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1. 3 Cells: The Living Units: Part A

2. (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

3. Membrane Junctions • Three types: • Tight junction • Desmosome • Gap junction

4. Membrane Junctions: Tight Junctions • Prevent fluids and most molecules from moving between cells • Where might these be useful in the body?

5. 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

6. Membrane Junctions: Desmosomes • “Rivets” or “spot-welds” that anchor cells together • Where might these be useful in the body?

7. 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

8. Membrane Junctions: Gap Junctions • Transmembrane proteins form pores that allow small molecules to pass from cell to cell • For spread of ions between cardiac or smooth muscle cells

9. 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

10. Membrane Transport • Plasma membranes are selectively permeable • Some molecules easily pass through the membrane; others do not

11. Types of Membrane Transport • Passive processes • No cellular energy (ATP) required • Substance moves down its concentration gradient • Active processes • Energy (ATP) required • Occurs only in living cell membranes

12. 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

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

14. Passive Processes: Simple Diffusion • Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer PLAY Animation: Diffusion

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

16. 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

17. Facilitated Diffusion Using Carrier Proteins • Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) • Binding of substrate causes shape change in carrier

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. Passive Processes: Osmosis • Water concentration is determined by solute concentration because solute particles displace water molecules • Osmolarity: The measure of total concentration of solute particles • When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

24. (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

25. (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

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

27. Tonicity • Tonicity: The ability of a solution to cause a cell to shrink or swell • Isotonic: A solution with the same solute concentration as that of the cytosol • Hypertonic: A solution having greater solute concentration than that of the cytosol • Hypotonic: A solution having lesser solute concentration than that of the cytosol

28. (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

29. Summary of Passive Processes • Also see Table 3.1