Let us study or revise what is photosynthesis.
Photosynthesis we all know can be carried
out by all green photosynthetic organisms
which have chlorophyll in them.
So, including the autotrophs such organisms
are autotrophs.
Now, autotrophs means what the organisms which
are capable of producing organic compounds
by using light energy and fixing the carbon
dioxide.
Now, an over view, so we all know in general
what happens in photosynthesis, in the presence
of light energy it drives, it produces sugars
and oxygen from carbon dioxide and water.
So, what how does it happen inside the leaf?
Why do plants look green?
Rest of the light is absorbed and only green
is transmitted and reflected that is why they
look green.
So, what happens in photosynthesis, you know
that it happens in chloroplast in plants.
Now, chloroplast are their membranes are rich
in a coloured, green coloured pigment which
is called as chlorophyll.
Now, they have a number of granular stacks
which are called as thylakoids .
Now, this light reactions they happen in the
membrane of these thylakoids, these granular
structures.
So, and the carbon fixation happens at the
stroma, the the carbon diffuses the carbon
dioxide diffuses in the stroma through the
chloroplast, and it is transparent to lights,
light can easily pass through the chloroplast
membrane.
So, what happens?
The entire process of photosynthesis is divided
into two parts as I said light reactions and
dark reactions.
In light reactions, the light energy is captured
by the photosystems, so they these are called
as reaction centres . So, these reaction centres
they have some 100 molecules few hundred molecules
of coloured pigments including chlorophyll.
So, the reaction centres in these photo systems,
photosystem 1 and photosystem 2, which is
which forms the a cyclic phosphorylation cycle.
Now, in bacteria you will find cyclic phosphorylation
. So, I am now going to talk about the light
reactions of photosynthesis, where the water
is broken down to produce oxygen the photo
photons are captured so as to drive ATP and
NADPH synthesis .
So, now stroma is the liquid part of the chloroplast
and grana are the stacks of the thylakoid.
So, this entire light reaction of capturing
the photons and then passing on the electron
excited electrons to the electron transport
chain everything is present at the membrane
of the thylakoid . So, what happens that as
soon as the photon the light is incident on
these reaction centres, I will show you a
picture.
The two reaction centres I said photosystem
1 and photosystem 2 during evolution photosystem
1 came first, and then photosystem 2, but
with adaptations to plants to make it more
economical or more efficient, the two photosystems
both were brought in together.
Therefore, photosystem 2 comes first and then
the photosystem 1 in that acyclic phosphorylation.
So, when the photons are incident, the light
energy is incident on the photos.
So, first comes the photosystem 2 because
it was discovered probably later.
So, photosystem 2 it is incident, then there
are number of pigment molecules, now the energy
which is absorbed by the pigment molecules
the it keeps on getting transferred to neighbouring
pigment molecules, still it reaches the reaction
centre .
What is this reaction centre, it is a chlorophyll
A molecule which can absorb light at a wavelength
of 600 nano metres . So, it absorbs the energy
and then which drives this energy drives oxidation
reduction reaction, breaking down the water
molecule releasing, oxygen and the electrons.
Now, this excited electron is then taken up
captured by the neighbouring electron acceptor
molecule, specific electron acceptor molecule.
And then it subsequently keeps transferring
the electrons in the electron transport chain
which is present at the thylakoid membrane
because of which the hydrogen ion gets released
in the thylakoid lumen.
So, now once the hydrogen ion concentration
in the thylakoid lumen increases, it has to
be the electrochemical gradient is there.
So, this electrochemical gradient through
chemiosmosis is then exploited by the ATPs
which is present in the thylakoid membrane
now this is ATPs then generates ATP by driving
the hydrogen ions out in the stroma.
The hydrogen ions which are then let out in
the stroma, they then combined with the cofactor
NADP plus forming NADPH . So, this is how
the light reactions give rise to ATP and the
NADPH.
Now, this ATP and the NADPH both are to be
utilised for capturing CO 2 to convert it
into sugars.
