Passive Transport PDF

Transport Across Membranes: Overcoming the Permeability Barrier Overview of Membrane Transport  Passive Transport  Along a gradient – without net energy input  Passive diffusion  Facilitated diffusion  Active Transport  Against a concentration gradien...

Transport Across Membranes: Overcoming the Permeability Barrier Overview of Membrane Transport  Passive Transport  Along a gradient – without net energy input  Passive diffusion  Facilitated diffusion  Active Transport  Against a concentration gradient Important Considerations for Transport  Solute properties  Relative solute concentrations  Availability of specific transmembrane proteins  *Availability of an appropriate energy source The Movement of a Solute Across a Membrane Is Determined by Its Concentration Gradient or Its Electrochemical Potential  The movement of a molecule that has no net charge is determined by its concentration gradient  Simple diffusion and facilitated diffusion involve exergonic movement “down” the concentration gradient (negative ΔG)  Active transport involves endergonic movement “up” the concentration gradient (positive ΔG) The Electrochemical Potential  The movement of an ion is determined by its electrochemical potential, the combined effect of its concentration gradient and the charge gradient across the membrane  The imbalance of ionic charges across a membrane creates a charge gradient, or membrane potential (Vm), across the membrane Simple Diffusion: Unassisted Movement Down the Gradient O2, CO2 and Erythrocytes  Oxygen and CO2 gas traverse the lipid bilayer readily by simple diffusion  Erythrocytes take up oxygen in the lungs, where oxygen concentration is high, and release it in the body tissues, where oxygen concentration is low  The movement of CO2 uses the same mechanism, but relative concentrations are reversed Osmosis Is the Diffusion of Water Across a Selectively Permeable Membrane  Water molecules, being uncharged, are not affected by the membrane potential  If two solutions are separated by a selectively permeable membrane, permeable to the water but not the solutes, the water will move toward the region of higher solute concentration  This movement is called osmosis  Relative concentration of solutes between cytoplasm and extracellular solution is called osmolarity hypertonic isotonic hypotonic Plant cells must maintain internal pressure (turgor pressure) to force their plasma membrane against the cell wall. To do so, the interior of the cell must be _________________ to their environment. A) Hypotonic B) Isotonic C) Hypertonic Cell Response to Osmolarity  Cells tend to shrink or swell as the solute concentration of the extracellular medium changes  For example, an animal cell in an isotonic solution (same solute concentration inside and outside the cell) will shrink and shrivel if moved to a hypertonic solution  The same cell will swell and perhaps burst (or lyse) if placed in a very hypotonic solution Cells with Cell Walls  Cells of plants, algae, fungi, and many bacteria have cell walls that keep a cell from swelling and bursting in a hypotonic solution  Instead, the cells become very firm from the built-up turgor pressure  This results from the inward movement of water  In a hypertonic solution, the plasma membrane pulls away from the cell wall by a process called plasmolysis Cells Without Cell Walls  Cells without cell walls solve the osmolarity problem by pumping out inorganic ions, reducing the intracellular osmolarity  This minimizes the intracellular osmolarity and the difference in solute concentration between the cell and the surroundings The Rate of Simple Diffusion Is Directly Proportional to the Concentration Gradient  Thermodynamically, simple diffusion is always an exergonic process, requiring no input of energy  Kinetically, the net rate of transport for a substance is proportional to its concentration difference across the membrane vinward .  v inward = rate of diffusion in moles/sec-cm2  Δ[S] = [S] outside – [S] inside  P = permeability coefficient, which depends on:  Nature of the membrane  Size, shape, polarity of the solute Facilitated Diffusion: Protein-Mediated Movement Down the Gradient  Most substances in the cell are too large or too polar to cross membranes by simple diffusion  These can move in and out of cells only with the assistance of transport proteins through facilitated diffusion  Carrier Proteins  Channel Proteins Carrier Proteins Alternate Between Two Conformational States  The alternating conformation model states that a carrier protein is an allosteric protein that alternates between two conformational states  (Shape 1) In one state, the solute-binding site of the protein is accessible on one side of the membrane  (Shape 2) The protein shifts to the alternate conformation, with the solute-binding site on the other side of the membrane, triggering solute release Carrier Proteins Are Similar to Enzymes in Their Specificity and Kinetics  Carrier proteins are analogous to enzymes  Facilitated diffusion involves binding a substrate on a specific solute-binding site  The carrier protein and solute form an intermediate  After conformational change, the “product” is released (the transported solute)  Carrier proteins are regulated by external factors Uniport Transport Coupled Transport Transport by GLUT1 (uniport) The glucose concentration gradient is maintained in liver cells due (in part) to phosphorylation of internal glucose. Which of the following processes also directly contributes to maintenance of the glucose concentration gradient? A) ion exchange B) dephosphorylation C) transcription D) polymerization E) osmosis The Erythrocyte Anion Exchange Protein: An Antiport Carrier  The anion exchange protein (also called the chloride-bicarbonate exchanger) facilitates reciprocal exchange of Cl– and HCO3– ions only  Exchange will stop if either anion is absent  The ions are exchanged in a strict 1:1 ratio The Erythrocyte Anion Exchange Protein: An Antiport Carrier Reversible Specific Dependent on concentration gradient Channel Proteins Facilitate Diffusion by Forming Hydrophilic Transmembrane Channels  Channel proteins form hydrophilic transmembrane channels that allow specific solutes to cross the membrane directly  There are three types of channels: aquaporins, ion channels and porins Aquaporins: Transmembrane Channels That Allow Rapid Passage of Water  Movement of water across cell membranes in some tissues is faster than expected given the polarity of the water molecule  Aquaporin (AQP) was discovered only in 1992  Aquaporins allow rapid passage of water through membranes of erythrocytes and kidney cells in animals, and root cells and vacuolar membranes in plants Aquaporin Structure Ion Channels: Transmembrane Proteins That Allow Rapid Passage of Specific Ions  Ion channels, tiny pores lined with hydrophilic atoms, are remarkably selective  Because most allow passage of just one ion, separate proteins are needed to transport Na+, K+, Ca2+, and Cl–, etc.  Selectivity is based both on binding sites involving amino acid side chains and on a size filter  Direction of ion movement depends on the electrochemical gradient Functions of Ion Channels  Ion channels play roles in many types of cellular communication, such as muscle contraction and electrical signaling of nerve cells  Ion channels are also needed for maintaining salt balance in cells and airways linking the lungs  A chloride ion channel, the cystic fibrosis transmembrane conductance regulator (CFTR), helps maintain the proper Cl– concentration in lungs; defects in the protein cause cystic fibrosis CFTR Protein Cl- and H2O regulation Porins: Transmembrane Proteins That Allow Rapid Passage of Various Solutes  Pores on outer membranes of bacteria, mitochondria, and chloroplasts are larger and less specific than ion channels  The pores are formed by multipass transmembrane proteins called porins  The transmembrane segments of porins cross the membrane as β barrels Structure of Porins The graph of the rate of facilitated diffusion versus substrate concentration is not linear like the graph of simple diffusion versus substrate concentration. Why? a. In facilitated diffusion there is a limited amount of active protein, so that at higher substrate concentrations, the protein becomes saturated. b. In facilitated diffusion the membrane proteins are chemical catalysts, and this is simply the plot you make to study a catalyst. c. In facilitated diffusion the proteins are not consumed by the process in which they participate (catalysis or transport). d. Actually, the facilitated diffusion graph is linear because it is still passive transport.