Chapter 7Chapter 7

Transport of Ions and Small Transport of Ions and Small Molecules Across Cell Molecules Across Cell MembranesMembranes

By Christi HainesBy Christi Haines

Molecule and Ion Movement Molecule and Ion Movement is Mediated by Selective is Mediated by Selective Membrane Transport Membrane Transport Proteins.Proteins. The phospholipid bilayer is The phospholipid bilayer is

impermeable to most water impermeable to most water soluble molecules, ions, and soluble molecules, ions, and water itself.water itself.

A model for passive diffusion of small A model for passive diffusion of small hydrophobic molecules across a pure hydrophobic molecules across a pure phospholipid bilayerphospholipid bilayer

Membrane Proteins Membrane Proteins Mediate Transport of Mediate Transport of Molecules and All Ions Molecules and All Ions across Biomembranesacross Biomembranes Channel proteins transport water Channel proteins transport water

or specific types of ions and or specific types of ions and hydrophilic molecules down their hydrophilic molecules down their concentration gradient (facilitated concentration gradient (facilitated diffusion).diffusion).

ATP Pumps and the ATP Pumps and the Intracellular Ionic Intracellular Ionic EnvironmentEnvironment Types of Pumps:Types of Pumps: P-Class PumpsP-Class Pumps V-Class PumpsV-Class Pumps F-Class PumpsF-Class Pumps ABC SuperfamilyABC Superfamily

Active transport by ATP-powered Active transport by ATP-powered pumpspumps

Nongated Ion Channels and Nongated Ion Channels and the Resting Membrane the Resting Membrane PotentialPotential The plasma membrane contains The plasma membrane contains

channel proteins that allow the channel proteins that allow the principal ions to move thru them principal ions to move thru them at different rates down their at different rates down their concentration gradients.concentration gradients.

Voltage (electric) Potential = Voltage (electric) Potential = 70mV70mV

Nernst EquationNernst Equation

The magnitude of the sodium The magnitude of the sodium equilibrium potential is given by equilibrium potential is given by the Nernst equation:the Nernst equation:

EENa = Na = 0.059 log0.059 log10 10 [Na[Na11]/[Na]/[Narr]]

Membrane Potential Membrane Potential Depends on Resting KDepends on Resting K++ ChannelsChannels The outward flow of KThe outward flow of K++ ions thru ions thru

resting Kresting K++ channels is the major channels is the major determinant of the inside determinant of the inside negative-membrane potential.negative-membrane potential.

Cotransport by Symporters Cotransport by Symporters and Antiportersand Antiporters

Cotransporters use enery stored Cotransporters use enery stored in electrochemical gradients of in electrochemical gradients of NaNa++ and H and H++

Transported molecule and Transported molecule and cotransported molecule move in cotransported molecule move in same direction = symportsame direction = symport

Move in opposite directions = Move in opposite directions = antiportantiport

NaNa++-linked symporters import -linked symporters import amino acids and glucose into amino acids and glucose into many cellsmany cells

Movement of WaterMovement of Water

Osmotic pressure causes water to Osmotic pressure causes water to move across membranesmove across membranes

Osmotic pressure is the Osmotic pressure is the hydrostatic pressure required to hydrostatic pressure required to stop the net flow of water across stop the net flow of water across a membrane separating solutions a membrane separating solutions of different compositions. of different compositions.

Different Cells have Various Different Cells have Various Mechanisms for Controlling Mechanisms for Controlling Cell VolumeCell Volume In a hypotonic solution (concentration In a hypotonic solution (concentration

of solutes is of solutes is lowerlower than in the cytosol) than in the cytosol) cells swell due to the osmotic flow of cells swell due to the osmotic flow of water inward.water inward.

In a hypertonic solution In a hypertonic solution (concentration of solutes is (concentration of solutes is higherhigher than in the cytosol) cells shrink as than in the cytosol) cells shrink as systolic water leaves the cell by systolic water leaves the cell by osmotic flow.osmotic flow.

Aquaporins Increase the Aquaporins Increase the Water Permeability of Cell Water Permeability of Cell MembranesMembranes Aquaporins function as water Aquaporins function as water

channelschannels

The structure of aquaporin, a water The structure of aquaporin, a water channel protein in the erythocyte channel protein in the erythocyte plasma membraneplasma membrane

Transport across epithelia: the Transport across epithelia: the intestinal epithelium is highly intestinal epithelium is highly polarizedpolarized

Proposed model for operation of Proposed model for operation of the two-Nathe two-Na++/one-glucose /one-glucose symportersymporter

Transepithelial movement of glucose Transepithelial movement of glucose and amino acids requires multiple and amino acids requires multiple transport proteinstransport proteins

Voltage Gated Ion Channels Voltage Gated Ion Channels and the Propagation of and the Propagation of Action Potentials in Nerve Action Potentials in Nerve CellsCells Specialized regions of neurons Specialized regions of neurons

carryout different functionscarryout different functions:: Cell BodyCell Body AxonsAxons Axon terminalsAxon terminals DendritesDendrites

Opening and Closing of Opening and Closing of Voltage Gated NaVoltage Gated Na++ and K and K++ Channels Generate Action Channels Generate Action PotentialsPotentials Voltage Gated NaVoltage Gated Na++ Channels – Channels –

closed in resting neurons, closed in resting neurons, depolarization caused channels to depolarization caused channels to open allowing Naopen allowing Na++ to enter to enter

Voltage Gated KVoltage Gated K+ + Channels – Channels – repolarization opens Krepolarization opens K++ channels channels allowing Kallowing K++ to leave the cell to leave the cell

Myelination Increases the Myelination Increases the Velocity of Impulse Velocity of Impulse ConductionConduction The presence of a myelin sheath The presence of a myelin sheath

around an axon increases the around an axon increases the velocity of impulse conduction to velocity of impulse conduction to 10-100 meter/second. 10-100 meter/second.

Neurotransmitters and Neurotransmitters and Receptor and Transport Receptor and Transport Proteins in Signal Transmission Proteins in Signal Transmission at Synapsesat Synapses Neurotransmitters are packaged in Neurotransmitters are packaged in

membrane-bound synaptic vessicles in membrane-bound synaptic vessicles in the axon terminusthe axon terminus

Arrival of an action potential at axon Arrival of an action potential at axon termini in presynaptic cells triggers termini in presynaptic cells triggers secretion of neurotransmitterssecretion of neurotransmitters

Binding of neurotransmitters by Binding of neurotransmitters by receptors on post-synaptic cells leads to receptors on post-synaptic cells leads to changes in their membrane potential.changes in their membrane potential.

Neurotransmitters are removed from the Neurotransmitters are removed from the cleft after stimulating post-synaptic cellscleft after stimulating post-synaptic cells

Questions?Questions?