Membrane Transport

1. Membrane Transport Chapter 6

2. Take in nutrients O2 energy substrates building materials cofactors Dispose of wastes CO2 Urea Cells Need to Exchange Materials with the Extracellular Fluid

3. Cells Must Control Movements of Materials • Need to maintain complexity inside the cell • Must regulate type and amount of material entering and leaving the cell

4. Plasma Membrane • Selectively Permeable • some materials can pass readily, others cannot Fig. 3.2

5. Membrane Permeability • Size • the smaller the particle, the more permeable • small molecules (O2, CO2,H2O) can • large molecules (protein, DNA) cannot • Lipid Solubility • YES: non-polar molecules (O2, cholesterol), • NO: charged atoms/molecules (Na+, Cl-, HCO3-), large polar molecules (glucose)

6. Membrane Transport • Requires: • Permeability of the membrane • A driving force • Passive Transport • movement of particles alonga gradient • does not require energy expenditure • Active Transport • movement of particles against a gradient • requires energy expenditure

7. Some Important Terms • Solution • mixture of two(+) substances that is uniform at the molecular level • Solute • particles (molecules or ions) present in a solution • Solvent • phase (generally a liquid) in which particles are dissolved (H2O) • Concentration • amt. solute dissolved in a given volume of solution or solvent

8. Passive Membrane Transport • Simple Diffusion • movement of particles along a concentration gradient • Osmosis • diffusion of water across a semi-permeable membrane • Facilitated Diffusion • movement of particles along a concentration gradient through a carrier protein

9. Diffusion • Molecules and ions in a solution are in a constant state of motion • Tend to diffuse - become evenly dispersed throughout the solution • Diffusion = movement of particles in a solution due to random thermal motion Fig. 6.2

10. Diffusion and Concentration • Solute particles diffuse from regions of high concentration to regions of low concentration • “Down” a concentration gradient (high  low) • Continues until equilibrium is reached Fig. 6.2

11. Gas Diffusion in Cells Fig. 6.3

12. Diffusion and Ions • Ions = charged particles • Like charges repel, opposites attract • Differences in charge between two areas = electrical gradient • Ions move along an electrical gradient until charges are balanced

13. Diffusion and Ions

14. Diffusion and Ions Membrane impermeable to (-) NOTE: Electrical equilibrium may require movement against the concentration gradient

15. Electrochemical Gradient • Net movement of ions due to the combined effects of the electrical gradient and the concentration gradient • Equilibrium may be achieved across a membrane at a point of unequal concentrations and charges

16. Diffusion and Membrane Transport • Lipid bilayer determines what substances can readily pass through the membrane • if bilayer is permeable, substance can diffuse through • if bilayer is impermeable, no diffusion even if gradient exists

17. Diffusion and Membrane Transport • Substances to which the membrane is impermeable must pass via alternate means • Facilitated Diffusion - movement across the cell membrane through a carrier protein • Channel Proteins - allow flow of ions across the cell membrane • Both allow regulation of flow Fig. 6.14 Fig. 6.4

18. Factors Affecting Rate of Diffusion • magnitude of the gradient •  gradient,  rate • permeability of the membraneto the substance •  permeability,  rate • temperature of the solution •  temperature,  rate • the surface area of the membrane through which diffusion is taking place •  SA,  rate

19. Osmosis • Net diffusion of water across a semi-permeable membrane • diffusion of the solvent, not the solute Figs. 6.5, 6.6

20. Osmosis • For osmosis to occur: • the membrane must be permeable to water and impermeable to at least one of the solutes in the solution • there must be a difference in solute concentration between the two sides of the membrane

21. Osmotic Pressure • Osmosis results in changes in volume on either side of the membrane • Changes in volume could be stopped by applying an equal and opposite force • would effectively stop osmosis Figs. 6.6, 6.7

22. Osmotic Pressure • Osmotic pressure = amount of pressure that would have to be exerted in order to prevent osmosis • measure of how strongly a solution “draws water into itself” •  [solute] ,  osmotic pressure ofthe solution

23. Facilitated Diffusion • Many molecules large and/or polar molecules are needed for metabolism • cannot pass through lipid bilayer • Shuttled across membrane by carrier proteins • Facilitated diffusion – carrier-mediated transport along the conc. gradient • no energy expended by the cell Fig. 6.14

24. Properties of Carrier Proteins in Facilitated Diffusion • Specificity– transport only one or a few different substances • possess special bind sites • Saturation – limited rate of transport • at high concentrations no further increase in transport rate will accompany increases in the conc. gradient • Reversible- direction of movement across membrane is influenced by solute concentration • If [Solute]out > [Solute]in mvmt is from out in • If [Solute]in > [Solute]out mvmt is from in  out • If [Solute]out = [Solute]in net diffusion = 0 Fig. 6.13

25. Active Membrane Transport • Requires energy expenditure by the cell (use of ATP) • Active Carrier Mediated Transport - use membrane proteins to move materials againsta gradient • Vesicular Transport - move large amounts of material into and out of the cell

26. Active Carrier-Mediated Transport • A carrier-mediated transport system that moves a substanceagainst its EC gradient across a cell membrane • requires ATP usage • pumps substances from low to high concentrations

27. Example: Ca2+pump • Ca2+ binds to protein • ATP breakdown causes protein to change shape AND affinity for Ca2+ • Ca+ ejected on opposite side of the membrane Fig. 6.16

28. Example: Na+/ K+ pump • Pumps Na+ out and K+ in • 3 Na+ out per 2 K+ in • Generates concentration gradients • Generates electrical gradient Fig. 6.17

29. ACMT vs. Facilitated Diffusion • Similarities • Carrier Protein Mediated • Exhibit Chemical Specificity • Differences • ACMT requires energy (ATP) • Binding affinity of carrier changes in ACMT • does not change for facilitated diffusion - gradient determines net movement

30. Types of Active Carrier-Mediated Transport • Primary Active Transport • hydrolysis (breakdown) of ATP directly required for the function of the carrier • e.g. Ca2+ pump, Na+/K+ pump Figs. 6.16 & 6.17

31. Types of Active Carrier-Mediated Transport • Secondary Active Transport (Coupled Transport) • energy needed for movement of a substance against gradient is provided by the movement of another substance along its gradient • Example: Na+-glucose cotransport • indirectly requires ATP via Na+/K+ pump (establishes gradient) Figs. 6.18 & 6.19

32. Vesicular Transport • Transport of vesicle contents across cell membranes • “bulk transport” - move large amounts of material • very large molecules can be moved this way • Two types of movement • exocytosis - movement of material out of the cell • hormones, neurotransmitters, etc. • endocytosis - movement of material into the cell • cellular debris, bacteria, etc. Fig. 6.20