1308 Cells Truc t Fun Ct

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Cell Structure and Function


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Cell Structure

In 1655, the English scientist Robert Hooke coined the

term cellulae for the small box-like structures he saw

while examining a thin slice of cork under a

microscope.


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Basic Cell Structure All cells have the following basic structure:

A thin, flexibleplasma membrane

surrounds the entire cell.

The interior is filled with a semi-fluid

material called thecytoplasm.

Also inside are specialized structures

called organelles and the cells genetic

material.


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Generalized Eukaryotic Cell


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Visualizing Cells


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Prokaryotic Cells

Simplest organisms

Cytoplasm is surrounded by plasma membrane and

encased in a rigid cell wall composed of peptidoglycan.

No distinct interior compartments

Some use flagellum for locomotion, threadlike structuresprotruding from cell surface


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Eukaryotic Cells

Characterized by compartmentalization byan endomembrane system, and the

presence of membrane-bound organelles.

central vacuole vesicles

chromosomes

cytoskeleton cell walls


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Animal cell anatomyAnimal Cell


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Membrane Function

All cells are surroundedby a plasma membrane.

Cell membranes arecomposed of a lipidbilayer with globularproteins embedded in thebilayer.

On the external surface,carbohydrate groups joinwith lipids to formglycolipids, and withproteins to form

glycoproteins. Thesefunction as cell identitymarkers.


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Fluid Mosaic Model

In 1972, S. Singer and G. Nicolson proposed the Fluid

Mosaic Model of membrane structureExtracellular fluid

CarbohydrateGlycolipid

Transmembraneproteins

Glycoprotein

Peripheralprotein

Cholesterol

Filaments ofcytoskeleton

Cytoplasm


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Phospholipids

Glycerol

Two fatty acids

Phosphate group

Hydrophilic

heads

Hydrophobic

tails

ECF WATER

ICF WATER


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Phospholipid Bilayer

Mainly 2 layers of phospholipids; the non-polar tails

point inward and the polar heads are on the surface.

Contains cholesterol in animal cells.

Is fluid, allowing proteins to move around within the

bilayer.

Polarhydro-philicheads

Nonpolarhydro-phobic

tails

Polarhydro-philicheads


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Steroid Cholesterol

Effects on membrane fluidity within

the animal cell membrane

Cholesterol


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Functions of Cell Membranes

Regulate the passage of substance

into and out of cells and between cellorganelles and cytosol

Detect chemical messengers arriving

at the surface

Link adjacent cells together bymembrane junctions

Anchor cells to the extracellularmatrix


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Outside

Plasmamembrane

Inside

Transporter Cell surfacereceptorEnzyme

Cell surface identity

marker

Attachment to the

cytoskeletonCell adhesion

Functions of Plasma Membrane Proteins


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Membrane Transport

The plasma membrane exhibits

selective permeability - It allows somesubstances to cross it more easily

than others


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Passive Transport

Passive transport is diffusion of a

substance across a membrane withno energy investment

4 types

Simple diffusion

Dialysis

Osmosis

Facilitated diffusion


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Solutions and Transport

Solution homogeneous mixture of

two or more components Solvent dissolving medium

Solutes components in smaller quantitieswithin a solution

Intracellular fluid nucleoplasm andcytosol

Extracellular fluid Interstitial fluid fluid on the exterior of thecell within tissues

Plasma fluid component of blood


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Diffusion Across a Membrane

The membrane has pores large enough for the molecules to pass

through.

Random movement of the molecules will cause some to passthrough the pores; this will happen more often on the side with more

molecules. The dye diffuses from where it is more concentrated to

where it is less concentrated

This leads to a dynamic equilibrium: The solute molecules continue

to cross the membrane, but at equal rates in both directions.

Net diffusion Net diffusion Equilibrium

Diff i A M b


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Diffusion Across a Membrane

Two different solutes are separated by a membrane that is

permeable to both

Each solute diffuses down its own concentration gradient.

There will be a net diffusion of the purple molecules toward the left,

even though the total solute concentration was initially greater on

the left side

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

Th P bilit f th Li id Bil


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The Permeability of the Lipid Bilayer

Permeability Factors

Lipid solubility Size

Charge

Presence of channels and transporters

Hydrophobic molecules are lipid soluble and canpass through the membrane rapidly

Polar molecules do not cross the membranerapidly

Transport proteins allow passage of hydrophilicsubstances across the membrane

P i T t P


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Passive Transport Processes

3 special types of diffusionthat involve movement of

materials across asemipermeable membrane

Dialysis/selective diffusionof solutes

Lipid-soluble materials

Small molecules that

can pass throughmembrane poresunassisted

Facilitated diffusion -substances require aprotein carrier for passive

transport Osmosis simple diffusion

of water


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Osmosis

Diffusion of the solvent across asemipermeable membrane.

