The Photo in Photosynthesis

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The Photo in Photosynthesis
To understand the major thrust of
photosynthesis, ask yourself this question what
does it take for a plant to synthesize a
carbohydrate? If you said it takes energy and a
carbon source, then you know the basic outline of
photosynthesis. Energy comes from the sun in the
form of photons, carbon as CO2 from the air.
Now, ask yourself what remarkable system must a
plant have to harness radiant energy of a photon?
And, what remarkable system must the plant have
to trap CO2 and use this as a carbon source to
make glucose, sucrose and starch? This tutorial
answers the first question.
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How should you picture the energy in light?
Physicists tell us that light is both a wave and
a particle that is manifested as electromagnetic
and electric fields. Whatever that means,
remember that light energy depends on its
frequency, which is defined as the number of wave
length units that can be fitted into a space of
one second. The more units, the greater the
frequency (click 1). Light energy is measured in
units called photons. The amount of energy in a
photon varies, but not in a continuous manner.
Rather photon energy occurs in steps or packets
called quanata (click 1). The steps are defined
by the equation that relates energy to frequency
of light (click 1). The h in the equation is
Plancks constant and has a value of 6.626 x
10-34 Joules x sec. The Greek symbol v or nu
refers to frequency.
2 per second
E h?
In the illustration on the right, the upper curve
has a shorter wave length (measured as red) than
the lower curve. Thus, based on the formula, one
could say that the upper curve has twice the
energy of the lower curve. More importantly, a
photon of light with a greater frequency can
impart more energy to the plant. But, the plant
must be receptive to the energy. Click 1 to go
on.
1 per second
One second
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Chlorophyll, a green pigment, allows plants to
absorb light energy. Energy absorption,
however, must be consistent with allowable (basal
to excited state) electron transitions within the
chlorophyll molecule (click 1). Because these
transitions are not continuous, a plant obtains
energy only at certain frequencies of light.
Energy insufficient to reach an excited state is
not absorbed (click 1). Similarly, energy that
drives an electron past one energy level but is
insufficient to reach a second is not absorbed
(click 1). To be absorbed, the energy must be
sufficient to reach only allowable energy states
(click 1). This simple rule of quantum physics
is all you need to know to understand an
absorption spectra of chlorophyll (click 1).
2
excited states
1
ground state
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Higher plants have two photocenters, P680 and
P700, so designated by the wavelength that
gives maximum absorption or O2 evolution.
Assume you wish to design a photosystem in a
plant. You need a membrane-bound enclosure with
a hollow center (click 1). Next, you need a
photocenter that serves as an electron source
(click 1). To replace the departing electron
this center must be capable of extracting
electrons from H2O, which results in O2 (click
1). To preserve energy you need a cytochrome
system that pumps protons (click 1) and an
electron carrier to bring electrons to the
cytochromes (click 1). Since the electrons
ultimate destination is NADP, you need another
photosystem to reduce NADP to NADPH (click 1).
This center must boost the electron to a higher
reduction potential. You will need another
carrier to bring electrons to this center (click
1). Finally, the protons that are pumped in to
the hollow lumen can be used to drive the
synthesis of ATP. For that you need an ATP
synthase complex (click 1)
NADP
NADPH H
Lumen
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Before we leave the light reaction, there is
just one more important point to consider. The
light reaction is supplying the plant with ATP
and NADPH. It is important that these two energy
sources be balanced. To accomplish this, PSI has
a switch that shunts excited electrons back to
cytochrome bf. The switch is controlled by the
ratio of NADP/NADPH. To understand the
regulation, consider what happens when NADP is
high (click 1). This means there is plenty of
NADP to take electrons from ferrodoxin (click
1). But, if NADP is low (because NADPH is
high), the electron are shunted back to
cytochrome bf to make more ATP (click 1). This
is how chloroplasts maintain energy balance.
ATP
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Test Your Understanding
Look at the absorption spectrum of chlorophyll.
Can you can see absorption bands representing the
2 photocenters?
Yes, but the wavelengths are not exactly at 680
or 700 nm. There are two peaks in and around the
near infrared region. Obviously the major
absorbing factors in the center at these
wavelengths are chlorophyll a and b molecules.
The presence of proteins, which are also part of
the center will alter the position of the peaks.

Why is there only one water-splitting center?
Both centers lose electrons. Removing the
electron from P680 (PSII) makes the chlorophyll a
very powerful oxidant. The power is demonstrated
by the ability to pull electrons away from a H2O
molecule. P700 (PSI) has the luxury of having an
electron from PSII fill the void, and hence water
is not needed.
Assume one mole of photons struck the P680 center
at two wavelengths, 250nm and 700nm. Which ?
will give more energy to the plant, and how much
more?
250nm will give more. A mole of photons at 250nm
is equivalent to 479 kJ. One mole of photons at
700 nm is equivalent to 171 kJ. At 250 nm , 308
more kJ strike the plant. This computes out to
be almost 3 times more energy at the lower wave
length of light. See Voet solutions, p SP-12