Difference between Plant cells and Animal Cells

Plant Cell		Animal Cell
Cell wall is the outermost layer		Cell membrane is the outermost layer
Has definite shape		Does not have definite shape
Plastids are present		No plastids
No centrioles		Centrioles are present
vacuoles are large (may even be 95% of the cell volume) and many		vacuoles are small, few or absent

Difference between Eukaryotic cells and Prokaryotic cells

Eukaryotes	Prokaryotes
Large in size (10 µm to 20 µm)	Small in size (1 µm to 10 µm)
Outer covering – Cell wall in plants Cell membrane in animals	Outer covering is the cell envelope
Membrane bound cell organelles are present	There are no membrane bound cell organelles
Have true nucleus with nuclear membrane, chromatin reticulum, and nucleolus	No true nucleus. Instead there is the nucleoid
Cell divides mainly by mitosis and meiosis	Cell divides mainly by amitosis

Eukaryotic Cells

includes all

• protists
• plants
• animals
• fungi

There is extensive compartmentalisation of cytoplasm due to the presence of membrane bound organelles like:

• the endoplasmic reticulum (ER)
• the golgi complex
• lysosomes
• mitochondria
• microbodies
• vacuoles

Animal cells have a non-membrane bound organelle called centrioles which helps in cell division.
Possess an organised nucleus with nuclear envelope.
Have a variety of complex locomotory and cytoskeletal structures.
Genetic material is organised into chromosomes.

There is also the ribosomes which are present in prokaryotes as well.

Nucleoid

Nucleoid is an irregular shaped clear region in the cytoplasm of prokaryotic cells which has nuclear material without a nuclear membrane and where the genetic material is localized.
The genome of prokaryotic organisms generally is a circular, double-stranded piece of DNA, of which multiple copies may exist at any time.
A genophore is the DNA of a prokaryote.This is commonly referred to as a prokaryotic chromosome. The term chromosome is misleading for a genophore because the genophore lacks chromatin

Plasmid and episome

A plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA.

An episome is a portion of genetic material that can exist independent of the main body of genetic material (called the chromosome) at some times, while at other times is able to integrate into the chromosome.

Examples of episomes include insertion sequences and transposons. Viruses are another example of an episome. Viruses that integrate their genetic material into the host chromosome enable the viral nucleic acid to be produced along with the host genetic material in a nondestructive manner. As an autonomous unit (i.e., existing outside of the chromosome) however, the viral episome destroys the host cell as it commandeers the host's replication apparatuses to make new copies of itself.

Another example is an hfr cell (also called hfr strain) which is a bacterium with a conjugative plasmid (often the F-factor) integrated into its genomic DNA. Hfr is the abbreviation for high frequency recombination

An episome is distinguished from other pieces of DNA that are independent of the chromosome (i.e.,plasmids) by their large size.

Plasmids are different from episomes, as plasmid DNA cannot link up with chromosomal DNA. The plasmid carries all the information necessary for its independent replication. While not necessary for bacterial survival, plasmids can be advantageous to a bacterium. For example, plasmids can carry genes that confer resistance to antibiotics or toxic metals, genes that allow the bacterium to degrade compounds that it otherwise could not use as food, and even genes that allow the bacterium to infect an animal or plant cell. Such traits can be passed on to another bacterium.

Inclusion Bodies

Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie freely in the cytoplasm. Eg:

• Phosphate granules
  
• Sulphur granules
  (The phosphate granules and sulphur granules are together called metachromatic granules because when they are treated with dyes they appear as coloured)
  
• Cyanophycean granules
  
• Glycogen granules
  
• Gas vacuoles are found in blue green and purple and green photosynthetic bacteria

Prokaryotic Ribosome

In prokaryotes, ribosomes are associated with the plasma membrane. They are about 15 nm by 20 nm in size (nanometre = 10^-9 m).

These are granular structures composed of ribonucleic acid (RNA) and proteins, and not surrounded by any membrane.

Made of two subunits

• 50S unit
  
• 30S unit
  

But it adds up to 70S. Here, S is the Svedberg's unit and stands for the sedimentation coefficient which is indirectly a measure of density and size.

The Eukaryotic ribosomes are similar but are 80S and have subunits 60S and 40S. But chloroplasts and mitochondria in eukaryotes have 70S ribosomes.

