what I'd like to do now is just
review a couple of experiments that
show
the importance of absorption of
different wavelengths of light by the accessory pigments
and the chlorophyll in the photosystems and
also to show an experiment which
demonstrates the importance of having
both photosystems
active in chloroplast so let's
look at that right now so the
first experiment
I'd like to go over is one in which a
filamentous algae which has long cells
and long filamentous shapes to it
can be exposed to different wavelengths of
light so for example
here we can shine blue light on this
part of the
filament of the algae here we can shine
green light on the
filament of the algae in here we can
illuminate this part with red light
so we're able to actually differentially
illuminate
the photosynthetic algae along a length
of a particular filament and then what
can be added are
oxygen seeking O2 seeking bacteria
amd these bacteria
can chemotax that is they can
move they will migrate using flagella
to areas of high O2 concentration
and what do you think happens
well the O2
sensing bacteria accumulate at very
high
cell densities
in this area of the filament
and also in this area of the filament they
accumulate at high cell densities
they migrate in other words they
chemotax to these areas
but only relatively few are found
in intermediate areas and how would you
interpret that knowing what you
know about photosynthesis well you
realize that the absorption spectra for
the
pigments found in the photosystems
absorb in the blue range very little in
the green range
and again the red range so these
absorption
we look at the absorption at various
wavelengths
of light and here we plot wavelengths of light
this would be bluish light this would be red light
we see that there's high oxygen
production then
in this area of the filament and this area
of the filament because the oxygen sensing
bacteria
move to those areas and so oxygen is being
produced here in oxygenic photosynthesis
 
and that is because of the absorption
spectra
of the photosynthetic pigments and
we've already shown you
absorption spectra graphs of that
so this is a very nice bioassay doesn't
involve
any biochemistry at all its a bioassay
using behavior of bacteria to
show that photosynthesis in the algae
is high and in when illuminated by blue
light and red light but not
by immediate wavelengths of light
again the absorption peaks of the
photosynthetic pigments in the
these areas now in another experiment
which shows the importance of both photo
systems
in chloroplast photosynthesis
it's called the a synergistic effect
or the enhancement effect
if you look at if we look time here
we do an experiment
and we look at rates of photosynthesis
here we'll call it photosynthetic rates
and you illuminate
cells with light at particular times lets say at this time
this time you eliminate with
far red light.. we'll get in an increase in photosynthesis
and then we turn that far red light off
photosynthesis drops of and then if at another time
we illuminate with red light we see
photosynthesis picking up again
until we turn that red light
off so here is on.. off
of far red light and here's on a red light
and off a far red light and again photosynthesis decreases
now if at this point we illuminate with
both
red and far
red light we see a big boost in
photosynthesis
the synergistic effect in photosynthesis in here
we turn that
light off both far red
and red light so what we see though is that
the amount the rates of photosynthesis
occurring with
far red light and the rates of photosynthesis occurring
with red light are
not as great if you added them
together
to account for illumniation with both
wavelengths of light that is this great
increase here which is more than the sum of
the rates of photosynthesis
from either of those lights alone this is a
synergistic effect
 
of both wavelengths of light on photosynthetic rates
and this basically gives us the importance
synergistic effect or this
enhancement effective of using both
red and far red light shows the
importance of both photosystems
because here we are activating
photosystem I
and here we are activating photosystem II
remember photosystem II has a reaction center
with the peak absorbance of 680
I'm sorry I have these reversed this
would be
oh no I had that right photosystem II remember that photosystem II has a
reaction center with an absorbance peak
at 680 nanometers
whereas photosystem I has a reactions center with an
absorbance peak
with the reaction center with an absorbance peak at 700 that is far red
light
and P680 represents red light
so here we see this synergistic effect
of utilizing both photosystems
in producing photosynthesis
so this is a nice demonstration then of
both photosystems I
and II acting together to
produce photosynthesis
in the light dependent reactions
of course these are all
light dependent reactions
and now we're ready to tackle what happens to
the molecules produced by the light dependent reaction that is
ATP lots of it though chemiosmosis
and NADPH
through electron-transfer through
oxidation NADP
we can now see how these molecules are
utilized
in the light independent reactions
that is the fixation of carbon
from CO2 and fixation of carbon
from CO2 into organic
molecules like glucose
C6H12O6
this then is the goal of photosynthesis to produce food
using the light energy the
harvesting of light energy by
these photosynthetic light dependent
reactions
so that's what we will tackle
next
are the light independent reactions
alright now we've covered the light
dependent reactions of photosynthesis lets move on
to the Calvin cycle
for the light independent reactions
these are the carbon fixation reactions and
we know that to build carbohydrates cells
need energy course and we
drive that energy from ATP
that is produced by the light dependent
reactions and mechanisms we've already
talked about
and we need reduction potential provided
by
reduced NADP or NADPH
and this comes from photosystem I
as we have seen so
what is the biochemical pathway
that fixes carbon its called the
Calvin cycle
named after Melvin Calvin, a plant
physiologist
and this is the pathway that fixes
carbon
it occurs in the stroma on the
chloroplast remember that
we have a chloroplast with an outer membrane
here
and an inner membrane
and then we have the thylakoids
they're stacked into grana... the
outside of the thylakoids we have this
stroma
S-T-R-O-M-A, stroma
so this is occurring in the stroma, the Calvin cycle is in the stroma
of the chloroplast and as we've discussed
ATP and ADP are used for the energy
that drives these biochemical reactions
so the trick is to incorporate carbon from
gases carbon dioxide into organic
molecules
and in that way to build food to
build sugars that
will sustain all life on Earth
essentially
and CO2 incorporated in organic
molecules occurs in the very first step
of the Calvin cycel so the first step
involves taking
carbon and fixing it in an organic molecule
and this is the reaction of that first
step of the Calvin cycle so ribulose-
bis-phosphate which has five carbons in it
we can refer to this as
RuBP
RuBP + 1 carbon
from CO2 is put into a 6 carbon
intermediate that is then converted into two
molecules
of phosphoglyerate
3-phosphoglyerate and that
has three carbons
in each of those so
we have taken a five carbon compound
added a single carbon from carbon dioxide
produced transiently a six carbon
molecule that is then cleaved into two
3 carbon molecules so you can see
that we've taken
the one carbon from carbon dioxide and
put it into an organic molecule
that carbon exists as one of the
carbons in one of these three carbon
3-phosphoglyerate molecules
and the enzyme that catalyzes this
reaction is known as rubisco
R-U-B-I-S-C-O
Rubisco, one of the most abundant enzymes
on the planet because of all the photosynthesis that is occurring
on Earth and that
is rubisco and we will see that
rubisco this enzyme
which is a large enzyme consisting of many subunits
has certain properties that make
this
process carbon fixation not entirely
efficient and
that's kind of an artifact of evolution
we'll consider
shortly but not right now let's move on
to the Calvin cycle
itself and look at the entire cycle
and thats where we will pick up with the next
part in this lecture will look at the
Calvin cycle and
look at its organization into
3 major phases
