CELL BIOLOGY - CLUTCH CH. 9 - TRANSPORT ACROSS MEMBRANES.

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2 K + K + K + K + CELL BIOLOGY - CLUTCH CONCEPT: PRINCIPLES OF TRANSMEMBRANE TRANSPORT Membranes and Gradients Cells must be able to communicate across their membrane barriers to materials with the environment Membranes are semi-permeable and only allow certain molecules to cross - Small nonpolar molecules (oxygen, carbon dioxide) can rapidly cross - Uncharged polar molecules can pass if small, but cannot if large - Charged molecules and ions cannot pass Cell membranes allow for internal cellular concentrations to from external concentrations - Concentration gradients: Concentrations of molecules differ on either side of a membrane - Electrical potentials: Net charge of environment differ on either side of a membrane - Electrochemical potential: Combined effect of concentration gradient and electrical potential - Membrane potential: Difference between the concentration gradient and electrical potential The overall net charge must be balanced EXAMPLE: Concentration and electrical gradients across a membrane Na 2+ Na 2+ Membrane Na 2+ Na 2+ K + Na 2+ Na 2+ K + Passive and Active Transport Molecules the membrane barrier through two ways: passive transport and active transport Passive transport moves molecules with a gradient (high concentration to an area of low concentration) - Simple diffusion can occur if the molecule needs no assistance in passing the membrane Page 2

3 CELL BIOLOGY - CLUTCH - Facilitated diffusion occurs if the molecule needs assistance in crossing the membrane - Diffusion requires no energy input Active transport moves molecules against a gradient (from low concentration to high concentration - Active transport requires energy usually from ATP hydrolysis EXAMPLE: Simple vs. Facilitated diffusion Passive Diffusion Facilitated Diffusion Three classes of transmembrane proteins transport molecules the membrane Channels provide a portal for molecules to pass - Only let molecules of specific size or electrical charge pass the membrane Transporters are highly selective in allowing molecules to pass - Transfer only molecules that fit into specific binding sites ATP powered pumps require energy from ATP to transport molecules EXAMPLE: Examples of the three classes of transmembrane proteins Channel Transporter ATP-powered pump Page 3

4 PRACTICE 1. Which of the following gradients describes an electrical charge difference across a membrane? a. Concentration gradients b. Electrical gradients c. Electrochemical gradients d. Membrane potential 2. Facilitated diffusion differs from simple diffusion because why? a. Facilitated diffusion requires energy to move substances across the membrane b. Facilitated diffusion requires a protein to move substances across the membrane c. Simple diffusion requires energy to move substances across a membrane d. Simple diffusion requires a protein to move substances across the membrane Page 4

5 3. True or False: Active diffusion moves molecules from an area of low concentration to an area of high concentration. a. True b. False Page 5

6 CELL BIOLOGY - CLUTCH CONCEPT: PASSIVE TRANSPORT: DIFFUSION AND OSMOSIS Diffusion Diffusion is movement of molecules equilibrium (depends on free energy) Simple diffusion is the unassisted (passive) movement of molecules Limited to small, uncharged, nonpolar molecules - Partition coefficient measures ratio of nonpolar solubility in a nonpolar solvent and water - Greater lipid solubility = faster diffusion Requires energy input (exergonic) Moves molecules from areas of high concentrations to areas of low concentration EXAMPLE: Simple diffusion across Facilitated diffusion is the movement of molecules Moves molecules from areas of high concentrations to areas of low concentration - Kinetics can be measured using the michaelis-menten equation used for enzymes Assistance is provided through two classes of proteins: channel proteins and carrier proteins - Channel proteins move molecules by providing a channel through which they pass - Carrier proteins move molecules by undergoing conformational changes Page 6

7 EXAMPLE: Carrier and channel proteins Transport proteins are further classified by how molecules they transport at once - Uniport proteins transport one molecule at a time. Fastest method of facilitated diffusion - Symport proteins transport two molecules in the same direction - Antiport proteins transport two molecules in opposite directions Facilitated diffusion only transports specific molecules EXAMPLE: Comparison of uniporter, symporters, and antiporters Page 7

