What is a Cell? Definition, Type, Theory, Diagram,

If you want to know about the cell from the beginning, then this article is for you because, in this post, the cell has been told from the very beginning, that too in very simple language, and I assure you. If you read this article completely, then you will get a lot of knowledge about the cell. So brother, without wasting time, let’s start.

Table of Contents

What is a cell?

“A cell is the unit of life”.

Now a question may be arising in your mind: why is the cell the unit of life, Because life started from a single cell, whether it is a microorganism or a very large organism, all living beings have cells. Because if there is no cell then it is non-living and if there is a cell then it is alive or those in which cells are found are alive.

For example, humans, animals, microorganisms, plants, etc., and those that do not have cells are non-living. For example – pens, stone, wood, iron, etc.

So far, we have learned why the cell is the unit of life. Now we will talk about what is called a cell.

What is the cell definition?

Definition of the cell –

The cell is the basic structural and functional unit for living organisms. Because the cell is capable of independent existence and performing all the essential functions of life. Independent existence means that only a single cell can perform all the functions that are characteristic of living things.

For example, respiration, nutrition, excretion, etc.

what is plant cell.
Plant cell

Let’s understand with an example –

How many cells is a human made of? You know that a human is made of many cells, that is, a human is a multicellular organism that performs all the necessary functions such as respiration, nutrition, excretion, movement, reproduction, etc.

You must have heard the name Amoeba. Amoeba is an organism that is made up of a single cell. And performs all the essential functions of life. Like – nutrition, excretion, locomotion, reproduction, etc. That is, despite being a single cell, Amoeba lives its life independently.

That’s why the cell is called an independent living structure and is also capable of performing all the essential functions of life. Now you must be thinking about why the cell is the structural and functional unit for living beings.

Because it is the cell that performs the structural and functional functions of the organisms. Now whether it is human or animal or plant, any living being.

Who discovered the cell?

First, Robert Hooke discovered the cell in 1665, which was a dead cell. After that, the living cell was first seen by Anton van Lievenhack in 1674.

Who discovered the nucleus?

The nucleus was discovered by Robert Brown.

What is cell theory?

In 1838, German botanist Matthias Schleiden, after studying many plants, said that plants are made up of different types of cells and these cells join together to form tissues.

After that, in 1839, a British zoologist, Theodore Schwann, after studying animals, reported that a membrane is being found outside the cells, which in today’s time is called protoplasm membrane, cell membrane, or plasma membrane.

Both of them studied and gave the cell theory that organisms are made up of many cells and these cells are covered from the outside by a cover called the cell membrane.

But his theory failed, why failed because when he was asked that you have been told that organisms are made up of many cells, but with the development of these organisms, the new cells that are being formed, where these cells are coming from. His theory failed to explain this, and the cell theory remained incomplete.

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After that, in 1855, scientist Rudolf Virchow said that cells divide, that is, cells that are already cells divide to form new cells. Only then this cell theory was completed.

What is found inside the cell?

The following things are found inside the cell.

Protoplasm is found inside the cell.

What is protoplasm?

Cell membrane, cytoplasm, and nucleus together make up the protoplasm, Or protoplasm is made up of cell membrane, cytoplasm, and nucleus.

The cell wall was found inside the cell, the cell membrane was found inside it and a fluid called cytoplasm was found inside it, and the nucleus was found inside the cytoplasm, and chromosomes were found inside the nucleus.

Chromosomes are made of DNA, apart from the nucleus, other cell organelles are found inside the cytoplasm, namely – endoplasmic reticulum, mitochondria, globules, Golgi complex, microbodies, lysosomes, and contractile vacuole. All these are found in eukaryotic cells, while membrane-bound organelles are not found in prokaryotic cells.

Another cell organelle is found inside the cell, which is named ribosome, but this cell organelle is found inside both prokaryotic and eukaryotic cells. The membrane is not found in ribosomes. The ribosome is not only found in the cytoplasm, it is also found in mitochondria, rough endoplasmic reticulum, and chloroplast.

In animal cells, a membrane-less cell organelle is found, which is called a centrosome. Two centrosomes join together to form a centrosome, and this helps in cell division.

What is life?

Friends, if someone asks you what life is, then you have to tell them that life is made up of biochemical reactions and these reactions take place in the protoplasm of the cell.

