© 2013 pearson education, inc. membrane transport: active processes two types of active processes...

© 2013 Pearson Education, Inc.

Membrane Transport: Active Processes

• Two types of active processes– Active transport– Vesicular transport

• Both require ATP to move solutes across a living plasma membrane because– Solute too large for channels– Solute not lipid soluble– Solute not able to move down concentration

gradient


Active Transport

• Requires carrier proteins (solute pumps)– Bind specifically and reversibly with

substance

• Moves solutes against concentration gradient– Requires energy


Active Transport: Two Types

• Primary active transport– Required energy directly from ATP hydrolysis

• Secondary active transport– Required energy indirectly from ionic

gradients created by primary active transport


Primary Active Transport

• Energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane

• E.g., calcium, hydrogen, Na+-K+ pumps


Primary Active Transport

• Sodium-potassium pump– Most well-studied– Carrier (pump) called Na+-K+ ATPase– Located in all plasma membranes– Involved in primary and secondary active

transport of nutrients and ions


Sodium-Potassium Pump

• Na+ and K+ channels allow slow leakage down concentration gradients

• Na+-K+ pump works as antiporter– Pumps against Na+ and K+ gradients to

maintain high intracellular K+ concentration and high extracellular Na+ concentration

• Maintains electrochemical gradients essential for functions of muscle and nerve tissues

• Allows all cells to maintain fluid volume


Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP.

Slide 1

Extracellular fluid Na+

Na+–K+ pump

K+

ATP-binding site

Cytoplasm

1 Three cytoplasmic Na+ bind to pumpprotein.

K+ released

6 Pump protein binds ATP; releases K+ to the inside, and Na+ sites are ready to bind Na+ again. The cycle repeats.

2 Na+ binding promotes hydrolysis of ATP.The energy released during this reactionphosphorylates the pump.

K+ bound

5 K+ binding triggers release of thephosphate. The dephosphorylated pumpresumes its original conformation.

K+

4 Two extracellular K+ bind to pump.

3 Phosphorylation causes the pump tochange shape, expelling Na+ to the outside.

Na+ bound

Na+ released

P

P

P

Pi



Slide 2


Na+–K+ pump

K+

Cytoplasm


ATP-binding site



Slide 3


Na+–K+ pump

K+

Cytoplasm



Na+ bound

P

ATP-binding site



Slide 4


Na+–K+ pump

K+

Cytoplasm




Na+ bound

Na+ released

P

P

ATP-binding site



Slide 5


Na+–K+ pump

K+

Cytoplasm



K+



Na+ bound

Na+ released

P

P

P

ATP-binding site



Slide 6


Na+–K+ pump

K+

Cytoplasm



K+ bound


K+



Na+ bound

Na+ released

P

P

P

Pi

ATP-binding site



Slide 7


Na+–K+ pump

K+

Cytoplasm


K+ released



K+ bound


K+



Na+ bound

Na+ released

P

P

P

Pi

ATP-binding site


Figure 3.10 Primary active transport is the process in which solutes are moved across cellmembranes against electrochemical gradients using energy supplied directly by ATP.

PLAYPLAY A&P Flix™: Resting Membrane Potential


Na+–K+ pump

K+

Cytoplasm


K+ released



K+ bound


K+



Na+ bound

Na+ released

P

P

P

Pi

ATP-binding site


Secondary Active Transport

• Depends on ion gradient created by primary active transport

• Energy stored in ionic gradients used indirectly to drive transport of other solutes


Secondary Active Transport

• Cotransport—always transports more than one substance at a time– Symport system: Substances transported in

same direction– Antiport system: Substances transported in

opposite directions


Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary activetransport.

Extracellular fluid

Na+-glucosesymporttransporterloads glucosefrom extracellularfluid

Na+-glucosesymport transporterreleases glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm

Primary active transport

The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient

for Na+ entry into the cell.

Secondary active transport

As Na+ diffuses back across the membranethrough a membrane cotransporter protein, itdrives glucose against its concentration gradientinto the cell.