8 EXAMPLE: Glucose transporter (GLUT1 uniport) moves glucose from an area of high concentration to lower concentration EXAMPLE: Sodium Calcium Antiporter regulates muscle contraction Sodium Calcium Osmosis Osmosis is the diffusion of water across semi-permeable membranes Water movement is dependent on concentrations - Water moves from lower solute concentration (high water) to higher solute concentration (low water) - When in a hypotonic solution (low solute) cells swell, whereas in hypertonic (high solutes) they shrink - Isotonic solutions have similar solute concentrations in cells and their environment Page 8

9 Osmotic pressure is the pressure required to stop water flow across membranes Aquaporins are channel proteins that allow to cross the membrane - Move water through the channel by forming hydrogen bonds with amino acids to displace other water - No conformational changes are need leads to fast transport - Different cell types and organisms have different amounts of aquaporins controls permeability levels EXAMPLE: Comparison of water movement in hypertonic, isotonic, and hypotonic solutions PRACTICE 1. Which of the following describes the diffusion of water across a membrane? a. Simple Diffusion b. Facilitated Diffusion c. Osmosis d. Uniporters Page 9

10 2. True or False: The plasma membrane by itself is impermeable to all charged molecules. a. True b. False 3. Which of the following transport proteins transports two molecules across the membrane in the same direction? a. Uniport b. Symport c. Antiport d. Biport Page 10

11 4. True or False: Carrier proteins that transport molecules via facilitated diffusion require energy from ATP. a. True b. False 5. Isotonic is a term that describes what? a. The cytoplasmic side of a membrane has a higher solute concentration b. The extracellular side of a membrane has a higher solute concentration c. Both sides of a membrane have equal solute concentrations Page 11

12 CONCEPT: TRANSPORTERS AND ATP-DRIVEN PUMPS Transporters Transporters transport molecules across the membrane through conformational changes Can modulate passive (Glucose uniport GLUT1) or active transport Three classes of active transporters exist - ATP-Driven pumps: Use energy from ATP to drive transport - Coupled pumps: Use energy from concentration gradient of one molecule to drive transport of another - Sometimes referred to as indirect active transport - Symports move the two molecules in the same direction - Antiports move the two molecules in opposite directions - Light-driven pumps: Use energy from light to drive transport across a membrane EXAMPLE: ATP-powered pump Coupled Pumps Light Driven Pumps NA 2+ glucose symporter allows for glucose into the cytosol even when concentrations are high Uses energy from sodium moving down its gradient to trigger glucose uptake - Binding of sodium enhances the binding of glucose - but both are required for transport Found on apical surface of gut epithelial Different transporters exist on basolateral surface (passive glucose uniporters) Page 12

13 - Releases glucose into blood stream EXAMPLE: Sodium Glucose Symporter Bacteriorhodopsin uses energy to pump H + ions Found in archaea H. halobium that lives in the Great Salt Lake in Utah Contains retinal a molecule that senses light - After light hits the retinal, it causes a proton to move to the cell exterior EXAMPLE: Bacteriorhodopsin pumps H + out of the cell ATP-Driven Pumps The four classes of ATP-driven pumps transport molecules a gradient using energy from ATP P pumps are phosphorylated in the process of pumping ions across the plasma membrane V pumps transport H+ ions across a vacuolar membrane - Maintaining acidity in lysosomes by pumping H + into the lumen F pumps work in reverse by using H + gradients to drive ATP synthesis Page 13

14 ABC transporters pump small molecules across the cell membrane (largest group of the four classes) - Multi-drug resistance protein (MDR) provide drug resistance by pumping drugs out of cells - ABC4 can act as a phospholipid flippase EXAMPLE: P pump V pump F pump Examples of ATP-Driven Pumps ATP drive pumps generate ionic gradients that drive important cellular processes The Ca 2+ pump drives muscle relaxation - This P class pump is found in the sarcoplasmic reticulum (specialized form of endoplasmic reticulum) - Responsible for causing muscle relaxation by pumping calcium from cytosol into SR lumen - Calcium and ATP binding cause conformational change that opens and releases calcium into SR lumen EXAMPLE: Calcium Cytosol Lumen Page 14

15 The Na 2+ K + pump moves sodium and potassium concentration gradients - Cytosol: High K + and low Na + - Extracellular space: Low K + and high Na 2+ - Pumps three Na 2+ ions out and two K + ions into the cell per ATP molecule hydrolyzed - Creates steep concentration gradients of sodium across the plasma membrane EXAMPLE: Sodium Potassium Pump PRACTICE 1. Which of the following is not considered a transporter? a. V ATP pump b. Light driven pumps c. Coupled pumps d. Ion channels Page 15

