Hi,
This is Dr. Neela Bakore,
And we are starting a new chapter that is
PHOTOSYNTHESIS.
When we talk of photosynthesis there are two
words which make this one word photosynthesis.
Photo is for light and synthesis is building
up or to make something,
So something is being synthesized with the
help of light.
So this is what we are starting now that is
PHOTOSYNTESIS.
So using light, green plants synthesize their
own food.
This is what photosynthesis is.
We will discuss this in detail.
Earlier, like when this concept when this
idea was discussed.
Amongst the scientist.
Some scientists thought that all that organic
matter which is synthesize by the plants is
actually coming from the,
The raw material is coming from the soil.
So two scientist that is Aristotle 
and Theophrastus.
Thought and proposed that the organic matter
which is synthesized by the plant is coming
from soil.
So their concept was, that the raw material
required by the plant to synthesize organic
material comes from soil.
Very soon this concept was disproved a very
simple experiment was conducted.
Where they took a potted plant.
Before they took this plant, they measured
the weight of the soil.
The part which was kept inside the pot.
And the weight of the plant.
And after few years when they re-weight this
two things,
There was a minor change in the weight of
that soil,
But a major increase in the mass of the plant.
If everything was coming from the soil, this
difference should have been equivalent.
That means if hundred kg's is increased in
plants,
Hundred kg's should have been decreased or
lowered in soil.
But that did not happen.
So, that simple experiment disproved their
concept,
That was initial thing which was given by
Aristotle and Theophrastus.
Van Helmont another Scientist changed this
theory a little bit because the original concept
was that everything is coming from water,
But when we have a potted plant or when we
have a plant growing in soil we have to water
it.
We have to supply water to that plant.
So Van Helmont said that this raw material
which the plant uses to synthesis organic
matter comes from water.
So his concept was that raw material comes
from water.
Instead of soil it is coming from water.
Both these theories were actually disproved
later on when the complete process of photosynthesis
was understood.
One more Scientist Von Neil explained that
there are certain bacteria which use hydrogen
sulfide H2S in
Place of water.
And when they use this hydrogen sulfide to
fix carbon the basic reaction which was proposed
was that carbon dioxide reacts with this hydrogen
sulfide.
Basically reaction is this hydrogen from hydrogen
sulfide is used to reduce carbon dioxide.
This is how the organic matter or glucose
was synthesized and what was given out as
the by product was this sulfur.
So this is what he observed in sulfur bacteria
which use hydrogen sulfide instead of water.
So what is the basic use of water?
Water is giving hydrogen, and this hydrogen
is required to use this carbon dioxide.
Everything into organic matter.
And the part of this hydrogen sulfide other
then hydrogen that a sulfur is given out as
a byproduct.
So his contribution was that he said instead
of water some organism use hydrogen sulfide
and sulfur is given out as a byproduct.
Two more Scientists Ruben and Kamen.
Ruben and Kamen actually demonstrated that
the oxygen which is given out as a byproduct
in photosynthesis comes from water.
So the reaction is same instead of H2S
H2O that is water is taken hydrogen from this
is used to reduce this carbon dioxide and
plants synthesis glucose and the oxygen which
is given out is this oxygen and they proved
it by using radioactive oxygen.
So they used oxygen 18 and the oxygen 18 was
used to make this water molecule and when
they examine the oxygen which was liberated
during this process was also the same isotope
of oxygen.
So that proved that oxygen which is given out
is coming from water.
We will talk about few more scientists as
we go through the process.
But these are some basic things which were
done by initial Scientists and then some reactions
and processes were also given out so we will
discuss those also.
How do we define photosynthesis?
Photosynthesis is defined as a process by which
plants or green plants synthesize organic matter
or food with the help of carbon dioxide and
water.
In presence of sunlight 
and pigments.
So the pigment which is required mainly is
chlorophyll so we normally write sunlight
and chlorophyll.
So the process is green plants by using carbon
dioxide and water in presence of sunlight
and chlorophyll,
synthesize their own food which is an organic
matter normally glucose but we talk about
organic matter.
Anything else which is given out,
Oxygen is given out as a byproduct.