1 2

Slide 1



Slide 2

Extracellular fluid

Na+-K+

pump

Cytoplasm




1



Slide 3

Extracellular fluid

Na+-glucosesymporttransporterloads glucosefrom extracellularfluid

Na+-glucosesymport transporterreleases glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm




Secondary active transport

As Na+ diffuses back across the membranethrough a membrane cotransporter protein, itdrives glucose against its concentration gradientinto the cell.

1 2


Vesicular Transport

• Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles

• Requires cellular energy (e.g., ATP)


Vesicular Transport

• Functions:– Exocytosis—transport out of cell – Endocytosis—transport into cell

• Phagocytosis, pinocytosis, receptor-mediated endocytosis

– Transcytosis—transport into, across, and then out of cell

– Vesicular trafficking—transport from one area or organelle in cell to another


Endocytosis and Transcytosis

• Involve formation of protein-coated vesicles

• Often receptor mediated, therefore very selective

• Some pathogens also hijack for transport into cell

• Once vesicle is inside cell it may– Fuse with lysosome– Undergo transcytosis


Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 1 Coated pit ingests substance.

Coat proteins are recycled to plasma membrane.

1

Protein coat(typicallyclathrin)

Protein-coated vesicle deta- ches.

Transportvesicle

EndosomeUncoated endocyticvesicle

Transport vesicle containing

Uncoated vesicle fuses with a sorting vesicle called an endosome.

Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis).

Extracellular fluid

Plasma membrane

Cytoplasm

Lysosome

2

membrane compone-nts moves to the plasmamembrane for recycling.

3

5 4

6


Slide 2Figure 3.12 Events of endocytosis mediated by protein-coated pits.

Coated pit ingests substance. 1


Extracellular fluid

Plasma membrane

Cytoplasm





Extracellular fluid

Plasma membrane


2Cytoplasm





Extracellular fluid

Plasma membrane


2Cytoplasm


3





Extracellular fluid

Plasma membrane


2Cytoplasm


3



4





Extracellular fluid

Plasma membrane


2Cytoplasm


3



4

Transportvesicle

membrane compone-nts moves to the plasmamembrane for recycling.

5 Transport vesicle containing



Coated pit ingests substance.


1



Transportvesicle



Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis).

Extracellular fluid

Plasma membrane

Cytoplasm

Lysosome

2

3

4

6

Transport vesicle containing membrane compone-nts moves to the plasmamembrane for recycling.

5


Endocytosis

• Phagocytosis– Pseudopods engulf solids and bring them into

cell's interior– Form vesicle called phagosome

• Used by macrophages and some white blood cells– Move by amoeboid motion

• Cytoplasm flows into temporary extensions• Allows creeping


Figure 3.13a Comparison of three types of endocytosis.

Receptors

Phagosome

PhagocytosisThe cell engulfs a large particleby forming projecting pseudopods ("false feet") around it and enclosing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain inthe vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein coated but has receptors capable of binding to microorganisms or solid particles.


Endocytosis

• Pinocytosis (fluid-phase endocytosis)– Plasma membrane infolds, bringing

extracellular fluid and dissolved solutes inside cell

• Fuses with endosome

– Most cells utilize to "sample" environment – Nutrient absorption in the small intestine – Membrane components recycled back to

membrane


Figure 3.13b Comparison of three types of endocytosis.

Vesicle

PinocytosisThe cell "gulps" a drop of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.


Endocytosis

• Receptor-mediated endocytosis– Allows specific endocytosis and transcytosis

• Cells use to concentrate materials in limited supply

– Clathrin-coated pits provide main route for endocytosis and transcytosis

• Uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins


Receptor-Mediated Endocytosis

• Different coat proteins– Caveolae

• Capture specific molecules (folic acid, tetanus toxin) and use transcytosis

• Involved in cell signaling but exact function unknown

– Coatomer• Function in vesicular trafficking


Figure 3.13c Comparison of three types of endocytosis.

Vesicle

Receptor-mediated endocytosisExtracellular substances bind to specific receptor proteins, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.


