CellsCloser Look at Cell Membranes Chapter 4b
Cell Membranes Show Selective Permeability O2, CO2, and other small nonpolar molecules;and H2O C6H12O6, andother large, polar (water-soluble) molecules;ions such as H+, Na+, K+,CI-, Ca++; plus H2O hydrogen-bonded to them X
Membrane Crossing Mechanisms Diffusion across lipid bilayer Passive transport Active transport Endocytosis Exocytosis
Transport Proteins • Span the lipid bilayer • Interior is able to open to both sides • Change shape when they interact with solute • Play roles in active (ATP or energy dependent) and passive transport (along electrical or concentration gradients)
Passive Transport • Flow of solutes through the interior of passive transport proteins down their electrical or concentration gradients • Passive transport proteins allow solutes to move both ways • Does not require any energy input
Passive Transport solute
Active Transport • Net diffusion of solute is against concentration gradient • Transport protein must be activated • ATP gives up phosphate to activate protein • Binding of ATP changes protein shape and affinity for solute
High solute concentration Active Transport Low solute concentration • ATP gives up phosphate to activate protein • Binding of ATP changes protein shape and affinity for solute P ATP ADP P P P
Bulk Transport Exocytosis Endocytosis
Osmosis • Diffusion of water molecules across a selectively permeable membrane (water can but solute cannot cross it) • Direction of net flow is determined by water concentration gradient (always from higher to lower H20 concentration) • Side with the most solute molecules has the lowest water concentration
Osmolarity • Refers to relative solute concentration of two fluids • Hyperosmotic - having more solutes • Iso-osmotic - having same amount • Hypo-osmotic - having fewer solutes
Tonicity • The effects of relative differences in solute concentration of two fluids or effects of other complex factors on H2O movement across cell membranes. They all refer to the external fluid. • Hypertonic - external fluid pulls H2O out of cells; the cell volume decreases (shrinkage occurs) • Isotonic - it has no effect on cell volume • Hypotonic - external fluid favors H2O entry into cells; the cell volume increases (swelling occurs)
Tonicity and Osmosis 2% sucrose water 10% sucrose 2% sucrose
selectively permeable membrane between two compartments water molecule protein molecule Fig. 5.21, p. 88
HYPOTONIC SOLUTION HYPERTONIC SOLUTION Increase in Fluid Volume compartment 1 compartment 2 membrane permeable to water but not to solutes fluid volume increases In compartment 2
Pressure • Pressure is force (F) applied over a surface area. It is defined as • P = F/unit area over which it is applied • Gases are highly compressible. A decrease in volume (V) or an increase in temperature (T) or an increase in gaseous matter (n) will increase the pressure (P=nRT/V).
Pressure • Liquids and solids are highly incompressible (very large amounts of pressure are required to slightly change the volume). Therefore a very tiny change in the volume of a compressed fluid or solid will cause an extremely large change in pressure. Example: Try to expand the film of water between two slides by pulling them apart-it is hard!
Pressure and Osmosis • Hydrostatic pressure • Pressure exerted by fluid on the walls that contain it and on the selective membrane • The greater the solute concentration of the fluid, the greater the hydrostatic pressure needed to oppose the osmotic entry of H20 • Osmotic pressure (hydrostatic) • Amount of pressure necessary to prevent further increase of a solution’s volume, an increase caused by the net entry of H20
Bioelectricity • The membranes of all normal living cells are electrically polarized, usually with the cytoplasmic side of the plasma membrane negatively charged relative to extracellular side. Polarization means separation of positive and negative charges across a cell membrane.
Na + Na + Na + Na+ Na + Na + Na + Na+ Na+ Na + Na+ Na + Na+ outside Na + Na+ Na + Na + Na+ Na + plasma membrane _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Protein-(COO-)nOrganic-PO-24 Cl- Other Anions inside K+ K+ K+
Charge separation across cell membranes is electrical work • Like charges repel and opposite attract each other by way of electrostatic forces • Oppositely charged ions in solution tend to be as close to each other as their thermal kinetic energy will allow. At equilibrium their tendency to come together is just balanced by their tendency to drift apart due to motion. • This equilibrium defines electrical neutrality, characterized by a minimal average distance separating the oppositely charged ions.