So the processes of synthesizing organic matter
by using carbon dioxide and water two things
are required for this reaction to takes place,
That is sunlight and a pigment which is going
to capture this sunlight and a byproduct is
given out which is oxygen.
So this is a simple process in terms of
its definition but when we come to the actual
reaction, we see how many reactions takes place.
There is one more way in which this complete
process of photosynthesis can be explained.
It is conversion of solar energy 
into chemical energy,
And using this chemical energy to fix carbon.
So it is basically carbon fixation that we
are talking of.
So solar energy or photonic energy or photic
energy is converted into chemical energy.
That is the ATP form and using that energy
to fix carbon.
This is little technical explanation,
This is a simple explanation the way the process
takes place.
Let us talk about the magnitude of photosynthesis.
Magnitude means the vastness the intensity
how much is what we are talking about.
About seventy to eighty billion tons,
We are talking in billion and tons of organic
matter is synthesized.
In how much time?
This is per year.
This much of organic matter is synthesized.
What is organic matter made up of carbon as
one important element.
So here we are actually using carbon to synthesize
so much of organic matter.
Out of this total synthesis 90 percent contribution
is of aquatic plants.
So out of this magnitude of seventy to eighty
billion tons of organic matter which is synthesize.
90 percent is by aquatic plants.
That is mainly which are found in oceans.
And only 10 percent contribution is of the
terrestrial plants.
This is in terms of the carbon which we are
actually fixing.
So the amount of organic matter.
How much of light energy are we using?
The estimate is that out of all the light
energy that falls on the earth surface.
These plants are able to use only 0.2%
of the total light energy that comes on the earth's surface.
So how much is coming to the earth whatever
quantity that is out of which 0.2% and
only using this much 70-80 billion tons of
organic matter is synthesized so now we get
an idea that only this much is being used
and billions of tons of organic matter is
synthesized.
So this is the magnitude of photosynthesis
that we are talking of.
Now coming back to this definition we used
two things here that is carbon dioxide and water,
which the plants used to synthesis organic matter.
So these two that is carbon dioxide
and water are considered as the raw material for photosynthesis.
Are two carbon dioxide and water.
Carbon dioxide is available in the air and
from air it is taken through stomata by the plants.
Water is taken from the soil, it is absorbed
by the root hair.
So these two raw materials are coming from
air and soil.
So this from air and water is from or taken
from the soil.
There are two conditions which are required.
One is light or solar energy so the sources
sun and the fourth condition is the pigment,
That is chlorophyll.
Which is present in the plant itself.
So this is in plant.
So these are four things which are necessary
for photosynthesis to take place there are
two raw materials that is carbon dioxide and
water.
And there are two conditions one is sunlight
and the other is pigment.
Now we are writing chlorophyll but we will
see in sometime that there are many pigments
which help in photosynthesis,
But the most important one is chlorophyll.
So chlorophyll is written here.
So four things necessary,
Now let us talk about the site of photosynthesis
where does it take place?
We have seen the structure of chloroplast.
So chloroplast is the site,
But we will quickly go over the structure.
So to see exactly where, what happens is photosynthesis.
So let us take the site of photosynthesis.
So site means exact place where photosynthesis
is taking place.
The complete structure which helps in photosynthesis
is chloroplast.
So the site is written as, site of photosynthesis
is chloroplast.
Now where is this chloroplast and how is the
structure,
Though we have discussed this in detail in
the chapter of cell structure.
Let us quickly talk about this.
If you see a leaf,
In the leaf if we take a section of the leaf,
There is upper epidermis which has flattened
cells.
The lower epidermis which has the tiny openings
called stomata .
Upper epidermis is covered with a layer of
cucticle.
And lower epidermis has stomata.
In between these two layers there parenchymatus cells.
These cells they are arranged in two layers
or two types, the cells which are on the outer
side or near the upper epidermis,
They are compactly arranged,
This is known as palisade parenchyma compactly
arranged.
This is known as palisade parenchyma.
This is the area where we would have the vascular
tissue that is xylem and phloem.
And rest of the cells they are loosely packed.
These are also parenchymatus cells but they are
not compactly arranged.
This is known as spongy parenchyma.
So these cells which are present here they
are arranged in two distinct bands,
One which is towards the upper epidermis they
are compactly arranged called palisade parenchyma
A palisade can also be written in PALLI or
single L also the lower cells are called spongy
parenchyma.
