Structure of cells

Prokaryotes Eukaryotes
Greek Word: Pro means Before Eu means Well or Truly
Karyon means Kernel or nucleus

Single celled Mostly multicellular

(Exception amoeba, yeast)

Membrane bound nucleus: No Yes

Organelles: No Membrane bound organelles
Distinct Chromosomes: No Yes
Structure of Prokaryotic Cell
 Eukaryotes

Animal Plant
cell

These cells are larger than prokaryotes and contain internal membrane
structures called organelles and cytoskeleton consist of microtubules and
microfilaments which play important role in cell organization and its shape
Structure of Prokaryotic Cell
 Gram Staining
Developed by Christian Gram in 1884 to differentiate according to the staining
potential of bacterial cell wall

Bacterial cell wall is made of polysaccharide, contains water and help bacteria
from desiccation in extreme conditions

Gram positive Gram negative

bacteria
 Staining procedure
Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan
(50–90% of cell envelope), and as a result are stained purple by crystal violet,
whereas gram-negative bacteria have a thinner layer (10% of cell envelope), so do
not retain the purple stain and are counter-stained pink by the Safranin.

There are four basic steps of the Gram stain:

1. Applying a primary stain (crystal violet) to a heat-fixed smear of a bacterial

culture. Heat fixation kills some bacteria but is mostly used to affix the bacteria
to the slide so that they don't rinse out during the staining procedure.

2. The addition of iodide, which binds to crystal violet and traps it in the cell

3. Rapid decolorization with ethanol or acetone

4. Counterstaining with safranin

 Structure and Function of Prokaryotic Cell
Prokaryotic cells are primitive and relatively simple cells

They are believed to have appeared about 3.5 billion years ago when life
originated on earth

They are mostly single celled and lack all intracellular membranes and membrane
bound organelles like ER, Golgi, Lysosomes, Mitochondria and chloroplasts

Due to the absence of nuclear membrane, hereditary material lies naked in the
cytoplasm

Characteristics of Prokaryotic Cell:

1. These cells are very small in size about the size of mitochondria

2. Distinct nucleus that is nucleolus and nuclear envelop is absent

3. Hereditary material is a single chromosome which is a single circular double

stranded DNA. It is known as nucleoid

4. Basic proteins like histones are absent

5. Cellular covering is plasma membrane that differs from eukaryotic cells in
following features:
a. It does not have sterols
b. Ratio of protein to phospholipid is high in bacteria (2:1) than in
eukaryotes (1:1)

6. Cell wall or capsule present outside the plasma membrane is non-cellulosic. It

is formed of carbohydrates and amino acids

7. Membrane bound organelles are absent

8. Plasma membrane in some cases is folded inwards and forms mesosomes and
chromatophores

9. Prokaryotic ribosomes are 70S type

10. Prokaryotic cells may contain flagella and pilli

11. Their cytoplasm does not exhibit streaming movement

Structure of Prokaryotic Cell
 Capsule
The cell capsule is a very large structure of some prokaryotic cells, such as
bacterial cells

It is a polysaccharide layer that lies outside the cell envelope of bacteria

The capsule—which can be found in both Gram-negative bacteria and Gram-

positive bacteria—should not be confused with the second lipid membrane (or
bacterial outer membrane), which contains lipopolysaccharides and lipoproteins
and is found only in Gram-negative bacteria

When the amorphous viscid secretion (that makes up the capsule) diffuses into
the surrounding medium and remains as a loose undemarcated secretion, it is
known as slime layer

It usually consists of polysaccharides but can be composed of other materials

(e.g., polypeptide (D-glutamic acid) in B. anthracis)

The capsule is considered a virulence factor because it enhances the ability of

bacteria to cause disease (e.g. prevents phagocytosis)

The capsule can protect cells from engulfment by eukaryotic cells, such as
macrophages
A capsule-specific antibody may be required for phagocytosis to occur

Capsules also contain water which protects the bacteria against desiccation

They also exclude bacterial viruses and most hydrophobic toxic materials such as
detergents

There are 14 different capsule types, which each impart their own specific
antigenicity

Immunity to one capsule type does not result in immunity to the other types

Capsules also help cells adhere to surfaces

 Cell Envelop – Cell wall
The cell envelope comprises the plasma membrane and cell wall of a bacteria

Most bacterial cell envelopes fall into two major categories: a Gram-positive type
and a Gram-negative type

Regardless of its Gram status, a bacterium might have an exterior polysaccharide-

containing structure called a capsule for further protection of the cell envelope

As in other organisms, the bacterial cell wall provides structural integrity to the
cell

In prokaryotes, the primary function of the cell wall is to protect the cell from
internal turgor pressure caused by the much higher concentrations of proteins
and other molecules inside the cell compared to its external environment

