Chapter 12 Transport Across Cell Membranes PDF

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Chapter 12: Transport Across Cell MembranesPrinciples of Membrane Transport Lipid Bilayers are Impermeable to Ions and Most Uncharged Polar Molecules● The smaller and more hydrophobic molecules will diffuse most rapidly ■ Gases can diffuse with no problem ● Protein channels allow large, polar molecu...

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Chapter 12: Transport Across Cell Membranes

Principles of Membrane Transport Lipid Bilayers are Impermeable to Ions and Most Uncharged Polar Molecules ● ● ●

The smaller and more hydrophobic molecules will diffuse most rapidly ■ Gases can diffuse with no problem Protein channels allow large, polar molecules to cross bilayer Ions cannot pass through membrane even if they are small

The Ion Concentrations Inside a Cell Are Very Different from Those Outside ●

Na is abundant outside cell and K is abundant inside cell

Differences in the Concentration of Inorganic Ions Across a Cell Membrane Create a Membrane Potential ● ● ● ●

Membrane potential is created when there is an excess of positive or negative charge A balanced exchange of anions and cations is a resting membrane potential (-20 — -200 mV) Cell interior is more negative than exterior Membrane potential allows cells to power cell activities

Cells Contain Two Classes of Membrane Transport Proteins: Transporters and Channels ● ● ● ●

Channels allow passing of specific molecules Most protein channel are multi pass allowing hydrophilic molecules to pass by avoiding phobic bilayer Channels sort through size and electric charge (any appropriate size and charge) Transporters have specific binding sites

Solutes Cross Membranes by Either Passive of Active Transport ●

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Molecules will flow DOWNHILL from high to low concentration which is a passive transport because no energy is required ■ All channels and many transporters Molecules moving against concentration gradients are coupled with events that provide energy and is called active transport ■ Pumps harness energy sources for transport

Both the Concentration Gradient and Membrane Potential Influence the Passive Transport of Charged Solutes ● ●

Interior is negative and exterior is positive so the membrane potential likes to draw positive in and negative out Concentration gradient is the “osmosis phenom” and the membrane potential is the electric difference between in and out ■ Net driving force becomes electrochemical gradient ■ When 2 factors work together there, rules apply as normal ■ When 2 factors are opposite, rules are opposite ■ K is high inside cell than outside cell so K should “osmosis” to outside cell but because the membrane potential and concentration gradient are opposite, K tends to stay inside cell instead of flow outside cell where it should be

Water Moves Passively Across Cell Membranes Down its Concentration Gradient - A Process called Osmosis ● ●

Water is small and uncharged so can easily diffuse across bilayer but slowly Aquaporins are water pores

Transporters and Their Functions Passive Transporters Move a Solute Along Its Electrochemical Gradient

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Glucose transporter is passive ■ Protein crosses bilayer at least 12x ■ Has many conformations and can switch about ■ Ex. Binding sites for glucose are exposed to exterior and can switch to sites to interior ■ When glucose low inside cell, molecule binds to exterior sites and conformation changes and brings glucose into cell. When glucose low outside cell, molecule binds to interior sites of liver and conformation change brings it outside cell ■ Has electro of 0 so direction is based on concentration gradient alone (“osmosis”) ○ Highly selective in D glucose and not mirror L glucose

Pumps Actively Transport a Solute Against Its Electrochemical Gradient ●

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Transmembrane pumps carry out active transport in 3 main ways ■ ATP driven pumps hydrolyzed ATP to drive transport uphill ■ Coupled pumps link the uphill transport of one to the downhill transport of another ■ Light-Driven pumps use sunlight energy to drive uphill (in bacterial) Seen in ATP driven NA pump ■ Na is driven out of cell against electrochemical gradient ■ Na then flows back into cell downhill with electrochemical gradient ■ Influx of Na provides energy for other molecules to flow into cell against electrochemical gradient Energy gets harnessed from the pumping of on molecule for another

The Na+ Pump in Animal Cells Uses Energy Supplied by ATP to Expel Na+ and Bring in K ● ●

Na+ pump requires 30% + of total ATP consumption Na+K+ uses ATP to drive Na+ out and K+ in

Ca2+ Pumps Keep the Cytosolic Ca2+ Concentration Low ● ●

Kept at low concentrations in the cytosol Can bind to different proteins within cell to alter activities

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Influx of Ca2+ into cell through Ca channels trigger activities like muscle contraction

Coupled Pumps Exploit Solute Gradients to Mediate Active Transport ● ● ● ●

Downhill movement of one solute can produce energy to power the uphill transport of another solute which are called coupled pumps If pump move solutes in the same direction it is called a symport If pump moves solutes in the opposite direction is is called an antiport A pump that moves one solute type is called a uniport

The Electrochemical Na+ Gradient Drives Coupled Pumps in Plasma Membrane of Animal Cells ●

