CELL THE UNIT OF LIFE: CELL STRUCTURE – 01 For Class 11th and AIPMT


Hi,
I am Dr. Neela Bakore. In this section we are going to start with
a cell. CELL – THE UNIT OF LIFE.
Let us talk about some historical background, Like when the cell was discovered who discovered
it and all those things. It was discovered by Robert Hooke.
He actually observed a Cork slide. So when he observed the cork slide.
He actually saw hexagonal compartments. Cork as we know is a dead tissue.
So these walls, they were probably the cell wall and there was nothing inside because
it was a dead tissue. So it looked like a compartment and that is
why he called it cell which means a small enclosed compartment.
Cork slide was observed by him. The cork was of a plant which is quercus.
The scientific name of the plant was quercus. So Robert Hooke discovered the cell.
He discovered. But the cell that he discovered was a dead
cell. So now, the other scientist who discovered
the living cell for the first time. Was Anton Von Lewin Hawk.
He observed living cell. So discovery of cell, the credit goes to Robert
Hooke. Because, he was the first one who saw the
cell. Though it was dead cell.
And the credit of observing the living cell for the first time goes to Lewin Hawk.
Robert Hooke is known as or he has been named father of Cytology.
Cytology is the branch of science, Which deals with the study of cell.
Lewin Hawk is named as Father of bacteriology. Because of the cells which he observed for
the first time. He observed bacteria, Protista, protozoan,
many types of cells. So, one important scientist is Robert Hooke
the other one is Anton Von Lewin Hawk. Now let us talk about few more scientists,
Another scientist was Dujardin. He observed a fluid like material in the cell
which he called Sarcode. Which was the fluid like material,
Later on Porkinje, observed the same substance in many other cells and named it protoplasm.
Another scientist was Robert Brown who discovered the nucleus.
So these are important scientist and their contributions in the cell related things.
I.e. is first observed the cell. Dead cell.
Lewin Hawk observed the living cell. Dujardin and Porkinje JE sorry.
Porkinje gave the term protoplasm. And Robert Brown discovered the nucleus.
Robert Hooke published the entire information about his discovery and observation and everything
in his book called Micrographia. So these are some basic Historical things
which we need to know before we get started with the cells.
Now as we are talking of cells. Let us talk about the shapes.
What is the shape of a cell. There are various shapes which we have observed
so far. It can be a cube like cell.
Cubical cell, which is seen in germinal epithelium, Which we also called cuboidal epithelium.
Another shape is long column like which we see in columnar epithelium.
Slide modification of this is the goblet cell. So this is again a tall columnar cell.
The goblet cell. Irregular shape is also very common is cell.
The WBC’s that is white blood corpuscles. They are irregular in shape.
If we see the smooth muscle fiber they are spindle shaped.
In sponges, there is a special type of cell which has a circular path,
As well as a small path which appears like a collar.
And a cilium arising from here. These cells are known as coenocytes.
Rectangular cells the skeletal muscles cells. Which can be, If it is skeletal muscle it
is multinucleated. So this is
skeletal muscle cell. Rectangular if we see the shape of RBC.
In case of human RBC, because human or mammalian RBC’s are enucleated,
They are biconcave. That means in the center there is no nucleus
and that’s why we see depression on both the sides.
So it is biconcave. But if you see the RBC of frog.
Then the RBC of frog is bulging in the middle. Reason, there is a big prominent nucleus which
is present in the middle region. So this is the RBC of frog.
The basic idea is to see how many various shapes are present or seen in cells.
These are few examples. There are many more.
Like neurons irregular shape. Let u talk about the size.
What is the size of a cell? As the shapes are variable,
Sizes are also variable. The smallest cell is PPLO or it is also known
as myco plasma. PPLO the full form is Pluro Pneumonia Like
Organism. It is placed between viruses and bacteria.
This is the smallest cell. The size is approximately .1 micro meters.
And the largest cell is ostrich egg. We have seen the size of the hens egg.
Like normal about two and a half inches. Ostrich egg is about seven, seven and a half
or nice inches. It is a big cell.
So smaller cell of PPLO and largest cell of Ostrich egg.
Now the next thing is types. Here we have seen the shapes, the sizes.
Now we are talking about the types of cells. We divide all the cells into two categories.
