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Cell
• Regarded as the basic structural and
functional unit of life.
• Cell is a mass of cytoplasm enclosed by a
plasma membrane consisting of certain
organelles besides the nucleus or nuclear
material, capable of self replicating.
Development of Cell Theory
The development of optical lenses and their
combination in compound microscope led to the
establishment of the cell theory.
In 17th Century
• Robert Hooke (1665) first observed honeycomb
like structures in a thin slice of cork and he called
them as ‘cells’.
• Leewenhock (1674) discovered free cells and
observed some organizations within cells,
particularly nucleus.
In 19th Century
• In 1838-39, Scheilden and Schwann both
postulated that the cell is the basic unit of
structure and function in a living organism.
• Schwann studied both animal and plant
tissues. His work with connective tissues (bone
and cartilage) led him to modify the evolving
theory by including the idea that living things
are made up of cells and its products.
• Virchow confirmed Nageli’s principle of
cellular basis of life’s continuity and stated in
latin “omnis cellula e cellula”.
• Loius Pasteur (1865) gave experimental
evidence to support Virchow’s extension of
the cell theory.
Cell Theory
The modern version of cell theory states that:
1. All living organisms (plants, animals & microbes)
are made up of one or more cells and cell
products.
2. All metabolic reactions in unicellular and
multicellular reactions takes place in cell.
3. Cells originate only from the pre existing cells by
division and duplication.
4. The smallest clearly defined unit of life is the
cell.
Compartmentalisation within the cells
• Many membrane bound compartments present
in the cytoplasm are the membrane bound
organelles.
• Part of cytoplasm excluding the compartments
are called cytosol.
• Microfilaments, centrioles, microtubules,
intermediate filaments & ribosomes are not
considered compartments.
• Lack of intracellular compartmentalisation shows
the primitive nature of the cell.
Types of Cells
• Cells are of two types Eukaryotic and Prokaryotic
• Based on
i. Organisation of membranes
ii. Variety of cytoplasmic membrane
iii. Complexity of nuclear region
• The terms were suggested by Hans Ris in 1960s.
Prokaryotic Cells
1. Most primitive single membrane cells,
2. Present in bacteria and blue green algae and
they are known as prokaryotes.
3. Comparatively small, 1-10 µm
4. Consists of three main coverings:Coverings
Outer
covering
Cytoplasm
Nuclear
region
Outer Covering
Cell Membrane
Cell wall
Slimy capsule
Cell Memrane
•
•
•
•
•
•
•
Thin and Flexible
Composed of lipids and proteins
Controls the movement of molecules in & out of the cell
In some bacteria mesosomes are present
Photosynthetic lamellae are present
Also thought to play a role in replication
Prokaryotic cell plan is not compartmentalised
Cell Wall
•
•
>
>
Range in thickness from 1.5-100µm
GRAM POSITIVE Bacteria
Single thick layer of peptidoglycan and teichoic acid
Peptidoglycan is the complex of oligosaccharide chains
and short peptides also called mucopeptide or murein
> Teichoic acid consists of glycerol or ribitol based
subunits linked by phosphate groups
• GRAM NEGATIVE Bacteria
Peptidoglycan
Periplasmic space
Outer membrane
> Three layers> Peptidoglycan layer is thin(25Å) &lies next to plama
membrane
> Periplasmic space contains proteins & lies outside
peptidoglycan
> Outer membrane consist of a lipid bilayer which is
permeable and contains porins.
Slimy Capsule
• Consists largely of polysaccharides but may
have polypeptides and other compounds also.
• It protects the cell against dessication,
antibiotics & virus attacks.
Cytoplasm
• Contains proteins, lipids, glycogen and inorganic
ions; enzymes for biosynthetic reactions, tRNA
and mRNA.
• Ribosome is the only cytoplasmic organelle found
in prokaryotic cell (20-30nm); forms
polyribosomes at the time of protein synthesis. It
is 70 S & composed of 23 S, 16 S & 5 S rRNAs and
50 different proteins
• Cytoplasm do not show streaming movements.
Phagocytosis, Pinocytosis & exocytosis do not
occur.
• May contain deposits of polysaccharides or
inorganic phosphates
Nucleoid
• Not enclosed by a nuclear envelope, such a nuclear
material is called nucleoid
• Single chromosome that is greatly coiled which shows
many special features:
₋ Short, simple & attached to cell membrane at 1 point
₋ Single copy of chromosome
₋ dsDNA, circular not associated with proteins
₋ Amount of DNA is 0.23 nm in length
₋ Transcription & translation occur in the cytoplasm;
mRNA does not require processing
₋ Absence of nucleolus & formation of mitotic
apparatus; cell membrane helps to segregate
replicated products.
₋ Prokaryotic chromosomes also called
prochromosomes.