Now a question must be arising in your mind that

What are biochemical reactions?

Biochemical reactions are called metabolism or metabolic reactions. There are two types of metabolism.

  1. Anabolism
  2. Catabolism
What is anabolism?

Those biochemical reactions in which something is created are called anabolism. It is also called constructive reaction.

6CO2 + 6H2O ——> C6H12O6 + 6O2

For example, plants make their food by photosynthesis. In this process, plants react with water and carbon dioxide in the presence of light to form glucose and extract oxygen. If glucose is formed in it, then this reaction is called anabolism.

What is catabolism?

That biochemical reaction in which something breaks down is called catabolism. It is also called a decomposition reaction.

Like – when we humans drink glucose, this glucose goes into our cells and there it breaks down in the presence of oxygen, after breaking down carbon dioxide, water, and energy are formed. And by using this energy, we do all the work.

Now you are seeing that glucose is breaking down, that is, it is getting decomposed. So this type of reaction is called a reduction or decomposition reaction.

How many types of cells?

There are two types of cells.

  1. Prokaryotic cell
  2. Eukaryotic cell

What is a prokaryotic cell? 

Prokaryotic is made up of two words. Pro + karyon in which pro means primitive and karyon means nucleus.

Definition of prokaryotic cell –

Those cells in which a primitive nucleus is found, or a distinct nuclear envelope is not found, or a nucleus is not found, then such cells are called prokaryotic cells.

prokaryotic cell, bacterial cell

How many types of prokaryotic cells?

There are three types of prokaryotic cells.

There are bacteria, blue-green algae (cyanobacteria), and mycoplasma, and mycoplasma is placed within a group called Pleuro Pneumonia Like Organisms (PPLO).

Now you must be wondering why mycoplasma has been kept in PPLO.

Because the cell wall is found in bacteria and blue-green algae and the cell wall is not found in mycoplasma. Due to the lack of cell walls in mycoplasma, its shape is not fixed. Therefore, in prokaryotic cells, those that do not have a definite shape are placed in the PPLO group.

Prokaryotic cells are much smaller than eukaryotic cells. And it is simpler than the eukaryotic cell, due to which the prokaryotic cell divides very rapidly. Than a eukaryotic cell.

Which is the more found of the three prokaryotic cells?

Most bacterial cells are found. And you must have heard the name of most bacteria out of these three because their number is maximum.

Also, read –

What is the animal kingdom? Complete notes.

What is the plant kingdom? Full Information.

What is biology’s definition and branches?

What is a cell wall? Complete notes

How many types of bacteria?

There are four types of bacteria.

  1. Bacillus
  2. Coccus
  3. Vibro
  4. spiral

All prokaryotic cells have a cell wall, except for mycoplasma, I have already told you this and after the cell wall, there is a cell membrane, and inside the cell membrane, there is a thick liquid fluid called cytoplasm.

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Inside this cytoplasm, there is genetic material (DNA) in a naked state, and it is not surrounded by a nuclear membrane because there is no nucleus in a prokaryotic cell, when there is no nucleus then where will the nuclear membrane be from?

Now, you must not be understanding one thing: what is the meaning of naked DNA? So let’s understand it.

The nucleus is found inside the eukaryotic cell and DNA is found inside this nucleus. This DNA is 2.2 meters long and is wrapped with histone protein. This is called the packaging of DNA.

Now, let’s assume that histone proteins are the coat of DNA. And when DNA is not clothed, that means it is not wrapped with histone protein, then it will be called naked.

So the naked state means that the DNA in the prokaryotic cell is not wrapped with histone proteins. And this DNA is called genomic DNA. It is also called a single chromosome.

Why is it called a single chromosome? Let us first know that

What is a chromosome?

When small pieces of DNA are linked in a cross-like structure, this structure is called a chromosome. In this, two pieces are joined together.

what is chromosome

The length of genomic DNA is 1.1 meters, And one end is connected to the other end in a circle. Now it is only one DNA with both ends joined together. It can be called a single chromosome or not. So that, it is called a single chromosome.

In addition to the circular genomic DNA in bacteria, there is another circular DNA called a plasmid.

Now you must be wondering what is the difference between genomic DNA and plasmid. The plasmid is also circular and has a single chromosome. So let’s know this too.

