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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College

C H A P T E R

Copyright © 2010 Pearson Education, Inc.

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Cells: The Living Units: Part B


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

• 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

Copyright © 2010 Pearson Education, Inc. 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


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


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


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


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


Endocytosis

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

• Macrophages and some white blood cells

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. 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

Copyright © 2010 Pearson Education, Inc. Figure 3.14a

1 The membrane-bound vesicle migrates to the plasma membrane.

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

The process of exocytosisExtracellular

fluid

Plasma membraneSNARE (t-SNARE)

Secretoryvesicle

VesicleSNARE(v-SNARE)

Molecule tobe secretedCytoplasm

Fusedv- and

t-SNAREs

3 The vesicleand plasma membrane fuse and a pore opens up.

4 Vesiclecontents are released to the cell exterior.

Fusion pore formed


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


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


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

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