In living systems the solvent is

always water, so biologistsgenerally define osmosis as the

diffusion of water across asemipermeable membrane:

O i


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Lower

concentration

of solute (sugar)

Higher

concentration

of sugar

Same concentration

of sugar

Selectively

permeable mem-

brane: sugar mole-

cules cannot pass

through pores, but

water molecules canMore free water

molecules (higher

concentration)

Water molecules

cluster around

sugar molecules

Fewer free water

molecules (lower

concentration)

Water moves from an area of higher

free water concentration to an area

of lower free water concentration

Osmosis

Osmosis

O ti P


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Osmotic Pressure

Osmotic pressure of a solution is the

pressure needed to keep it inequilibrium with pure H20.

The higher the concentration of

solutes in a solution, the higher its

osmotic pressure.

Tonicity is the ability of a solution tocause a cell to gain or lose water

based on the concentration of solutes

T i it


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Tonicity

If 2 solutions have equal [solutes], they are called

isotonic

If one has a higher [solute], and lower [solvent], is

hypertonic

The one with a lower [solute], and higher [solvent], is

hypotonic

Hypotonic solution Isotonic solution Hypertonic solution

H2OH2O H2O H2O

Lysed Normal Shriveled


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Water Balance In Cells With Walls

Plant cell. Plant cells

are turgid (firm) and

generally healthiest in

a hypotonic environ-

ment, where the

uptake of water is

eventually balanced

by the elastic wallpushing back on the

cell.

(b)

H2OH2OH2OH2O

Turgid (normal) Flaccid Plasmolyzed


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My definition of Osmosis

Osmosis is the diffusion of wateracross a semi-permeable membrane

from a hypotonic solution to a

hypertonic solution

F ilit t d Diff i


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Facilitated Diffusion

Diffusion of solutes through a semipermeable membrane with the

help of special transport proteins i.e. large polar molecules and ions

that cannot pass through phospholipid bilayer. Two types of transport proteins can help ions and large polar

molecules diffuse through cell membranes:

Channel proteins provide a narrow channel for the substance to pass

through.

Carrier proteins physically bind to the substance on one side ofmembrane and release it on the other.

EXTRACELLULAR

FLUID

Channel protein Solute

CYTOPLASM

Carrier proteinSolute


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Facilitated Diffusion

Specific each channel or carrier

transports certain ions or molecules

only

Passive direction of net movementis always down the concentration

gradient

Saturates once all transportproteins are in use, rate of diffusion

cannot be increased further

Active Transport


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Active Transport

Uses energy (from ATP) to move a

substance against its naturaltendency e.g. up a concentration

gradient.

Requires the use of carrier proteins

(transport proteins that physically bind

to the substance being transported).

2 types: Membrane pump (protein-mediated active

transport)

Membrane Pump


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Membrane Pump

A carrier protein uses energy from

ATP to move a substance across amembrane, up its concentration

gradient:

The Sodium potassium Pump


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One type of active transport system

The Sodium-potassium Pump

2. Na+ binding stimulates

phosphorylation by ATP.

1. Cytoplasmic Na+ binds

to the sodium-potassium

pump.

6. K+ is released and Na+

sites are receptive again;

the cycle repeats.

3. Phosphorylation causes the

protein to change its conformation,

expelling Na+ to the outside.

4. Extracellular K+ binds to the

protein, triggering release of the

Phosphate group.

5. Loss of the phosphate

restores the proteins

original conformation.

P

EXTRACELLULAR

FLUID

Na+

CYTOPLASM

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

P ATP

Na+Na+

Na+

P

ADP

K+

K+

K+

K+K+

K+

[Na+] high[K+] low

P iP i

Coupled transport


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Coupled transport

2 stages:

Carrier protein uses ATP to move a substance across the

membrane against its concentration gradient. Storing energy.

Coupled transport protein allows the substance to move down its

concentration gradient using the stored energy to move a

second substance up its concentration gradient:

Review: Passive And Active Transport Compared


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Review: Passive And Active Transport Compared

Passive transport. Substances diffuse spontaneously

down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell.

The rate of diffusion can be greatly increased by transport

proteins in the membrane.

Active transport. Some transport proteins act

as pumps, moving substances across a

membrane against their concentrationgradients. Energy for this work is usually

supplied by ATP.

Diffusion. Hydrophobic

molecules and (at a slow

rate) very small uncharged

polar molecules can diffuse

through the lipid bilayer.

Facilitated diffusion. Many

hydrophilic substances diffuse

through membranes with the

assistance of transport proteins,

either channel or carrier proteins.

ATP

Bulk Transport


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Bulk Transport

Allows small particles, or groups of

molecules to enter or leave a cellwithout actually passing through the

membrane.

2 mechanisms of bulk transport:

endocytosis and exocytosis.

