cell structure v

8/14/2019 CELL STRUCTURE V

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Membrane compartments of theMembrane compartments of the

cellcell

CytoskeletonCytoskeleton

Svetlana Lutsenko, Dept. Biochemistry and Mol. BologySvetlana Lutsenko, Dept. Biochemistry and Mol. Bology

MRB 624, Ext. 4-6953, [email protected] 624, Ext. 4-6953, [email protected]


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Reading for the Cytoskeleton portion:

Alberts Essential Cell Biology pp.513-532


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1. Transport of various molecules,including proteins, DNA and RNA

through large protein-lined pores.

Specificity of transport is based on

size exclusion

3. Simple diffusion and protein-mediated transport of ions and small

molecular weight nutrients, such an

amino-acids and sugars

5. Vesicular transport is mediated by

membranes that bud off as a vesiclesfrom their original compartment and

carry out specific cargo (ions,

proteins, neurotransmitters) to

another compartment

Mechanisms by which molecules can pass through membranesMechanisms by which molecules can pass through membranes


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Cellular Compartments and TheirCellular Compartments and Their

FunctionsFunctions

Cytosol ::many biosynthetic pathway;synthesis of cytosolic protein

Nucleus :: DNA and RNA synthesis

Mitochondria :: energy metabolism,urea cycle

Endoplasmic Reticulum: ProteinSecretion; Synthesis of Membrane

Proteins

Golgi :: Distribution and Modification of

Secreted Proteins

Lysosomes :: Degradation of proteinsand other molecules

Peroxisomes: Degradation of CertainFatty Acid


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NucleusNucleus

The nucleus is a membrane

bound structure that

contains the cell's hereditary

information and controls the

cell's growth and

reproduction.

It is commonly the mostprominent organelle in the

cellhttp://biology.about.com/library/weekly/aa032300a.htm
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.brigadoon.com/%7Eschaffer/biology/nucleus.htmlhttp://biology.about.com/gi/dynamic/offsite.htm?site=http://www.brigadoon.com/%7Eschaffer/biology/nucleus.html


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Nuclear MembranesNuclear Membranes

The nuclear content is located in thenuclear lumennuclear lumen and is surrounded by a

double membrane ornuclear envelopenuclear envelope,

composed ofinner membraneinner membraneandouterouter

membranemembrane.

The outer membrane is contiguous with

the ER

The nuclear membrane contains nuclearnuclear

porespores, which provide selective access into

and out of the nuclear lumen

The inner membrane has a protein lining

called the nuclear lamina, which binds to

chromatin and other nuclear components.


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The Nuclear Pore ComplexesThe Nuclear Pore Complexesform aform a continuous aqueous channel between cytoplasm & nucleoplasm

-30 different proteins found in pore; basic subunit

repeated 16 times. Central granule" now called

"transporter" functions to move molecules through

pore. There is a ring of proteins that anchors pore

to N.E. and the "basket" of fibers with unknown

function

-no obvious motor proteins were found

Proposed mechanism -transport factors with cargo destined to pass through nuclear pore bind to

pore increasing their local concentration. Higher local concentration allows diffusion across the

diffusion barrier channel

nuclear pore complex is

large about 120 million

Daltons


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The NucleolusThe Nucleolus

The region of the nucleus where portions of chromosomes that contain genescoding for ribosomal RNA are transcribed and ribosomal subunits are

assembled

http://biology.about.com/library/weekly/aa032300a.htm

Stretch of DNA with rRNA genes

nucleolar organizing region =

(NOR)

Ribosomal proteins are synthesized

in cytoplasm and transported into

the nucleus

These proteins self associate withappropriate rRNA during rRNA

synthesis forming immature

ribosomal subunits

Ribosomes finish self assembly in

cytoplasm


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Endoplasmic ReticulumEndoplasmic Reticulum


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The Endoplasmin Reticulum (ER) is an extensive,The Endoplasmin Reticulum (ER) is an extensive,

extra-nuclear membrane system with the followingextra-nuclear membrane system with the following

functions:functions:

ER is a home for various enzymes involved in protein folding , drug

detoxification, membrane lipid biosynthesis, cholesterol and fatty acidmetabolism

ER is an entry point for protein sorting. Targeting of these proteins is

mediated by signal sequence. The membrane proteins are inserted into

the ER membrane in their proper orientation. Secreted proteins aretranslocated into ER lumen and then transported to the destination

place


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The rough ER manufactures membranes and secretory proteins.

The rough and smooth ER are usuallyinterconnected and the proteins andmembranes made by the rough ER move

into the smooth ER to be transferred toother locations.

In leukocytes the rough ER produces antibodies.

In pancreatic cells the rough ER produces insulin.

The cytoplasm has a reducing environment,

while ER lumen is oxidizing . Thisdifference is generated by unequal

distribution of trypeptide glutathione and

is essential for formation of disulfide bonds

in proteins and for proper folding


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MitochondriaMitochondria


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Mitochondria (singular: mitochondrion) are the sites of

aerobic respiration, and generally are the major energy

production center in eukaryotes

The number of mitochondria range from one to thousands per cells.

They are often positioned in cells nearest to sites of energy utilization

One of the richest sources of mitochondria is a hummingbird flight

muscle


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Mitochondria are a double membrane organelle in which the

inner membrane is in-folded to form cristae.

