d. fundamentals of cell movement€¦ · figure 16-81b molecular biology of the cell (© garland...

D. Fundamentals of Cell Movement

What cell types move?

• Prokaryotes must find food, evade toxins• Free‐living ciliar and flagellar eukaryotes• Plants don’t have motile cells but can demonstrate both

rapid and slow movements due to cell activity

• Animals have both ciliar and flagellar cells• We also have cells that crawl, rather than swim

Many cells during development and growthWhite blood cells responding to infectionWound healing cells

• Muscular movements in animals result from individual cell movements

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What strategies do cells employ to move?

• Swimming through liquids: oars and propellers

• Crawling on solid surfaces: grab‐pull‐release

• Selectively contracting some cells but not others: some use motor proteins, others water volume

• Even ‐ growing more cells, or letting some die, to move the entire structure closer or farther away!

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Figure 1-18a Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

Many prokaryotic cells have a structure composed of a membrane‐bound motor complex driving propeller‐like movement of the extracellular flagellum

The flagellum is composedof the helical protein flagellin

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The helical structure of flagellin allows for two kinds of movement: coordinated linear vs. stationary ‘tumbling’

RECEPTOR CHEMOTAXIS

senses correct direction: will swim in a straight line for a longer time before tumbling

senses wrong direction: will tumble sooner and try a new direction at random.

finds the location with the highest concentration of anattractant (lowest of repellent )

Even at high concentrations, can distinguish very small differences in concentration.

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Figure 15-73 Molecular Biology of the Cell (© Garland Science 2008)

Two levels of regulation:1. Signal transductio to motor2. Control of receptor activation

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Ciliar and Flagellar Eukaryotes

• The Basic Mechanism– Complex microtubular structures extend out from the cell body under the plasma membrane

– They extend out from basal bodies rather than centrisomes

– Immobilized dynein pulls retrograde and bends the microtubule

– Relaxation or a counter pull creates waving

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Figure 1-32 Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

Free‐living eukaryote Didiniumhas two fringes of cilium used for swimming

Here it is phagocytosing anothereukaryotic cell as prey

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Airway Epithelium

‐ G.I. epithelium‐ Fallopian tubes‐ Epidydimus

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Flagellar Animal Cells

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Figure 16-81a Molecular Biology of the Cell (© Garland Science 2008)

The structure of microtubules in both cilia and flagella are theclassic 9+2:

An external ring of 9 doubletsaround 2 full microtubules

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Basal body structure is a ring of nine (9) triplets

Same as the centriole

Microtubules are nucleated from γ‐tubulinand are capped and stabilized long‐term.

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Figure 16-81b Molecular Biology of the Cell (© Garland Science 2008)

They work as a unit by being held together with ‘radial spoke’ and ‘nexin’ proteins.

As dyneins attached to one doublet attempt to walk on the adjacent one they all bend.

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Mechanisms of Waving

• In long flagellum, sequential peristaltic contractions cause a whip‐like back and forth motion

• In short cilia, alternating side‐to‐side contractions or simple relaxations cause waving

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Cell Migration or “Crawling”

• The Basic Mechanism– Triggered by signals from outside the cell– Actin‐myosin based movement– Requires attachments to outside to pull against– Gotta’ drag all of the cell contents along for the ride

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In embryo development and wound healing, epithelial cellscan migrate as sheets.

In general, these types ofmigrations are combinationsof cell division and directedmigration.

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Chemotaxis

Circumferential receptors

Rho‐family GTPases (monomeric)

Rho‐dependent kinases

1. Actin monomer nucleotide exchange2. Actin fiber polymerization and disassembly3. Myosin motor ATPase activity

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Cell type‐specific migration receptor

Rho family monomeric GTPase

Rho‐dependent kinase

Circumferential distribution of migration‐inducing signaling cascades

Cell capable of migration

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Source of signal

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actin fibers

actin growth blocked

Leading edge extension is driven by actin polymerization.

Cell membrane is physically pushed forward by actin1. Core of all structures is very dense actin network2. Completely exclude membrane enclosed organelles.

Leading edge contains everything needed for migration.1. a cut piece without organelles will continue to migrate

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Actin treadmilling – actin is an ATPase

g-actin adds to the f-actin chain as ATP-actin

Comes off as ADP-actin.