E d t i


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Endocytosis

The plasma membrane envelops

small particles or fluid, then seals onitself to form a vesicle or vacuole

which enters the cell:

Phagocytosis

Pinocytosis

Receptor-Mediated Endocytosis -

Three Types Of Endocytosis


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Three Types Of Endocytosis

EXTRACELLULAR

FLUID

Pseudopodium

CYTOPLASM

Food or

other particle

Food

vacuole

1 m

Pseudopodium

of amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium via

phagocytosis (TEM).PINOCYTOSIS

Pinocytosis vesicles

forming (arrows) ina cell lining a smallblood vessel (TEM).

0.5 m

In pinocytosis, the cell

gulps droplets of

extracellular fluid into tiny

vesicles. It is not the fluid

itself that is needed by thecell, but the molecules

dissolved in the droplet.

Because any and all

included solutes are taken

into the cell, pinocytosis

is nonspecific in the

substances it transports.

Plasma

membrane

Vesicle

In phagocytosis, a cell

engulfs a particle by

Wrapping pseudopodiaaround it and packaging

it within a membrane-

enclosed sac large

enough to be classified

as a vacuole. The

particle is digested after

the vacuole fuses with a

lysosome containinghydrolytic enzymes.

PHAGOCYTOSIS

Process of Phagocytosis


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Process of Phagocytosis

Receptor mediated Endocytosis


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0.25 m

Ligand

Coat protein

Coated

pit

Coatedvesicle

Receptor

A coated pit

and a coated

vesicle formedduring

receptor-

mediated

endocytosis

(TEMs).Plasma

membrane

Coat

protein

Receptor-mediated endocytosis

enables the cell to acquire bulk quantities

of specific substances, even though those

substances may not be very concentrated

in the extracellular fluid. Embedded in the

membrane are proteins with specific

receptor sites exposed to the extracellular

fluid. The receptor proteins are usually

already clustered in regions of themembrane called coated pits, which are

lined on their cytoplasmic side by a fuzzy

layer of coat proteins.

Extracellular substances (ligands) bind

to these receptors. When binding occurs,

the coated pit forms a vesicle containing

the ligand molecules. Notice that there are

relatively more bound molecules (purple)

inside the vesicle, other molecules

(green) are also present. After this

ingested material is liberated from the

vesicle, the receptors are recycled to the

plasma membrane by the same vesicle.

Receptor-mediated Endocytosis

Exocytosis


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Exocytosis

The reverse of endocytosis

During this process, the membrane of a vesiclefuses with the plasma membrane and its

contents are released outside the cell:

Cell Junctions


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Cell Junctions

Long-lasting or permanent connections betweenadjacent cells, 3 types of cell junctions:

Tight junctions prevent

fluid from moving

across a layer of cells

Tight junction

0.5 m

1 m

Space

between

cellsPlasma membranes

of adjacent cells

Extracellular

matrix

Gap junction

Tight junctions

0.1 m

Intermediate

filamentsDesmosome

Gap

junctions

At tight junctions, the membranes of

neighboring cells are very tightly pressed

against each other, bound together by

specific proteins (purple). Forming continu-

ous seals around the cells, tight junctions

prevent leakage of extracellular fluid across

A layer of epithelial cells.

Desmosomes (also called anchoring

junctions) function like rivets, fastening cells

Together into strong sheets. Intermediate

Filaments made of sturdy keratin proteins

Anchor desmosomes in the cytoplasm.

Gap junctions (also called communicating

junctions) provide cytoplasmic channels from

one cell to an adjacent cell. Gap junctions

consist of special membrane proteins that

surround a pore through which ions, sugars,

amino acids, and other small molecules may

pass. Gap junctions are necessary for commu-

nication between cells in many types of tissues,

including heart muscle and animal embryos.

TIGHT JUNCTIONS

DESMOSOMES

GAP JUNCTIONS

The Nucleus And The Nuclear Envelope


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Nucleus

NucleusNucleolusChromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pore

Rough ER

Pore

complex

Surface of nuclear

envelope.

Pore complexes (TEM). Nuclear lamina (TEM).

Close-up of

nuclear

envelope

Ribosome

1 m

1 m

0.25 m

The Nucleus And The Nuclear Envelope Repository for genetic material called chromatin - DNA and proteins

Nucleolus: holds chromatin and ribosomal subunits- region of intensiveribosomal RNA synthesis

Nuclear envelope: Surface of nucleus bound by two phospholipid bilayermembranes - Double membrane with pores

Nucleoplasm: semifluid medium inside the nucleus

Ch


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Chromosomes DNA of eukaryotes is divided into linear

chromosomes. Exist as strands of chromatin, except

during cell division

Histones associated packaging proteins

Ribosomes


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Ribosomes Ribosomes are RNA-protein complexes composed of two

subunits that join and attach to messenger RNA.