Difference between genomic DNA and plasmid –

Genomic DNA is the guide for all the functions of the cell. Let’s understand it better.

What is the director of the eukaryotic cell is its nucleus. Who told me this was a scientist named Hammer ling? He studied a cell named Acetabularia which is a green alga and found that who is controlling all the functions of a cell, is its nucleus.

Difference between genomic DNA and plasmid
Difference between genomic DNA and plasmid

And what happens inside this nucleus is DNA. So in the end what controls all the functions of a cell is its DNA, and the DNA which controls or directs or determines all the functions of the cell is called Genomic DNA.

Whereas, what is a plasmid does not control all the functions of the cell. So, brother, the question arises of what the plasmid does.

Functions of plasmid –

Plasmid DNA shows special characteristics in bacteria. There are many types of plasmid DNA. They also have many functions, one of which is that they are resistant to antibiotics, and this function is performed by R-plasmid.

You must not have understood this thing very well, so let us understand it in simple language.

Small genes are found inside the plasmid. What is a gene? Small sections of DNA are called genes.

Now let’s assume that this plasmid is R-plasmid, it is called Resistance Plasmid. And what does this R-plasmid show resistance? Antibiotics against whom. Or, those plasmids which show resistance against antibiotics are called R-plasmids.

Let’s say there is a bacterium and it has a plasmid and that plasmid has two genes. There is one gene showing resistance against ampicillin and another showing resistance against tetracycline.

Now you will think about what is ampicillin and tetracycline, both of them are antibiotics.

Now let’s say that this bacteria got into me and I got sick, now I went to the doctor and the doctor gave me an antibiotic and the name of that antibiotic is Ampicillin. And I ate this antibiotic;

Similarly, after eating tetracycline, one will not die because the bacteria have genes that act against it. Then after that, what the doctor gave me was streptomycin. I ate it now to tell whether the bacteria will die or not this time it will die because this time the bacteria do not have a gene or plasmid that acts against streptomycin.

What is mesosome?

In prokaryotic cells, the cell membrane folds inwards to form a structure called mesosome, and it is of three types vesicles, tubules, and lamellae.

Its function is to aid in respiration, form cell walls, aid in the multiplication of DNA, aid in the division of daughter cells, and increase the number of enzymes.

It forms a specific structure in a prokaryotic cell.

Prokaryotic cell envelope and its modifications –

The covering of prokaryotic cells, especially in bacteria, is made up of three layers. Glycocalyx, cell wall, and cell membrane. Together, these three levels protect the cell.

Like – protects the cell from bursting. Stops waste particles from going, etc. Their cell membrane is semipermeable.

What are gram-positive and gram-negative bacteria?

There was a scientist whose name was Christian Gram, he used a method which was named Gram Stain.

They took two beakers and put different types of bacteria in both beakers. After that, a dye was first put on them and the name of that dye was Crystal Violet. Now what color will this dye put on both of them mean it will make it anointing, it means to get colored.

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After that, both these types of bacteria were washed first with water and then with alcohol.

After that, they observed that one type of bacteria remained stained and the other type of bacteria discolored.

Now those on which the color remained were called Gram-positive and those on which the color was removed were called Gram-negative.

Now, here a question arises why does the color remain on gram-positive bacteria, and why did the color get discolored from gram-negative bacteria?

Gram-negative bacteria had a high amount of lipids (oil or fat) in their cell wall due to which the color got discolored after washing while gram-positive bacteria had very few amounts of lipids or oil or fat in their cell wall Because of this the color remained on them.

That’s it, brother, it’s about its cover, now we will know about its transformation.

What is the flagellum?

Some bacterial cells move, that is, they move, and some bacterial cells do not move. Those that move have a thin tail-like structure called the flagellum. It is attached to the cell cover, which is made up of three parts.

Filament, hook, and basal body. The filament is the largest part of the flagellum that extends outward from the surface of the cell. This helps with speed. And there are pili and fimbriae structures on the surface of the cell. Which does not help with their speed.

Now the question arises, what do Pili and Fimbriae?

What do Pili and Fimbriae?

Pili – It is made of a special protein named the Pili protein. It helps to attach one bacterium to another bacterium.

Fimbriae – These help the bacteria to adhere to the rocks and host tissues.