Exocytosis

• Usually activated by cell-surface signal or change in membrane voltage

• Substance enclosed in secretory vesicle

• v-SNAREs ("v" = vesicle) on vesicle findt-SNAREs ("t" = target) on membrane and bind

• Functions– Hormone secretion, neurotransmitter release,

mucus secretion, ejection of wastes


Figure 3.14 Exocytosis. Slide 1

Extracellularfluid

Plasma membraneSNARE (t-SNARE)

The process of exocytosis

Secretoryvesicle Vesicle

SNARE(v-SNARE)

Molecule tobe secretedCytoplasm

1 The membrane-bound vesiclemigrates to theplasma membrane.

2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).

Fusedv- and

t-SNAREs

3 The vesicle and plasma membranefuse and a poreopens up.

Fusion pore formed

4 Vesicle contents are released to thecell exterior.


Slide 2Figure 3.14 Exocytosis.

Extracellularfluid




SNARE(v-SNARE)





Extracellularfluid




SNARE(v-SNARE)




Fusedv- and

t-SNAREs



Extracellularfluid




SNARE(v-SNARE)




Fusedv- and

t-SNAREs


Fusion pore formed



Extracellularfluid




SNARE(v-SNARE)




Fusedv- and

t-SNAREs


Fusion pore formed

4 Vesicle contents are released to thecell exterior.


Figure 3.14b Exocytosis.

Photomicrograph of a secretoryvesicle releasingits contents by exocytosis (100,000x)


Table 3.2 Active Membrane Transport Processes (1 of 2)


Table 3.2 Active Membrane Transport Processes (2 of 2)


Generation of a Resting Membrane Potential

• Resting membrane potential (RMP)– Produced by separation of oppositely charged

particles (voltage) across membrane in all cells

• Cells described as polarized

– Voltage (electrical potential energy) only at membrane

• Ranges from –50 to –100 mV in different cells– "–" indicates inside negative relative to outside


Selective Diffusion Establishes RMP

• Electrochemical gradient established– Electro (charge); chemical (ion concentration)

• K+ diffuses out of cell through K+ leakage channels, proteins cannot inside cell membrane more negative

• K+ attracted back as inner face more negative

• K+ equalizes across membrane at –90 mV when K+ concentration gradient balanced by electrical gradient = RMP


Figure 3.15 The key role of K+ in generating the resting membrane potential.

1 K+ diffuse down their steepconcentration gradient (out of the cell)via leakage channels. Loss of K+

results in a negative charge on theinner plasma membrane face.

2 K+ also move into the cellbecause they are attracted to thenegative charge established on theinner plasma membrane face.

3 A negative membrane potential(–90 mV) is established when themovement of K+ out of the cell equalsK+ movement into the cell. At thispoint, the concentration gradientpromoting K+ exit exactly opposes theelectrical gradient for K+ entry.

Extracellular fluid

Potassiumleakagechannels

Protein anion (unable tofollow K+ through themembrane)Cytoplasm

++

+ + + ++ +

––

–––––

–

Slide 1





Extracellular fluid



++

+ + + ++ +

––

–––––

–

Slide 2






Extracellular fluid



++

+ + + ++ +

––

–––––

–

Slide 3






3 A negative membrane potential(–90 mV) is established when themovement of K+ out of the cell equalsK+ movement into the cell. At thispoint, the concentration gradientpromoting K+ exit exactly opposes theelectrical gradient for K+ entry.

Extracellular fluid



++

+ + + ++ +

––

–––––

–

Slide 4


Selective Diffusion Establishes RMP

• In many cells Na+ affects RMP– Attracted into cell due to negative charge

RMP to –70 mV – Membrane more permeable to K+ than Na+, so

K+ primary influence on RMP

• Cl– does not influence RMP—concentration and electrical gradients exactly balanced


Active Transport Maintains Electrochemical Gradients

• Na+-K+ pump continuously ejects 3Na+ from cell and carries 2K+ in

• Steady state maintained because rate of active transport equal to and depends on rate of Na+ diffusion into cell

• Neuron and muscle cells "upset" RMP by opening gated Na+ and K+ channels


Cell-Environment Interactions

• Cells interact directly or indirectly by responding to extracellular chemicals

• Always involves glycocalyx– Cell adhesion molecules (CAMs)– Plasma membrane receptors– Voltage-gated channel proteins