Prokaryotic cells and Eukaryotic cells. Prokaryotic term is derived from two words.
Pro means primitive and carrion means nucleus. The cells which have primitive nucleus.
Now what is a primitive nucleus. Primitive nucleus is one where there is no
membrane surrounding the genetic material. So it is basically naked genetic material
which is found in the cell. The example of this is Bacteria.
So it is primitive plus as there is no nuclear membrane.
Other membranes are also absent. So no membrane bound structure.
No organelle eukaryotic again is derived from two words.
Eu means advance and carrion again means nucleus. So here the nucleus is advanced.
We find these eukaryotic cells in Fungi, Plants and animals.
That means the cells which are found in Fungi, plants and animals are eukaryotic cells.
And the cells which are in bacteria Or bacteria means unicellular,
So these cells are prokaryotic cells. Now let us talk about one more important
historical event that is cell theory. Cell theory was given by two scientists.
Schleigen and Schwann. They put forth three important points which
was considered as the cell theory . First point was that all living organisms
are made-up of cells. It could be cell or cells.
So unicellular as well as multi cellular organs were covered under this point.
The second postulate or the second point which was put forth was that cell is the structural
and functional unit of living organisms. Third point was that the functioning of an
organism whatever organisms functions are the contribution of all the cell which make
up the body of the organism. So functions of organisms are due to the cells
which the body is made up of. Now this cell theory was, modified later on
when one more scientist contributed one point here.
The scientist was Rudolf Virchow. He added one point.
He said all the cells arise from pre-existing cell.
So this became the fourth postulate and after fourth postulate the cell theory was termed
as modern cell theory. So if we are talking of only cell theory there
were only three initial postulates given by Schleigen and Schwann.
After the fourth postulate was added by Rudolf Virchow the cell theory was termed as modern
cell theory. The point was the cell arise from pre-existing
cells. And so the cell theory become modern cell
theory. So contribution in modern cell theory is of
three scientist, Schleigen and Schwann which was the original
one and then the Rudolf Virchow who added the fourth point.
Rudolf Virchow he added this point in his own language.
And that was written as Omnis Cellular A Cellular. Which was later on translated into English
which meant cells arise from pre-existing cells. Now in this further part we would try to see
the structure of prokaryotic cell and eukaryotic cell.
So this was just the basic of what cell is? And how different shape, sizes and types of
cells exist. So now let us see the prokaryotic and eukaryotic
cells. Let us see the structure of a prokaryotic
cell. We are basically talking about the structure
of prokaryotic or we will take the example as bacterial cell.
So the structure is of a bacteria. There are various shapes in which these bacteria
exist. We will talk about this a little later.
But the most common shape is oval. Which we also termed as rod shaped structure.
So this is the outer most membrane which we call the plasma membrane.
Outside this plasma membrane there is a cell wall in bacteria this cell wall is made up
of a substance called peptido glycon. So this outer layer is cell wall.
Made up of peptido glycon. Peptido glycon means a part is protein and
glycon is for carbohydrate. It’s a protein carbohydrate combination.
Let us complete this cell wall. At certain places we find that the plasma
membrane is thrown into in folds. There are places where the membrane has folding,
But these folding are on inner sides. These are known as mesosome.
Mesosomes have respiratory enzymes. So they work or they help in respiration.
So basically they are working as mitochondria of eukaryotic cells.
The complete inner compartment is filled with a fluid which is cytoplasm.
In this cytoplasm the genetic material. This is the prokaryotic cell.
So genetic material is without any membrane. That means there’s no nuclear membrane.
Circular genetic material DNA normally this DNA is found attached to the plasma membrane
at one point. It is detached from the plasma membrane when
the cell has to divide. So it remains attached to the one side of
the plasma membrane most of the times. It is circular DNA and there is no membrane.
As we said prokaryotic means pro is primitive and carrion is nucleus.
So this is naked DNA which is found. So this is the DNA.
This DNA without nucleus is actually known as nucleoid.
So in eukaryotes we call it nucleus and prokaryotes we call nucleoid.
Which means basically only naked DNA. Other things there are no membrane bound structure
in here. There some ribosomes.
Ribosomes are non membranes bodies. These ribosomes which are scattered in the
cytoplasm. They are 70S type.
