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Cell Organelles
LS1101 : Introduction to Biology
Anindita Bhadra
E-mail: abhadra@iiserol!ac!in
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Cells "nder a #icrosco$e
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A %enerali&ed Cell
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The cell/plasma membrane is the thin nearly invisible structure that
cytoplasm of the cell.
It also connects the endoplasmic reticulum, and the nuclear membr
In eukaryotic cells, organelles are bounded by plasma membranes.
The cell membrane encloses the cell, defines its boundaries, and maessential differences between the cytosol and the extracellular envir
All biological membranes have a common general structure: each i
film of lipid and protein molecules, held together mainly by noncov
interactions.
'he Cell #embrane
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'he Cell #embrane
ell membranes are structures, and most
molecules are able to
in the plane of the m
The (luid mosaic mmembrane was first
!. ". !inger and #. $
&'().
*hospholipids
*roteins*hospholipids
*roteins
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'he )luid #osaic #odel
+ydrocarbon tai
*hosphate group
double bonded o
epending on the temperature and lipid composition, phospholipid
transition between a -gel or more solid state0 and a li1uid or mor
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In a living organism, the composition of the phospholipid bilayer i
membrane is generally maintained in a li1uid state, which maximi3
proteins and the dynamic capabilities of the membrane.
The lipid molecules are arranged as a continuous double layer abou
The lipid bilayer serves as a relatively impermeable barrier to the p
water2soluble molecules.
5loating around in the cell membrane are different kinds of protein
generally globular proteins. They are not held in any fixed pattern
float around in the phospholipid layer.
'he )luid #osaic #odel
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'y$es o( *roteins:
&. Integral: trans2membrane proteins6
span the hydrophobic interior
a. hannels
b. arriers
). *eripheral: not embedded6 attached
to the surface
a. 7n3ymatic activity
b. !tructure
)unctions o( membr
&. Transport
). 7n3yme
8. 9eceptor sites
. Intercellular ;unctio
4. ell2cell recognitio
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arbohydrates c
associated with t
surface of the m
Cholesterol2 sti
membrane by co
phospholipids
%lycoli$ids2 sig
%lyco$roteins2
attached chain o
antibodies0
Other #olecules
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Channel *roteins:form small openings for
diffuse through
Carrier *roteins:regulate transport and dif
+ece$tor *roteins:molecular triggers that s
responses such as release of hormones or op
channel proteins0
Cell +ecognition *roteins:=arker proteins
the cell to other cells
En&ymatic *roteins:carry out metabolic re
#embrane *roteins
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Carrier $roteins are peripharal protein
not extend all the way through the mem
bond and drag molecules through the b
and release them on the opposite side,
'rans$ort *roteins
Channel $roteins extend through the bilipid layer.
They form a pore through the membrane that can
move molecules in several ways, including diffusion,
which re1uires no energy.
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Sym$orts also use the process of diffusi
a molecule that is moving naturally into diffusion is used to drag another molecu
5or example, glucose hitches a ride with
'rans$ort *roteins
!ome proteins actively use energy from the A'*sin the cell to drag
from area of low concentration to areas of high concentration work
against diffusion0 an example of this is the sodium/potassium pump
energy of a phosphate shown in red0 is used to exchange sodium a
potassium atoms.
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#arer *roteins
They play an important role in organ transplants. If the marker prot
transplanted organ are different from those of the original organ the
re;ect it as a foreign invader.
#arer $roteins extend across the ce
and serve to identify the cell.
The immune system uses these protei
friendly cells from foreign invaders.
They are as uni1ue as fingerprints.
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Endocytosis
An important group of peripheral membrane proteins are water2so
en3ymes that associate with the polar head groups of membrane ph
>ne well2understood group of such en3ymes are the phospholipas
hydroly3e various bonds in the head groups of phospholipids.
These en3ymes have an important role in the degradation of damacell membranes.
$arge molecules that are manufactured in the cell are released thro
membrane by the reverse process, called e,ocytosis.
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The cell membran
engulf structures ttoo large to fit thro
pores in the memb
This process is kn
endocytosis.
In this process the
itself wraps around
and pinches off a v
the cell.
Endocytosis
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Cell Organelles
LS1101 : Introduction to Biology
Anindita Bhadra
E-mail: abhadra@iiserol!ac!in
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'he Cell )actory
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'he Cell )actory
The cell membran
boundary to the cregulates what en
the cell.
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'he Cell )actory
The cytoplasm or
the factory floor, all the organelles.
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'he Cell )actory
The cytoskeleton is re
maintaining the shapethe beams and pillars o
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Cytoseleton
The cytoskeleton is a system of protein filaments in the cytoplasm
cell that gives the cell shape and the capacity for directed movemen
Intermediate (ilamentsprovide mechanical strength.