All the cells which are in between these two
layers upper epidermis and lower epidermis
are known as mesophyll cells.
Chloroplast is present in all these.
Number of chloroplast depends on the intensity
of the light.
If intensity is more the chloroplast number
would be less if intensity is less chloroplast
number would be more,
Plus the arrangement of chloroplast is also
variable.
If intensity is high the chloroplast get vertically
arranged that means if this the epidermis.
Intensity, sunlight, intensity is very high.
The chloroplast which is inside this cells.
If I just draw chloroplast, the chloroplast
would get arranged in this manner.
If intensity is high.
And if intensity is low then they would get
horizontally arranged.
So that maximum light can fall on them and
can be absorbed.
So number and arrangement depends on the intensity
of the light.
In these mesophyll cells are present the chloroplast.
So if we enlarge one cell.
We have made these cells and because we draw
the plant cells in a geometric shape a large
vacuole and I am just making one chloroplast
here,
Now if we magnify this chloroplast a double
membrane structure, both the membranes are
smooth, no foldings.
And there are some membranous structure called
thylakoids,
And these thylakoids, they are arranged in
stacks.
So these stacks of thylakoids are known as
grana.
These grana are interconnected and they are
also connected to the inner membrane of the
chloroplast.
Plus this chloroplast has its own circular
DNA which is known as the plastidome it is
also known as CT DNA, Chloroplast DNA.
This matrix which is present here is known
as stroma,
And this stack of thylakoids is known as a
granum
And multiple are known as grana.
Plus there are some ribosomes which are also
found in this stroma.
These ribosomes are 70 S type so this the structure
which we have already discussed in the chapter
of cell.
Inside the thylakoid are present the pigments.
Now we draw these pigments here.
This is the green pigment chlorophyll.
But if we just magnify this thylakoid then
we would see that these thylakoids have these
kind of membranes and in these thylakoid membranes
are present four protein complexes or four
complexes we can say.
One is called Photo System One Complex,
PS 1 for Photo System. So, PS 1 complex.
PS2 complex,
ATP is that is enzyme which is helping in
ATP synthesis, ATPase complex,
And there is one more which is called cytochrome complex.
Cytochromes are electron acceptors so they
would help in transfer of electrons from one
side to other side of the membrane.
So there are four types of complexes which
are seen.
The membranes which are exposed,
They have photosystem 1 and ATPase complex.
So there are two complexes which are here
on the exposed part,
This is PS1 complex and ATP as complex.
The membrane which is inner,
That is this membrane which is not exposed
it has PS2 complex.
And cytochrome is everywhere.
Because Cytochrome has to help in electron
transfer,
One more important thing is these stroma lammillae
they have only PS1 complex.
So they have PS1 complex whereas the thylakoid
membrane as we have seen they have both PS1
and PS2 complexes.
We will see what exactly PS1
And PS2 are,
So thylakoid have these complexes and what
is this complex made up of PS1 and PS2,
They are made up of chlorophyll pigment and
some accessory pigments,
That means there are some helping pigments
also.
So this the structure.
Now when we are taking of this site where
is what happening?
In the thylakoid membrane where the chlorophyll
is present.
So this thylakoid or we can also call it grana,
Because these stack make the grana.
This is the site for light reaction,
So granum is for light reaction,
Reason it has chlorophyll,
So the light solar energy would be absorbed
by chlorophyll only.
So the reaction which is light dependent would
take place in grana or in thylakoid membrane.
Wherever the chlorophyll pigments are stroma
is the site for dark reaction.
So stroma for dark reaction and grana for
light reaction.
When we say photosynthesis which gets completed
in two parts,
One is light reaction and other is dark reaction.
Light reaction is in thlyakoid where chlorophyll
pigment is present.
And dark reaction takes place in this stroma
where all enzymes needed for the reaction
are present,
And that is why when we talk of site,
We write the whole structure or whole organelle,
Because one part is in one structure that
is grana and the other part dark reaction
is in stroma.
So this is the site of photosynthesis.
Now let us take certain important things which
are required for photosynthesis like the pigments.
And then we will talk about the light.
Which type of light helps and which type of
pigments help.
So pigments first...
