Cells: The Living Units: Part B

1. 3 Cells: The Living Units: Part B

2. Membrane Transport: Active Processes • Two types of active processes 1) Active transport 2) Vesicular transport • Both require ATP to move solutes across a living plasma membrane because solute: • too large for channels • not lipid soluble • not able to move down concentration gradient © 2013 Pearson Education, Inc.

3. 1) Active Transport • Requires carrier proteins • Bind specifically and reversibly with substance • Moves solutes against concentration gradient © 2013 Pearson Education, Inc.

4. Active Transport: Two types • Primary active transport • Required energy directly from ATP hydrolysis • Secondary active transport • Required energy indirectly from ionic gradients created by primary active transport © 2013 Pearson Education, Inc.

5. Primary Active Transport • Hydrolysis of ATP  change in shape of transport protein  ‘pumps’ solutes (ions) across membrane e.g. sodium-potassium pump (Na+-K+ ATPase) • located in all plasma membranes • involved in transport of nutrients and ions • Pumps against Na+ and K+ gradients to maintain high intracellular K+ concentration and high extracellular Na+ concentration • Maintains electrochemical gradients essential for functions of muscle and nerve tissues © 2013 Pearson Education, Inc.

6. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 2 Extracellular fluid Na+ Na+–K+ pump K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. © 2013 Pearson Education, Inc.

7. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 3 Extracellular fluid Na+ Na+–K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. © 2013 Pearson Education, Inc.

8. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 4 Extracellular fluid Na+ Na+–K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. Na+ released P 3 Phosphorylation causes the pump to change shape, expelling Na+ to the outside. © 2013 Pearson Education, Inc.

9. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 5 Extracellular fluid Na+ Na+–K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. Na+ released P K+ 3 Phosphorylation causes the pump to change shape, expelling Na+ to the outside. P 4 Two extracellular K+ bind to pump. © 2013 Pearson Education, Inc.

10. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 6 Extracellular fluid Na+ Na+–K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. Na+ released K+ bound P Pi K+ 3 Phosphorylation causes the pump to change shape, expelling Na+ to the outside. 5 K+ binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. P 4 Two extracellular K+ bind to pump. © 2013 Pearson Education, Inc.

11. Figure 3.10 Primary active transport is the process in which solutes are moved across cellmembranes against electrochemical gradients using energy supplied directly by ATP. Extracellular fluid Na+ Na+–K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na+ bind to pump protein. P K+ released 6 Pump protein binds ATP; releases K+ to the inside, and Na+ sites are ready to bind Na+ again. The cycle repeats. 2 Na+ binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. Na+ released K+ bound P Pi K+ 3 Phosphorylation causes the pump to change shape, expelling Na+ to the outside. 5 K+ binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. P 4 Two extracellular K+ bind to pump. © 2013 Pearson Education, Inc.

12. Secondary Active Transport • Depends on ion gradient created by primary active transport • Energy stored in ionic gradients used indirectly to drive transport of other solutes © 2013 Pearson Education, Inc.

13. Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary activetransport. Slide 1 Extracellular fluid Glucose Na+-glucose symport transporter releases glucose into the cytoplasm Na+-glucose symport transporter loads glucose from extracellular fluid Na+-K+ pump Cytoplasm Primary active transport The ATP-driven Na+-K+ pump creates a steep concentration gradient for Na+ 1 2 Secondary active transport As Na+ diffuses back across the membrane, it also brings glucose into the cell. © 2013 Pearson Education, Inc.

14. 2) Vesicular Transport Transport of large particles, macromolecules & fluids across membrane in sacs called vesicles • Types: 1) Exocytosis—transport out of cell 2) Endocytosis—transport into cell 3 types: phagocytosis, pinocytosis, receptor-mediated endocytosis 3) Transcytosis—transport into, across and then out of cell 4) Vesicular trafficking—transport from one organelle in cell to another

15. 1) Exocytosis • Usually activated by cell-surface signal or change in membrane voltage • Substance enclosed in secretory vesicle • Functions: • Hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes © 2013 Pearson Education, Inc.

