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CELL THE UNITAll living
An
organisms possesslte
organism consists
and are made
of one or more
OF LIFE
up ot basic unit
structure called cell.
cells.
Accordingly there are two types of
(i) organisms . () Unicellular
Multicellular organisms e.g..plants. animals etc. organisms e.g Amoeba, Diatomse
WHAT IS A CELL? ns etc
Unicelluiar organisms are
capable of
) Independent existence
(6)Performing the essential functions of life.
Anything less than a complete structure of a cell
does not ensure
Thus, cell is the
fundamental structural and independent living.
Robert Hooke studied and functional unit of alliving organisms.
discovered the cell from
thin slice of cork
a
Anton Von but that was the
Leeuwenhoek was the first person who observed few 'dead cell.
protozoa, spermatozoa and red blood living cells capable of
coipuscles under
CELL THEORY his own
designed microscope. moving.
such as
bactena
In 1838, Matthias
Schleiden, a German botanist studied a
composed of different kinds of cells which form the large number of
plants and observed that
tissues of the plant. all plants are
At abeut the
same tirne, another
Scientist Theodor
of animal cells and schwann (1839), a British
reported that cells had a thin outer
layer which is now
zoologist, studied different
types
He also
conciuded, based on his studies
known as the
Plasma membrane'.
on piant tissues, that the presence of cell
the plant cels.
wall is a
unique character of
On the basis of
this, he proposed a
their products. hypothesis that the bodies of plants and animals are
composed of cells and
Schleiden and Schwann together formulated
the cell theory.
This theory however, did not explain as to
hownew cells were formed.
Rudolf Virchow (1855) first
explained that cells divided and new cells are
(Omnis cellula-e cellula). He modified the formed from
pre-exIsting
hypothesis of Schleiden and Schwann to give cell theory a final
ceis
Celi theory is understood as snape
) All iving organisms are
composed of celis and products of cells.
) All cells arise from
pre-existirng cells.
(m) Actvites of an organism are the
outcome of sum total of activities and
interactions of ts construen oei
Viruses exception
are an to cell
theory as they are not made up of cells.
They are composed of nucleoprotein partidles.
Therefore, they are not considered either living or non-living.
,AN OVERVIEW OF CELL
Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus
are prokaryotic.
In both prokaryotic and eukaryotic cells, semi-Aluid matrix clled cytoplasm occupies the volume of the cell
The cytoplasm is the main arena of cellular activities in both the plant and animai cells.
Besides nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the
Endoplasmic Reticulum (ER), the Golgi complex, ysosomes, mitochondria, microbodies and vacuoles.
The prokaryotic cells lack such membrane-bound organelles.
Ribosomes are non-membrane bound organelles found in all cells-both eukaryotic as well as prokaryotic.
Within the cell, ribosomes are found not onily in the cytoplasm but also within the two organelles - chloroplasts
(in plants) and mitochondria and on rough ER.
Animal cells contain another non-membrane bound organelle called centriole vhich helps in cell dVISIon.
Cells differ greatly in size, sthape and activities For example
Size
) Mycoplasma (Smallest c e l l ) - 0.3 um in length
(i) Bacteria 3-5 um
(im) Human RBC umi in diameter
The largest isolated single cel! egg of ostrich.
Nerve cells are some of trhe longest cells.
Cofuinar epithelikil cells
Vhite blood cells
tlong and narrow)
Red blood cclls
(round and bicoicave
amoeboic
(Branched and long
Mesopliyl cells
(round atd oval)
A trachetd
(elongated)
Figure: Diagram shewing ilcrent shapcs of the cclls
, PROKARYOTIC CELLS
PPLO(Pleuropneumonia
aryoic cells are represented by bacteria, blue-green algae, wycopzsna and PPLO (Pleuropn
like organi sms
ey are generally smaller in size and multiply much faster ithan the eukaryolic cells.
y ay vary greatly in shape and size but exhibit a similar Dasic celuar Organisation
Typleal bacteria
2Jum
PPLO
faloul 0. I jpm)
Viruscs
A ty]ical eukaryotl¢ cell
10-20 jum (0.02-0.2 un)
Figure: Diagram shOwiing comparison of cukaryotic cell with other oxgaisms
Most prokaryotes have a cell wal Surrounaing the cell membrane.
Except mysoplasma iithout cel wall and known as Joker of olant kingdom). (MycepXalma
Thefiuid matrix filling the cellis the cytoplasm.
There is nO well-defined nuclEus
The genetic material is basically naked. rot enveloped by a nuclear membranE. kido
ln addition to the
genomic DNA (the single chromos ome/circular DNA), many bacteria have small.circularDNA
Outside the genomic DNA
These smaller DNA are cailed piasmios
The plasmid DNA confers certain unique pher.otypic characters to such bacteria.
one such characteris resistance to antibiotics,
Theplasmid DNAisused to moniior bacterialtransformation withforeign DNA.
Cell Envelope and its Modificaticns
Most prokaryotic cells, particularly the bacterial cells, have a chemicaliy complex cell envelope.
The cell envelope consists of a tightly bound three-layered structure ie., the outermost glycocaiyx followed by the
cell wall and trie plasma membrane.
Glycocalyx is the outermostlayercomprisingacoating of mucous or polysaccharides inacromolecules, which
protects the cells and alsc helps in adhesion.
This layer differs in thickness and chemical composition in different bacteria.
Some have a loose sheath called slime layer, which protects the cell from loss of water and nutrients
Others may have a thick and tough covering known as capsule.
The capsule and slime layer are made up of polysaccharides, but may sometimes contain proteins
also
The capsule is responsible for giving gummy and sticky character to the cel.
It allows bacterium to hide from host's immune system.
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