16 2. True or False: Transporters always require energy to move solutes across a membrane? a. True b. False 3. Which of the following classes of ATP-drive pumps can synthesize ATP when reversed? a. P pumps b. V pumps c. F pumps d. ABC Transporters Page 16

17 4. The sodium-potassium pump works by doing what? a. Pumping one sodium ion into the cell, while pumping one potassium ion out of the cell b. Pumping one sodium ion out the cell, while pumping one potassium ion into the cell c. Pumping three sodium ion into the cell, while pumping two potassium ion out of the cell d. Pumping three sodium ion out the cell, while pumping two potassium ion into the cell Page 17

18 CONCEPT: ION CHANNELS AND MEMBRANE POTENTIALS Ion Channels Ion channels form transmembrane pores that allow for passive transport of small, polar molecules Ion channels are gated, meaning that they are not continuously - Voltage-gated channels open depending on differences in charge across a membrane - Ligand-gated channels in response to binding of a ligand molecule - Mechanically-gated channels open in response to mechanical force Ion channels are and are permeable to a specific ion - Contain a selectivity filter inside a narrow pore which ions must be able to pass - Ions must disassociate from water in order to pass and only the targeted ion will be able to do this Ion channels move molecules in response to electrical gradients (charge gradients) across the membrane EXAMPLE: Voltage-Gated Ligand-Gated Membrane Potential Membrane potential is the in environment between the intracellular and extracellular environments Differences in molecular concentration or charge - Resting membrane potential is when the flow of + and ions across the membrane is balanced - The charge is balanced, but doesn t necessarily mean it rests at no net charge The Nernst equation allows for the calculation and quantification of this difference Na 2+ K + pump (transporter) creates a large concentration gradient of Na 2+ and K + across the membrane Page 18

19 - K + leak channels open and close randomly to passively transport of K + to restore the membrane potential EXAMPLE: Membrane Potential Across a Membrane The patch-clamp technique measures the activity of individual ion channels Micropipette isolates a small patch of membrane containing a single ion channel - Analyzes the flow of ions through the channel EXAMPLE: The patch clamp technique Page 19

20 PRACTICE 1. Which of the following is not considered a type of gated ion channel? a. Voltage gated b. Ligand gated c. Mechanically gated d. Electrical gated 2. True or False: Ion channels require energy to transport substances across a membrane. a. True b. False Page 20

21 3. K + leak channels are important for doing what? a. Keeping a steady level of K + inside the cell b. Keeping a steady level of K + outside the cell c. Maintaining a strict concentration gradient across the membrane d. Maintaining membrane potential 4. True or False: When the concentration of ions is exactly the same on either side of the membrane, then the cell is at a resting membrane potential. a. True b. False Page 21

22 CELL BIOLOGY - CLUTCH CONCEPT: ION CHANNELS, ACTION POTENTIALS, AND NEURONS Neuron Structure Neurons are nerve cells that function by receiving, integrating, and signals Neurons have a few distinctive physical features - Cell body (nucleus) is the core of the neuron - Axons are the long extension that conduct electrical signals away from the cell body - Diameter controls speed; Larger diameter = faster speed - Dendrites are the several shorter branching extension that radiate from the cell body to receive signals - The nerve terminal is the end of the axon that branches to pass the neurons message to many cells EXAMPLE: Structure of a neuron Axons contain distinctive features - Myelin is a protective cell covering formed around the axon from glial cells, and schwaan cells - Myelin sheath insulates the axon so ions do not leak out of the membrane - Nodes of Ranvier are patches of ion channels that interrupt the sheath for neuron signaling - Signal is passed between each Node of Ranvier down a neuron Page 22

23 EXAMPLE: Myelin sheath and Nodes of Ranvier The junction between two neurons contains physical features - Synapse: Junction through which the signal is transmitted - Pre-synaptic cell: Cell that contains the signal - Post-synaptic cell: Cell that receives the signal - Synaptic cleft: Space between the pre and post synaptic cell. Electrical signals cannot cross EXAMPLE: Anatomy of a synapse Pre-synaptic cell Post-synaptic cell Synaptic Cleft Action Potentials Action potentials are traveling waves of electrical excitation that can carry messages between neurons Travel very, up to 100 meters per second - Travel without weakening the message Page 23