George Palade first observed ribsomes under electron microscope as dense particles in 1953

Ribosomes are the site of protein synthesis.

Sometimes, several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. These ribosomes translate the mRNA into proteins.

Cell Envelope and its modifications

Most prokaryotic cells, especially bacteria have chemically complex cell envelope which consists of a tightly bound 3 layered structure.

• Glycocalyx (outermost)
  
• Cell wall
  
• Plasma membrane
  

Though each has different functions, all of them act together as a protective unit.

Bacteria can be

• Gram positive (those that take up gram stain) (contains teichoic acid) (eg: Staphylococci, Streptococci, Bacillus, Clostridium) [see wikipedia article about cell wall of Gram +ve]
  
• Gram negative (those that do not take up gram stain) (contains Lipopolysaccharides (LPS), also known as lipoglycans) (eg: Escherichia coli, Salmonella, Shigella) [See wikipedia article about cell wall of Gram -ve]
  

on the basis of differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram.

Glycocalyx differs in composition and thickness. It actually is a network of polysaccharides that project from cellular surfaces. It can be:

• Slime layer (a loose sheath) (The function of the slime layer is to protect the bacteria cells from environmental dangers such as antibiotics and desiccation). (autoclaving or flushing with boiling water are the only certain methods of decontamination)
• Capsule (thick and tough) (prevents phagocytosis, ie engulfing solid particles by the cell membrane to form an internal phagosome)

Cell wall determines the shape of the cell and provides structural support (prevents bursting or collapsing) It is made up of peptidoglycan (also called murein)

Plasma Membrane is semi-permeable and interacts with outside world. It is similar to eukaryotic plasma membrane

The mesosome is a special membraneous structure formed by the extension of plasma membrane into the cell. They may be in the form of

• Vesicles
  
• Tubules
  
• Lamella
  

They help in

• Cell wall formation
  
• DNA replication and distribution to daughter cells
  
• Respiration
  
• Secretion
  
• Increasing surface area of plasma membrane and enzymatic content
  

(But actually whether they perform all these functions is in doubt [:D])

In some prokaryotes like cyanobacteria, there are other membraneous extensions called chromatophores that contain pigments. (Chromatophores are responsible for skin colouring in cold blooded animals too)

Motility:
Bacteria may be motile or non-motile. If motile, they will have thin filamentous extensions from the cell wall called flagella.

Flagellum : is different from eukaryotic flagellum. Is composed of three parts:

• Basal body (embedded in cell envelope)
  
• Hook (embedded in cell envelope)
  
• Filament (longest portion and extends from cell surface to the outside)
  

Based on the arrangement of flagella there can be bacteria of type:

• Monotrichous : single flagellum at one end
  
• Lophotrichous: many flagella at one end
  
• Amphitrichous : flagella at both ends
  
• Peritrichous: flagella all over the body.
  

Besides flagella there are structures, that do not play a role in motility:

• Pili: elongated tubular structures made of a special protein (oligomeric pilin proteins)
  (Pili connect a bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the cytoplasms of the cells. This enables the transfer of plasmids between the bacteria. An exchanged plasmid can code for new functions, e.g., antibiotic resistance.)
  (During bacterial conjugation, a sex pilus emerging from one bacterium ensnares the recipient bacterium, draws it in, and eventually triggers the formation of a mating bridge, which establishes direct contact, merging the cytoplasms of two bacteria via a controlled pore. This pore allows for the transfer of bacterial DNA from the bacteria with the pilus (donor) to the recipient bacteria. Through this mechanism of genetic transformation, advantageous genetic traits can be disseminated amongst a population of bacteria. Not all bacteria have the ability to create sex pili, however sex pili can form between bacteria of different species.)
• Fimbriae: small bristle like fibres sprouting out of the cell. (helps bacteria attach to rocks in streams or to host tissues) (these are actually small pili that help to attach bacteria together when the pili is used for connecting)
  

Prokaryotic Cell

Eg:

• bacteria,
  
• cyanobacteria,
  
• mycoplasma (PPLO: Pleuro Pneumonia Like Organisms – discovered by Edmond Nocard)(lack a cell wall, so is pleomorphic, ie, has the ability to change shape, Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis.)
• rickettsiae (has no cell wall, so, is a pleomorphic organism too) (The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.)
  Rickettsia species are carried as parasites by many ticks, fleas, and lice, and cause diseases such as typhus, rickettsialpox, Boutonneuse fever, African Tick Bite Fever, Rocky Mountain spotted fever, Australian Tick Typhus, Flinders Island Spotted Fever and Queensland Tick Typhus ^[2] in human beings.
  They have also been associated with a range of plant diseases.
  Like viruses, they only grow inside living cells

Prokaryotes are smaller than eukaryotes, and multiply more rapidly than eukaryotes.