Site of protein synthesis

Assembled in nucleoli

ERRibosomes Cytosol

Free ribosomes

Bound ribosomes

Largesubunit

Small

subunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 m

Endoplasmic reticulum (ER)

E d b S t


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Endomembrane System

Compartmentalizes cell, channeling passage

of molecules through cells interior. Endoplasmic reticulum

Rough ER - studded with ribosomes

Smooth ER - few ribosomes

R h ER


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Rough ER Rough ER is especially abundant in cells that secrete proteins.

As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the

cisternal space through a pore formed by a protein complex in the ER membrane.

As it enters the cisternal space, the new protein folds into its native conformation. Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is

attached.

Secretory proteins are packaged in transport vesicles that carry them to their next stage.

Rough ER is also a membrane factory.

Membrane-bound proteins are synthesized directly into the membrane.

Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.

As the ER membrane expands, membrane can be transferred as transport vesicles to other

components of the endomembrane system.

S th ER


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Smooth ER The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.

Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.

These include the sex hormones of vertebrates and adrenal steroids.

In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as

alcohol and barbiturates.

Smooth ER stores calcium ions.

Muscle cells have a specialized smooth ER that pumps calcium ions from

the cytosol and stores them in its cisternal space.

When a nerve impulse stimulates a muscle cell, calcium ions rush from

the ER into the cytosol, triggering contraction.

Th G l i t


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The Golgi apparatus The Golgi apparatus is the shipping and receiving center for cell

products.

Many transport vesicles from the ER travel to the Golgi apparatusfor modification of their contents.

The Golgi is a center of manufacturing, warehousing, sorting, andshipping.

The Golgi apparatus consists of flattened membranous sacscisternaelooking like a stack of pita bread.

The Golgi sorts and packages materials into transport vesicles.

F ti Of Th G l i A t


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Functions Of The Golgi Apparatus

TEM of Golgi apparatus

cis face

(receiving side of

Golgi apparatus)

Vesicles move

from ER to GolgiVesicles also

transport certainproteins back to ER

Vesicles coalesce to

form new cis Golgi cisternae

Cisternal

maturation:

Golgi cisternae

move in a cis-

to-trans

direction

Vesicles form and

leave Golgi, carrying

specific proteins toother locations or to

the plasma mem-

brane for secretionVesicles transport specificproteins backward to newer

Golgi cisternae

Cisternae

trans face

(shipping side ofGolgi apparatus)

0.1 0 m1

6

5

2

3

4

Golgi

apparatus

Membrane Bound Organelles


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Membrane Bound Organelles

Lysosomes vesiclecontaining digestiveenzymes that break downfood/foreign particles

Vacuoles food storageand water regulation

Peroxisomes - containenzymes that catalyze the

removal of electrons andassociated hydrogenatoms

(a) Phagocytosis: lysosome digesting food

1 m

Lysosome contains

active hydrolytic

enzymes

Food vacuole

fuses with

lysosome

Hydrolytic

enzymes digest

food particles

Digestion

Food vacuole

Plasma membrane

Lysosome

Digestive

enzymes

Lysosome

Nucleus

Mitochondria


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Mitochondria Sites of cellular respiration, ATP synthesis

Bound by a double membrane surrounding fluid-filled matrix.

The inner membranes of mitochondria are cristae The matrixcontains enzymes that break down carbohydrates and

the cristae house protein complexes that produce ATP


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Cytoskeleton The eukaryotic cytoskeleton is a network of

filaments and tubules that extends from thenucleus to the plasma membrane that supportcell shape and anchor organelles.

Protein fibers

Actin filaments

cell movement

Intermediate filaments Microtubules

centrioles

Centrioles


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Centrioles

Centrioles are shortcylinders with a 9 + 0

pattern of microtubule

triplets.

Centrioles may beinvolved in microtubule

formation and

disassembly during cell

division and in the

organization of cilia and

flagella.


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Cilia and Flagella


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Cilia and Flagella

Cilia (small and numerous) and flagella (large and single)have a 9 + 2 pattern of microtubules and are involved in

cell movement. Cilia and flagella move when the microtubule doublets

slide past one another.

Each cilium and flagellum has a basalbodyat its base.

Cilia and Flagella


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(a) Motion of flagella. A flagellumusually undulates, its snakelike

motion driving a cell in the same

direction as the axis of the

flagellum. Propulsion of a human

sperm cell is an example of

flagellatelocomotion (LM).

1 m

Direction of swimming

Cilia and Flagella

(b) Motion of cilia. Cilia have a back-

and-forth motion that moves the

cell in a direction perpendicular

to the axis of the cilium. A dense

nap of cilia, beating at a rate ofabout 40 to 60 strokes a second,

covers this Colpidium, a

freshwater protozoan (SEM).

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