Charge separation across cell membranes is electrical work • Moving the ions apart beyond this minimal distance requires expenditure of energy (ATP) by pumps like the Na+, K+ ATPase which traverses the plasma membrane. Once the charges are separated, electrical energy is stored as they are kept apart by the membrane’s impermeable insulating lipid bilayer. This stored energy per unit test charge is called the cell membrane potential and ranges from -40 to -100 mV.
3 Na+ pumped out K+ leaks out intersitial fluid outside membrane neuron’s plasma membrane Na+,K+ ATPase cytoplasm next to the membrane Cl- leaks in Na+ leaks in 2 K+ pumped in Fig. 30.4, p. 489
What polarizes the plasma membrane? -1,2 • 1) Electrogenic activity of the Na+, K+ pump that kicks out 3 Na+ ions for every 2 K+ ions moved into the cell. This generates a deficiency of positive charges on the cytoplasmic side of the cell membrane or a negative potential. • 2) Impermeable anions (large negatively charged proteins, organic phosphates, Cl- ions, etc.) which are partially neutralized by intracellular K+ ions.
INTERSTITIAL FLUID K+/Na+ pump Inactivated voltage-gated Na+ channel Open voltage-gated Na+ channel lipid bilayer of plasma membrane trigger zone CYTOPLASM Fig. 30.3, p. 489
What polarizes the plasma membrane?- 3a • 3) An outward K+ concentration gradient coupled to a high resting selective permeability to K+ ions and a low resting selective permeabilty to Na+ ions. • Consider a totally impermeable membrane separating two solutions, each with a different concentration of KCl, which almost completely dissociates into K+ and Cl- ions in each solution.
polarizes the plasma membrane? - 3b • Continued... • Let one solution called “external” contain 5 mM KCl • Let the other solution called ”internal” contain 145 mM KCl • The difference in concentration of both K+ and Cl- ions across the membrane is called an ionic concentration gradient, which is 145 mM / 5 mM = approx. 30 fold for each of the two ions. • Suppose we now poke many holes in the membrane, so that they will all selectively allow only K+ ions to pass through, but not Cl- ions or any other ion.
What polarizes the plasma membrane - 3c • Continued... • The K+ ions will tend to diffuse from the higher concentration (internal solution) to the lower concentration (external solution). The same tendency will apply to Cl- ions. However only K+ ions are allowed to cross the K+ selective membrane. • As each K+ ion goes through the pore it leaves behind a Cl- ion that begins to pull back on it due to electrostatic attraction between the separating opposite charges.
What polarizes the plasma membrane? - 3d • Thus, as K+ ions move across they line up along the opposite (outer) side of the membrane in an area confined very close to the membrane and cannot move into the bulk of the external solution by virtue of the backward pull by internal Cl- ions, which also line up similarly along the internal side of the membrane. • Finally a true equilibrium is established when the negative membrane potential pulling on the K+ ions inward just balances the tendency of the outward concentration gradient to push the K+ ions outward.
What polarizes the plasma membrane? - 3e • No energy input is required to maintain an equilibrium. Surprisingly about 40,000 separated charges lined along each side of the K+ selective membrane can ward off the mixing of 145 - 5 = 140 millimoles/liter of K+ ions in the 2 bulk solutions (1 millimole = 1*1O20 ions!), thus keeping the concentration gradient across the membrane essentially unchanged. The K+ equilibrium potential is about -100 mV with the inside negative relative to the outside. An abbreviation for the K+ equilibrium potential is EK.
