Lecture 5 Cell Membrane Transport

Transcription

1 Lecture 5 Cell Membrane Transport Overcoming the Cell Barrier The cell membrane is a barrier, but: Nutrients must get in Products and wastes must get out Permeability determines what moves in and out of a cell A membrane is: Impermeable if it lets nothing in or out Freely permeable if it lets anything pass Selectively permeable if it restricts movement Cell membranes are selectively permeable: Allow some materials to move freely Restrict other materials PLAY Membrane Transport: Fat- and Water-Soluble Molecules Restricted Materials Selective permeability restricts materials based on: Size Electrical charge Molecular shape Lipid solubility 1

2 Diffusion in Solutions All molecules are constantly in motion Molecules in solution move randomly Random motion causes mixing Concentration is the amount of solute in a solvent Concentration gradient: More solute in one part of a solvent than another Solutes move down a concentration gradient: Molecules mix randomly Solute spreads through solvent Eliminates concentration gradient PLAY Membrane Transport: Diffusion Factors Affecting Diffusion Rates Distance the particle has to move Molecule size: Smaller is faster Temperature: More heat, faster motion Gradient size: The difference between high and low concentration Electrical forces: Opposites attract, like charges repel Osmosis Osmosis is the movement of water across the cell membrane Osmotic Pressure is the force of a concentration gradient of water Equals the force (hydrostatic pressure) needed to block osmosis 2

3 Diffusion vs. Osmosis Effects of Osmosis on Cells Tonicity how a solution s osmolarity affects cell volume Isotonic solutions with the same solute concentration as that of the cytosol Hypertonic solutions having greater solute concentration than that of the cytosol; water leaves the cell causing crenation (shrinkage) Hypotonic solutions having lesser solute concentration than that of the cytosol; water enters the cell causing swelling and potential lysis Hydrostatic and Osmotic Pressure Hydrostatic pressure = water pressure Filtration is the passage of water and solutes through a membrane by hydrostatic pressure Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area Osmotic pressure can create an important counter force against hydrostatic pressure 3

4 KEY CONCEPT Concentration gradients tend to even out In the absence of a membrane, diffusion eliminates concentration gradients When different solute concentrations exist on either side of a selectively permeable membrane, osmosis moves water through the membrane to equalize the concentration gradients Transport Through Cell Membranes Transport through a cell membrane can be: Active (requiring energy and ATP) Passive (no energy required) 3 categories of transport Diffusion (passive) Carrier-mediated transport (passive or active) Vesicular transport (active) Diffusion and the Cell Membrane Diffusion can be simple, channel, or carrier mediated Channel & carrier mediated diffusion is: Specific: to size, charge, & interaction with the channel Subject to saturation: making the channels rate limiting PLAY Membrane Transport: Facilitated Diffusion 4

5 Active Transport Active transport proteins: Move substrates against concentration gradient Require energy, such as ATP Ion pumps move ions (Na +, K +, Ca +, Mg 2+ ) Na + -K + Exchange Pump moves both of these ions at the same time, each in the opposite direction (called antiport or countertransport) Proton Pump uses photosynthesis or food energy to create a proton concentration gradient that then is used to manufacture ATP PLAY Active Transport Sodium-Potassium Exchange Pump Active transport, carrier mediated: 1 ATP moves 3 Na + out 2 K + in This creates an electrical potential across the membrane Called the Transmembrane Potential Transmembrane Potential Voltage across a membrane Resting membrane potential the point where K + potential is balanced by the membrane potential Ranges from 20 to 200 mv Results from Na + and K + concentration gradients across the membrane Differential permeability of the plasma membrane to Na + and K + Steady state potential is maintained by active transport of ions 5

6 Proton Pump (in Mitochondrial Membranes) Expends metabolic energy to pump protons across membranes PLAY Proton Pump Types of Active Transport Primary active transport: hydrolysis of ATP phosphorylates the transport protein causing conformational change Secondary active transport: use of an exchange pump (such as the Na + -K + pump) indirectly to drive the transport of other solutes Symport system two substances move across a membrane in the same direction (also called cotransport) Antiport system two substances move across a membrane in opposite directions (also called countertransport) Vesicular Transport Also called bulk transport Transport of large particles and macromolecules across plasma membranes Directional Descriptive Terms Exocytosis moves substance from the cell interior to the extracellular space Endocytosis enables large particles and macromolecules to enter the cell Receptor-mediated Pinocytosis Phagocytosis Functional Descriptive Terms Transcytosis moving substances into, across, and then out of a cell Vesicular trafficking moving substances from one area in the cell to another Phagocytosis pseudopods engulf solids and bring them into the cell s interior 6

7 Receptor-Mediated Endocytosis Receptors (glycoproteins called clathrin) bind target molecules (ligands) Coated vesicle (endosome) carries ligands and receptors into the cell Pinocytosis Pinocytosis (cell drinking) Endosomes drink extracellular fluid Phagocytosis Phagocytosis (cell eating) pseudopodia (psuedo = false, podia = feet) engulf large objects in phagosomes 7

8 Exocytosis Is the reverse of endocytosis Summary The 7 methods of transport 8