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Transcript of 3
PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College
C H A P T E R
Copyright © 2010 Pearson Education, Inc.
3
Cells: The Living Units: Part B
Copyright © 2010 Pearson Education, Inc.
Membrane Transport: Active Processes
• Two types of active processes:
• Active transport
• Vesicular transport
• Both use ATP to move solutes across a living plasma membrane
Copyright © 2010 Pearson Education, Inc.
Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against a concentration gradient
• Types of active transport:
• Primary active transport
• Secondary active transport
Copyright © 2010 Pearson Education, Inc.
Primary Active Transport
• Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membrane
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc.
Vesicular Transport
• Transport of large particles, macromolecules, and fluids across plasma membranes
• Requires cellular energy (e.g., ATP)
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
Endocytosis and Transcytosis
• Involve formation of protein-coated vesicles
• Often receptor mediated, therefore very selective
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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.
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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+)
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc. 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
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Cell-Environment Interactions
• Involves glycoproteins and proteins of glycocalyx
• Cell adhesion molecules (CAMs)
• Membrane receptors
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc.
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