lecture 15 plasma membrane transport active transport pp72-79

Lecture 15plasma membrane transport

Active transportpp72-79

Active Processes• Whenever a cell uses the bond energy of ATP

to move solutes across the membrane, the process is referred to as active.

• Substances moved actively across the plasma membrane are usually unable to pass in the necessary direction by passive transport processes. The substance may be too large to pass through the channels, incapable of dissolving in the lipid bilayer, or unable to move down its concentration gradient.

Membrane Transport: Active Processes

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

• Both use ATP to move solutes across a living plasma membrane

Active transport• like carrier-mediated facilitated diffusion,

requires carrier proteins that combine specifically and reversibly with the transported substances. However, facilitated diffusion always honors concentration gradients because its driving force is kinetic energy. In contrast, the active transporters or solute pumps move solutes, most importantly ions (such as Na+, K+, and Ca2+), “uphill” against a concentration gradient. To do this work, cells must expend the energy ofATP.

Figure 3.7b

Lipid-insoluble solutes (such as sugars or amino acids)

(b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein

Active Transport

• Requires carrier proteins (solute pumps)• Moves solutes against a concentration

gradient• Types of active transport:

– Primary active transport– Secondary active transport

Primary Active Transport

• Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membrane

Primary Active Transport

• Sodium-potassium pump (Na+-K+ ATPase)– Located in all plasma membranes– Involved in primary and secondary active transport of

nutrients and ions– Maintains electrochemical gradients essential for

functions of muscle and nerve tissues

Figure 3.10

Extracellular fluid

K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats.

Binding of Na+ promotesphosphorylation of the protein by ATP.

Cytoplasmic Na+ binds to pump protein.

Na+

Na+-K+ pump

K+ released

ATP-binding siteNa+ bound

Cytoplasm

ATPADP

P

K+

K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation.

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

Extracellular K+ binds to pump protein.

Na+ released

K+ bound

P

K+

PPi

1

2

3

4

5

6

Figure 3.10 step 1

Extracellular fluid


Na+

Na+-K+ pump

ATP-binding site

Cytoplasm

K+

1

Figure 3.10 step 2


Na+ bound

ATPADP

P

2

Figure 3.10 step 3


Na+ released

P

3

Figure 3.10 step 4


P

K+

4

Figure 3.10 step 5


K+ bound

Pi

5

Figure 3.10 step 6


K+ released

6

Figure 3.10

Extracellular fluid




Na+

Na+-K+ pump

K+ released

ATP-binding siteNa+ bound

Cytoplasm

ATPADP

P

K+




Na+ released

K+ bound

P

K+

PPi

1

2

3

4

5

6

Secondary Active Transport

• Depends on an ion gradient created by primary active transport

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

Secondary Active Transport

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

direction– Antiport system: Two substances transported in opposite

directions

Symport

• carries 2 or more solutes thru the membrane simultaneously in the SAME direction

• Cotransport = process

Antiport

• Carriers 2 or more solutes in the OPPOSITE directions

• Counter transport = process

Figure 3.11

The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell.

As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradientinto the cell. (ECF = extracellular fluid)

Na+-glucosesymporttransporterloadingglucose fromECF

Na+-glucosesymport transporterreleasing glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm

Extracellular fluid

1 2

Figure 3.11 step 1


Na+-K+

pump

Cytoplasm

Extracellular fluid

1

Figure 3.11 step 2


As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradientinto the cell. (ECF = extracellular fluid)

Na+-glucosesymporttransporterloadingglucose fromECF

Na+-glucosesymport transporterreleasing glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm

Extracellular fluid

1 2

Vesicular Transport

• Transport of large particles, macromolecules, and fluids across plasma membranes

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

Vesicular Transport

• Functions:– Exocytosis—transport out of cell – Endocytosis—transport into cell– Transcytosis—transport into, across, and then out

of cell– Substance (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

Types• Endocytosis – vesicular process that brings

matter INTO the cell

• Exocytosis – vesicular process that releases matter OUTSIDE the cell

Exocytosis

Figure 3.12a

Vesicular Transport

• Transcytosis – moving substances into, across, and then out of a cell

• Vesicular trafficking – moving substances from one area in the cell to another

• Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior

Vesicular Transport

• Fluid-phase endocytosis – the plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell

• Receptor-mediated endocytosis – clathrin-coated pits provide the main route for endocytosis and transcytosis

• Non-clathrin-coated vesicles – caveolae that are platforms for a variety of signaling molecules

Figure 3.12

Coated pit ingestssubstance.

Protein-coatedvesicledetaches.

Coat proteins detachand are recycled toplasma membrane.

Uncoated vesicle fuseswith a sorting vesiclecalled an endosome.

Transportvesicle containing

membrane componentsmoves to the plasma

membrane for recycling.

Fused vesicle may (a) fusewith lysosome for digestionof its contents, or (b) deliverits contents to the plasmamembrane on theopposite side of the cell(transcytosis).

Protein coat(typicallyclathrin)

Extracellular fluid Plasmamembrane

Endosome

Lysosome

Transportvesicle

(b)(a)

Uncoatedendocytic vesicle

Cytoplasm

1

2

3

4

5

6

Figure 3.12 step 1




Cytoplasm

1

Figure 3.12 step 2





Cytoplasm

1

2

Figure 3.12 step 3






Cytoplasm

1

2

3

Figure 3.12 step 4







EndosomeUncoatedendocytic vesicle

Cytoplasm

1

2

3

4

Figure 3.12 step 5







Endosome

Transportvesicle


Cytoplasm

1

2

3

4

5 Transportvesicle containing



Figure 3.12 step 6





Fused vesicle may (a) fusewith lysosome for digestionof its contents, or (b) deliverits contents to the plasmamembrane on theopposite side of the cell(transcytosis).



