Membrane Transport

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

2. 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

3. 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

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

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

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 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

14. 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

15. (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

16. (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

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

18. 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

19. (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

20. Summary of Passive Processes • Also see Table 3.1