Solute Movement Across Cell Membranes PDF

Solute Movement Across Cell Membranes There are five different ways by which substances cross cell membranes: 1. 2. 3. 4. 5. Simple diffusion through the lipid bilayer Simple diffusion through an aqueous channel Diffusion of ions through ion channels Facilitated diffusion (binding is involved) Acti...

Solute Movement Across Cell Membranes There are five different ways by which substances cross cell membranes: 1. 2. 3. 4. 5. Simple diffusion through the lipid bilayer Simple diffusion through an aqueous channel Diffusion of ions through ion channels Facilitated diffusion (binding is involved) Active transport 1.Simple Diffusion Through the Lipid Bilayer • Diffusion requires both a concentration gradient and membrane permeability • Lipid permeability is determined by the molecular size and polarity of a solute Q) How we measure the polarity?  Polarity is quantified by the partition coefficient, or the ratio of solubility in a nonpolar solvent to that in water. It shows the ability of a drug to cross the cell membrane • Small molecules penetrate the lipid bilayer more rapidly than larger ones • Polar molecules, like sugars and amino acids, have poor membrane penetration • The greater the lipid solubility, the faster the penetration Fig 4.34: The relationship between partition coefficient and membrane permeability. Questions Q) Which substance has more permeability? Sucrose or Caffeine? Caffeine? Q) How polar molecules (sugar and amino acids ) cross the cell membrane if they have poor permeability? They have special mechanism that mediates their entry into the cells Q) Molecule S1 and S2 have same partition coefficient but S2 is little smaller than S1. Based on this information which one you think is able to penetrate the lipid bilayer more rapidly? Why? S1 because it is smaller 2.Diffusion Through the Lipid Bilayer via Aqueous Channels Fig 4.35: The effects of differences in the concentration of solutes on opposite sides of the plasma membrane • Diffusion of water through a semipermeable membrane is called osmosis where water diffuses from areas of lower solute concentration to areas of higher solute concentration • Cells swell in hypotonic solution, shrink in hypertonic solutions, and remain unchanged in isotonic solutions. 2. Diffusion Through the Lipid Bilayer via Aqueous Channels • Plants utilize osmosis in different ways as they are usually hypertonic compared to their fluid environment • There is a tendency for water to enter the cell, causing it to develop an internal (turgor) pressure that pushes against its surrounding wall • In hypertonic solutions the plant cell undergoes plasmolysis, and the plant loses its support and wilts • A family of small integral proteins, aquaporins, allow the passive movement of water across the plasma membrane in plants and animals  It’s central channel is lined primarily by hydrophobic amino Fig 4.36: The effects of osmosis on a plant cell acid residues and is highly specific for water molecules  Aquaporins are prominent in kidney tubules or plant roots https://www.science.org/doi/10.1126/science.aao2440 Aquaporins in kidney https://www.researchgate.net/figure_fig2_303416704 3.The Diffusion of Ions through Membranes • Ions cross membranes through ion channels – Ion channels are selective and bidirectional (two directions), allowing diffusion in the direction of the electrochemical gradient • Superfamilies of ion channels have been characterized by patch-clamping experiments • The voltage across the membrane can be maintained (clamped) at any value, and the current originating in the small patch of membrane surrounded by the pipette can be measured • Fig: shows a glass micropipette is enclosing a patch of the membrane containing a single ion channel. Pipette is wired as an electrode (a microelectrode), a voltage Fig 4.38: can be applied across the patch of the membrane enclosed by the pipette and the responding flow of ions through the membrane channel can be measured. Measuring ion conductance by patchclamp recording https://www.youtube.com/watch?v=mVbkSD5FHOw Revision Is water transport active or passive? –why? Passive- concentration gradients What kind of proteins are involved in water transport and found in the PM? Aquaporins – an integral protein Ability of a drug to cross the cell membrane is called? Partition coefficient 3. The Diffusion of Ions through Membranes • Most ion channels can exist in either an open or a closed conformation, and are called gated. The three major categories of gated channels are: • 1. Voltage‐gated channels: Conformational state depends on the difference in ionic charge on the two sides of the membrane. (e.g, local anesthesia) https://www.youtube.com/watch?v=wLSSf2NRbqk • 2. Ligand‐gated channels: Conformational state depends on the binding of a specific molecule (ligand), which is usually not the solute that passes through the channel. Ligand-gated channels can either open or close after ligand binding to the outer or inner surface of the channel. (e.g, General anesthesia, GABA, nicotine) • 3. Mechano‐gated channels: Conformational state depends on mechanical forces (e.g., stretch tension) that are applied to the membrane. E.g., Specific cation channels can be opened by stereocilia movement on the hair cells of the inner ear in response to sound or motions of the head. Fig: https://www.news-medical.net/health/Importance-of-Ion-Channels-in-the-Body.aspx What is the practical application of ion channel in medicine? Procaine and Novocaine (sodium channel blocker and act by closing ion channels in the membrane of sensory cells and neurons) Closing of theses ions channels results in inhibition of action potential to the affected cells (Skin or teeth) and brain will not get the information of the stimululs. 