Topic 8: MEMBRANE TRANSPORT (lectures 11-12) OBJECTIVES: 1. Have a basic appreciation of the chemical characteristics of substances that impact their ability to travel across plasma membranes. 2. Know the factors that regulate the rate of passive diffusion of non-polar (lipidsoluble) and slightly polar (but uncharged) substances across the plasma membrane. 3. Be able to differentiate the following terms: isosmotic, hyperosmotic, hypo-osmotic, isotonic, hypertonic and hypotonic. 4. Be able to compare and contrast the processes of membrane transport by facilitated diffusion and active transport. 5. Understand the basic types of bulk membrane transport. Consider the structure of the plasma membrane (fig. 8.6)- phospholipid bilayer with peripheral and integral proteins. Substances that must traverse these membranes (in and out of the cell): (1) Water*, gases (O 2, CO 2 ), inorganic ions (Na +, K +, Cl -, Mg 2+, PO 4 2-, Ca 2+, HCO 3 -, ammonia/ammonium) (2) small molecular weight organic molecules- sugars, amino acids, small lipids and lipid precursors, urea) (3) large organic molecules (proteins, lipids) and cell fragments Permeability = defines how easily of molecule will pass through a substance Permeability of phospholipid bilayers: (1) impermeable to charged substances (inorganic ions, charged organic molecules etc); interior of membrane is very hydrophobic and resists penetration by charged atoms and molecules (2) permeable to lipid-soluble (non-polar) molecules; permeability decreases as size of molecule increases (here we mean molecular volume). (3) also generally permeable to small slightly polar but uncharged molecules like water *and dissolved gases (O 2, CO 2 ); these molecules are literally small enough to pass beween phospholipid molecules. (4) Impermeable to proteins *NOTE: water also passes through special protein complexes in cell membranes called AQUAPORINS. 1
Passive Diffusion Membrane transport of small lipid-soluble (non-polar) molecules and small slightly polar but uncharged molecules like water and dissolved gases (O 2, CO 2 ) is by a process known as passive diffusion [transport] (passive diffusion - the random movement of molecules from an area of high concentration to an area of low concentration). Fig. 8.10- a concentration gradient exists (concentration gradient = difference in concentration between two areas or compartments). In these examples, the solutes diffuse downhill from an area of high concentration to an area of low concentration. Eventually, the concentrations on both sides become equal (no concentration gradient exists) and there is no net passive diffusion. The rate of passive transport or diffusion (J) is given by the following equation: J = P x A x C where J = rate of passive transport across the membrane ( expressed as amount of material transported per unit time) P = permeability of the membrane for the diffusing molecule; this is a property of the molecule (lipid solubility and/or molecular size) A = the surface area of the membrane C = concentration gradient (the difference in concentration across the membrane Thus, rates of passive transport are linearly related to P, A and C. Passive transport of water: Osmosis (NOTE: TEXT DEFINITIONS ON PAGE 146 FOR HYPO-, HYPER- and ISOTONIC ARE INCORRECT!!!!!!!!!!!!!) Water flows osmotically from an area of high water concentration to an area of lower water concentration. Water concentration is a function of the total solute concentration ( = amount of dissolved materials). The higher the solute concentration, the lower the water concentration and vice versa. isosmotic - solutions which have the same total solute concentration (hence same water concentration) hyperosmotic- a solution which has a higher total solute concentration (lower water concentration) than another solution. hypo-osmotic- a solution which has a lower total solute concentration (higher water concentration) than another solution. 2
fig. 8.11 (replace tonic with osmotic ) - two compartments separated by a water permeable, solute impermeable membrane; water flows from the hypo-osmotic solution to the hyperosmotic solution until the system reaches equilibrium. At the point the solutions are isosmotic (no water concentration gradient exists). tonicity - refers to how a solution influences the volume of a cell when it is placed in the solution (fig. 8.12). isotonic solution- the cell neither shrinks nor swells; 0.9% NaCl solution (0.9 g NaCl + 99.1 ml water) is called isotonic saline because red blood cells do not change volume when suspended in it. hypotonic solution- cell swells; its volume increases due to water influx. In animal cells this may lead to rupture or lysis of the cell. In plants, the cells become turgid due to the presence of the wall. hypertonic solution- cell shrinks due to water efflux (loss). Animal cells shrivel and plant cells undergo plasmolysis (plasmolysis = plasma membrane pulling away from cell wall) In general, all cells have specific ways in which they avoid large changes in cell volume. Movement of highly polar, charged atoms and small molecular organic moleculesdownhill movement (with a concentration gradient). facilitated diffusion- passive diffusion of molecules through the plasma membrane as mediated by specific proteins known as transporters (organic molecules) or channels (typically inorganic ions). Fig. 8.14- cartoon of a transporter/channel; substance cannot pass through apolar internal region of bilayer. Transporter creates an alternative pathway which bypasses this region. Transporters are typically very large, complex integral proteins. 1. transporters/channels depend on an existing concentration gradient 2. transporters/channnels can be saturated by the material that is being transported much like an enzyme becomes saturated with substrate. This is due to the fact that there are a finite number of transporter/channel molecules embedded in the membrane. 3. organic molecule transporters are typically open for transport at all times. 4. inorganic ion channels may be open at all times but many show the phenomenon of gating. That is, they open or close in response to specific chemical, electrical and even mechanical stimuli. 3
Comparison of facilitated diffusion vs. passive diffusion- Movement of highly polar, charged atoms and small molecular organic molecules- uphill movement (against a concentration gradient). Active transport- the uphill, membrane transport of substances from an area of lower concentration to an area of higher concentration. (1) mediated by transport proteins which are integral proteins; these proteins are often referred to as pumps (2) also show saturation (3) uphill transport (which is thermodynamically unfavorable) is coupled to some thermodynamically favorable process - ATP hydrolysis; in this case the pumps are often referred to as ATPases 4
fig. 8.17 H + ATPase fig. 8.15 Na + :K + ATPase - cotransport: uphill transport is coupled to downhill transport of another substance (fig. 8.18, sucrose-h + cotransporter); this is the primary way in which low molecular weight organic molecules are bioconcentrated in cells. Fig. 8.16- reviews membrane transport processes. Bulk transport across membranes (fig. 8.19)- phagocytosis- cellular eating; large particles pinocytosis- cellular drinking; volumes of extracellular fluid engulfed receptor-mediated endocytosis- extracellular ligand (a molecular that binds to another molecule) binds to a membrane receptor triggering the cell to take up ligandreceptor complex 5