Plasmids
• Small circular DNA besides nucleoid called
plasmids
• 1000-30,000 bps
• Generally code for non essential proteins
• Some encode proteins needed by organism to
resist antibiotics or other toxic materials
Flagella
• 150Å thick & 10-15µm long
Two main
parts
Filament
Basal
body
Filament
Composed of spiral chains
of subunits of protein
named flagellin
Basal Body
•Comprises of shaft & hook
•M ring is embedded in the
plasma membrane
Pilli
• Short, rod-like, non-motile processes called
pilli/fimriae
• Formed of protein pilin
• Usually less than 10 nm thick & under 1 µm
long
• Used for attachment to surfaces
Eukaryotic Cell
• Double membrane or two-envelope system; primary
membrane surrounds the cell and secondary
membrane envelope the nucleus and other
organelles
• SIZE: Range from 10-100 µm
size is determined by 2 main factors:
i.
requirement of nutrients and oxygen
ii. Regulating ability of its nucleus
Types of cells
Size
Sporozoites of Plasmodium
vivax
2 µm
Ostrich egg
175 X 120 mm
Acetabularia
100 mm
Human cells
20-30 µm
• SHAPE: spherical, oval, disc-like, cuboidal,
columnar, polygonal, spindle-like, irregular, etc.
o
Shape is often correlated with its function
o
Shape depends on:
i. Function
ii. Age
iii. Viscosity
iv. Cell wall
v. External pressure
vi. Internal and external skeleton
• NUMBER:
– Organisms which have their body made up of a
single cell are called unicellular/acellular
– Organisms composed of large no. of cells are
termed as multicellular organism
Eukaryotic
Cell
Cell
Membrane
Cytoplasm
Nucleus
Cell Membrane
• Thin, elastic, living covering called cell
membrane/plasma membrane/plasmalemma
• Gives form and shape to the cell & is
composed of a lipid-protein complex
• Controls the movement of particles through
the cell membrane by the processes
endocytosis and exocytosis
• Certain protists, fungi, all plant cells have thick
rigid non-living covering called cell wall
Cytoplasm
(cytoplasmic matrix/cytosol/hyaloplasm)
• It is a semi-fluid, homogenous, translucent
substance present b/w nucleus and plasma
membrane
• Often show streaming movements/cyclosis
• Contains two types of structures:
– Organelles
– Inclusions
Organelles
• Endoplasmic Reticulum: network of
membrane-bound channels whose main role
is to transport materials in the cell.
• Ribosomes: minute riboprotein particles lying
free or attached to ER & nuclear envelope. 80S
ribosomes composed of 28S, 5.8S, 5S & 18S
rRNAs and 80 different proteins. 70S in
mitochondria & plastids.
• Microfilaments: fine protein fibrils often found
in bundles in the cytoplasm. It plays a role in
all movements of the cell.
• Microtubules: fine proteins tubules present in
cytplasm, centrioles, cilia , flagella, nerve
fibres and mitotic spindle. They bring about
intra cellular movements.
• Intermediate filaments: solid, unbranched,
supporting fibres in the cytoplasm. They are
composed of proteins called keratin, desmin &
vimentin.
• Vacuoles: small, membrane-bound, bubble
like sacs contianing cell sap
• Golgi Apparatus: pile of flattened sacs often
near the centrosome and has mainly the
secretory role.
• Lysosomes: membrane bound organelle
comprising of enzymes for intracellular
digestion.
• Mitochondria: cylindrical vesicles filled with
fluid matrix which convert chemical energy
into biologically useful form. e.g. ATP
• Plastids: these are the fluid filled vesicles of 3
types- chloroplasts, lecoplasts &
chromoplasts.
• Centrioles: cylinder like organelles which help
arrange mitotic apparatus during cell division.
Inclusions
They are incapable of growth and considered as
non living.
• Starch Grains: present in plant cells only. They
are common storage organs.
• Aleurone Grains: protein materials deposited
in plant cells in the outer endosperm cells of
cereals.
• Glycogen Granules: polysaccharide particles
stored in liver & muscle cell of animals.
• Fat Droplets: present in fat storing
cells(adiposites) of animals, endosperm of
castor, coconut oil and cotyledons of
groundnut and mustard seeds.
• Inorganic Crystals: occur in plant cells. Crystals
of calcium & silica are common.
Nucleus
• Genetic material is covered in a nuclear envelope
having pores for exchange of particles.
• Chromosomes are long complex & have no connection
with the plasma membrane
• Two copies of each chromosome hence they are
diploid.
• DNA is ds, helical &linear having histone & non-histone
proteins
• Replication of DNA occurs only during the S phase of
the cell
• Transcription takes place in the nucleus and translation
in cytoplasm