#icrotubules determine the positions of membrane bound organel
intracellular transport.
Actin (ilaments determine the shape of the cell?s surface and are n
whole2cell locomotion.
A large number of accessory $roteins are associated with these fil
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Cytoseleton
7ach type of cytoskeletal filament is constructed from smaller prot
The cell is able to build large cytoskeletal structures by the repetiti
large numbers of the small protein subunits.
@ecause these subunits are small, they can diffuse rapidly within cy
whereas the assembled filaments cannot.
In this way, cells can undergo rapid structural reorgani3ations, disa
filaments at one site and reassembling them at another site far away
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Cytoseleton
7ach type of cytoskeletal filament is constructed from smaller prot
The cell is able to build large cytoskeletal structures by the repetiti
large numbers of the small protein subunits.
@ecause these subunits are small, they can diffuse rapidly within cy
whereas the assembled filaments cannot.
In this way, cells can undergo rapid structural reorgani3ations, disa
filaments at one site and reassembling them at another site far away
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'he Cell )actory
The endoplasmic retic
assembly line of the ce
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Endo$lasmic +eticulum
The 79 consists of membranous channels that wind through the c
It is the transport network for molecules targeted for certain modi
specific final destinations, as opposed to molecules that are destin
freely in the cytoplasm.
There are two types of 79, rough and smooth. 9ough 79 has riboto it, and smooth 79 does not.
9ough 79 produces proteins, smooth 79 produces steroids.
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'he Cell )actory
The ribosomes are the wo
on the assembly line of th
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+ibosomes
The ribosomes are the sites of protein synthesis in the cell.
They are mostly attached to the 79, but some are also free, in the
A ribosome, formed from two subunits locking together, functions
1. 'ranslateencoded information from the cell nucleus provided
ribonucleic acid m9%A0,/. Lin together amino acids selected and collected from the cyt
transfer ribonucleic acid t9%A0,
. E,$ortthe polypeptide produced to the cytoplasm where it w
functional protein.
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The proteins produced by the free ribosomes are mostly used with
while those produced by the ribosomes on the 79 are transported
In a mammalian cell there can be as many as 10 million ribosome
!everal ribosomes can be attached to the same m9%A strand, this
called a $olysome.
9ibosomes have only a temporary existence. hen they have syn
polypeptide the two sub2units separate and are either re2used or br
+ibosomes
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9ibosomes can ;oin up amino acids at a rate of )BB per minute.
9ibosomes are organelles composed of ribosomal proteins ribopr
ribonucleic acids ribonucleoproteins0.
A eukaryotic ribosome is composed of ribosomal 9%A and about
and has a molecular mass of about ,)BB,BBB a.
9ibosomes are found in prokaryotic cells, eukaryotic cells, chloro
mitochondria.
+ibosomes
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There are about &B billion protein molecules in a mammalian cell
produce most of them.
The proteins and nucleic acids that form the ribosome sub2units ar
nucleolus and exported through nuclear pores into the cytoplasm.
The two sub2units are une1ual in si3e and exist in this state until ruse.
The larger sub2unit has mainly a catalytic function6 the smaller su
decoding one.
+ibosomes
'h C ll ) t
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'he Cell )actory
The #olgi apparatus is th
packaging department of
% i A
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'he %olgi A$$aratus
The #olgi apparatus looks similar to the 79. It is a set of ( or C fl
saccules between the 79 and the cell membrane.
It is involved in the formation of lysosomes and other en3yme2con
cellular inclusions, and in the formation of secretory granules in c
#olgi apparatus acts as a condensation membrane for the concentrproducts produced elsewhere into droplets or granules by losing w
transported to the cell surface for export.
The principal function of #olgi complex is secretion.
'h % l i A t
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#olgi apparatus has been shown to be a great intracellular centre o
formation in some cell types.
#olgi body in endocrine cells helps in secretion of hormones.
#olgi bodies of plant cells synthesi3e all polysaccharides such as
hemicellulose and microfibrils of a2cellulose. These are packagedsecretion.
#lycoproteins are formed in the #olgi complex by the attachment
carbohydrate to the protein products of the endoplasmic reticulum
'he %olgi A$$aratus
'he Cell )actor
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'he Cell )actory
Desicles in the cell act as
factories.
i l
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esicles
Desicles are double membrane bound sacs carrying different produ
part of the cell to another.
They are often associated with the #olgi apparatus.
They also carry food, en3ymes, waste products.
They are involved in storage, transport and phagocytosis.
$ysosomes and peroxisomes are two kinds of vesicles found in th
'he Cell )actory
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'he Cell )actory
The =itochondria are the
of cells.
#it h d i
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#itochondrion
The mitochondrion is the site of cellular res$iration.
