Welcome back to the lecture series on Bioenergetics
of Life Processes. So, we have started with
photosynthesis. So, today we will proceed
with photosynthesis. One of the interesting
things in photosynthesis is, I have already
talked to you that it consists of two photo
systems; and one of the byproduct of photosynthesis
is evolution of oxygen and light is essential
as we have talked, in the very first lecture
of water or are the prerequisite for photosynthesis
to happen.
One interesting aspect which was realized
pretty early in photosynthesis is that, there
is not a linear relationship between what
is the instant light falling on the leaves
and what is the oxygen evolution what does
that mean? That means, say for example, I
say because it is a energetic process, someone
has to keep a balance sheet to understand
that how much solar energy is being used and
how much oxygen is being evolved and what
is the efficiency.
So, it was believed that say for example,
x quanta of light or x quanta of photon is
falling on a surface, then a corresponding
y or x quanta of oxygen will be evolved there
should be an some stoichiometry; it was observed;
that it is not kind of a very linear like
you know, one photon, one oxygen like not
like that. It is something like after several
photons the oxygen is getting evolved. In
other word, you need a good number of photons
to fall on the chlorophyll molecule, before
the oxygen is being getting evolved. That
set up a very interesting idea initial phases
and eventually hypotheses are now kind of
it is believed is that, not all chlorophyll
are involved in evolution of oxygen apparently.
Suppose we look at the whole structure of
the leaf there are millions of chlorophyll
molecules in one cell.
So, it is not that every chlorophyll will
lead to the energetics reaction of evolution
of oxygen. Because why we are talking the
evolution of oxygen because that is your output,
that is how you are evaluating how much what
is the efficiency of the system it seems like
there are reaction centers in other word let
us put it in perspective..
So, this is our week 4 lecture 1 and this
is essentially or lectures 16 W 4 L 1. So,
what does that mean is say for example, we
are talking about leaf surface and light is
falling which is ok. So, we are evaluating
based on the oxygen evolving out ok. So, it
has been observed after quite a good number
of photons which are hitting upon, you have
one oxygen molecule which is getting involved.
In other word the photon absorbed by many
chlorophyll leads to photons absorbed by many
chlorophyll leads to evolution of single molecule
of 
oxygen ok.
So, that brings us to a concept called reaction
center in the chlorophyll molecules. That
means, say for example, this is all a location
of several chlorophyll molecules are sitting
there and say for example, light falls on
this. So, the energy kind of get transferred
there are several forms of energy which are
getting transferred and one of them out of
this will be the reaction center, from where
electron will be emitted out.
So, in 1932 Robert Emerson and William Arnold
measured the oxygen yield of photosynthesis
in chlorella ok. Chlorella is a easy model
system to work with chlorella cells which
is a green algae it is a unicellular, unicellular
green algae ok. These unicellular green algae
when exposed to light flashes lasting for
few microseconds they expected to find that
the yield per flash would increase with the
flash intensity. Until each chlorophyll molecule
absorbs a photon which would then be used
for the dark reactions, but the experimental
observations was entirely unexpected.
What was the unexpected aspect on it? It was
a saturating light flash led to the production
of only one molecule of oxygen per 2500 chlorophyll
molecule.
So in other word if this is the site where
there are 2500 chlorophyll molecules which
is kind of estimated based on the per unit
density of the chlorophyll molecule, it was
observed and this is single flash of light
single flash . So, one can calculate the energy
what you are giving single flash of light,
what they observe is single oxygen which is
getting emitted out. So, in other word if
you draw a graph it will come something like
this, in the y axis sorry in the x axis you
have light intensity and in the y axis you
have rate of photosynthesis and this is the
limiting value .
So, this gave birth to the concept of photon
absorbed by many chlorophyll photons absorbed
by many chlorophyll is funnel to a reaction
center. Now what is critical is? What is the
reaction center and how to identify? As of
now to the best of my knowledge no one can
really predict which one is the reaction center,
and how this reaction center is being determined
among all this same cast of chlorophyll molecule
is also a very unresolved and a mysterious
question, but it does exist ok. The energy
level of chlorophyll at the reaction center
is lower than the other chlorophyll ok. So,
they in other word the energy level out here
is lower. So, the way it works is something
like this, if you kind of think it like that
these are the chlorophyll which are sitting
at ground state.