When they come to the detail of the organelle we will talk about 70 S and 80 S.
80 S type of ribosomes are found in eukaryotes and 70 S are found in prokaryotes plus the
organelle which are there in eukaryotes. In some bacteria or in some prokaryotic cells.
There is a circular DNA. This is apart from the name genetic material.
This is called plasmid. If plasmid is present ,
So now here there are two possibilities either it is present or absent.
So if it is present then we will call or consider that the cell of the positive string and if
plasmid is absent. Then it will be considered as of negative
string. So basically this is just to let us know whether
the bacterium is of positive string or of negative string.
Other then this there is nothing else inside the cell.
These bacterial cells or prokaryotic cells may be flagellate or non-flagellate.
If they flagellate they can have one to many tuft of flagella everywhere Flagella.
Here we are drawing only one flagella. It arises from the cytoplasm and it’s a
long thread like structure. This is the flagella.
So the structure is very simple. During adverse conditions what we find is
outside this cell wall. There can be some jelly like material,
Thick jelly like material which makes a capsule. This capsule is just musilagenerous thing.
Which protects the cell from desiccation or drying up.
So this would be seen only during unfavorable condition.
Otherwise only the cell wall and plasa membrane. Now when we talk of prokaryotes or when we
come to kingdom monera. We would talk about gram positive and gram
negative type of bacteria. At that time we will also discuss.
What is the difference in the cell walls or the layers of the cells.
And it is a gram positive or a negative. So this is a basic structure.
Now, let us come to eukaryotic cells. And we will drawing two together.
Eukaryotic cells. As we said plants and animal cells are eukaryotic.
So we will draw one plant cell and one animal cell.
So simultaneously we will also be able to understand the difference between the two
cells. Plants cells are normally of fixed shape.
They are either hexagonal or polygonal or squarish.
We have picked hexagonal cell here. This outer membrane which we have drawn here
is plasma membrane. In case of an animal cell it can be of various
shapes again and here we are choosing some oval type of cell.
So this membrane is plasma membrane. Common thing in both the cells.
Animal cells as well as plant cell. This is the plant cell.
So let us write down plant cell here and animal cell here.
In plant cell outside the plasma membrane there is one more layer which is made up of
cellulose. And it is called the cell wall.
Cellulose is polysaccharide which makes a this cell wall.
Which is found in plant cell it is absent in animal cells.
So this is one distinguishing feature between plant cell and animal cell.
In prokaryotes, there is cell wall, But that is made up of peptidoglycon.
So this cell wall made up of cellulose. Not present in animal cells.
Only in plant cells. In case of plant cells.
The largest structure that we see inside is a big vacuole.
This is the biggest structure which is visible in a plant cell.
So this large structure is vacuole. In case of animal cells there are vacuoles
but they are smaller in size. They are many more in number but they are
smaller in size. Because of this large vacuole in a plant cell.
The nucleus gets shifted to one side. So in plant cells we find that the nucleus
is shifted to one side. Whereas in case of animal cells this nucleus
is normally and centrally placed. It is large and is centrally placed.
The nuclear membrane here, And here in the plant cell we see the nucleus
is slightly eccentric. It is slightly in one side.
Whereas in case of animal cells. It is centrally placed and large
So in plant cell the largest structure is vacuole whereas in case of animal cell the
largest structure which is visible is the nucleus.
Inside the nucleus there are chromatin fibers. Nucleolus same is the case in animal cells.
Chromatin fibers, nucleolus normally the number of nucleolus is one so these,
This entire structure is the nucleus. Chromatin fibers now, inside the nucleus there
is again a fluid which is known as nucleoplasm. Here also nuceleoplasm.
So in plant and animal cell this basic structure of nucleus is same because it is a typical
eukaryotic cell with a distance defined nuclear membrane and all other organelles.
now the other organelle which are present in these cells.
One is mitochondria. So mitochondria is present in both.
In case of animal cells also the number of mitochondria can be variable depending upon
the activity level of the cell. If the cell is more active it would have more
number of mitochondria if it is less active it will have less.
Plant cells are less active as they don’t have from their place or the metabolism is
little slower. So the number of mitochondria here are less.
