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.
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
Planck’s constant and has a value of 6.626 x 10-34 Joules x sec. The Greek symbol v or
“nu” refers to frequency.
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.
2 per second
1 per second
One second
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
Chlorophyll b
Absorption
Intensity
excited states
1
ground state
Chlorophyll a
300
400
500
600
700
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 electron’s 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)
QH2
+
PS
II
Fd
Q
Cyt bf
PC
NADP
PSI
NADPH + H+
O2
2H2O
4H+
Stroma
Lumen
CFo
ADP + Pi
CF1
ATP
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.
NADP+
QH2
PS
II
Q
Fd
Cyt bf
PC
PSI
+
NADP
NADPH
NADPH
O2
2H2O
4H+
Stroma
Lumen
CFo
ADP + Pi
CF1
ATP
ATP
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