What is a Cell? Definition, Types, Structure, Functions

If you are curious about cells and want to learn about them from the ground up, this article is perfect for you! will be exploring the world of cells in a clear and easy-to-understand way. By the end, you will have a solid foundation in cell biology. So, let’s dive in without wasting any time!

Table of Contents

What is a cell?

A cell is the fundamental unit of life. All living things from microorganisms to giant organisms are composed of cells. Without cells, there would be no life.

Examples of living things with cells:

Humans, animals, microorganisms, plants, etc.

Examples of non-living things without cells:

Pens, stones, wood, iron, etc.

Why is the cell the unit of life?

Calls are capable of independent existence and performing all the essential functions of life, such as respiration, nutrition, and excretion.

How many cells does a human have?

A human is a multicellular organism meaning it’s made up of trillions of cells that work together to carry out all necessary functions like respiration, nutrition, excretion, movement, and reproduction.

Amoeba: A single celled organism.

Amoeba is a unicellular organism meaning it consists of a single cell that performs all vital life functions like nutrition, excretion, nutrition, locomotion, and reproduction. This demonstrates how a single cell can sustain life independently.

Why is the cell the structural and functional unit of life?

Cells are responsible for the structure and function of all living organisms from humans and animals to plants.

what is plant cell.
Plant cell

Who discovered the cell?

Robert Hooke’s first objective cell and 1665 but they were dead cells. The first living cell was seen by Anton von Leeuwenhoek in 1674.

Who discovered the nucleus?

The nucleus was discovered by Robert brown.

What is cell theory?

In 1838 Matthias Schleiden a German botanist, proposed that all plants are made up of different types of cells that join together to form tissues.

Shortly after in 1839, Theodore Schwann a British zoologist observed a membrane surrounding cells in animals. This membrane is known as the plasma membrane or cell membrane.

Their combined work led to the cell theory which states that all organisms are composed of cells and that cells are covered by a membrane. However, this theory initially lacked an explanation for how new cells arise during organism development.

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In 1855, Rudolf Virchow completed the cell theory by proposing that all cells come from pre-existing cells through cell division.

What’s inside a cell?

The main components of a cell are:

  • Cell membrane: controls what enters and leaves the cell.
  • Cytoplasm: A jelly-like substance that fills the cell and contains various organelles.
  • Nucleus: the control center of the cell containing genetic material (DNA).

Prokaryotic vs Eukaryotic cells

There are two main types of cells.

  1. Prokaryotic cell: a simpler cell that Lacks a true nucleus and membrane-bound organelles. Examples include bacteria and archaea.
  2. Eukaryotic cell: a more complex cell with a membrane-bound nucleus and other organelles. Examples include animal plant and fungal cells.

Structure of a prokaryotic cell

Prokaryotic cells have a simple structure compared to eukaryotic cells. Here are their key features:

  • The cell wall: (except for mycoplasma) provides rigidity and protection.
  • Cell membrane: controls what enters and leaves the cell.
  • Cytoplasm: contains the cell’s genetic material (DNA) in a free-floating state (not enclosed within a nucleus).
  • Ribosomes: produce protein (these are found in both prokaryotic and eukaryotic cells).
prokaryotic cell, bacterial cell

How many types of prokaryotic cells are there?

There are three main types of prokaryotic cells: bacteria, cyanobacteria (formerly known as blue-green algae), and mycoplasma. Mycoplasma is also placed within a group called Pleuropneumonia-like organisms (PPLO).

You might be wondering why mycoplasma my age classified as a PPLO.

You might be wondering why mycoplasma is classified as a PPLO.

The reason is that bacteria and cyanobacteria have a cell wall, while mycoplasma does not. This lack of a cell wall gives mycoplasma its flexible shape. Therefore, the PPLO group includes prokaryotic cells that lack a rigid cell wall.

Prokaryotic cells are significantly smaller than eukaryotic cells.

Their simpler structure allows them to divide much faster than eukaryotic cells.

Which of the three types of prokaryotic cells is most common?

Bacteria are the most abundant type of prokaryotic cell. You’ve likely heard of more bacteria than the other two types because they exist in such vast numbers.