The outer membrane is a fairlysimple phospholipid bilayer,

containing porins,proteins that

render it permeable to molecules

of about 10 kilodaltons or less.

Ions, nutrient molecules, ATP,ADP, etc. easily pass through the

outer membrane and enterthe

intermembrane space

The inner membrane is more

complex and contains respiratorychains and transporters

The matrix lies within the inner membrane. The access to this compartment

often requires specific transporters


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Four possible localization for mitochondrial enzymes


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The Golgi ComplexThe Golgi Complex


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Located near cell nucleus, consists of flattened,

membrane-bounded sacs (cisternae) forming a stack

Each stack has:

cis-face is an entry face - adjacent to ER to acceptincoming vesicles

trans-face is an exit face points towards plasma membrane, produces vesicles for

forward flow


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The function of the Golgiis to transport and processsecreted and membrane proteins from ER to the cell surface

Cis removal of mannose,

phosphorylation

Medial removal of mannose,

addition of N-acetylglucosamine

Trans Removal of galactose,

addition of sialic acid

TGN addition of sialic acid,

Sorting

Cisternae segregated intoconvex ("cis"), medial

(middle), and concave

("trans") compartments.

ER

cys

medial

trans

TGN


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Vesicular TransportVesicular Transport


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The Endocytic PathwayThe Endocytic Pathway

Endosomes and LysosomesEndosomes and Lysosomes


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LysosomesLysosomes

Lysosomes are active in recycling the cell's organic material and in the intracellular

digestion of macromolecules.Lysosomes contain various hydrolytic enzymes that are capable of digesting nucleic

acids, polysaccharides, fats and proteins.The inside of a lysosome is acidic.In humans, a variety of inherited conditions can affect lysosomes. These defects are

called storage diseases and include Pompe's disease and Tay-Sachs disease. People with

these disorders are missing one or more of the lysosomal hydrolytic enzymes.
http://www.pompe.org.uk/http://www.pompe.org.uk/http://www.ninds.nih.gov/patients/disorder/taysachs/taysachs.htmhttp://www.ninds.nih.gov/patients/disorder/taysachs/taysachs.htmhttp://www.ninds.nih.gov/patients/disorder/taysachs/taysachs.htmhttp://www.ninds.nih.gov/patients/disorder/taysachs/taysachs.htmhttp://www.pompe.org.uk/http://www.pompe.org.uk/


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The CytoskeletonThe Cytoskeleton


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Cytoskeleton is a network of protein

filaments in the cytoplasm

Main functions:

Supports large volume of the cytoplasm

Participates in large-scale movements associated

with the changes in cell shape and cell motility

Provides machinery for organelle transport,

chromasome segregation during mitosis, and celldivision

M j t f t k l tM j t f t k l t


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Actin filaments Microtubules

Intermediate filaments

Major components of cytoskeletonMajor components of cytoskeleton


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The cytoskeletal filamentsThe cytoskeletal filaments

Common Features :

Linear polymers of protein subunits

Actin ( ~8 nm in diameter)

Intermediate Filaments ( ~10 nm in diameter)

Microtubules ( ~24 nm in diameter)

Filaments are dynamic, i.e. they can assemble

and disassembleHighly conserved


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Intermediate Filaments

Intermediate filaments enablecells to withstand mechanical

stress when cells are stretched.

They can span the entire

cytoplasm and are anchored to

the plasma membrane.


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The Microtubule CytoskeletonThe Microtubule Cytoskeleton

Also penetrates the entire

volume of the cell

Whereas actin fibers areconcentrated at the periphery,

most microtubules radiate from

a central location in the cell

Main functions: intracellulartransport and mitosis


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Microtubules provide anorganizational structure in aninterphase cell and separatechromosomes in a dividing cell

http://www.circs.neu.edu/external/Frank.Gibbons/spindle.html

MicrotubulesMicrotubules


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Microtubules Provide Tracks for Transport


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MicrotubulesMicrotubules are long hollow cylinders made oftubulin

Protofilaments are linear chains of tubulin

dimers, a parallel bundle of 13 protofilaments

forms a microtubule

There are three kinds of tubulins, each with

many subtypes:

-tubulin-tubulin and -tubulin-tubulin form / tubulindimers and represent the basic building block of

microtubules

-tubulin-tubulin is involved in more specializedprocesses, such as nucleation

Microtubules have a GTP cap stabilizing the

ends.


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Motor

Proteins

Motor proteins bind to

microtubules and move

by cycles of

conformational changesusing energy from ATP.

One end of the proteincan bind to specific

cellular components.


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Actin is the most common protein in

the cytoplasm

Actin filaments are concentrated

beneath the plasma membrane and

give the cell mechanical strength

Assembly of actin filaments can

determine cell shape and cause cell

movement

Association of actin filaments withmyosin can form contractile structures

Actin filaments = microfilamentsActin filaments = microfilaments

S i


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SpectrinSpectrinPrincipal component of the cytoskelton(protein meshwork underlying surface ofthe red cell)

Maintains structural integrity of the redcell (e.g. biconvave shape)

Long thin flexible rod

Necessary as red cells go through smallcapillaries

cell structure v

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