3

1. 3 Cells: The Living Units: Part A

2. Erythrocytes Fibroblasts Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Nerve cell Skeletal Muscle cell (e) Cell that gathers information and control body functions Smooth muscle cells (b) Cells that move organs and body parts Sperm Macrophage (f) Cell of reproduction Fat cell (c) Cell that storesnutrients (d) Cell that fights disease Figure 3.1

3. Nuclear envelope Chromatin Nucleolus Nucleus Smooth endoplasmic reticulum Plasma membrane Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements • Microtubule • Intermediate filaments Peroxisome Figure 3.2

4. Extracellular fluid (watery environment) Cholesterol Polar head of phospholipid molecule Glycolipid Glycoprotein Carbohydrate of glycocalyx Outward- facing layer of phospholipids Integral proteins Filament of cytoskeleton Peripheral proteins Bimolecular lipid layer containing proteins Inward-facing layer of phospholipids Nonpolar tail of phospholipid molecule Cytoplasm (watery environment) Figure 3.3

5. (a) Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Figure 3.4a

6. (b) Receptors for signal transduction Signal A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. Receptor Figure 3.4b

7. (c) Attachment to the cytoskeleton and extracellular matrix (ECM) Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Figure 3.4c

8. (d) Enzymatic activity Enzymes A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. Figure 3.4d

9. (e) Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. CAMs Figure 3.4e

10. (f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Figure 3.4f

11. Microvilli Plasma membranes of adjacent cells Intercellular space Basement membrane Interlocking junctional proteins Intercellular space (a) Tight junctions:Impermeable junctions prevent molecules from passing through the intercellular space. Figure 3.5a

12. Microvilli Plasma membranes of adjacent cells Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Linker glycoproteins (cadherins) (b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers. Figure 3.5b

13. Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (connexon) (c) Gap junctions: Communicating junctions allow ions and small mole- cules to pass from one cell to the next for intercellular communication. Figure 3.5c

14. Passive Processes • What determines whether or not a substance can passively permeate a membrane? • Lipid solubility of substance • Channels of appropriate size • Carrier proteins PLAY Animation: Membrane Permeability

15. Passive Processes • Simple diffusion • Carrier-mediated facilitated diffusion • Channel-mediated facilitated diffusion • Osmosis

16. Extracellular fluid Lipid- soluble solutes Cytoplasm (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer Figure 3.7a

17. Passive Processes: Facilitated Diffusion • Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which: • Exhibit specificity (selectivity) • Are saturable; rate is determined by number of carriers or channels • Can be regulated in terms of activity and quantity

18. Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7b

19. Facilitated Diffusion Using Channel Proteins • Aqueous channels formed by transmembrane proteins selectively transport ions or water • Two types: • Leakage channels • Always open • Gated channels • Controlled by chemical or electrical signals

20. Small lipid- insoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7c

21. Passive Processes: Osmosis • Movement of solvent (water) across a selectively permeable membrane • Water diffuses through plasma membranes: • Through the lipid bilayer • Through water channels called aquaporins (AQPs)

22. Water molecules Lipid billayer Aquaporin (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7d

23. (a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged Left compartment: Solution with lower osmolarity H2O Solute Solute molecules (sugar) Membrane Figure 3.8a

24. (b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move Left compartment Right compartment H2O Solute molecules (sugar) Membrane Figure 3.8b

25. Importance of Osmosis • When osmosis occurs, water enters or leaves a cell • Change in cell volume disrupts cell function PLAY Animation: Osmosis

26. (a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9

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

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

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

30. 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

31. 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

32. Extracellular fluid Na+ Na+-K+ pump K+ ATP-binding site Cytoplasm 1 Cytoplasmic Na+ binds to pump protein. Figure 3.10 step 1

33. Na+ bound P ATP ADP 2 Binding of Na+ promotesphosphorylation of the protein by ATP. Figure 3.10 step 2

34. Na+ released P 3 Phosphorylation causes the protein tochange shape, expelling Na+ to the outside. Figure 3.10 step 3

35. K+ P 4 Extracellular K+ binds to pump protein. Figure 3.10 step 4

36. K+ bound Pi 5 K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation. Figure 3.10 step 5

37. K+ released 6 K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats. Figure 3.10 step 6

38. 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

39. Extracellular fluid Na+-K+ pump Cytoplasm 1 The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. Figure 3.11 step 1

40. 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 step 2

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

42. 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

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

44. 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

45. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm Figure 3.12 step 1

46. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm 2 Protein-coatedvesicledetaches. Figure 3.12 step 2

47. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm 2 Protein-coatedvesicledetaches. 3 Coat proteins detachand are recycled toplasma membrane. Figure 3.12 step 3

48. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm 2 Protein-coatedvesicledetaches. 3 Coat proteins detachand are recycled toplasma membrane. Endosome Uncoatedendocytic vesicle 4 Uncoated vesicle fuseswith a sorting vesiclecalled an endosome. Figure 3.12 step 4

49. 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. Figure 3.12 step 5

50. 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 step 6