This we will see how does it happen . So,
oxygen if you see it is an incidental by product
because water is available, it captures break
breaks down water and it is an by product
which happens in photosynthesis .
So, if you see the entire picture whatever
I said would be very clear.
These are the different, so you must have
seen the electron transport even in the chain
in your mitochondria . So, it is similar to
that .
So, the different steps I will repeat what
happens in the thylakoid membrane the chlorophyll
is organised along with other molecules to
from the two photosystems . Now, if you will
observe that the photosystem 1 it was it is
called as P700, photosystem 2 is P680 which
means there is a difference in the maximum
absorption wavelength.
Now, it is a fact that both the reaction centres
are identically same chlorophyll molecules
A, but still there is a difference in the
wavelength.
Why do you think?
This difference in absorption properties happens
because it is in conjugation with other proteins
present in the reaction centre . So, because
it combines with the other proteins in the
two photosystems, there is a difference in
the absorptive properties.
Then the photosystems are what as I said these
are light harvesting units.
So, again to repeat there are two photosystems.
When a photon of light, it will struck, strike
the photosystems, the electrons are released.
These electrons being high energy states,
they are transferred to a nearing electron
acceptor molecule which again transfers it
to the electron transport chain present in
the membrane, thereby releasing the hydrogen
ions in the thylakoid lumen.
This causes electrochemical gradient which
is then exploited by AT phase to produce ATP,
and the hydrogen ions sent back or diffusing
through the membrane into the stroma, then
are used to reduce the NADP plus to form NADPH.
Let us talk about the dark reactions.
So, this was the picture which I showed in
the beginning.
So, we have now spoken about the light reactions.
Light reactions have given rise to ATP and
NADPH.
Now, dark reaction is called as Calvin cycle.
Now, why dark reactions in this reaction the
CO 2 is captured.
This CO 2 is combined with ribulose 1, 5-bisphosphate
. Now, in order to combine this, you need
an enzyme which is called as rubisco, it is
the most abundant organic molecule present
on the earth . So, this enzyme then combines
the two to give you two stable, one unstable
6 carbon molecule which immediately breaks
down to give you two stable molecules of phosphoglyceric
acids.
Now, because these are the first stable products
of the Calvin cycle, this cycle is also called
as C 3 cycle .
So, in general Calvin cycle, we have three
portions, one is carboxylation, then reduction
phase, and after the reduction phase regeneration
phase.
So, what happens in the carboxylation phase
the carbon dioxide is reduced by rubisco,
which is ribulose bisphosphate carboxylase.
So, it combines carbon dioxide with ribulose
1, 5-bisphosphate to give you two molecules
of phosphoglyceric acid.
Now, this phosphoglyceric acid is then reduced
using ATP and NADPH and with if you remember
your gluconeogenesis which is reverse of glycolysis,
some of those it is similar to those reactions
such reactions takes place to convert your
phosphoglyceric acid to 6 molecules of 3 phosphoglycerate.
Now, in this then one molecule is then diverted
to form your sugars, and the remaining five
is then again in the regeneration phase.
Now, why it is called regeneration phase,
because again the precursor molecular ribulose
bisphosphate has to be regenerated.
So, now, these five molecules are again regenerated
to give you back ribulose bisphosphate which
can then subsequently again combined with
CO 2 to give you PGA.
So, which enzymes are involved rubisco which
is rubisco 1, 5-bisphosphate carboxylase.
Then it catalyses the addition of CO 2 to
ribulose 1, 5-bisphosphate as I said forming
the first stable two molecules of phosphoglyceric
acid.
Then comes the reduction phase I discussed
three phosphoglyceric acid is reduced to glyceraldehyde-3-phosphate.
If you remember glyceraldehyde-3-phosphate
is gives rise to is a part of glycolysis . So,
it is then reduced to give you glyceraldehyde-3-phosphate
can then give you sugars . And there ATP and
NADPH is needed to reduce PGA to glyceraldehyde-3-phosphate.