24 Voltage-gated cation channels mediate action potentials - The membrane potential in a resting neuron is -60mV (intracellular space is more negatively charged) - Opening of specific channels can result in rapidly changing to +40mV - Triggers the continually opening of other cation channels which travel down the neuron Potassium channels help return the neuron to a resting membrane potential - Delayed K + channel: Returns neuron to its original state and prepares it to fire again - Rapidly activating K + channel: Removes relationship between rate of firing and intensity - Ca 2+ activated K + channels: Increase the delay between one action potential and next EXAMPLE: An action potential traveling through an neuron Page 24

25 Steps to Neuronal Signaling Signal propagation between neurons occurs in 7 steps 1. A neuron receives a signal which triggers opening voltage-gated Na 2+ channels - Results in massive depolarization (influx of positive charge due to sodium flowing into cells) - These channels become inactivated within a millisecond to prevent continuous Na 2+ transport 2. The depolarization triggers more voltage-gated ion channels to open - The depolarization travels down the axon 3. Voltage-gated K + channels are delayed in opening - High positive charge inside cell results in K + transport out of the cell - Helps restore the neuron to its resting state 4. The traveling wave of depolarization reaches the nerve terminal EXAMPLE: Steps of an action potential Page 25

26 CELL BIOLOGY - CLUTCH 5. The electrical signal is converted into a chemical signal because the synaptic cleft cannot pass electrical signals - Neurotransmitters are the chemical signals a. Neurotransmitters sit in vesicles near the nerve terminal plasma membrane b. When depolarization arrives, it triggers opening of voltage-gated calcium channels c. Influx of calcium causes fusion of vesicles with neurotransmitters and release into synaptic cleft 6. Neurotransmitter binds receptors on the post-synaptic membrane - Trigger the cell to fire an action potential and cycle is repeated 7. Neurotransmitter is immediately removed through one of two methods - Enzymes in the post synaptic cell destroy it - It is pumped back into the pre-synaptic cell for re-use EXAMPLE: Steps of neurotransmitter release Page 26

27 Neurotransmitter signaling Neurons must be able to complex combinations of neurotransmitters Neurotransmitters fall into two classes: excitatory or inhibitory - Excitatory neurotransmitters trigger an action potential in post-synaptic cell - Acetylcholine in neuromuscular junction - results in muscle contraction - Inhibitory neurotransmitters trigger Cl - channels which make depolarization harder (Ex: GABA) Some toxins and drugs target neurotransmitters EXAMPLE: Excitatory vs. Inhibitory neurotransmitters Excitatory Inhibitory Huge neuronal receive large combinations of neurotransmitters - Combinations of excitatory and inhibitory signals - Each neuron contains its own set of receptors and ion channels - Neurons have to combine and interpret all of these signals Synaptic plasticity is when the magnitude of a neuron s response depends on how much its been used in past EXAMPLE: Networks of competing neurotransmitters Page 27

28 PRACTICE 1. Match the following neuron structure with its definition i. Cell Body ii. Myelin Sheath iii. Nerve Terminal iv. Nodes of Ranvier v. Synapse a. Junction through which the signal is transmitted b. End of the axon that branches to pass the neurons message to many cells c. Core of the neuron d. Insulates the axon so ions do not leak out of the membrane e. Patches of ion channels that interrupt the sheath for neuron signaling 2. What neuron structure is responsible for ensuring that ions do not leak out of the axon membrane? a. Cell body b. Dendrites c. Myelin sheath d. Nodes of Ranvier Page 28

29 3. The opening of which type of channel causes depolarization of the neuron? a. Voltage gated K + channels b. Voltage gated Na 2+ channels c. Voltage gated Ca 2+ channels d. K + leak channels 4. An influx of calcium at the synapse causes what to happen? a. Neurotransmitter release b. Neurotransmitter binding c. Depolarization d. Opening of the voltage gated K + channels Page 29

30 5. Which of the following neurotransmitter types blocks Cl - channels and makes depolarization harder? a. Excitatory b. Inhibitory 6. Potassium channels are mainly responsible for what? a. Mediating action potentials b. Returning the neuron to a resting membrane potential c. Releasing neurotransmitters d. Depolarization Page 30