Ribosomes and inclusion bodies –

In prokaryotic cells, ribosomes are attached to the cell membrane. And its size is 15 to 20 nanometers. It is made up of subunits. There is a small subunit called 30S and a large subunit called 50S, and these two join together to form the 70S prokaryotic ribosome.

Ribosomes are a membrane-less structure. Two things are found inside the ribosome. The first protein and the second rRNA (ribosomal RNA).

What is a polyribosome?

When many ribosomes bind to one m-RNA (messenger RNA) to form a chain, then this chain is called a polyribosome.

Which is called the factory of protein.

Ribosomes are called the factory of protein. Because ribosomes make proteins.

Inclusion body –

In prokaryotic cells, some substances are left that are not used, then these remaining substances gather in a structure in the cytoplasm, this structure itself is called the inclusion body.

Who is the remaining substance

Phosphate granules, cyanophycin granules, and glycogen granules. The gas reservoir is found in blue-green, violet, and green photosynthetic bacteria.

So friends till now we have been studying prokaryotic cells, and we also studied them thoroughly.

Now we will study the eukaryotic cell, so let’s start studying.

What is a eukaryotic cell?

Those cells in which a well-developed nucleus or nuclear envelope is found, then such cells are called eukaryotic cells.

In which type of organism are eukaryotic cells found?

Eukaryotic cells are found in protozoans, fungi, plants, and animals.

Both membrane-bound and membrane-less organelles are found in the cytoplasm of eukaryotic cells.

Membrane cell organelles – Mitochondria, Golgi complex, Lysosome, Endoplasmic reticulum, Chloroplast, Nucleus.

The membrane-less organelle is the ribosome. You have read this in prokaryotic cells.

In eukaryotic cells, the cytoplasm moves, that is, the cytoplasm moves inside. A cellular skeleton is also found in it.

Now, what is meant by a cellular skeleton? See, the cells of plants, fungi, and Protista are found in their cells and this cell wall maintains their cell shape, but the cells of the Animalia kingdom are not found in them. So who will maintain their cell shape, something is needed that will maintain their shape.

So some fibers are found in animal cells, which are called microscopic fibers. These fibers form the cell skeleton, which maintains the shape of the cell.

Now we will talk about plant cells and animal cells, so let’s study them also.

So most of all we see only plants and animals. Both of these are eukaryotic cells. Yet there is a difference between these two cells.

Difference between plant cells and animal cells –

Plant cellAnimal cell
They have cell wall.They do not have cell wall.
Chloroplast is found in these.Chloroplast is not found in these
There is a big vacuole in them.There is no vacuole in them.
They do not have a centrosome.There is no vacuole in them.

Frequently Asked Questions –

Which is the smallest cell?

Mycoplasma is 0.3 micrometer.

Which is the largest cell?

The largest cell is the egg of the ostrich.

Which is the longest cell?

The longest cell is a nerve cell.

Which is the smallest cell in a human?

The smallest cell in a human is the sperm.

What is the largest cell in a human?

The largest cell in an ovum.

What is the powerhouse of the cell?

Mitochondria are called the powerhouse of the cell.

What is the cell wall?

The covering of the cell which maintains the shape of the cell is called a cell wall. The cell wall is found in all types of cells except animal cells. It is located between the glycocalyx and the plasma membrane.

Types of cells –

There are two types of cells: prokaryotic cells and eukaryotic cells.

Who discovered the living cell?

The living cell was discovered by the scientist Anton Van Lievenhack in 1674.

Who discovered the cell and how?

Robert Hooke first discovered the cell in 1665 in the bark of the plant.

Who discovered the dead cell?

Robert Hooke discovered the dead cell.

Names of the organelles –

Mitochondria, Golgi complex, Lysosome, Endoplasmic reticulum, Chloroplast, Nucleus, and Ribosome.

The outermost layer of the animal cell.

cell membrane.

What is cytoplasm?

Inside the cell, there is a thick fluid called cytoplasm.

How many chromosomes are found in a human cell?

46 chromosomes are found. There are 23 pairs.

When was the cell discovered?

In 1665.

Where does protein synthesis take place inside the cell?

In ribosome.

What is the scientific name of goldfish?

The scientific name of the goldfish is Carassius auratus.

What is the cell wall made of in a plant cell?

It is made of Cellulose.


I Hope friends, you would have liked the information given about cells, if you like it, then share it with your friends too so that they too can benefit from it.


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