Mycoplasma and the PPLO group

Mycoplasma is a type of bacteria that Lacks A cell wall resulting in a constantly changing shape. Because of this unique characteristic, it’s classified within the pleuropneumonia-like organism (pplo) group.

Prokaryotic cell size and reproduction

Prokaryotic cells are generally much smaller than eukaryotic cells and reproduce much faster through a process called binary fission.

Types of bacteria

There are four main shapes of bacteria:

  1. Bacillus: rod-shaped
  2. Coccus: spherical
  3. Vibrio: Comma shaped
  4. Spirillum: Spiral-shaped

As you might recall, not all prokaryotic cells have a cell wall. Mycoplasma is the exception.

Following the cell wall (if present), there’s a cell membrane. Inside the cell membrane lies a thick, jelly-like liquid called cytoplasm.

Within this cytoplasm, the genetic material (DNA) exists in a “naked” state. It’s not surrounded by a nuclear membrane because prokaryotic cells lack a nucleus altogether. Since there’s no nucleus, a nuclear membrane wouldn’t be present.

Let’s clear up the concept of naked DNA.

Eukaryotic cells have a nucleus that houses their DNA. This DNA is quite long, about 2.2 meters, and needs to be tightly packed to fit within the nucleus. Histone proteins help with this packaging process.

Think of histone proteins as spools that DNA wraps around. If DNA is not wrapped around these spools (histones) then it’s considered naked.

Therefore naked DNA refers to prokaryotic DNA that is not packaged with histone proteins. This DNA is typically referred to as genomic DNA and exists as a single circular chromosome.

What is a chromosome?

A chromosome is a long molecule of DNA that contains genetic information. In bacteria chromosomes are circular pieces of DNA. The two ends of the DNA molecule are joined together to form a circle.

what is chromosome

Difference between Genomic DNA and Plasmid

Genomic DNA: This is the main DNA molecule in a cell and contains all the essential instructions for cell function.

Plasmid: a plasmid is a small circular piece of DNA that is separate from the genomic DNA. Plasmids often carry genes that provide bacteria with advantages such as antibiotic resistance.

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

Functions of plasmids:

Plasmid in bacteria: special DNA with unique functions

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Plasmids are special DNA molecules found in some bacteria. They come in various types and serve numerous functions. One important function of antibiotic resistance is mediated by R-plasmid (resistance plasmids). Understanding plasmid and antibiotic resistance

Let’s break this down:

  1. Genes: these instructions encoded in DNA and plasmids often carry small genes.
  2. R-plasmids: these are plasmids that confer antibiotic resistance to bacteria. They don’t resist against whom but rather against specific antibiotics.

Example: Plasmids and Multiple Antibiotic Resistance

Imagine a bacterium with a plasmid containing two genes. One gene provides resistance to ampicillin, while the other offers resistance to tetracycline (both being antibiotics).

How does plasmid affect antibiotic treatment?

If you get sick from this bacteria a doctor might prescribe an ampicillin resistance gene, but the antibiotic won’t be effective. The same goes for tetracycline

However, if the doctor prescribes streptomycin an antibiotic the bacteria lack resistance against (no gene on the plasmid for it) the bacteria will likely die. This highlights the importance of using the correct antibiotic to fight infections.

Mesosome: Infoldings in Prokaryotic Cells

In prokaryotic cells, the cell membrane can fold inwards to create a structure called a mesosome. Mesosomes come in three main forms: vesicles, tubules, and lamellae.

These infoldings are thought to play several roles in the cell, including:

  • Respiration: They may aid in processes related to cellular respiration.
  • DNA replication: They might be involved in facilitating DNA replication.
  • Cell wall formation: they might contribute to the formation of the cell wall.
  • Increased enzyme activity: The increased surface area provided by mesosomes might allow for a higher concentration of enzymes.

While the exact function of mesosomes is still being explored; they are believed to be important structures within prokaryotic cells.

What are gram-positive and gram-negative bacteria?

The story of gram stain: a method to classify bacteria

A Scientist named Christian Gram developed a method for classifying bacteria known as the Gram stain.

The process involves pressing different types of bacteria in separate beakers. First, a dye called crystal violet is applied to both samples staining the bacteria.

Next both samples are washed with water and then with alcohol. Here, “anointing” can be replaced with “stained.”