What polarizes the plasma membrane? - 3f • If instead the external solution had 150 mM NaCl, the internal one had 15 mM NaCl, and the membrane were made selectively permeable to Na+ ions (only), the 10 fold inward (from outside towards the inside) concentration gradient would generate an analogous Na+ equilibrium potential of opposite polarity or of +60 mV with the inside positive relative to the outside. This potential is abbreviated as ENa • Real plasma membranes are not 100% selective for either K+ or Na+. Thus their permeability (P) to K+ versus their permeability to Na+ (or PK/PNa) will determine whether the cell’s membrane potential is closer to EK (-100 mV) or to ENa (+60mV).
What polarizes the plasma membrane?- 4a • 4) Positive outward ionic currents do. Currents flow in circles. Ions either flow through channels or complete circuits by repelling like charges on the opposite side of the insulating lipid bilayer. In outward currents, as positive ions try to re-enter the cell, they abut against the outer side of the lipid bilayer and repel internal positive ions facing them. As the repelled ions finish the circuit, internal negative charges are left facing external positive charges across the bilayer.
What polarizes the plasma membrane? - 4b • Outward currents thus induce positive charges outside or negative charges inside, enhancing cell membrane polarization. Conversely, in inward currents, as positive ions try to exit the cell, they abut against the cytoplasmic side of the lipid bilayer, neutralize the first negative cytoplasmic charges they meet and then repel any positive ions facing them on the external side of the lipid bilayer.
What polarizes the plasma membrane? 4c • The repelled external ions again complete the circuit leaving behind external negative charges facing internal positive charges. But this time the cell membrane’s polarization has reversed (the membrane has depolarized). Thus outward positive currrents polarize (also repolarize or hyperpolarize), and inward positive currents depolarize the membrane.
What polarizes the plasma membrane - 4d • Since outward currents generated by passive outward leakage of K+ are larger than the inward Na+ currents generated by the passive leakage of Na+ into the cells of excitable tissues (neurons or muscle), their resting cell membrane is polarized (the inside negative relative to the outside).
What determines the direction of current flow? -1,2 • 1) Concentration gradients of ions across the cell membrane. These are generated by the activity of active pumps such as the Na+,K+ pump. • 2) Electrical gradients across the cell membrane. These are generated by the separation of opposite charges across the lipid bilayer.
What determines the direction of current flow? - 3 • 3) Electrochemical gradients which are a combination of concentration and electrical gradients. • In neurons and skeletal muscle, the concentration of Na+ outside is 145 mM and inside about 15 mM. This 10 fold inward concentration gradient acts as a driving force for Na+ entry into the cell.
What determines the direction of current flow? - 4 • In neurons and skeletal muscle, the resting membrane potential lies between -70 and -100 mV inside relative to the outside. This negative membrane potential also acts as a driving force force for the entry of Na+ ions which are attracted by the inside negativity and repelled by the outside positivity.
What determines the direction of current flow? - 5 • Thus in muscles and neurons both concentration and electrical gradients across the cell membrane favor Na+ entry. Combined they constitute an inward electrochemical gradient and driving force for the entry of Na+. The same is not the case for K+ ions.
What determines the direction of current flow? - 6 • The outside concentration of K+ is 5 mM and the inside concentration about 150 mM. This 30 fold outward concentration gradient favors the loss of K+ from the cell. • On the other hand the electrical gradient of -70 to -100 mV inside relative to the outside tends to retain K+ ions inside the cell, since they are positively charged.
What determines the direction of current flow? - 7 • Thus the outward driving force due to the outward K+ concentration gradient is nearly balanced by inward driving force due to the negative membrane potential, providing only a small net outward driving force for K+ ions.
What determines the direction of current flow? - 8 • However, if the membrane were to depolarize, as it does at the peak of an action potential, the electrical driving force would become outward adding its contribution to the strong outward driving force due to the outward K+ concentration gradient. This would greatly increase outward K+ current, which would repolarize or even transiently hyperpolarize the cell.
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STIMULUS trigger zone interstitial fluid Na+ cytoplasm Na+ Na+ K+ K+ K+ K+ K+ K+ 3Na+ Na+ Na+ Na+ Na+ Na+ Na+ 2K+ Fig. 30.5, p. 490-91