Passive DiffusionTopic 8: MEMBRANE TRANSPORT (lectures 11-12)OBJECTIVES:1. Have a basic appreciation of the chemical characteristics of substances that impacttheir ability to travel across plasma membranes. 2. Know the factors that regulate the rate of passive diffusion of non-polar (lipid-soluble) and slightly polar (but uncharged) substances across the plasmamembrane.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 facilitateddiffusion and active transport.5. Understand the basic types of bulk membrane transport.Consider the structure of the plasma membrane (fig. 8.6)- phospholipid bilayer withperipheral and integral proteins. Substances that must traverse these membranes (in and out of the cell):(1) Water*, gases (O2, CO2), inorganic ions (Na+, K+, Cl-, Mg2+, PO42-, Ca2+, HCO3-,ammonia/ammonium)(2) small molecular weight organic molecules- sugars, amino acids, small lipids andlipid precursors, urea)(3) large organic molecules (proteins, lipids) and cell fragmentsPermeability = defines how easily of molecule will pass through a substancePermeability of phospholipid bilayers:(1) impermeable to charged substances (inorganic ions, charged organic moleculesetc); interior of membrane is very hydrophobic and resists penetration by chargedatoms and molecules(2) permeable to lipid-soluble (non-polar) molecules; permeability decreases as sizeof molecule increases (here we mean molecular volume).(3) also generally permeable to small slightly polar but uncharged molecules likewater *and dissolved gases (O2, CO2); these molecules are literally small enough topass beween phospholipid molecules.(4) Impermeable to proteins*NOTE: water also passes through special protein complexes in cell membranes calledAQUAPORINS.1Passive DiffusionMembrane transport of small lipid-soluble (non-polar) molecules and small slightly polarbut uncharged molecules like water and dissolved gases (O2, CO2) is by a processknown as passive diffusion [transport] (passive diffusion - the random movement ofmolecules from an area of high concentration to an area of low concentration). Fig. 8.10- a concentration gradient exists (concentration gradient = difference inconcentration between two areas or compartments). In these examples, the solutesdiffuse “downhill” from an area of high concentration to an area of low concentration.Eventually, the concentrations on both sides become equal (no concentration gradientexists) 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 membraneThus, 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 ISOTONICARE INCORRECT!!!!!!!!!!!!!)Water flows osmotically from an area of high water concentration to an area of lowerwater concentration. Water concentration is a function of the total solute concentration( = amount of dissolved materials). The higher the solute concentration, the lower thewater 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.2fig. 8.11 (replace “tonic” with “osmotic”) - two compartments separated by a waterpermeable, solute impermeable membrane; water flows from the hypo-osmotic solutionto the hyperosmotic solution until the system reaches equilibrium. At the point thesolutions are isosmotic (no water concentration gradient exists).tonicity - refers to how a solution influences the volume of a cell when it is placed in thesolution (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 changevolume 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 dueto 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 molecules-downhill movement (with a concentration gradient).facilitated diffusion- passive diffusion of molecules through the plasma membrane asmediated 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 apolarinternal region of bilayer. Transporter creates an alternative pathway which bypassesthis region. Transporters are typically very large, complex integral proteins.1. transporters/channels depend on an existing concentration gradient2. transporters/channnels can be “saturated” by the material that is being transportedmuch like an enzyme becomes saturated with substrate. This is due to the fact thatthere are a finite number of transporter/channel molecules embedded in themembrane.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 ofgating. That is, they open or close in response to specific chemical, electrical andeven mechanical stimuli.3Comparison of facilitated diffusion vs. passive diffusion- Movement of highly polar, charged atoms and small molecular
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