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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. 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 protein and Na + sites are ready to bind Na + again. The cycle repeats. Binding of Na+ promotes phosphorylation of the protein by ATP. Cytoplasmic Na + binds to pump protein. Na + Na + -K + pump K + released ATP-binding site Na + bound Cytoplasm ATP ADP P K+K+ K + binding triggers release of the phosphate. Pump protein returns to its original conformation. Phosphorylation causes the protein to change shape, expelling Na + to the outside. Extracellular K + binds to pump protein. Na + released K + bound P K+K+ P PiPi
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 gradient into the cell. (ECF = extracellular fluid) Na + -glucose symport transporter loading glucose from ECF Na + -glucose symport transporter releasing glucose into the cytoplasm Glucose Na + -K + pump Cytoplasm Extracellular fluid 12
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 ingests substance. Protein- coated vesicle detaches. Coat proteins detach and are recycled to plasma membrane. Uncoated vesicle fuses with a sorting vesicle called an endosome. Transport vesicle containing membrane components moves to the plasma membrane for recycling. 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). Protein coat (typically clathrin) Extracellular fluid Plasma membrane Endosome Lysosome Transport vesicle (b) (a) Uncoated endocytic vesicle Cytoplasm
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) Phagocytosis The 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) Pinocytosis The 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 recycled to plasma membrane (c) Receptor-mediated endocytosis Extracellular 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, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). The process of exocytosis Extracellular fluid Plasma membrane SNARE (t-SNARE) Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm Fused v- and t-SNAREs 3 The vesicle and plasma membrane fuse and a pore opens up. 4 Vesicle contents are released to the cell exterior. Fusion pore formed
Copyright © 2010 Pearson Education, Inc. Summary of Active Processes Also see Table 3.2 ProcessEnergy SourceExample Primary active transportATPPumping of ions across membranes Secondary active transport Ion gradientMovement of polar or charged solutes across membranes ExocytosisATPSecretion of hormones and neurotransmitters PhagocytosisATPWhite blood cell phagocytosis PinocytosisATPAbsorption by intestinal cells Receptor-mediated endocytosis ATPHormone 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 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. Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm Extracellular fluid
Copyright © 2010 Pearson Education, Inc. 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
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