After washing scientists observed that the stain remained on one type of bacteria but disappeared from the other.

Bacteria that retained the stain were classified as gram positive while those that lost the stain were classified as gram-negative.

The reason behind the stain: Cell wall differences

The reason for this difference lies in the cell wall composition of the bacteria. Gram-negative bacteria have a cell wall in rich (fats). During the alcohol wash, these lipids dissolved allowing the stain to be removed.

On the other hand, gram-positive bacteria have a cell wall with much less lipid content. This allows the crystal violet stain to remain bound to the cell wall even after the alcohol wash.

Flagella: the propellers of bacteria

Not all bacteria can move. Those that can have a thin tail-like structure called a flagellum. it’s attached to the cell envelope, which consists of three main parts:

  1. Filament: this is the longest part of the flagellum and extends outward from the cell surface. It’s the primary structure responsible for propulsion.
  2. Hook: This curved structure connects the filament to the basal body.
  3. Basal body: embedded in the cell envelope the basal body acts like a motor, rotating the flagellum to propel the bacterium.

It’s important to note the difference between flagella and pilli/fimbriae. While flagella are for movement pili and fimbriae are hair-like structures on the cell surface that help bacteria adhere to the surface or other cells. They don’t contribute to swimming speed.

Pili and Fimbriae: tiny hair-like structure with big roles

Pili and fimbriae are both hair-like appendages found on the surface of many bacteria. While they may look similar they have slightly different functions.

  • Pili (singular: pilus): These are typically longer and less numerous than Fimbriae. Pili are made up of a Special protein called pilin. Their primary function is to help bacteria conjugate which is the process of transferring genetic material between two bacterial cells.
  • Fimbriae (singular: Fimbriae): These are shorter and more numerous than pili. Fimbriae are also composed of proteins but not necessarily pilin. Their main function is to help bacteria adhere to the surface, including Rock, host tissues, or even other bacteria. This attachment allows bacteria to colonize a specific environment or interact with the host cell.
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Also, read –

What is the animal kingdom? Complete notes.

What is the plant kingdom? Full Information.

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What is a cell wall? Complete notes

Ribosome and inclusion body in prokaryotic cells

Ribosomes are tiny structures found within prokaryotic cells responsible for protein synthesis. Unlike some cellular components they lack is surrounding membrane. Each ribosome is roughly 15 to 20 nanometers in size and is made up of two subunits:

  1. Small subunit (30S): this subunit plays a crucial role in the initiation of protein synthesis.
  2. Large subunit (50S): this subunit is responsible for the elongation and termination stages of protein synthesis.

These two subunits come together to form the complete functional 70S ribosome.

What’s inside a ribosome?

Ribosomes primarily contain two main components:

  1. Proteins: This protein provides structure support and helps with the various steps of protein synthesis.
  2. Ribosomal RNA (rRNA): this special type of RNA molecule plays a catalytic role in protein assembly.

Polyribosome: teamwork in protein synthesis

A polyribosome also known as a polysome is not a single chain. Instead, its complex structure is formed when multiple ribosomes are attached to a single messenger RNA (mRNA) molecule. Imagine mRNA as a set of instructions and ribosomes as tiny factories that build protein based on those interactions.

Having multiple ribosomes working on the same mRNA molecule simultaneously can significantly increase its protein production rate. It’s like having multiple construction crews working on different sections of the same building at the same time.

Which is called the protein factory.

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

Inclusion body

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

Examples include phosphate granules cynophycin granules and glycogen granules.

Who is the remaining substance?

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

So friends, till now we have been studying prokaryotic cells, and we have 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 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-bound cell organelles include mitochondria, Golgi complex, lysosome, endoplasmic reticulum, chloroplast, and nucleus.

The membrane-less organelles are the ribosome. You have read about 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 cellular skeleton? See the cells of plants fungi and Protista have a cell wall that maintains their cell shape, but the cells of the Animalia Kingdom do not have this. So something is needed to maintain their shape.

Difference between Plant cells and Animal cells

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

Microscopic fibers are found in animal cells which form the cell skeleton and maintain the shape of the cell.

now we will talk about plant cells and animal cells so let’s study them as well.

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

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 a 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.


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