Rate of hydrolysis controls rate of treadmilling

ATP

ATPATPATPATPATPATPATP

ADP

profilin

ADP

ATP

cofilin

ADP

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Ultimately, the length of the f‐actin remains constant but it moves forward

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Attachment: Microfilament Connections Depend on Migratory Surface

All the usual suspects: focal adhesions, adherens junctions

ATP-actin can bind to f-actin chain and to anchor proteins

Internal binding of cadherins/integrins allows external binding

ADP-actin loses binding to f-actin and anchor protein

ATP ATPATPATP ATP ATPATP

ATPADP

Direction of travel

Detachment is as important as attachment for movement!

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Migrating cells tend to follow ECM and/or cell tracts towards their target.

The original integrins and/or cadherins on the cell surface determine these tracts.

eg. Fibronectin and cadherins outside of the cell.

What you could bind to when stationary is what you can bind to when migratory (until you change gene expression)

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ATP ATPATPATP ATP ATPATP

ATPADP

Direction of travel

cellular components

myosin motors

Traction: Movement of the Cell Body Across Attached Leading EdgeThe actin structure performs a scaffolding function.

Myosin pulls against actin bound to extracellular components.

Myosins transport many cellular components directly.

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Figure 16-87c Molecular Biology of the Cell (© Garland Science 2008)

One of the principal cargos of myosins that are involved in migration are intermediate filaments and microfilaments

‐ nearly everything is already bound to them!

IF (blue)

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Myosin pulls so hard that it realigns the ECM proteins.

This sets up “game trails”wherein the first cell blazesa trail that is easier for the next cell to follow.

The later cells reach the destination faster than those that went ahead.

cell 1

cell 2

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Realignment of microtubules in the direction of travel allows streaming of mitochondria into the leading edge

Every g‐actin placed into f‐actinand every myosin powerstrokerequires a fresh ATP!

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The special case of extravasation

• Circulating WBC must get out of the vessel• Combines activation of the WBC with ‘Cell Rolling’, ‘Adhesion’ and ‘Diapedesis’1. The presence of environmental cues associated with injury

and infection change endothelial surface selectins

2. These catch closely matched WBC surface oligosaccharides and make them roll to a stop on endothelial surface

3. The white blood cell then activates an integrin that binds tightly to ICAM on endothelial cells

4. Diapedesis uses basic migratory mechanisms along with WBC shape change to squeeze between endothelium

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Intercellular Diapedesis

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TranscellularDiapedesis

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Molecular shortening of the sarcomere shortens the cell because the filaments attach to the plasma membrane

Each of the cells, or fibers, is attached by ECM to the other cells in its fiber bundle, or fasicle, and pulls on them

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Few to many muscle fasicles make up a muscle, such as the quadriceps, all joined by connective tissue. When cells contract the force is transfered directly to these extracellular structures

Transference of that force through the tendon to the bone produces motion

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Nastic movements are non‐directional responses to stimuli. The movement can be due to changes in turgor and the rate or frequency of these responses increases as intensity of the stimulus increases.

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Thignonasty/seismonasty: response to touch

Photonasty: response to light

Nyctinasty: movements at night or in the dark

Chemonasty: response to chemicals or nutrients

Hydronasty: response to water

Thermonasty: response to temperature

Geonasty/gravinasty: response to gravity

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Gravitropism: response to gravity

Chemotropism: response to chemicals

Heliotropism: response to sunlight

Hydrotropism: response to water

Phototropism: response to lights or colors

Thermotropism: response to temperature

Electrotropism: response to an electric field

Thigmotropism: response to touch or contact

Host tropism: response to pathogens

Tropic movements are growth or turning in response to an environmental stimulus that is dependent on the direction of the stimulus. Tropisms may be either positive (towards) or negative (away from) the stimulus.

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The Endomembrane System and Intracellular Trafficking

What does a cell do when its mitochondria or lysosomes wear out?

How does it keep the lysosomal enzymes from digesting everything in the process?

How does a cell change the receptors on its plasma membrane when necessary?

How does a cell duplicate EVERYTHING when it’s time to divide?

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Fig. 7‐3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Remember what the cytosol and membranes are made up of....

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Fig. 7‐9ac

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

And....

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Fig. 7‐9df

(d) Cell‐cell recognition

Glyco‐protein

(e) Intercellular joining (f) Attachment tothe cytoskeletonand extracellularmatrix (ECM)Carbohydrates often play important roles on

the plasma membrane. Covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

And....

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And....

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How does the cell get all of these hydrophobic molecules to their appropriate locations –

Right through the middle of an aqueousenvironment?!

