Welcome back to the lecture series on by energy.
Yesterday we talked about in our previous
classes about photo system 1 & photo system
2 how photo system 1 photo system 2 varies,
I told you then in the space of location of
places they are located at different locations
there is one more thing to add up to that
both for system one and system two stays at
a different kind of reduction potential or
redox potential each one of these four systems
have a different ability to lose electrons.
One lose faster than the other there how such
things are being achieved so the wave four
systems develop is that say for example our
system may have a mix of chlorophyll a and
chlorophyll b but that number can be varied
so for example if I say each photo system
is made up of 100 molecules of chlorophyll
so there may be a 4 system where they will
be say so one option is at 50- 50 so50 chlorophyll
A 50 chlorophyll B.
But anything which ships say for example one
has said 10 chlorophyll-A other will have
a 90 chlorophyll B or the and think the other
way rivers one has 10 chlorophyll b + 100
chlorophyll-a now look at the spectrum here
back to the spectrum but I agree for you.
Now based on that number the spectra the overall
light-absorbing ability is going to shift
again look at the spectra carefully look at
this respect I so this is the zone for chlorophyll
the pink one is the further chlorophyll b
and the green one is for chlorophyll a, now
say for example in your system you have a
photo system where there are out of 100 molecules
just this is just for your understanding sake
I am telling hundred molecules.
They do not think there is millions of such
molecules in chapters in there but they have
a finite number 44 system one and photo system
2 but their proportionate number of chlorophyll
A and B are different in for system one there
may be a different proportion of chlorophyll
A and chlorophyll B similarly in photo system
2 there will be a different proportion of
chlorophyll a and chlorophyll b.
And these differences completely change their
optical properties they are light absorbing
potential and the way and their redox potential
these are apart from so there are three things
now I am adding from the previous two lectures
point one we have two photo systems and we
will talk about how this all being discovered
actually it was discovered like that only
that way I'm telling you the experiment the
way I'm telling is they giving you this explanation.
Point one, there are two different water systems
for system 1 for system two and as I will
proceed further they are indicated by different
wavelengths p680 p700 will come to that that
is basically telling and there is correspondence
so that essentially telling you that which
one is which one for system 1 for system two
second thing they are especially located at
different location third important aspect
is that they have different proportion of
chlorophyll a and chlorophyll b, fourth they
are standing at different redox potentials.
So in other words their light absorbing power
light absorbing ability and electron addiction
ability is entirely different these three
or four concepts I really wish get clear in
your brain because that will be a big help
to understand the whole basic architecture
of photosynthesis okay so from here let us
coming back to the slide.
So we talked about the dye-sensitized solar
cell we will be dealing with it you remember
we talked about the reaction centers so this
is the topic what we will be dealing now is.
Photons absorbed okay absorbed by many chlorophyll
CHL stand for chlorophyll funnels it into
a reaction Center a reaction center so in
the last class I told you that light is falling
is not that at every point there is a reaction
taking place each one of the chlorophyll molecules
are getting oxidized by leaving an electron
sitting the next molecule but the real reaction
Center is only unique and I also mentioned
in previous classes that you really cannot
locate the reaction sitter but how that was
discovered.
Well put on a statement for you that will
help you to realize that how that was being
discovered so basically the output of photosynthesis
so before you get into that just before they
put the statement there the output of photosynthesis
is measured, so if you look at look back at
the basic reaction co2 + H2O CH2 O + oxygen
so the output is measured by the output of
oxygen, so if you have a probe by virtue of
which.
You can measure the output of oxygen then
you can measure the rate of photosynthesis
so in other word so let us add one more thing
into it right so if I am giving this much
quanta of light say I give x quanta of light
then what is the y mole of oxygen which is
evolved so this is how the efficiency of photosynthesis
photosynthetic efficiency is being determined,
okay.
Now what is important here so moving to the
next slide out here?