4. Facilitated Diffusion • In many cases, the diffusing substance binds selectively to a membrane-spanning protein, called a facilitative transporter. • Solute binding triggers a conformational change to expose it to the other membrane surface and diffuse down its concentration gradient. • Facilitated transporters can mediate the movement of solutes in both directions and depends on the relative concentration of the substance on both sides. • Facilitated diffusion is similar to an enzymecatalyzed reaction since it is specific for the molecules transported. Fig 4.44: Schematic model of facilitated diffusion. The alternating conformation of a carrier that exposes the glucose binding site to either the inside or the outside of the membrane 4. Facilitated Diffusion • Transporters can be regulated and exhibit saturation-type kinetics • It is important in transporting polar solutes, like sugars and amino acids, that do not penetrate the lipid bilayer • The glucose transporter (GLUT) is an example of facilitated diffusion: – When insulin levels are low, responsive cells contain relatively few glucose transporters (GLUT), GLUT4 (isoform of GLUT), on their plasma membrane Fig 4.45: Kinetics of facilitated diffusion compared to simple diffusion – Rising insulin levels stimulates the movement of transporters to the cell surface where they can bring glucose into the cell Fig 15.26:Regulation of glucose uptake 5. Active Transport • Cells maintain an imbalance of ions across the plasma membrane, which cannot occur by either simple or facilitated diffusion – Equilibrium means death for cells! • Gradients are generated by active transport, which depends on integral membrane protein “pumps” to bind a solute and move it across the membrane in a process driven by changes in the protein’s conformation • Coupled energy input is needed like ATP hydrolysis, absorbance of light, electron transport, or the flow of other substances down their gradients. Revision: Solute Movement Across Cell Membranes 1. Simple diffusion through the lipid bilayer- partition coefficient 2. Simple diffusion through an aqueous channel- aquaporins 3. Diffusion of ions through ion channels-voltage, ligand and mechano-gated channels 4. Facilitated diffusion (binding is involved)- GLUT4 and glucose transport 5. Active transport – gradient and coupled with energy Lipid permeability is determined by the molecular size and polarity 5. Active Transport Primary Active Transport: Coupling Transport to ATP Hydrolysis Fig 4.46: Steps Explanation 1. 2. 3. 4. 5. In step 1, the ATP pump is in E1 conformation and allows Na+ to binds inside of the cell. This leads to bind three Na+ ions and an ATP. Protein gate closes so Na+ ions can no longer flow back into the cytosol. Hydrolysis of ATP causes the change in conformation from E1 to E2 and this results in binding site more accessible to the ions in the extracellular compartment. Once the three Na+ ions are released the protein picks up two K+ ions. Binding of Potassium to the protein and dephosphorylation causes the protein to snap back to its original conformation and allow potassium ions to diffuse into the cells. 5. Active Transport Primary Active Transport: Coupling Transport to ATP Hydrolysis Fig 4.46: • The Na+/K+ ATPase (sodium-potassium pump) maintains a gradient with a ratio of Na+:K+ pumped is 3:2 per ATP molecule that is hydrolyzed • This excess K+ inside the cells is balanced by negative charges of proteins and nucleic acids; whereas the Na+ excess outside the cell is balanced by Cl- ions • The ATPase is a P-type pump, in which phosphorylation causes changes in conformation and ion affinity that allow transport against gradients: – E1: conformation: Ion binding sites are accessible to the inside of the cell – E2: Ion binding sites are accessible to the outside of the cell • • The sodium–potassium pump is found only in animal cells EX: Digitalis drug (Digoxin) is used to treat certain heart conditions such as strengthen the heart’s contraction by inhibiting the Na+/K pump (obtain from dried leaves of Foxglove plant)-binds to allosteric site of ATPase (inhibits Na+/K+ pump) leads to a chain of events that increases Ca+ availability inside the muscle cells of the heart and leads to increase in cardiac concentration . https://www.youtube.com/watch?v=ljm1JSubTOc Foxglove plant The Human Perspective Defects in Ion Channels and Transporters as a Cause of Inherited