Bacteria can be of four basic shapes

• bacillus (rod like)
  
• coccus (spherical)
  
• vibrio (comma shaped)
  
• spirillum (spiral)
  

But still the organization of all prokaryotic cells is similar.

All prokaryotes have

• cell wall surrounding cell membrane (forming the cell envelope)
  
• cytoplasm
  
• no well defined nucleus (naked genetic materials).
  Bacteria have a small circular DNA outside the genomic DNA (the single chromosome/circular DNA). These smaller DNA are called plasmids. The plasmid DNA decides phenotypic (observable) characters of the bacteria such as resistance to antibiotics. It is also used for monitoring bacterial transformation with foreign DNA.
  
• Ribosomes are the only organelles of eukaryotes that are present in prokaryotes too.
  
• Inclusions
  
• Mesosome (specialised differentiated form of cell membrane – infoldings of cell membrane)

An overview of cell

Eukaryotes have membrane bound distinct strctures called organelles like endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.

The non-membrane bound organelles found in eukaryotes and prokaryotes are Ribosomes (found in cytoplasm, chloroplasts (in plants), mitochondria, rough ER)
The non-membrane bound organelle found in animal cell is centriole which helps in cell divison
(so, animal cell has 2 non-membrane organelle, plant cell -1)


Cells differ in size, shape, activities

Bacteria could be 3 to 5 µm.
Mycoplasmas the smallest cells are only 0.3 µm long. ("Mycoplasmas the smallest cells are only 0.3 µm (micrometre) long.") (1 µm = 0.000001 m = 1×10⁻⁶ m)
The egg of an ostrich is the largest isolated single cell

RBC = 7.0 µm.
Nerve cells are the looooooooooooooooooooooonnnnnnnnnnnnngest cells

There may be disc like, polygonal, columnar, cuboid, thread like or even irregular cells. The shape vary with function

Contributions of Different Scientists

Anton von Leeuwenhoek first observed and described a live cell. (Robert Hook observed cork cell which was dead)
Robert Brown discovered the nucleus.

And all this and future discoveries were only due to the invention of microscope or anything in between simple microscope and the latest electron microscope


Matthias Schleiden (German botanist) examined plants and saw that all plants were composed of different cells that form tissues. (Sch'l'eiden and P'l'ant) (1838)


Theodore Schwann (British as in text, actually German zoologist) studied animal cells and found that cells had a thin outer layer (which is today called plasma membrane). He observed plant cells and found that cell wall was a unique character of plant cells. (1839) (Schw'ann' and 'an'imals)

Thus was put forward the hypothesis that the bodies of plants and animals are composed of cells and cell products.
Thus, Schleiden and Schwann together formulated the cell theory. But didn't explain how new cells were formed.

Rudolf Virchow (1855) explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified S&S's cell theory to:

• all living organisms are composed of cells and products of cells
• all cells arise from pre-existing cells
  

Difference between Unicellular and Multicellular Organisms

Unicellular	Multicellular
Consists of only one cell	Consists of groups of cells
The single cell performs all the functions of the organism	Each group of cells perform a particular function and they get organized into tissues, organs, organ systems, etc.
Eg: amoeba, bacteria	Eg: plants, animals

Cell: The Unit of Life

Cell - the structural and functional unit of life - called so because it gives structure to the organism and performs the functions of the organism.

Anything that consists of a cell at least (unicellular organisms) are capable of:

• independent existence and
• perfoming the essential functions of life

And anything less does not ensure these. So, cell is the structural and functional unit of all living organisms.

The difference between the living beings and non-living is the cell.

All organisms are made up of cells and based on the number of cells there are:

• Unicellular Organisms
• Multicellular Organisms