So, there is a light which is falling on it
and which takes this chlorophyll at a higher
state there is an energy transfer to the next
to the third, and when it comes to the reaction
center. So, this is where the reaction center,
it is sitting at a lower energy ok. The energy
level of the chlorophyll at the reaction center
is lower than the other chlorophyll which
enables a reaction center to trap the excitation
ok. So, this is what the figure is telling
you. The transfer of energy by direct electromagnetic
interaction between chlorophyll and then the
reaction center is very rapid occurring. So,
this is the timeline or time window we are
talking about picoseconds. So, to trap such
a reaction of that intensity is exceptionally
challenging ok. So, this is all about the
reaction center what you needed to know; that
kinds of in terms of energetic gives you an
idea what is the input energy and what is
the output input energy in terms of photons
ok.
So, this is the first point I want you to
kind of take into account, the second thing
is second aspect is oxygen evolved in photosynthesis.
PS is photosynthesis comes from water, this
is the second concept. So, if I put the two
equations side by side CO 2 plus 2 H 2 O where
light is falling you have CH 2 O which is
your carbohydrate oxygen plus H 2 O. Now if
you replace CO 2 with 2 H 2 S hydrogen sulfide
if you remember I talked about this, what
you are going to get is CH 2 O which is again
the carbohydrate plus 2 sulfur plus water
ok. So, this is the sulfur that is formed
by photosynthetic bacteria is analogous to
the oxygen, that is evolved in plants. So,
this is what are the sulfur bacteria does
and Van Nelle proposed a general formula for
photosynthesis, and the general formula for
photosynthesis is like this.
CO 2 plus 2 H2 A keep this in mind this A
in the presence of light forming CH 2 O plus
2 A that A which is coming from that side
plus H 2 O `.
Now, what is the C? So, CO 2 out here is your
hydrogen acceptor in other words CO 2 is getting
reduced hydrogen acceptor because it is accepting
hydrogen it is getting reduced to this situation.
So, this is a reduction reaction happening
here whereas, H 2 A this is your hydrogen
donor, this one is hydrogen donor this product
which is carbohydrate is your reduced acceptor
and 2 A is dehydrogenated donor this is that
dehydrogenated donor, which has donated the
hydrogen or the electron 
dehydrogenated donor ok. So, this is how the
reaction occurs and one can prove it very
easily if you have radioisotope like you know.
You have H 2 18 O I have different oxygen
isotope with CO 2 in the presence of light
you have CH 2 O and what you are getting is
18 O 2. So, if you see the mass of this one,
which will be higher and that is how you can
figure it out that oxygen is getting evolved
by splitting of water splitting of water.
So, it seems that nature learned long back
how to split water and if nature could split
water and could lead to the evolution of oxygen
and of course, the other product which is
been getting used up here is the hydrogen;
that means, for those who dream of having
a world of clean energy by hydrogen evolution
nature already knows the trick and that is
the beauty of nature. It has learned in its
own laboratory of billions of years by playing
with its own atoms how to split any molecule
and that is the charm of studying these things.
That you know the whole world is you know
looking towards energetics clean energy by
hydrogen economy you must have all seen somewhere
you know they want to store hydrogen that
is the cleanest form of energy and blah, what
is important is nature does it nature does
it very elegantly in stick just nature does
not produce hydrogen, in the air it utilizes
the hydrogen for all its reduction purpose.
So, this brings me to that point what I was
trying to highlight in the beginning that,
the world or life systems energy economy is
governed by a perennial electron donor. You
needed a perennial electron donor all the
time because if you see out here the world
was a harsh place lot of hydrogen sulfide
and the perennial electron donor at that time
was hydrogen sulfide. All those sulfur loving
bacteria and all those things survive some
billions of years ago. Still they could be
seen in anaerobic conditions, but then as
the earth was cooling down if you remember
one of my previous lectures I told you, as
the earth was cooling down hydrogen sulfide
and all these things were lowering down.
So, nature had to search for another perennial
electron source and that is where nature picked
up this wonderful molecule water nature picked
it up for its hydrogen supply or electron
supply, but simultaneously nature while splitting
this started evolving a very strong oxidant
called oxygen and that was that great point
in the history of chemical evolution or oxygen
took over and as soon as oxygen took over,
all these anaerobes were threatened and because
all iron 2 plus become f e 3 plus everything
which was there were getting oxidized higher
oxidation states were picking up. And that
is why this transition called the great transition
over another millions of years pretty much
all our ecosystem started depending on oxygen
and that was the rise of aerobic situation..