The bacterial cell wall differs from that of all other organisms by the presence of
peptidoglycan (poly-N-acetylglucosamine and N-acetylmuramic acid), which is
located immediately outside of the cytoplasmic membrane

Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the
determination of cell shape

It is relatively porous and is not considered to be a permeability barrier for small

substrates
 Cell Envelop – Cell Membrane
The cell membrane or plasma membrane is similar to eukaryotic cells

It separates the interior of all cells from the outside environment

The basic function of the cell membrane is to protect the cell from its
surroundings

The cell membrane is selectively permeable to ions and organic molecules and
controls the movement of substances in and out of cells

It consists of the phospholipid bilayer with embedded proteins

According to the fluid mosaic model of S. J. Singer and G. L. Nicolson (1972),

which replaced the earlier model of Davson and Danielli, biological membranes
can be considered as a two-dimensional liquid in which lipid and protein
molecules diffuse more or less easily

Cell membranes are involved in a variety of cellular processes such as cell

adhesion, ion conductivity and cell signalling and serve as the attachment surface
for several extracellular structures, including the cell wall and intracellular
cytoskeleton

It also contains respiratory chain enzymes in prokaryotes

The bacterial plasma membrane is modified to form varied structures like
Mesosomes

1. Mesosomes: These are folded invaginations in the plasma membrane of

bacteria. These may be tubular, flattened disc-like or curved. They contain
enzymes of electron-transport system. They help in respiration, secretion,
synthesis of material for cell wall and separation and distribution of
chromosomes to daughter cells

Diagram of the arrangement of lipid molecules to form a lipid

bilayer. The polar head groups separate the hydrophobic tails from
the aqueous cytosolic and extracellular environments
 Extracellular structures
Pilus

These are protein tubes that extend out from the outer membrane in many
members of the bacteria

They are generally short to medium in length and present on the bacterial cell
surface in low numbers

A few organisms (e.g. Myxococcus) use pilus for motility

They are involved in the process of bacterial conjugation where they are called
conjugation pili or "sex pili“

Type IV pili (non-sex pili) also aid bacteria in gripping surfaces

Flagella

Perhaps the most recognizable extracellular bacterial cell structures are flagella

Flagella are whip-like structures protruding from the bacterial cell wall and are
responsible for bacterial motility (i.e. movement)

The arrangement of flagella about the bacterial cell is unique to the species
Common forms include:

Monotrichous - Single flagellum (A)

Lophotrichous - A tuft of flagella found at one of the cell

Pole (B)

Amphitrichous - Single flagellum found at each of two

opposite poles (C)

Peritrichous - Multiple flagella found at several locations

about the cell (D)

The bacterial flagellum consists of three basic components: a whip-like filament, a

motor complex, and a hook that connects them

The filament is approximately 20 nm in diameter and consists of several

protofilaments, each made up of thousands of flagellin subunits

The bundle is held together by a cap and may or may not be encapsulated
 Eukaryotic cells
Cell membrane - forms the outer covering of the cell, and is semi-permeable

Cytoplasm - is a gel-like matrix where all the other cell organelles are suspended
inside the cell

Nucleus - contains the hereditary material DNA and directs the activities of the cell

Centrioles - organize the microtubules assembly during cell division

Endoplasmic Reticulum - are a network of membranes composed of rough and

smooth endoplasmic reticulum

Golgi complex - is responsible for storing, packaging of cellular products

Lysosomes - are enzyme sacs, that digest cellular wastes

Microtubules - are hollow rods, function primarily as support and shape to the cell

Mitochondria - is the site for cellular respiration and producers of energy

Ribosomes - are made of RNA and proteins, and are sites for protein synthesis

Nucleolus - is the structure within the nucleus and helps in synthesis of

ribosomes

Nucleopore - is the tiny hole in the nuclear membrane, allows the movement of
nucleic acids and proteins in/out of the cell

Cell membrane

It is a semi-permeable barrier, allowing

only a few molecules to move across it

Electron microscopic studies of cell

membrane shows the lipid bi-layer model
of the plasma membrane, it also known as
the fluid mosaic model

The cell membrane is made up of phospholipids which has polar (hydrophillic)

heads and non-polar (hydrophobic) tails

Cytoplasm

The fluid matrix that fills the cell is the cytoplasm

The cellular organelles are suspended in this matrix of the cytoplasm

This matrix maintains the pressure of the cell, ensures the cell doesn't shrink or
burst