Disease Defects in ion Channels Quick Review Quick Review Q) What is p pump? Driven by phosphorylation Q) What is NOT true about Digitalis? a) It strengthens the heart’s contraction by inhibiting the Na+/K+ pumps b) It is steroid and obtained from the foxglove plant and has been used for 200 years as the treatment for heart disease c) The use of Digitalis in turns increases the Ca+2 availability inside the muscle cells of the heart d) It is steroid and obtained from the foxglove plant and has been used for 200 years as the treatment for sickle cell anemia 5. Active Transport Primary vs Secondary Active Transport • In each of the previous cases, chemical energy, in the form of ATP hydrolysis, is used to transport ions or small molecules and is called primary active transport (Remember- this transport occurs by the EXPENSE OF ATP) • If the generated electrochemical gradient is utilized to drive the formation of a gradient for another solute (ion or molecule), then this would be called secondary active transport (Remember- this transport not use ATP- instead gradient generated by the EXPENSE OF ATP is used to move the substance) In Secondary active transport substance can be moved in the same direction or opposite direction https://d2gne97vdumgn3.cloudfront.net/api/file/TZV6mBX1TImdMRlUgcpa Secondary Active Transport: Substance move from lower conc. to the higher conc. because of the gradient generated by another solute 5. Active Transport Co-Transport: Coupling Transport to Existing Ion Gradients • Secondary active transport of glucose is an example of symport, two transported species moving in the same direction • Antiporters, or exchangers, move two transported species in opposite directions • For example, cells can maintain a proper cytoplasmic pH by coupling the inward movement of Na+ with the outward movement of H+ http://previews.123rf.com/images/extender01/extender011507/extender01150700003/42176187Symport-and-antiport-types-of-cell-membrane-transport-systems-Stock-Vector.jpg Fig 4.49: Secondary transporter: the Na+ gradient helps to transport glucose by a Na+/glucose cotransporter 5. Active Transport Secondary Active Transport (or Co-Transport): Coupling Transport to Existing Ion Gradients • Potential energy stored in ionic gradients is utilized to perform work, including the transport of other solutes. • The movement of glucose across the apical plasma membrane of the epithelial cells, against a concentration gradient, occurs by cotransport with sodium ions – Na+ concentration is kept low by a Na+/K+ATPase pump. – Diffusion of sodium ions down a concentration gradient drives the cotransport of glucose molecules into the cell against a concentration gradient (moves 2 Na ions and one glucose) • Once inside, the glucose molecules diffuse through the cell and are moved across the basal membrane by facilitated diffusion Fig 4.49: Secondary transporter: the Na+ gradient helps to transport glucose by a Na+/glucose cotransporter Membrane Potentials • Irritability: response to external stimulation; all organisms have it! • The basis of this is the propagation of nerve impulses • Potential differences exist when charges are separated, and membrane potentials have been measured in all types of cells Fig 4.51: The structure of a nerve cell  Neurons are specialized cells for information transmission using changes in membrane potentials  Dendrites receive incoming information; the cell body contains the nucleus and metabolic center of the cell; the axon is a long extension for conducting outgoing impulses  Most neurons are wrapped by a lipid-rich myelin-sheath  The place where no myelin sheath is called Node of Ranvier and that is the site where action potential is generated- Why?  (Uninsulated and rich in ion channels) Membrane Potentials The Resting Potential • The resting potential is the membrane potential of a nerve or muscle cell, subject to changes when activated. • K+ gradients maintained by the Na+/K+ATPase are responsible for the resting potential. • The Nernst equation is used to calculate the voltage equivalent of the concentration gradients for specific ions. • In Fig 4.52 A- when both electrodes are on the outside of the cells- no potential difference is measured. • In Fig 4.52 B- When one electrode penetrates the PM the potential difference drops to -70 mV and approaches the equilibrium potential for potassium ions Fig 4.52: Measuring a membrane’s resting potential Membrane Potentials The Action Potential • When cells are stimulated, Na+ channels open, causing membrane depolarization • When cells are stimulated, voltagegated Na+ channels open, triggering the action potential • Na+ channels are inactivated immediately following an AP, producing a short refractory period when the membrane cannot be stimulated It is a period in which a nerve cell is unable to fire an action potential. • • Excitable membranes exhibit all-ornone behavior Fig 4.53: Formation of an action potential refractory period Membrane Potentials Explanation: The Action Potential LEAP: Less negative Excitation (Depolarization) Action Potential Time 1, left