The mitochondrion is a double membranebound structure filled
matri,.
The outer membrane is selectively permeable, ;ust like the cell me
The inner membrane is folded inward to provide extra surfaces fo
respiration.
These folds are known as cristae.
#itochondrion
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#itochondrion
The inner membrane is freely permeable only to oxygen, carbon d
water.
The inter2membrane space has an important role in the primary fu
mitochondria, which is oxidative phosphorylation.
The matrix contains dissolved oxygen, water, carbon dioxide, the intermediates that serve as energy shuttles, en3ymes etc.
=itochondria produce adenosine triphosphate AT*0 by systemati
extracting energy from nutrient molecules substrates0.
'he Cell )actory
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'he Cell )actory
The vacuoles are storage
cell.
acuole
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acuole
Dacuoles serve a variety of functions, including storage.
They have an irregular shape and are usually very large in plant ce
8B E 'BF of the cell volume.
They contribute to the rigidity of the plant using water to develop
pressure.
They also store nutrient and non2nutrient chemicals and break dow
molecules.
'he Cell )actory
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'he Cell )actory
The nucleus is the seat of con
2ucleus
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2ucleus
=ost eukaryotic cells contain a nucleus, which is seen under the m
dark mass inside the cell.
The nucleus is surrounded by a double membrane, known as the n
en3elo$e.
A fluid2filled space or $erinuclear s$ace is present between the twnuclear membrane.
The nuclear envelope is a highly regulated membrane barrier that
compartmentalisation of the nucleus from the cytoplasm.
2ucleus
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2ucleus
2uclear $ore com$le,es %*s0 are large proteinaceous channel
the nuclear membrane and allow transport of molecules in and ounucleus.
2ucleo$lasm, also known as karyoplasm, is the matrix present in
nucleus.
#enes are located in chromosomesinside the nucleus.
The nucleus is the site of %A re$licationand transcri$tion.
2ucleus
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2ucleus
The %A is transcribed into pre messenger 9%A pre m9%As0 in
nucleus. The mature m9%A after splicing is exported to the cytoptranslation.
The clearest substructure in the nucleus in most eukaryotes is the
which is the site of r%A transcription and ribosome biosynthesis
The nucleolus is the largest structure in the cell nucleus.
The nucleolus has numerous other functions including assembly o
recognition particles, modification of transfer 9%As and sensing c
2ucleus
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2ucleus
The nucleolus disappears when a cell undergoes division and is re
the completion of cell division.
2uclear s$ecles are highly dynamic, irregularly shaped nuclear
enriched with pre m9%A splicing factors.
>ne of the earliest ideas about the evolution of the eukaryotic cell
organelles formed when the plasma membrane grew into the cell a
walling off parts of the cell. This idea, called the autogenic hy$ot
correctly explain the origin of the nuclear membrane, endoplasmi
and #olgi apparatus.
2ucleus
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2ucleus
7xceptions to the rule:
#ultinucleated cells: =ammalian skeletal muscle cells6 plasmod
molds6 metastati3ing tumor cells.
Enucleated cells: =ammalian red blood cells.
olly was born from an enucleated oocyteG
Chromosomes
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Chromosomes
hromosomes are packaged forms of %A
inside the nucleus.
hromosomes are not visible in the cell?s
nucleusHnot even under a microscopeHwhen
the cell is not dividing.
=ost knowledge of chromosomes comes from
observations of %A during cell division.
The haploid human genome contains approximately 8 billion base
packaged into )8 chromosomes
Chromosomes
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Chromosomes
7ach chromosome has a constriction point
called the centromere, which divides the
chromosome into two sections, or -arms.
The short arm of the chromosome is labeled
the -p arm. The long arm of the
chromosome is labeled the -1 arm.
The location of the centromere on each
chromosome gives the chromosome its
characteristic shape, and can be used to help
describe the location of specific genes.
Chromosomes
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4istone $roteinsare involved in packaging chromosomal %A in
microscopic space of the eukaryotic nucleus.
The histone2%A complex is known as chromatin.
+istones are a family of small, positively charged proteins termed
+)@, +8, and +. %A is negatively charged, and thus binds to h
The basic repeating structural and functional0 unit of chromatin i
nucleosome, which contains eight histone proteins and about &<
%A
Chromosomes
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4istone $roteinsare not present in most prokaryotes.
*rokaryotes package their %A by supercoiling.
=ultiple proteins act together to fold and condense prokaryotic
particular, one protein called +, which is the most abundant prot
nucleoid, works with an en3yme called topoisomerase I to bind %
introduce sharp bends in the chromosome, generating the tension negative supercoiling.
>nce the prokaryotic genome has been condensed, %A topoisom
gyrase, and other proteins help maintain the supercoils.
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