So measurement of the dependence of the rate
of photosynthesis on the intensity of the
illumination that it increases linearly at
low intensities and reaches a saturating value
at high intensities, so this was discovered
that something very interesting graph for
you to look at so this was what I just now
explained the rate of photosynthesis 
okay and the light intensity, so what you
are seeing is that this is where the limiting
value is being approached.
And followed by this so as we talked about
the as saturating value is observed in a strong
light because chemical reaction utilizing
the absorb photon become rate limiting this
experiment provided the first intimation that
photosynthesis can be separated into light
reaction and a dark reaction, what who is
much earlier than that if this was one area
around 1932 there was an experiment which
was done by Robert Emerson 
and William Arnold.
This experiment was really critical what they
did that they as I showed you in the previous
slide measured oxygen as a function of s and
they use so chlorella what they have used
ok Chlorella this is basically a an algae
so they measured the oxygen by exposing the
flash of light measured oxygen with as I showed
you in the previous slide flash of light and
what the observed was very interesting.
The observation is one of the critical experiment
saturating light flash 
led to the 
production of only one molecule of oxygen
for 2,500 chlorophyll molecule this is that
critical statement which is significant, so
when 2,500chlorophyll molecules are illuminated
what you are seeing there is a single oxygen
molecule is being pretty it means that every
chlorophyll molecule is not leading to the
reaction so it means that so based on that
and several other experiment it was proposed
that what is essentially is happening is something
like this.
Out here so first again revisiting the reaction
Center concept where so these are the chlorophyll
molecule which are sitting at different reaction
Center chlorophyll a chlorophyll b respective
of it and when the light is falling like this
so it is kind of traveling through and then
is one particular Center which we call as
the reaction Center and the way it possibly
works one of the currently accepted theory
is that there is light the light energy which
and then there are hopping off energy transfer
taking place this level.
And eventually it reaches to the reaction
centers so and these are the chlorophyll which
are sitting at the ground state and these
are the chlorophyll which are sitting so these
are the chlorophyll at ground state and these
are the clothes which are sitting at the higher
state so this is
How by very simple experiment back in the
early nineteen hundred it was discovered that
the whole process is not one-to-one like one
light will fall on a chlorophyll and will
generate an oxygen it is not like that so
you need a cluster of chlorophyll molecules
to initiate the reaction of oxygen formation
and simultaneously these are the reactions
which laid the foundation for to kind of reaction
the light reaction and the dark reaction taking
place in the hole for synthetic apparatus
okay.
So now come back we will move on to the next
slide we will talk about one second okay.
So we talked about another concept so the
very basic reaction if you again go back to
the basic reaction I will keep on following
this basic reaction because that is where
lies all most of our understanding so this
is where the light is falling okay and what
we are talking about CH 20 + oxygen now the
question is which was posed was, how oxygen
is getting evolved?
Is that coming from carbon dioxide or is it
from water.
Again look at the reaction carefully you will
realize what I am trying to ask you so here
is oxygen which is getting involved how you
prove that this is coming from here or this
is coming from here it can come from both
sources right because both have oxygen you
can split the carbon dioxide it can release
oxygen you can split the water it can release
oxygen so there is no one has told the molecule
that you have to come from this spot, so how
to prove it.
Now we know of course in the beginning we
said that is a water is kind of getting a
split and oxygen is odd but how that reaction
took place how that was kind of being discovered
so the way it was done I will just put two
reactions in front of you, you remember I
told you that there are hydrothermal vents
where there are no sunlight it life does evolve
and those are the regions which are rich in
hydrogen sulfide those of you just forgotten
just go back to those lectures and realize.
Because it has some link and that is why I
might possibly made it a point to bring this
aspect for you that why I mentioned that thing
out there so let us put the two reactions
out here which will Help you to realize one
of the very seminal contribution so what we
are claiming is this okay oxygen evolved in
photosynthesis 
okay oxygen evolved in photosynthesis synthesis
come from water and how to prove it okay comes
from water, so in front of you I am putting
two reactions now Co2 carbon dioxide to 2H2O
waters in the presence of light making CH2O
+ oxygen plus you can have water whatever
okay fine.