box: The membrane in this region of the nerve cell exhibits the resting potential, in which only the K + leak channels are open and the membrane voltage is approximately −70 mV. Time 2, middle box: The depolarization phase: The membrane has depolarized beyond the threshold value, opening the voltage‐regulated sodium gates, leading to an influx of Na + ions. The increased Na + permeability causes the membrane voltage to temporarily reverse itself, reaching a value of approximately +40 mV. Time 3, right box: the repolarization phase: Within a tiny fraction of a second, the sodium gates are inactivated and the potassium gates open, allowing potassium ions to diffuse across the membrane and establish an even more negative potential at that location (−80 mV) than that of the resting potential. Propagation of Action Potentials as an Impulse Saltatory conduction: Propagation of an impulse by forming an action potential only at the nodes of Ranvier Speed Is of the Essence: Speed of neural impulse depends on axon diameter (the greater the diameter the lower the resistance and more rapid action potential) and whether the axon is myelinated. Nearly all of the Na+ ion channels of a myelinated neuron reside in the unwrapped gaps, or nodes of Ranvier, between adjacent Schwann cells or oligodendrocytes that make up the myelin sheath. The nodes of Ranvier are the only sites where action potentials can be generated, jumping from node to node. Fig 4.55: Saltatory conduction Jumping of impulse from node to node is called saltatory conduction. (impulse travel in myelinated axon is 20x more faster than the unmyelinated axon). Multiple sclerosis (MS) is a disease associated with deterioration of the myelin sheath that surrounds axons in various parts of the nervous system- start in young adulthood and patient has difficulty in walking, weakness in hands and vision problems : Propagation of an impulse by forming an action potential only at the nodes of Ranvier Let’s Label the Diagram (Check Test) Depolarization Action potential Repolarization Refractory Period Nodes of Ranvier Myelin Propagation of Action Potentials as an Impulse APs produce local membrane currents depolarizing adjacent membrane regions of the membrane that propagate as a nerve impulse. The large depolarization from an action potential creates a difference in charge along the inner and outer surfaces of the plasma membrane. Once triggered, a succession of action potentials passes down the entire length of the neuron without any loss of intensity, arriving at its target cell with the same strength it had at its point of origin. Q) Do you think stronger stimuli will produce a bigger impulse? All impulses traveling along a neuron exhibit the same strength, so stronger stimuli cannot produce bigger impulses, however the strength of stimuli can make a difference can active more nerve cells (scalding, hot, water versus warm water) Fig 4.54: Propagation of an impulse results from the local flow of ions unidirectionally Neurotransmission: Jumping the Synaptic Cleft Presynaptic neurons communicate with postsynaptic neurons at a specialized junction, called the synapse, across the synaptic cleft (a narrow gap of about 20 to 50nm) . Chemicals (neurotransmitters) released from the presynaptic cleft diffuse to receptors on the postsynaptic cell. Bound transmitter can depolarize (excite) or hyperpolarize (inhibit) the postsynaptic cell. Q) How neurotransmitter maintains in our body? Transmitter action is terminated by reuptake or enzymatic breakdown. Fig 4.56: The neuromuscular junction Neurotransmission: Jumping the Synaptic Cleft Neurotransmission: Jumping the Synaptic Cleft Sequence of events during synaptic transmission with acetylcholine as the neurotransmitter Fig 4.57: • Depolarization of pre-synaptic cell causes Ca2+ channels in membrane to open, Ca2+ stimulates fusion of vesicles with membrane. • Calcium ions diffuse from extracellular fluid into the terminal knobs of the neuron and elevated calcium triggers the fusion of synaptic vesicles to the plasma membrane. • Neurotransmitter bind to the receptor in the postsynaptic plasma membrane • Neurotransmitter binding to ion channel receptors can either stimulate or inhibit action potential (cation-selective channels –more positive membrane potential, anion-selective channels –more negative membrane potential) Action of Drugs on Synapses • A neurotransmitter (NT) can be eliminated by two ways: enzyme that destroy NT in the synaptic cleft and NT reuptake process • Milder inhibitors of acetylcholinesterase (enzyme that hydrolyze acetylcholine) are used to treat the symptoms of Alzheimer’s disease, which is characterized by the loss of acetylcholine-releasing neurons. • Many drugs act by inhibiting the transporters that sweep neurotransmitters out of the synaptic cleft such as antidepressants. • Inhibiting the reuptake of serotonin, mood disorders can be treated. • Cocaine, interferes with the reuptake of the dopamine in the synaptic cleft of the limbic system and produces a short-lived feeling of euphoria and a strong desire to repeat the activity.