Roles of Cell Adhesion Molecules

• Thousands on approximately every cell in body• Anchor to extracellular matrix or each other• Assist in movement of cells past one another• Attract WBCs to injured or infected areas• Stimulate synthesis or degradation of adhesive

membrane junctions• Transmit intracellular signals to direct cell

migration, proliferation, and specialization


Roles of Plasma Membrane Receptors

• Contact signaling—touching and recognition of cells; e.g., in normal development and immunity

• Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones, and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels)– Same ligand can cause different cell responses– Response determined by what receptor linked to

inside cell


Chemical Signaling

• Ligand binding receptor structural change protein alteration– Catalytic receptor proteins become activated

enzymes– Chemically gated channel-linked receptors open and

close ion gates changes in excitability– G protein–linked receptors activate G protein,

affecting an ion channel or enzyme, or causing release of internal second messenger, such as cyclic AMP


Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.

Ligand (1st messenger)

Receptor G protein Enzyme 2ndmessenger

Ligand* (1st messeng-er) binds to the receptor. The receptor changes shape and activates.

The activated receptorbinds to a G protein and acti-vates it. The G protein changesshape (turns “on”), causing it torelease GDP and bind GTP (an energy source).

Activated G proteinactivates (or inactivates) an effector protein by causing its shape tochange.

Effector protein(e.g., an enzyme)

Extracellular fluid

G proteinGDP

Intracellular fluid

Cascade of cellular responses (The amplification effect istremendous. Each enzyme

catalyzes hundreds of reactions.)

Activatedkinaseenzymes

Active 2ndmessenger

Inactive 2nd messenger

Activated effector enzymescatalyze reactions that produce2nd messengers in the cell.(Common 2nd messengers includecyclic AMP and Ca2+.)

Second messengersactivate other enzymes or ionchannels. Cyclic AMP typicallyactivates protein kinase enzymes.

Kinase enzymes activateother enzymes. Kinase enzymestransfer phosphate groups from ATPto specific proteins and activate aseries of other enzymes that triggervarious metabolic and structuralchanges in the cell.

* Ligands include hormones and neurotransmitters.

ReceptorLigand

1 2 3

4

5

6

Slide 1The sequence described here is like a molecularrelay race. Instead of a baton passed from runnerto runner, the message (a shape change) ispassed from molecule to molecule as it makes itsway across the cell membrane from outside toinside the cell.






Extracellular fluid

Intracellular fluid


ReceptorLigand

1

Slide 2






Extracellular fluid

Intracellular fluid


ReceptorLigand

1 The activated receptorbinds to a G protein and acti-vates it. The G protein changesshape (turns “on”), causing it torelease GDP and bind GTP (an energy source).

G proteinGDP

2

Slide 3






Extracellular fluid

Intracellular fluid


ReceptorLigand


G proteinGDP



2 3

Slide 4






Extracellular fluid

Intracellular fluid


ReceptorLigand


G proteinGDP



2 3

Active 2ndmessenger



4

Slide 5






Extracellular fluid

Intracellular fluid


ReceptorLigand


G proteinGDP



2 3

Active 2ndmessenger



4



5

Slide 6






The activated receptorbinds to a G protein and acti-vates it. The G protein changesshape (turns “on”), causing it torelease GDP and bind GTP (an energy source).



Extracellular fluid

G proteinGDP

Intracellular fluid

Cascade of cellular responses (The amplification effect istremendous. Each enzyme

catalyzes hundreds of reactions.)


Active 2ndmessenger




Kinase enzymes activateother enzymes. Kinase enzymestransfer phosphate groups from ATPto specific proteins and activate aseries of other enzymes that triggervarious metabolic and structuralchanges in the cell.


ReceptorLigand

1 2 3

4

5

6

Slide 7

© 2013 pearson education, inc. membrane transport: active processes two types of active processes...

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na bound na

na binding

extracellular fluid

primary active transport

na sites

expelling na

concentration gradients

cytoplasmic na bind