Endosome

Lysosome

Transportvesicle

(b)(a)


Cytoplasm

1

2

3

4

5

6

Transportvesicle containing



Endocytosis

• Phagocytosis—pseudopods engulf solids and bring them into cell’s interior– Macrophages and some white blood cells

Figure 3.13a

Phagosome

(a) PhagocytosisThe cell engulfs a large particle by forming pro-jecting pseudopods (“false feet”) around it and en-closing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the 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

• Fluid-phase endocytosis (pinocytosis)—plasma membrane infolds, bringing extracellular fluid and solutes into interior of the cell – Nutrient absorption in the small intestine

Figure 3.13b

Vesicle

(b) PinocytosisThe cell “gulps” drops 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—clathrin-coated pits provide main route for endocytosis and transcytosis– Uptake of enzymes low-density lipoproteins, iron,

and insulin

Figure 3.13c

Vesicle

Receptor recycledto plasma membrane

(c) Receptor-mediatedendocytosisExtracellular substances bind to specific receptor proteins in regions of coated pits, 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

• Examples: – Hormone secretion – Neurotransmitter release – Mucus secretion – Ejection of wastes

Summary of Active Processes

• Also see Table 3.2

Process Energy Source Example

Primary active transport ATP Pumping of ions across membranes

Secondary active transport

Ion gradient Movement of polar or charged solutes across membranes

Exocytosis ATP Secretion of hormones and neurotransmitters

Phagocytosis ATP White blood cell phagocytosis

Pinocytosis ATP Absorption by intestinal cells

Receptor-mediated endocytosis

ATP Hormone and cholesterol uptake

Thank you

Membrane Potential

• Separation of oppositely charged particles (ions) across a membrane creates a membrane potential (potential energy measured as voltage)

• Resting membrane potential (RMP): Voltage measured in resting state in all cells – Ranges from –50 to –100 mV in different cells– Results from diffusion and active transport of ions

(mainly K+)

Generation and Maintenance of RMP

1. The Na+ -K+ pump continuously ejects Na+ from cell and carries K+ back in

2. Some K+ continually diffuses down its concentration gradient out of cell through K+ leakage channels

3. Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell

Generation and Maintenance of RMP

4. Electrochemical gradient begins to attract K+ back into cell

5. RMP is established at the point where the electrical gradient balances the K+ concentration gradient

6. A steady state is maintained because the rate of active transport is equal to and depends on the rate of Na+ diffusion into cell

Figure 3.15

1

2

3

K+ diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K+ results in a negative charge on the inner plasma membrane face.

K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face.

A negative membrane potential(–90 mV) is established when the movement of K+ out of the cell equals K+ movement into the cell. At this point, the concentration gradient promoting K+ exit exactly opposes the electrical gradient for K+ entry.

Potassiumleakagechannels

Protein anion (unable tofollow K+ through themembrane)Cytoplasm

Extracellular fluid

Cell-Environment Interactions

• Involves glycoproteins and proteins of glycocalyx– Cell adhesion molecules (CAMs)– Membrane receptors

Roles of Cell Adhesion Molecules

• Anchor cells to extracellular matrix or to each other

• Assist in movement of cells past one another• CAMs of blood vessel lining attract white

blood cells 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 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)

• G protein–linked receptors—ligand binding activates a G protein, affecting an ion channel or enzyme or causing the release of an internal second messenger, such as cyclic AMP

Figure 3.16

1 Ligand (1st messenger) binds to the receptor.

The activated receptor binds to a G protein and activates it.

Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change.

Extracellular fluid

Intracellular fluid

GDP Active 2ndmessenger

Activatedkinaseenzymes

Effector protein(e.g., an enzyme)

Receptor

G protein

Ligand

Cascade of cellular responses (metabolic and structural changes)

Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell

Second messengers activate other enzymes or ion channels

Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses.

Inactive 2nd messenger

2 3

4

5

6

Figure 3.16 step 1


Extracellular fluid

Intracellular fluid

ReceptorLigand

Figure 3.16 step 2



Extracellular fluid

Intracellular fluid

GDP

Receptor

G protein

Ligand

2

Figure 3.16 step 3




Extracellular fluid

Intracellular fluid

GDP


Receptor

G protein

Ligand

2 3

Figure 3.16 step 4




Extracellular fluid

Intracellular fluid



Receptor

G protein

Ligand

Activated effector enzymes catalyze reactions that produce 2nd messengers in the cellInactive 2nd

messenger

2 3

4

Figure 3.16 step 5




Extracellular fluid

Intracellular fluid




Receptor

G protein

Ligand




2 3

4

5

Figure 3.16 step 6




Extracellular fluid

Intracellular fluid




Receptor

G protein

Ligand

Cascade of cellular responses (metabolic and structural changes)



Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses.


2 3

4

5

6

lecture 15 plasma membrane transport active transport pp72-79

Documents

binding of na

pump protein

expelling na

na sites

na releasedp3figure

na boundatpadpp2figure

protein carrier specific

active processeswhenever