Enzymes involved are what 3-phosphoglycerate
kinase these are the major enzymes and glyceraldehyde-3-phosphate
dehydrogenase.
Now, comes the regeneration phase where the
carbohydrates or fructose, and your glucose,
and how does it happens with using enzymes
trans transaldolase and transketolase.
If you remember these are enzymes involved
in pentose phosphate pathway.
So, again similar reactions of pentose phosphate
pathways are involved ones from glyceraldehyde-3-phosphate
you get fructose diphosphate.
If you remember your gluconeogenesis, cycle
of glycolysis .
So, now coming back to we were saying talking
about adaptations.
Now, C 3 plants which are generally all the
plants are follow C 3 metabolism.
Now, in C 3 plants, the light reactions and
the dark reactions both are taking place in
the same chloroplast, same cell.
So, but what happens as I discussed earlier
that in order to prevent excessive water loss,
the stomata is closed.
Now, once the stomata is closed the CO 2,
now is anyways getting utilised in carbon
fixation.
So, then a time comes when the concept relative
concentration of CO 2 to oxygen reduces.
As soon as this happens rubisco now starts
getting more associated with oxygen . Now,
what happens now it starts acting as an oxygenise,
and it will combine your ribulose bisphosphate
with oxygen.
Now, this oxygen when it drives this reaction
as oxygenise, it will give rise to I said
a toxic molecule which is called as phosphoglycolate.
I will be talking about this process.
Now, once phosphoglycolate is formed, it is
a highly toxic molecule.
So, the cell drives certain reactions to convert
this toxic molecule into a number of molecules
like glycine, and then glycine gets converted
to serine, and then serine thereby gives rise
to many other molecules.
Now, all these reactions takes place using
organelles, paroxysm and mitochondria.
Now, these reactions again happen at the expense
of cells energy.
So, ATP is involved, and there is a loss of
CO 2 when the glycine is getting converted
to serine in mitochondria.
So, effectively 30 percent of the carbon fixed
is lost in salvaging this molecule.
So, we will see what do plants to in that
case.
So, there is one adaptation which is called
as C 4 metabolism . So, in C 4 plants, the
photosynthesis takes place in chloroplast.
And I was mentioning that there are cells
which surround the vascular bundle.
These are called as bundle sheath cells.
Now, why it is called C 4?
Yes, so because the first stable compound
formed is malate or aspartic acid.
Now, this 4-carbon compound is then transported,
now this is light reaction, is then transported
to bundle sheath cells where it again breaks
down to release CO 2.
Now, because it is now very close to vascular
bundle, and in in the inner portions of the
leaf of the tissue, it is set to be secured.
Now, then once it releases CO 2 in the vicinity,
the relative concentration of CO 2 to oxygen
is then increased which then can cause rubisco
to again carry out the dark reactions . So,
this is what C 4 metabolism.
Now, so there is a spatial change in the light
and the dark reactions.
Now, then there is one other kind of metabolism
which is called as scam metabolism which is
found in succulent plants.
Now, why succulent plants, these leaves are
plump with lot of water content, now there
is a reason why do they become plump and with
lot of water content because of the four molecule
which was mallet.
Now, in these scam metabolism rather than
opening in early hours of the morning and
taking in CO 2 like C 4 plants and these carry
out the photosynthesis and capturing of CO
2 to form a 4-carbon molecule at night.
And in the morning, in the same cells, this
molecule is broken down and photosynthesis
or the dark reactions fixing of the CO 2 which
is released happens.
So, it is a temporal shift between the light
and the dark reactions.
So, what happens in photorespiration, very
hot days the stomata is closed.
Once the stomata is closed, there is no gas
exchange.
The CO 2 is continuously getting utilised
. Then again as I said relative concentration
of CO 2 to O 2 is reduced.
So, now, rubisco is combining with O 2 to
give you a toxic molecule phosphoglycolate.