16. Slide 1 The process of exocytosis Plasma membrane SNARE (t-SNARE) Fusion pore formed Extracellular fluid 3 The vesicle and plasma membrane fuse and a pore opens up. Secretory vesicle Vesicle SNARE (v-SNARE) 1 The membrane- bound vesicle migrates to the plasma membrane. Molecule to be secreted Cytoplasm 4 Vesicle contents are released to the cell exterior. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs © 2013 Pearson Education, Inc.

17. Figure 3.14b Exocytosis. Photomicrograph of a secretory vesicle releasing its contents by exocytosis (100,000x) © 2013 Pearson Education, Inc.

18. 2) Endocytosis and Transcytosis • Involves formation of protein-coated vesicles • Very selective • Once vesicle is inside cell, it may • Fuse with lysosome & get broken down • Undergo transcytosis © 2013 Pearson Education, Inc.

19. Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 1 1 Extracellular fluid Coated pit ingests substance. Plasma membrane Protein coat (typically clathrin) Cytoplasm 2 Protein-coated vesicle deta- ches. 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle Endosome 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. 5 Transport vesicle containing membrane compone -nts moves to the plasma membrane for recycling. Lysosome 6 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). © 2013 Pearson Education, Inc.

20. Endocytosis 1) Phagocytosis • Pseudopods engulf solids and bring them into cell's interior © 2013 Pearson Education, Inc.

21. Figure 3.13a Comparison of three types of endocytosis. Phagocytosis The cell engulfs a large particle by forming projecting pseudopods ("false feet") around it and enclosing 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. Receptors Phagosome © 2013 Pearson Education, Inc.

22. Endocytosis 2) Pinocytosis (fluid-phase endocytosis) • Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell • Most cells utilise to "sample" environment • Nutrient absorption in the small intestine • Membrane components recycled back to membrane © 2013 Pearson Education, Inc.

23. Figure 3.13b Comparison of three types of endocytosis. Pinocytosis The cell "gulps" a drop of extracellular-fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated. Vesicle © 2013 Pearson Education, Inc.

24. Endocytosis 3) Receptor-mediated endocytosis • Allows specific endocytosis and transcytosis • Cells use it to concentrate materials in limited supply • Clathrin-coated pits provide main route for endocytosis and transcytosis • Uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins © 2013 Pearson Education, Inc.

25. Figure 3.13c Comparison of three types of endocytosis. Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins, 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 © 2013 Pearson Education, Inc.

26. Table 3.2 Active Membrane Transport Processes (1 of 2) © 2013 Pearson Education, Inc.

27. Generation of a Resting Membrane Potential • Resting membrane potential (RMP) • Produced by separation of oppositely charged particles (voltage) across membrane in all cells • Cells described as polarized • Voltage (electrical potential energy) only at membrane • Ranges from –50 to –100 mV in different cells • "–" indicates inside negative relative to outside © 2013 Pearson Education, Inc.

28. Selective Diffusion Establishes RMP • Electrochemical gradient established • K+ diffuses out of cell through K+ leakage channels, proteins cannot •  inside cell membrane more negative • K+ attracted back as inner face more negative • K+ equalizes across membrane at –90 mV when K+ concentration gradient balanced by electrical gradient = RMP © 2013 Pearson Education, Inc.

29. Figure 3.15 The key role of K+ in generating the resting membrane potential. Slide 1 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 © 2013 Pearson Education, Inc.

30. Active Transport Maintains Electrochemical Gradients • Na+-K+ pump continuously ejects 3Na+ from cell and carries 2K+ in • Steady state maintained because rate of active transport equal to rate of Na+ diffusion into cell • Neuron and muscle cells "upset" RMP by opening gated Na+ and K+ channels © 2013 Pearson Education, Inc.

31. WHAT YOU NEED TO KNOW • Active transport processes • define and understand • provide examples • Principles & basics of RMP