Now Co2 + 2H2 now I replace this oxygen with
sulfur which is you remember now the hydrothermal
vent which is rich in hydrogen sulfide and
CH2O + 2 sulfur + H2O this was the reaction
which led to the understanding of a generic
reaction what is the generic reaction now
I am drawing the generic reaction for you,
is this oxygen H2O what we are claiming as
the currently the electron source H2S also
can be electrons.
So instead of H2O you put H2S and what you
are getting the output is sulfur so essentially
when in the beginning I told you that role
of when we are splitting the water it is just
an output which is coming out because it was
not really maybe it was never planned not
only what nature needed was an electron and
it was looking for a very I should say a source
for electron which is very easily available
one of the easily available source.
It may have found in water but he never taught
that this will led to the evolution of oxygen
and that was one of the biggest event which
happen in history when from the whole life
form moved from anaerobic life form to aerobic,
so essentially what we are telling that possibly
very early in the environment this was the
kind of reactions which were happening earth
which was rich in hydrogen sulfide rich in
all sorts of nasty gases and that is where
probably life evolved.
And at some point as earth was cooling down
as everything was coming down the hydrogen
sulfide level was going down and nature was
in a hunt for another perennial electron source
and nature's perennial electron source became
water we do not know how many billions of
years it took nature to make that transformation
from H2S to possibly water but what nature
did not anticipate it or maybe it anticipated
we do not know.
That the outcome of it will be evolution of
oxygen and that was a turning point because
the cause of the evolution of oxygen earlier
this is the world war which was anaerobic
and all the organisms which are surviving
were adapted to survive in the absence of
oxygen but with the evolving oxygen most of
the anaerobic species which were present on
the floor of earth slowly started getting
extinct because they could not withstand the
oxygen pressure or oxygen tension so possibly
it is at that juncture what we see the modern-day
world from non-oxygenated environment to oxygenated
environment or a ship Trump's in aerobic to
our current day Arabic world.
And this evolving oxygen leads to the evolution
of species which are adapted to oxygen environment
as to there are several microbial life forms
which grows in absence of oxygen in absence
of light deep inside the hydrothermal vent
and a life form very similar to Martian environment,
environment in the Mars where it is emulated
that that where it is believed that similar
environment exists as in the hydrothermal
vent where even in the absence of oxygen such
process takes place.
And that was the reason why I wanted to show
so the generic reaction now I will put the
light of this wisdom will put the generic
reaction and this is very critical for all
the biomass producing mechanism available
on earth h2 now I am putting a I am NOT putting
o or s in the presence of light leads to CH2O
+ 2A again I am just circling the a plus H2O
okay now this one is let us put is the hydrogen
accepted.
Leading to 
the biomass formation and this one the contrary
this particular molecule is your hydrogen
donor 
in other words this is the source for all
kind of reduction reaction and CH2O now I
am putting it in green because this is the
biomass what we are talking about is your
reduced acceptor because this is getting reduced
because of the addition of hydrogen to carbon
dioxide and this one is your dehydrogenated
donor why it is a dehydrogenated donor.
Because hydrogen has been removed whether
it was the H2S or whether it is a water sorry
hydrogenated donor very interesting to note
that.
One second let me the hydrogen donor h-2a
is H2O in green plants and H2S is in photosynthetic
sulfur bacteria so this is the second reaction
what I just now showed you this is basically
the sulfur bacteria 
and this is what is happening in the green
plant, these are the two critical things what
we are dealing with and oxygen evolution would
then be a necessary consequence of this dehydrogenation
process these dehydrogenation process what
Is happening out here leads to oxygen or sulfur
okay.
So the availability in 1949 specially of different
kind of heavier isotopes with the radioactivity
was already discovered these kind of processes
could be mapped with different kind of oxygen
isotopes it was being observed that indeed
the oxygen is getting evolved from the water
not from the carbon dioxide and that is why
this reaction is very important for you guys
to mark that how these things are happening
with this and we will take up each part of
it in our subsequent lecture.