Nucleus

Nucleus is the house for most of the cells genetic material- the DNA and RNA

The nucleus is surrounded by a porous

membrane known as the nuclear membrane

The RNA moves in/out of the nucleus through

these pores

Proteins needed by the nucleus enter through

the nuclear pores

The RNA helps in protein synthesis through transcription process

The nucleus controls the activity of the cell and is known as the control center

The nucleolus is the dark spot in the nucleus, and it is the location for ribosome
formation
Ribosomes

Ribosomes is the site for protein synthesis where the translation of the RNA takes
place
60S
As protein synthesis is very important to the
40S 60S 40S
cell, ribosomes are found in large number in
all cells

Ribosomes are found freely suspended in the cytoplasm and also are attached to
the endoplasmic reticulum

Endoplasmic reticulum

ER is the transport system of the cell

It transports molecules that need certain

changes and also molecules to their
destination

ER is of two types, rough and smooth

ER bound to the ribosomes appear rough and is the rough endoplasmic

reticulum; while the smooth ER do not have the ribosomes
Lysosomes

It is the digestive system of the cell

They have digestive enzymes helps in

breakdown the waste molecules and also
help in detoxification of the cell

If the lysosomes were not membrane

bound the cell could not have used the
destructive enzymes

Centrosomes

It is located near the nucleus of the

cell and is known as the 'microtubule
organizing center' of the cell

Microtubules are made in the centrosome

During mitosis the centrosome aids in

dividing of the cell and moving of the
chromosome to the opposite sides of the cell
Vacuoles

They are bound by single membrane and small

organelles

In many organisms vacuoles are storage

organelles

Vesicles are smaller vacuoles which

function for transport in/out of the cell

Golgi bodies

Golgi bodies are the packaging center of the cell

The Golgi bodies modify the molecules from

the rough ER by dividing them into smaller
units with membrane known as vesicles

They are flattened stacks of membrane-bound

sacs
Mitochondria

Mitochondria is the main energy source of the cell

They are called the power house of the cell because

energy(ATP) is created here

Mitochondria consists of inner and outer membrane

It is spherical or rod shaped organelle

It is an organelle which is independent as it has its

own hereditary material.

Peroxisomes

Peroxisomes are single membrane bound organelle that contain oxidative

enzymes that are digestive in function

They help in digesting long chains of fatty acids and amino acids and help in
synthesis of cholesterol
Cytoskeleton

It is the network of actin filaments, microtubules and microfilament fibres

They give structural support and maintain the shape of the cell

Cilia and Flagella

Cilia and flagella are structurally identical structures

They are different based on the function they perform and their length
Cilia are short and are in large number per cell while flagella are longer and are
fewer in number

They are organelles of movement

The flagellar motion is undulating and wave-like whereas the ciliary movement is
power stroke and recovery stroke
 Functions of a Animal Cell
The functions of animal cell is carried out by the different cell organelles

The organelles of the cell function as a unit and regulate the activities of the cell

The different cell organelles and their functions are as follows:

Cell Nucleus

Cell nucleus is referred to as the control center of the cell

The genetic material of the organism is present in the cell

The replication of DNA, and synthesis of RNA occurs in the nucleus of the cell

It also regulates the activities of the other cellular organelles

Mitochondria

The mitochondria is referred to as the power house of the cell

Its main function is to produce energy for cell by the process of cellular
respiration
The energy produced is ATP

Endoplasmic Reticulum

It is a network for transportation of certain substances in and out of the nucleus

Golgi Apparatus

It is involved with processing and packaging of the molecules that are

synthesized by the cells

The crude proteins that are passed on by the ER to the apparatus are developed
by the golgi apparatus into primary, secondary, and tertiary proteins

Ribosomes

The function of ribosomes is protein synthesis

Lysosomes

They are referred to as the suicide bags of the cell

They have digestive enzymes and are involved in clearing the in wanted waste
materials from the cell
They also engulf damaged materials like the damaged cells and invading
microorganisms and digest food particles

Vacuole

They are large storage organelles

They store excess food or water

The animal cells perform variety of activities by the aid of the cellular organelles

These cells function as a unit and the cells together form tissues

A group go tissues with similar function form an organ and a group of organ of
specific function to perform becomes and organ system

Thus, the microscopic cells form the basic unit for the activities and coordination
and help survival of the organism
Differences between Animal and Plant cell

Contd………..
 Similarities between animal and plant cell

Animal Cell Plant Cell

Cytoplasm Present Present

Endoplasmic Reticulum
Present Present
(Smooth and Rough)

Ribosomes Present Present

Mitochondria Present Present

Golgi Apparatus Present Present

Plasma Membrane Present Present

Microtubules/
Present Present
Microfilaments
May be found in some May be found in some
Flagella
cells cells
Nucleus Present Present
Prokaryotes Eukaryotes