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One Na+ Pump drags in glucose along with it, the binding of both are cooperative so the binding of on enhances the binding of another ■ When one is not bound the other will not bind Many Na+ pumps drag in other important molecules into the cells Some transports pump in Na+ downhill and pump H+ out of cell controlling cytosol pH

Electrochemical H+ Gradients Drive Coupled Pumps in Plants, Fungi, and Bacteria ● ● ● ● ● ● ●

Do not have Na+ pumps Rely on H+ pumps to import solutes H+ pumped out of cell creating a electrochemical proton gradient making an acidic pH in exterior Import of sugars and amino acids are mediated by symports In photosynthetics H+ gradients created by light driven H+ pumps H+ gradient is generated by H+ pumps in membrane using ATP hydrolysis to pump H+ out of cell Active H+ pumps also found within organelle membranes pumps H+ into organelle keeping cytosol neutral and organelle interior acidic

Ion Channels and the Membrane Potential Ion Channels Are Ion-Selective and Gated ● ●

Show ion selectivity that depends on diameter and shape of ion channel Ions in solution are surrounded by water molecules that must be shed so the ions can pass single file through the ion channels

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Ion channels are not always open as they are gated and the specific shape changes conformation of channel and it switches on

Membrane Potential is Governed by the Permeability of a Membrane to Specific Ions ● ● ● ● ● ● ● ●

Changes in membrane potential create electrical signalling Mediated by alterations in permeability of membranes to ions K+ is active ion interiorly K+ leak channels randomly open and closed K tends to flow out and so unbalanced negative charges creates membrane potential Resting potential Nernst Equation calculates theoretical resting potential Open ion channels for other ions change permeability

Ion Channels Randomly Snap Between Open and Closed States ●

Ion channels are all or none

Different Types of Stimuli Influence the Opening and Closing of Ion Channels ● ● ●

Voltage-Gated Channel where opening is controlled by membrane potential (voltage) Ligand-Gated Channel where opening is controlled by binding of ligand to site Mechanically-Gated Channel where opening is controlled by mechanical force (ear hair)

Voltage-gated Ion Channels Respond to the Membrane Potential

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Neurons, muscles, eggs Carry electrical signals Contain voltage sensors sensitive to membrane potential changes Threshold changes open, closed gates ■ Increases probability that the channel will open ■ 10% may be open during rest and 90% open during activation When one type of voltage gate opens, others can inactivate or activate

Ion Channels and Nerve Cell Signaling Action Potentials Allow Rapid Long-Distance Communication Along Axons ● ● ● ●

Signals create change in membrane potentials Neural circuit is formed when signals get passed on to other neurons Passive spread signal weakens over long distances Action potentials can travel long distances

Action Potentials Are Mediated by Voltage-Gated Cation Channels ● ● ●

Stimulated neurons have a membrane potential moving towards 0 If depolarization is large enough the voltage-gated Na+ channels will open Open channels allow Na+ to flow downhill **Must actually read this part**

Voltage-Gated Ca2+ Channels in Nerve Terminals Convert an Electrical Signal into a Chemical Signal ● ● ● ● ● ● ● ● ●

Signals are transmitted via synapses Presynaptic & Postsynaptic cells are separated by synaptic cleft 20nm across Electric signals are converted into chemical signals with neurotransmitters stored within synaptic vesicles Some synaptic vesicles fuse with plasma membrane to release transmitters into synaptic cleft Action potential arrival and transmitter secretion involves activation of Ca2+ voltage gated channel Action potential opens Ca2+ channels in presynaptic neuron Ca2+ rushes into cell ^ Ca content Membrane fusion is triggered to release transmitters Ca2+ channels convert electrical energy into chemical energy

Transmitter-Gated Ion Channels in the Postsynaptic Membrane Convert the Chemical Signal Back into an Electrical Signal ● ● ●

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The neurotransmitter released diffuses across the synaptic cleft and binds to neurotransmitter receptors on the postsynaptic neuron A change in the membrane potential occurs and triggers an action potential if big enough Neurotransmitters are removed from the synaptic cleft by enzymes, by reabsorption, or taken up by near by neurons ○ Limits the duration and spread of the signal (seizure) There a various types of neurotransmitter receptors

Neurotransmitters Can Be Excitatory or Inhibitory ● ●

Receptors for excitatory neurotransmitters are ligand gated cation channels for acetylcholine and glutamate Inhibitory neurotransmitters are GABA and glycine and are ligand Cl- channels and makes the plasma membrane harder to depolarize

Most Psychoactive Drugs Affect Synaptic Signalling by Binding to Neurotransmitter Receptors ●

Drugs attach to the specific receptor for the specific neurotransmitter and either increase or decrease the release and uptake of transmitter

The Complexity of Synaptic Signalling Enables Us to Think, Act, Learn, and Remember

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The presynaptic cell converts electric signal into a chemical signal in order to send it into the synaptic cleft The postsynaptic cleft takes up the chemical transmitters and generates a membrane potential (back to an electric signal) Different transmitters and their corresponding receptors allow the neuron to behave in various and certain ways (inhibit / excitatory)...