But it is still there are places where you
have the other modus operandi still operating
where H 2 S is being used and is being utilized
ok. Now coming back to another concept which
I wanted to highlight in this lecture is,
there is something called hill reaction which
is illuminated chloroplast evolved oxygen
and reduced electron acceptors.
So, this is one concept I am not going to
the detail of the reaction, how it has been
done, but just you needed to know the basic
message which is given. Illuminated chloroplast
illuminated chloroplast evolved oxygen and
reduce electron acceptor, and if you realize
reduce electron acceptor; that means, CO 2
becoming CH 2 O.
So, essentially this is that electron acceptor
if you just see the previous slide, this one
hydrogen acceptor ordains which is essentially
an electron acceptor. So, you can replace
that electron acceptor with another electron
acceptor and prove this fact and it evolves
oxygen which is a strong oxidant. There is
another interesting concept before I go into
the architecture of it is that two light reactions
interact in photosynthesis.
.
So, two light reaction interacts in photosynthesis
what does that? Apparently it looks like that
photosynthesis is governed at two different
spatial location, a single unit of photosynthesis
especially and this is; am talking about the
evolved plant models what, we are currently
more interested in to there are two specific
centers.
So, if you go back pretty one of the very
let me go back and yes out here.
Now if you look into this picture, if you
see it carefully. So, this is where the photosystems
are being placed. So, out here there are two
unique centers called photosystem 1 and photosystem
2. There are two unique photosystem model
modules which are there. So, investigation
of the dependence of the rate of photosynthesis
on wavelength of incident light led to the
discovery that, chloroplast contains two different
photo systems. The photosynthetic rate or
the rate of oxygen evolution, divided by the
number of quanta absorbed gives the relative
quantum efficiency of the process ok. So,
that is the energetic. For a single kind of
photo receptor the quantum efficiency is expected
to be independent of wavelength over its entire
absorption band, this is not the case in photosynthesis
the quantum efficiency of photosynthesis drops
sharply.
So, this part is very important the quantum
efficiency of photosynthesis drops sharply
at wavelengths longer than remember that longer
than 680 nanometer ok. Although the chlorophyll
still absorbs light in the range of 680 to
700 nanometer. However, the rate of photosynthesis
using long wavelength light can be enhanced
by adding light of shorter wavelengths such
as 600 nanometer. The photosynthetic rate
in the presence of both 600 and 700 nano meter,
light is greater than the sum of the rates
when two wavelengths are given separately.
This part is very important and I wanted to
kind of keep an attention. The photosynthetic
rate in the presence of both 600 nanometer
and 700 nanometer light is greater than the
sum of the rates when two wavelengths are
given separately ok.
So, if you give these two wavelengths together,
if you give 600 and 700 nanometer separately,
and add the efficiency in terms of oxygen
evolution or you give 600 nanometer plus 700
nanometer together and oxygen evolution. You
will see here the efficiency is more is efficiency
is more. So, that led this observation is
called a red drop and enhancement phenomena,
let one of the scientists in this field Emerson
to propose that photosynthesis requires the
interaction of two light reactions, both of
them can be driven by light of wavelength
less than 680 nanometer, but only one of them
by light of longer wavelength and the red
drop graph is something like this.
So, here you have the wavelength 400, 520,
600, 680 and here you have the quantum yield
of photosynthesis in terms of oxygen evolved
for photon, which is called quantum yield
or photosynthesis. The quantum yield of photosynthesis
is something very interesting to look at if
you look at the graph it is something like
this, this is that red drop.
So, if you add the 700 then something like
that this drop you would not see. This quantum
yield of photosynthesis drops abruptly when
the excitation wavelength is greater than
680 nanometer ok. So, this is something very
interesting which was observed once again,
which was once something like this slightly
more out here once again let me. Just redraw
this again for you. So, it is something like
this on and something like this red drop ok..
So, Emerson again I am repeating it Emerson
proposed that photosynthesis requires, the
interaction of two light reactions both of
them can be driven by light of wavelength
less than 680 nanometer, but , but only one
of them by light of longer wavelength. So,
this is where if you use only one and this
is what is going to happen there will be a
drop, both of them are together you will see
a higher efficiency.
So, I will close in here in the next class
we will move on to the actual light reaction
photosystem 1 and photosystem 2.
Thank you.