As compare to the animal cells. So this structure which we have drawn here
is mitochondria. Another important structure is golgi complex
in case of plants, Those golgi bodies or golgi complex is known Dictyosomes and it can be slightly away from the nucleus also.
Golgi complex away from the nuclus also. The number can be more.
More than one golgi complex can be there. In case of animals normally the number is
one. And whenever we draw golgi complex we have
to remember that the convex side should be facing the nuclear membrane.
Why this happens? That we will study when we come to organelle.
So here also we have shown the convex side towards th nucleus and concave side towards
theplasma membrane. Another structure which we find only in plant
cell is the plastids or chloroplasts. Chloroplasts are the structures which have
grana, stroma and in those thylakoids of grana there is this green pigment called chlorophyll.
So this is the chloroplast. Exclusively present in plants cell.
These are not found in animal cell. So , so far there are two differences plant
cells have cell wall, Animal cells don’t.
Plant cells have plastids like chloroplast, Animal cells don’t.
Now let us come to animal cells a structure which is present only in animal cells.
That structure is known as centrosome. Centrsome has two centrioles,
This structure is again closer to the nucleus and is found only in animal cells.
So right now we have three differences. Cell wall present in plants cell,
Plastids or chloroplast present in plant cells. And centrosome present in animal cell.
These are the three differences so far. One more structure which is seen,
They are spherical structures filled with enzymes.
These structures are known as lysosomes. The number of lizosomes are more in animal
cells. As they have to digest food.
Whereas in case plants cells the number of lysosomes are less because they synthesize
their own food. but they will be required to digest the food
when they store it. So they do have the lysososmes though the
number is less. So now the space which is here is filled with
a fluid which is cytoplasm in case of animal cells also we have the space completely filled
cytoplasm,. So this jelly like substance is cytoplasm
present in both. Plant Cell and Animal Cell.
Now this is the basic structure and as we have drawn both this structure together it
is easier to see the difference between the two.
In plant cell the shape is normally geometric. Whereas in case of animal cells it is slightly
ovoid or circular. Cell wall made up of cellulose.
Which gives a firmness to the cell plant cell no such cell wall present in animal cell.
A large vacuoles in case of plant cell, many small vacuoles in case of animal cells.
Nucleus eccentric shifted towards one side because of this vacuole.
Whereas in case of animal cells it is centrally placed.
Other organelle like mitochondria common. Golgi body common,
Plastids or chloroplast present in plant cell but not there.
One more organelle which we need to draw is endoplasmic reticulum it is the network of
tubules. Which normally runs between the nuclear membrane
and the plasma membrane there are two types of endoplasmic reticulums.
One is rough which we called aria or rough endoplasmic reticulum.
And other is smooth. So we will draw one of both.
So normally this is drawn in the form of a tube like thing.
But it is not exactly a tube when we come to the structure of endocrine or the sorry
the endoplasmic reticulum. We will see the exact structure and one more
which we can draw here. Now what makes one smooth and other one rough
is presence of ribosomes on it. If the membrane is studded with ribosomes
we will call it SER that is… Sorry this will be called RER Rough Endoplasmic
Reticulum, And the one which is without the ribosomes
is the smooth endoplasmic reticulum. Same would be in the plant cell.
So here again the endoplasmic reticulum. A network of tubes and let us draw one more,
the one which is studded with ribosomes that means ribosomes are attached on its surface
is the rough one RER and the one which is without ribosome is the smooth SER i.e. Smooth
Endoplasmic Reticulum. So this completes our structure of plant and
animal. So in eukaryotic cells we have seen the structure
of both the types of eukaryotic cell. The fungus or yeast for example is pretty
much similar to this,. Basic structure remains the same.
So now we have three cells to compare one is prokaryotic and these two eukaryotic cells.
You see in this prokaryotic cell there is nothing inside, no membrane bound structure.
There is one exception that is of blue green algae where there are some thylakoids which
we will talk about in Kingdom Monera and in case of plant cell basic pattern is same except
for couple of organelle. Which are present or absent in either of the
cells. So two types of cells prokaryotic and eukaryotic Now moving forward what we are going to do
is we will take individual structures and see the details of it.
That means we will talk of plasma membrane then cell wall, nucleus, vacuole, mitochondria.
Each structure these are called organelle. So now let us take a organelle one off one
by one.

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