Now, this toxic molecules has to be broken
down to prevent the cell damage.
So, now, this whole process is called as photo
respiration.
So, as I said see when the CO 2 levels drop
in the leaf less than 50 ppm, then the rubisco
gets acts starts acting as an oxygenise.
So, this is the reactions which take place
in photorespiration.
So, three organelles are involved chloroplast,
peroxisomes and mitochondria.
If you remember when I was discussing organelles
I asked you what do peroxisomes, what is the
role of peroxisomes . Peroxisomes play a crucial
role in photorespiration . So. as I said this
glycolate when formed it is then sent to peroxisomes
where the reaction to convert it into glycine
happens.
Now, this happens by utilising oxygen.
So, in the peroxisome, the oxygen is then
taken into convert your glycolate to glycine.
Now, once the glycine is formed, it is then
transported to mitochondria and then the glycine
is converted to serine.
Now, this reaction happens by taking in by
release of CO 2 . So, therefore, respiration.
So, as I said oxygenise and carboxylase is
it depends on their Km value.
The Km although for rubisco to combine with
CO 2 is will be very low or very high.
In general rubisco combines with CO 2 . So,
if it can combine with oxygen the Km value
for CO 2 will be higher or lower, lower, more
affinity towards CO 2 than with oxygen.
So, what in nature, the strategy could be
applied to prevent this process photorespiration,
either the plant should have avoided rubisco
there should have been another enzyme to carry
out photosynthesis or fixing CO 2 in an environment
shielded from O 2, this could be another choice.
The third choice could be use of an enzyme
that does not react with O 2 which is the
same.
Do not use rubisco use another enzyme which
does not react with O 2.
So, this is what happened.
CO 2 fixation we know it happens in the mesophyll
cells, palisade cells, spongy cells with air
gaps for gas exchange . So, now, CO 2 fixing
enzyme is not rubisco, the adaptation happened.
Then CO 2 fixing enzyme was then changed from
rubisco to PEP carboxylase.
Then rubisco still remains the same, but CO
2 fixing enzyme became for the first light
reactions it was changed to PEP carboxylase.
So, we will see how does the malate is formed.
So, in C 4 synthesis to prevent the wasteful
effect of photorespirations, plants like your
sugarcane and corn they adopt to this metabolism.
Now, what what happened their anatomy.
Even in if you compare the anatomy between
C 3 and C 4 plants, it is different . In C
4 plants, you will see there there are bundle
sheath cells around the vascular bundle, well
which is xylem and phloem.
While in your C 3 plants and they all mesophyll
cells are all palisade we saw coalminer cells,
and then they were spongy cells, so the chloroplast
in C 3 plants and all these cells can carry
out light and dark both the reactions.
So, now, C 4 pathway is this where it combine
the PEP carboxylase converts PEP capture CO
2 to make the first four carbon molecule which
is called as malate.
This malate is then transported to bundle
sheath cells in C 4plants where it is again
broken down to release CO 2.
Now, in this the rubisco is protected . It
is not now on the surface to, so that it can
be affected by the oxygen.
So, here again then rubisco combines with
co two in dark reactions to give sugar molecules.
Now, what happens in cam plants i have already
spoken about it, it is temporally different.
So, this CO 2 fixation into malate happens
early hours of the morning where CO 2 is taken
in converted into malate, and then this during
the day hours when the stomata is closed,
this malate is broken down to give you fixing
the CO 2, and then CO 2 combines with rubisco
to give sugars.
But then in cam plants at night photosynthesis
these reactions happen, the CO 2 is fixed
to give you malate, malate is stored in vacuoles.
And then in the morning in the malate is then
broken down to give you CO 2.
Now, this happens in succulent plants I said.
Now, why did I say succulent plants they are
more plump with lot of water, there is a large
amount of malate present.
So, lot of water is sucked in, so that is
the reason why they become succulent in nature.
And sometimes there is so much of pressure
that the malate is forced to be broken down
by such kind of plants.
