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Organization of Cell

The Basic Structural and Functional Unit

What is a Cell?

A fundamental, structural and functional unit of all living organisms

Anton von Leeuwenhoek first described the live cells.

Cell theory:

All living organisms are composed of cells and products of cells.

All cells arise from pre-existing cells.

Smallest cell: Mycoplasma (0.3 μm)

Largest cell: Ostrich egg

Longest cell: Nerve cell

Prokaryotic Cells

Represented by bacteria, blue-green algae, PPLO and Mycoplasma

Smaller and rapidly multiplying

Vary greatly in shape and size

Characteristic features:

Have cell wall surrounding the cell membrane

Absence of a well-defined nucleus

May have plasmids − small, circular, extra-chromosomal DNAs present in addition to the genomic DNAs; this confers characteristics like antibiotic resistance to bacteria, and help in bacterial transformation with foreign DNA.

Absence of organelles (only ribosomes are found in prokaryotes)

Have mesosomes (specialised differentiated cell membranes); these are infoldings of the cell membrane

Have inclusion bodies


Cell Envelope in Prokaryotes

Cell Envelope − Three-Layered Structure

Glycocalyx (Outermost): May be present in the form of loose sheath called slime layer in some bacteria, or as a thick and tough capsule in others

Cell wall (middle): Determines the shape of a cell and provides a strong structural support

Plasma Membrane (innermost): Semi-permeable and structurally similar to that of eukaryotes


Formed by the extension of the plasma membrane into the cell

These extensions are made up of vesicles, tubules and lamellae.

Functions: Cell wall formation; DNA replication and distribution; respiration and secretion processes; increase surface area of plasma membrane and enzymatic content

Prokaryotic Cell may be − Motile or Non-Motile

Motile: Have flagella

Non-motile: Lack flagella

Flagella has three parts: Filament, Hook, Basal body

Pili (tubular structures made of proteins) and fimbriae (bristle-like fibres) are also present along with flagella, but their function is attachment (to the substratum or the host cell).

Depending upon the cell envelope, bacteria are of two types:

Gram positive − take up gram stain

Gram negative − do not take up gram stain


Associated with plasma membrane

Made up of two subunits: 50S and 30 S = 70S

Site of protein synthesis

Polysome: When several ribosomes attach to a single mRNA

Help in the translation of mRNA into proteins

Inclusion Bodies

Reserve material in prokaryotic cells is stored in the form of inclusion bodies.

Suspended freely in the cytoplasm, e.g., phosphate granules or glycogen granules

Gas vacuoles: found in blue-green, purple and green photosynthetic bacteria

Let us find out some more differences between gram positive and gram negative bacteria.  

Gram Positive Bacteria Gram Negative Bacteria They retain the crystal violet stain during gram staining. They do not retain the crystal violet stain during gram staining. They lack the outer membrane. They possess the outer membrane. Peptidoglycan layer in cell wall is thick and multilayered. Peptidoglycan layer in cell wall is thin and single layered. Cell wall is smooth and is around 20-80 nm thick.  Cell wall is wavy and is around 5-10 nm thick. Cell wall contains very less lipid content. Cell wall contains high lipid content. Very few bacteria are pathogenic in nature. Most of the bacteria are pathogenic in nature. Examples include Bacillus, Streptococcus, Clostridium, etc. Examples include Chlamydia, Pseudomonas, Salmonella, etc.



Every cell has a nucleus, except some such as the RBCs of mammals and the sieve tube cells in vascular plants.

A cell usually has one nucleus, except some variations.

Nucleus is bound by a nuclear envelope which consists of two membranes with perinuclear space (10 − …

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