Transport Processes

1. Transport Processes AP 151

2. Membrane Transport Plasma membrane is selectively permeable Impermeable membrane - membrane though which nothing can pass Freely permeable membrane - any substance can pass through it Selectively permeable membrane - permits free passage of some materials and restricts passage of others Distinction may be based on size, electrical charge, molecular shape, lipid solubility Cells differ in their permeabilities; depending on what lipids and proteins are present in the membrane and how these components are arranged.

4. Types of Transport Processes Diffusion results from random motion of particles (ions, molec.) is a passive process Carrier-mediated transport Requires the presence of specialized integral proteins Can be passive or active Vesicular transport Movment of materials with small membranous sacs, or vesicles Always an active process

5. Membrane Permeability Diffusion through lipid bilayer Nonpolar, hydrophobic substances diffuse through lipid layer; these are �lipid soluble� or lipophilic (fat-loving) substances Diffusion through channel proteins water and charged hydrophilic solutes diffuse through channel proteins; these are lipid insoluble or lipophobic (fat fearing) substances Cells control permeability by regulating number of channel proteins

6. Simple Diffusion Net movement of particles from area of high concentration to area of low concentration due to their constant, random motion Difference between the high and low concentrations is a concentration gradient Diffusion tends to eliminate the gradient Also known as movement �down the concentra- tion gradient�

7. Diffusion Examples: Scent of fresh flowers, drop of ink coloring a glass of water, movement of oxygen and CO2 through cell membranes Simple diffusion � nonpolar and lipid-soluble substances Diffuse directly through the lipid bilayer Diffuse through channel proteins

8. Factors that Influence Diffusion Rates Distance - The shorter the distance, the more quickly [ ] gradients are eliminated Few cells are father than 125 microns from a blood vessel Molecular Size Ions and small molecules diffuse more rapidly Temperature - ? temp., ? motion of particles Steepness of concentrated gradient - The larger the [ ] gradient, the faster diffusion proceeds Membrane surface area - The larger the area, the faster diffusion proceed

9. Diffusion Across Membranes Simple Diffusion Lipophilic substances can enter cells easily because they diffuse through the lipid portion of the membrane Examples are fatty acids, steroids, alcohol, oxygen, carbon dioxide, and urea, Channel-Mediated Diffusion Membrane channels are transmembrane proteins Only 0.8 nm in diameter Used by ions, very small water-soluble compounds Much more complex than simple diffusion Are there enough channels available? Size and charge of the ion affects which channels it can pass through

10. Diffusion Through the Plasma Membrane

11. Effect of Membrane Permeability on Diffusion

12. Osmosis: A Special Case of Diffusion Each solute in the intra- and extracellular fluids diffuses as if it were the only material in solution. From more to less, i.e., down the [ ] gradient Some into the cytosol, others out of the cytosol Yet, total concentration of ions and molecules on either side of the membrane stays the same This equilibrium persists because a typical cell membrane is freely permeable to water. Whenever a solute concentration gradient exist, a concentration gradient for water also exists. Thus, the higher the solute concentration, the lower the water concentration.

13. Osmosis - By Definition Movement of water Across a selectively permeable membrane Down its concentration gradient (from high to low concentration) Toward the solution containing the higher solute concentration This solution has a lower water concentration Continues until water concentrations and solute concen-trations are the same on either side of the membrane

14. Effect of Membrane Permeability on Diffusion and Osmosis

15. Osmolarity and Tonicity Mole - the gram molecular weight of a substance 1 mole of Glucose =180; 1 mole of NaCl = 58.5 Molarity - the number of moles of solute per liter of solution 1.0 M glucose contains 180 g/L; 1.0 M NaCl contains 58.5 g/L Most body fluids are less concentrated than 1 M; use mM (millimolar) or �M (micromolar) concentrations --10-3 and 10-6, respectively. Osmolarity = the total solute concentration in an aqueous solution Osmolarity = molarity (mol/L) x # of particles in solutions A 1 M Glucose solution = 1 Osmolar (Osm) But a 1 M NaCl soln = 2 Osmolar because NaCl dissociates into 2 particles (Na and Cl) whereas Glucose does not A 1 M MgCl2 solution = what osmolarity???? __________ Physiological solutions are expressed in milliosmoles per liter (mOsm/L) blood plasma = 300 mOsm/L or 0.3 Osm/L

16. Tonicity Tonicity - ability of a solution to affect fluid volume and pressure within a cell depends on concentration and permeability of solute Isotonic solution solution with the same solute concentration as that of the cytosol; normal saline Hypotonic solution lower concentration of nonpermeating solutes than that of the cytosol (high water concentration) cells absorb water, swell and may burst (lyse) Hypertonic solution has higher concentration of nonpermeating solutes than that of the cytosol (low water concentration) cells lose water + shrivel (crenate)

17. Osmosis and Cells Important because large volume changes caused by water movement disrupt normal cell function Cell shrinkage or swelling Isotonic: cell neither shrinks nor swells Hypertonic: cell shrinks (crenation) Hypotonic: cell swells (lysis)

18. Effects of Tonicity on RBCs

19. Filtration Cell membrane works like a sieve Depends on pressure difference on either side of a partition Moves from side of greater pressure to lower Water and small molecules move through the pores of the membrane while large molecules don�t. Example: urine formation in the kidneys.

