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1. 3 Cells: The Living Units: Part B

2. Membrane Transport: Active Processes • Two types of active processes: • Active transport • Vesicular transport • Both use ATP to move solutes across a living plasma membrane

3. Active Transport • Requires carrier proteins (solute pumps) • Moves solutes against a concentration gradient • Types of active transport: • Primary active transport • Secondary active transport

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

5. 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

6. Extracellular fluid Na+ Na+-K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Cytoplasmic Na+ binds to pump protein. P ATP K+ released ADP 6 2 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. Na+ released K+ bound P Pi K+ 5 3 K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation. Phosphorylation causes the protein tochange shape, expelling Na+ to the outside. P 4 Extracellular K+ binds to pump protein. Figure 3.10

7. 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

8. 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

9. Extracellular fluid Glucose Na+-glucose symport transporter loading glucose from ECF Na+-glucose symport transporter releasing glucose into the cytoplasm Na+-K+ pump Cytoplasm 1 2 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) Figure 3.11

10. Vesicular Transport • Transport of large particles, macromolecules, and fluids across plasma membranes • Requires cellular energy (e.g., ATP)

11. 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

12. Endocytosis and Transcytosis • Involve formation of protein-coated vesicles • Often receptor mediated, therefore very selective

13. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm 2 Protein-coatedvesicledetaches. 3 Coat proteins detachand are recycled toplasma membrane. Transportvesicle Endosome Uncoatedendocytic vesicle 4 Uncoated vesicle fuseswith a sorting vesiclecalled an endosome. 5 Transportvesicle containingmembrane componentsmoves to the plasmamembrane for recycling. Lysosome 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). (b) (a) Figure 3.12

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

15. (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. Phagosome Figure 3.13a

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

17. (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. Vesicle Figure 3.13b

18. Endocytosis • Receptor-mediated endocytosis—clathrin-coated pits provide main route for endocytosis and transcytosis • Uptake of enzymes low-density lipoproteins, iron, and insulin

19. (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. Vesicle Receptor recycled to plasma membrane Figure 3.13c

20. Exocytosis • Examples: • Hormone secretion • Neurotransmitter release • Mucus secretion • Ejection of wastes

21. The process of exocytosis Plasma membrane SNARE (t-SNARE) Extracellular fluid Fusion pore formed 1 The membrane- bound vesicle migrates to the plasma membrane. Secretory vesicle Vesicle SNARE (v-SNARE) 3 The vesicle and plasma membrane fuse and a pore opens up. Molecule to be secreted Cytoplasm 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). 4 Vesicle contents are released to the cell exterior. Fused v- and t-SNAREs Figure 3.14a

22. Summary of Active Processes • Also see Table 3.2

23. 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+)

24. Generation and Maintenance of RMP • The Na+ -K+ pump continuously ejects Na+ from cell and carries K+ back in • Some K+ continually diffuses down its concentration gradient out of cell through K+ leakage channels • Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell

25. Generation and Maintenance of RMP • Electrochemical gradient begins to attract K+ back into cell • RMP is established at the point where the electrical gradient balances the K+ concentration gradient • A steady state is maintained because the rate of active transport is equal to and depends on the rate of Na+ diffusion into cell

26. 1 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. Extracellular fluid 2 K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 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 Figure 3.15

27. Cell-Environment Interactions • Involves glycoproteins and proteins of glycocalyx • Cell adhesion molecules (CAMs) • Membrane receptors

28. 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

29. 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