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The endomembrane system allows containment and movement of hydrophobic and dangerous materials

• Components of the endomembrane system:– Nuclear envelope– Endoplasmic reticulum– Golgi apparatus– Lysosomes– Peroxisomes– Vacuoles– Plasma membrane

• These are bridged by membrane vesicles

• This process is called vesicular transport

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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Fig. 6‐16‐3

Smooth ER

Nucleus

Rough ER

Plasma membrane

cisGolgi

transGolgi

1. Movement from the nucleus outward

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Fig. 6‐14

Nucleus 1 µm

Lysosome

Digestiveenzymes

Lysosome

Plasmamembrane

Food vacuole

(a) Phagocytosis

Digestion

(b) Autophagy

Peroxisome

Vesicle

Lysosome

Mitochondrion

Peroxisomefragment

Mitochondrionfragment

Vesicle containingtwo damaged organelles

1 µm

Digestion

2. Movement from theplasma membrane inward

3. Movement withinthe cytosol

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Like everything else in the cell this starts in the nucleus‐ER complex....The Nucleus delivers all RNAs (r,t,m) to ER for translation.

a. Nuclear pores need be very large, 8 protein subunits.

b. The outer membrane is continuous with the ER and may even have ribosomes on the nucleus proper.

c. Free ribosomes direct cytosolic translation.

1. Free and bound ribosomes are structurally identical

2. mRNA sequence directs them on or off the RER surface

3. “Free” is a relative term – they are anchored to the cytoskeleton

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Fig. 5‐26‐3

mRNA

Synthesis ofmRNA in thenucleus

DNA

NUCLEUS

mRNA

CYTOPLASM

Movement ofmRNA into cytoplasmvia nuclear pore

Ribosome

AminoacidsPolypeptide

Synthesisof protein

1

2

3

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Fig. 6‐11

Cytosol

Endoplasmic reticulum (ER)

Free ribosomes

Bound ribosomes

Large subunit

Small subunit

Diagram of a ribosomeTEM showing ER and ribosomes

0.5 µm

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What kinds of things are made on free ribosomes?

• Intermediate Filaments, Actin, Tubulin• Myosin, Kinesin, Dynein• Microfilament associated proteins• Microtubular associated proteins• 2nd messengers for signaling cascades• Glycolysis enzymes• Lots and lots of others....

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What kinds are made on bound ribosomes?

• Signal Receptors, Transporters, Channels• Cadherins, Integrins• Anchor protein complexes• Enzymes inside organelles• Secreted proteins• Lots and lots of others....

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Remember: Principle Functions of the ER

1. Lipid biosynthesis ‐ phospholipids, steroids, lipoproteins (HDL, LDL)‐ there is great cell‐specificity in lipid enzymes

2. Membrane-bound translation of mRNA- Amino‐terminus leader sequences direct placement into proper orientation

3. Initial integration of lipids and proteins - Lipids associate based on amino acid structures

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What info is in the Amino Terminus?

• If you are a transmembrane protein, the amino terminus will get you put in the transmembrane position in the ER

• If you are a luminal or secreted protein, the amino terminus will get you put into the lumen of the ER

• If you are a meshwork protein, the amino terminus will get you attached to the ER cytosolic face

• The information is interpreted by ribosomes and/or chaperonins

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Fig. 7‐8

N‐terminus

C‐terminusCYTOSOLICSIDE

NON‐CYTOSOLICSIDE

The amino terminus comes off the ribosome first and is thus the first through the ER membrane

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The Cytosolic v. Non‐Cytosolicrelationship is always maintained in the endomembrane system:

Once non‐cytosolic,

always non‐cytosolic, etc.

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Different amino acidside chainshave binding affinity for different ER lipids

The Proteins that are Placed into the ER Membrane Determine the Lipids that Assemble from the Mix

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Fig. 6‐16‐3

Nucleus

Plasma membrane

GolgiApparatus

Vesicles

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Budding, Transport , Targeting and Fusion

1. Vesicles bud off of a membrane due to coat proteins in the membrane meshwork

2. Vesicles are transported to their next membrane due to vesicle motor‐binding proteins

3. Vesicles are specifically targeted to their next membrane due to vesicle membrane proteins

4. Vesicles fuse to their destination membrane due to their lipid constituents

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Budding, Transport and Targeting are dependent on a combination of the transmembrane and meshwork proteins of the source membrane

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The three major families of coat proteins...

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Figure 13-7a, b Molecular Biology of the Cell (© Garland Science 2008)

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Figure 13-7c, d Molecular Biology of the Cell (© Garland Science 2008)

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Transportation and targeting of vesicles

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Table 13-1 Molecular Biology of the Cell (© Garland Science 2008)

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Fusion of two membranes depends on a reasonably good match between the lipids that they have assembled in the two sheaths of their bilayers.