20. Carrier Mediated Transport Many molecules cannot enter or leave cell by diffusion CMT utilizes proteins to carry solutes across cell membrane Characteristics of mediated transport: Specificity - each transport protein binds to and transports only a single type of molecule or ion Competition - results from similar molecules binding to the same protein. Saturation - rate of movement of molecules is limited by the number of available transport proteins

21. Membrane Carriers Uniporter carries only one solute at a time Symport carries 2 or more solutes simultaneously in same direction (cotransport) Antiport carries 2 or more solutes in opposite directions (countertransport) sodium-potassium pump brings in K+ and removes Na+ from cell Any carrier type can use either facilitated diffusion or active transport

22. Saturation of a Carrier Protein When the concentration of x molecules outside the cell is low, the transport rate is low because it is limited by the number of molecules available to be transported. When more molecules are present outside the cell, as long as enough carrier proteins are available, more molecules can be transported; thus, the transport rate increases. The transport rate is limited by the number of carrier proteins and the rate at which each carrier protein can transport solutes. When the number of molecules outside the cell is so large that the carrier proteins are all occupied, the system is saturated and the transport rate cannot increase.

23. CMT: Facilitated Diffusion Glucose and amino acids are insoluble in lipids and too large to fit through membrane channels Passive process, i.e. no ATP used Solute binds to receptor on carrier protein Latter changes shape then releases solute on other side of membrane Substance moved down its concentration gradient

24. CMT: Active Transport Uses ATP to move solutes across a membrane It is not dependent on a [ ] gradient Can move substances against their [ ] gradients - i.e. from lower to higher concentrations! Wow! Allows for greater accumulation of a substance on one side of the membrane than on the other. Carrier proteins utilized called ion or exchange pumps. Ion pumps: actively transport Na+, K+, Ca++, Cl- Exchange pumps: Na+-K+ pump

25. Types of Active Transport

26. Sodium-Potassium Pump

27. Functions of Na+ -K+ Pump Regulation of cell volume �fixed anions� attract cations causing osmosis cell swelling stimulates the Na+- K+ pump to ? ion concentration, ? osmolarity and cell swelling Heat production (thyroid hormone increase # of pumps; heat a by-product) Maintenance of a membrane potential in all cells pump keeps inside negative, outside positive Secondary active transport (No ATP used) steep concentration gradient of Na+ and K+ maintained across the cell membrane carriers move Na+ with 2nd solute easily into cell SGLT saves glucose in kidney

28. Secondary Active Transport Ions or molecules move in same (symport) or different (antiport) direction. Is the movement of glucose a symporter example or an antiporter example? This example shows cotransport of Na+ and glucose. A sodium-potassium exchange pump maintains a concentration of Na that is higher outside the cell than inside. Active transport. Na moves back into the cell by a carrier protein that also moves glucose. The concentration gradient for Na provides the energy required to move glucose against its concentration gradient.

29. Vesicular Transport Transport large particles or fluid droplets through membrane in vesicles uses ATP Exocytosis �transport out of cell Endocytosis �transport into cell phagocytosis � engulfing large particles pinocytosis � taking in fluid droplets receptor mediated endocytosis � taking in specific molecules bound to receptors

30. Vesicular TransportEndocytosis Packaging of extracellular materials in vesicles at the cell surface Involves relatively large volumes of extracellular material Requires energy in the form of ATP Three major types Receptor-mediated endocytosis Pinocytosis Phagocytosis

31. Receptor Mediated Endocytosis A selective process Involves formation of vesicles at surface of membrane Vesicles contain receptors on their membrane Vesicles contain specific target molecule in high concentration Clathrin-coated vesicle in cytoplasm uptake of LDL from bloodstream If receptors are lacking, LDL�s accumulate and hypercholesterolemia develops

32. Receptor Mediated Endocytosis

33. Vesicular TransportPinocytosis or �Cell-Drinking� Taking in droplets of ECF occurs in all human cells Not as selective as �receptor-mediated endocytosis� Membrane caves in, then pinches off into the cytoplasm as pinocytotic vesicle

34. Vesicular TransportPhagocytosis or �Cell-Eating�

35. Vesicular Transport: Exocytosis Secreting material or replacement of plasma membrane

36. Passive Membrane Transport � Review -

37. Active Membrane Transport � Review