
molecule is in an oxidized state until the electrons are replaced during photolysis.
What is photolysis?
C.B. van Niel used a radioactive tracer to determine where the O2 released in
photosynthesis originated from. It was originally thought that it came from CO2 and
that H2O was used in building sugar molecules. C.B. van Niel used a heavy
radioactive isotope of oxygen O18 to track the origin of oxygen released. This
isotope of oxygen was only released into the atmosphere if it had originated in H2O.
As a result, it was determined that H2O was used to replenish electrons and supply
H+ ions. Photolysis occurs when the energy from a photon of light with the energy
of visible light or higher is used to split a water molecule apart. This occurs in the
thylakoid membrane and is carried out by a poorly understood structure called the
water-splitting complex or the oxygen-evolving complex.
In order to produce one molecule of O2, two water molecules are broken down into
four H+ and four e-. The electrons are used to replace electrons lost to the primary
electron acceptors in the ETC.
Chemiosmosis
4) The electrons travel down the electron transport chain (ETC) within the thylakoid
membrane to photosystem I. Along the way, more H+ is pumped into the thylakoid space.
The ultimate electron acceptor of the electron transport chain is the reaction center of
photosystem I, replacing lost e-.
photosystem II (P680)  Primary acceptor  Plastoquinone (Pq)  cytochrome
complex  plastocyanin (Pc)  photosystem I (P700)
Chemiosmosis: 2 e- are passed from the primary electron acceptor to plastoquinone along
with 2 H+ molecules from the stroma. These combine to form PQH2 which diffuses across
the membrane and passes the 2 e- to the next protein in the chain, cytochrome complex,
while releasing the 2 H+ into the lumen or thylakoid space
2H+stroma + 2e- + PQ
PQH2
PQH2
PQ + 2H+lumen + 2e-
During photosynthesis the lumen becomes acidic, as low as pH 4, compared to pH 8 in
the stroma. This represents a 10,000 fold concentration gradient for protons across the
thylakoid membrane. Thus, the chemiosmotic potential between the lumen and stroma is
high enough to drive ATP synthesis using the ATP synthase. In this manner, the lightdependent reactions are coupled to the synthesis of ATP via the proton gradient.
The electrons are then passed to plastocyanin (another electron carrier). Pc delivers the
e- to photosystem I.
Meanwhile in Photosystem I…
5) A photon of light hits photosystem I, and the energy is transferred via a light-harvesting
complex to the PS I reaction center. This excites an electron of one of the two P700
chlorophyll a molecules in the reaction center. The photoexcited electron is then transferred
to the primary electron acceptor of PS I. The space vacated by the electron in PS I is
replaced by an electron from the ETC.
6) The electrons from PSI are passed down a second electron transport chain through the
protein ferrodoxin (Fd). NADP+ reductase transfers electrons from Fd to NADP+.
NADP+ is then reduced to NADPH.
NADP+ + 2H+ + 2e- → NADPH + H+
This consumes the H+ ions produced by the splitting of one water molecule, leading to a
net production of 1/2O2, ATP, and NADPH+H+ with the consumption of solar photons
and water.
In summary…
The light reactions use solar power to generate ATP and NADPH which provide chemical
energy and reducing power, respectively, to the sugar-making reactions of the Calvin Cycle.
Cyclic Photophosphorylation
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Each CO2 taken up by the Calvin cycle requires 3 ATP and 2 NADPH
molecules.
How many carbons are found in a glucose molecule?
How many ATP would be required to synthesize one glucose molecule?
How many NADPH would be required to synthesize one glucose molecule?
For every molecule of water split in photosynthesis, six protons are pumped into the lumen
for use in ATP production. Two protons come from the water molecule itself, two protons
come from the PQ shuttle, and two protons are pumped across the cytochrome b/f
complex.
Another way to look at this is for every two e- that enter the electron transport chain, four
protons are pumped across the thylakoid membrane. These are then added to the two
protons that were generated by the splitting of a water molecule which occurs in the lumen
side of the membrane near the reaction center.
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Each water molecule split in photosynthesis provides the two e- needed to form
one NADPH molecule. This means that for every O2 generated by the water
splitting complex, 2 NADPH molecules are formed.
The chemiosmosis driven by the four electrons used to form two NADPH
molecules as they pass through the cytochrome b6/f complex liberates only
enough energy to pump 12 protons into the interior of the thylakoid.
But in order to make 3 molecules of ATP, the ATPase in chloroplasts appears to
have 14 protons (H+) pass through it. This is because studies have shown that for
each ATP produced through photophosphorylation, approximately 4.5 protons are
required.
So there appears to be a deficit of 2 protons.
Since ATP is used faster than NADPH, when the ATP concentration gets lower,
the Calvin cycle slows down. Because
these are high energy molecules,
they cannot be stored for very
long, so the
plant has a need to slow NADPH
production and increase ATP
production to maximize efficiency.
It is at this point that the
chloroplast switches from
noncyclic to cyclic electron flow.
Notice that photosystem I
transfers the electrons from a
primary electron acceptor to Fd
which takes the electrons to the
plastoquinone found in the ETC
of photosystem II. The purpose
of this is to generate more ATP