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Fig. 7‐10

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

Membranes have distinct inside and outside faces

The asymmetrical distribution of proteins, lipids, and carbohydrates is determined when the membrane is built by the ER and Golgi apparatus

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Lipid and protein signatures

• Lipid signature is relatively assymetric• Protein signature is absolutely assymetric• Sugars are also absolutely assymetric


The lipids and proteins exposed on the cytosolic face of a membrane are different from those on the non‐cytosolic face.

Enough to easily tell them apart.

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Fig. 7‐5c

Cholesterol

Phospholipids Cholesterol Cerebrosides Sphingolipids

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Fig. 7‐8

N‐terminus

C‐terminusCYTOSOLICSIDE

NON‐CYTOSOLICSIDEAmino

acid sequences exposedon the cytosolicface are absolutely different from those on the non‐cytosolicface

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The Role of the Golgi Apparatus

• Process vesicular proteins and lipids through covalent modifications

• As the vesicle progresses from cis to trans it undergoes a distinctive series of changes

• When the product leaves the GA it has new transport, targeting and fusion information

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Different cisdomains receive distinct ER products

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Vesicles fuse and bud as they travel along the cisterna

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Many GA alterations but none more important than sugar assembly and glycosylation.

100’s of human enzymes give great cell‐specificity in glycosylation patterns

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Plasma Membrane Targeting, Transport and Fusion

• Constituitive Delivery and Secretion– Automatic transport from trans‐GA to PM– Occurs continuously as needed

• Regulated Secretion– Specialized vesicles dock under PM– Wait for a secondary signal to cause fusion

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Focal adhesions

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Lysosome Targeting, Transport and Fusion

• Enzymes are potentially deadly to the cell

• Optimal pH of enzyme activity is ~5.0, while the rest of the cell is maintained at ~7.2

• They are made in the ER and placed in the lumen, where they are inactive

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Lysosome Targeting, Transport and Fusion

• Co‐expressed with the standards: MBC, KBC, GA rab, lysosome rab

• Also with a mannose‐6‐phosphate receptor protein – sugar activated lysosome targeting

• Also with an inactive hydrogen ion pump –lipid fusion at lysosome ONLY activates it!

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The Endosomal System

• Inward flow of vesicular material

• Phagocytosis and pinocytosis

• Receptor removal and recycling

• ECM turnover and remodeling

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Fig. 7‐20cRECEPTOR‐MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 µm

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor‐mediatedendocytosis(TEMs)

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Mito Targeting, Transport and Fusion

• Remember: The Endosymbiotic Theory

• Their own DNA, divide by binary fission

• Two membranes– The outer membrane is 50:50 protein:lipid– The inner membrane is 80:20 protein:lipid

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The lipids and most proteins of the inner membrane arise from expression of genes maintained in the mitochondrion itself

Those of the outer membrane arise from expression of genes maintained in the nucleus and trafficked via ER and GA

What of the rate limiting step enzymes?

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Figure 12-28d Molecular Biology of the Cell (© Garland Science 2008)

TOM COMPLEX

Expressed on free ribosomes from nuclear mRNAs

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Neurotransmitter Release

Let’s put it all together....

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Neurons and muscle cells in animals and phloem cells in plants rely on electrical signaling.

• Electricity is the energy created by the movement of charged particles – it’s named for the example of electrons

• When a cell uses electricity it does it by allowing ions that it has concentrated by active transport to rush from one side of the membrane to the other through channel proteins

• The opening and closing of the channels determines when the electrical current is flowing

• Voltage is a measure of how many ions are on the move

• Membrane potential is a measure of how many ions have been actively concentrated across a membrane


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• Concentrated ions diffuse faster than uncharged molecules

• Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:

– A chemical force (the ion’s concentration gradient)

– An electrical force (the effect of the membrane potential on the ion’s movement)


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• An electrogenic pump is a transport protein that generates voltage across a membrane

• The sodium‐potassium pump is the major electrogenicpump of animal cells

• The main electrogenic pump of plants, fungi, and bacteria is a proton pump

• Mitochondria and chloroplasts use a proton pump to help make ATP


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• Build the axon electrical gradient• Build the synapse• Build and locate the neurotransmitter vesicles• Depolarize and repolarize the plasmamembrnae• Transduce the signal that causes vesicular fusion

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d. fundamentals of cell movement€¦ · figure 16-81b molecular biology of the cell (© garland...

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