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Questions and Answers
What characterizes selective permeability in biomembranes?
What characterizes selective permeability in biomembranes?
What type of transport does not require energy expenditure?
What type of transport does not require energy expenditure?
Which factor does NOT influence the rate of diffusion across a membrane?
Which factor does NOT influence the rate of diffusion across a membrane?
Which statement about hydrophobicity is correct?
Which statement about hydrophobicity is correct?
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Which transport mechanism uses energy directly from ATP?
Which transport mechanism uses energy directly from ATP?
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What characterizes facilitated diffusion?
What characterizes facilitated diffusion?
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Which type of transport protein uses ATP to move substances against their concentration gradient?
Which type of transport protein uses ATP to move substances against their concentration gradient?
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How does secondary active transport differ from primary active transport?
How does secondary active transport differ from primary active transport?
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What distinguishes gated ion channels from non-gated ion channels?
What distinguishes gated ion channels from non-gated ion channels?
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Which of the following correctly describes uniport transport?
Which of the following correctly describes uniport transport?
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Which ion is the most critical for maintaining the membrane potential?
Which ion is the most critical for maintaining the membrane potential?
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What causes the activation of the Na+/H+ antiporter?
What causes the activation of the Na+/H+ antiporter?
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What is generated by the activity of the Na+/K+ ATPase on the basolateral surface membrane?
What is generated by the activity of the Na+/K+ ATPase on the basolateral surface membrane?
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Which transport mechanism helps to decrease cytosolic pH?
Which transport mechanism helps to decrease cytosolic pH?
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What occurs during the outward movement of K+ ions through nongated K+ channels?
What occurs during the outward movement of K+ ions through nongated K+ channels?
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What is the function of the Na+/glucose symporter in the context of rehydration therapy?
What is the function of the Na+/glucose symporter in the context of rehydration therapy?
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What is the role of proton pump inhibitors in the stomach?
What is the role of proton pump inhibitors in the stomach?
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How does ouabain affect muscle contraction?
How does ouabain affect muscle contraction?
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Which of the following best describes the mechanism of the Na+/K+ pump as influenced by ouabain?
Which of the following best describes the mechanism of the Na+/K+ pump as influenced by ouabain?
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What condition is primarily treated by rehydration therapy involving water, sugar, and salt?
What condition is primarily treated by rehydration therapy involving water, sugar, and salt?
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Which of the following substances may be implicated in hypertension?
Which of the following substances may be implicated in hypertension?
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What is a key characteristic of uniport transport compared to simple diffusion?
What is a key characteristic of uniport transport compared to simple diffusion?
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What effect does the acid produced in the stomach have on digestion?
What effect does the acid produced in the stomach have on digestion?
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Which glucose uniport transporter is known to be expressed in erythrocytes?
Which glucose uniport transporter is known to be expressed in erythrocytes?
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What role does the Na+/Ca2+ antiporter play in muscle function?
What role does the Na+/Ca2+ antiporter play in muscle function?
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What does osmotic pressure relate to in terms of water movement across membranes?
What does osmotic pressure relate to in terms of water movement across membranes?
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What role do aquaporins play in cell membranes?
What role do aquaporins play in cell membranes?
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Which class of pumps is primarily responsible for generating and maintaining ion gradients across membranes?
Which class of pumps is primarily responsible for generating and maintaining ion gradients across membranes?
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What is a major function of the MDR1 pump in the ABC superfamily?
What is a major function of the MDR1 pump in the ABC superfamily?
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What distinguishes gated channels from ungated (leaky) channels?
What distinguishes gated channels from ungated (leaky) channels?
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What is a characteristic of the flippase mechanism used by the ABC superfamily pumps?
What is a characteristic of the flippase mechanism used by the ABC superfamily pumps?
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What factor significantly influences a substance's hydrophobicity when moving through a lipid bilayer?
What factor significantly influences a substance's hydrophobicity when moving through a lipid bilayer?
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Which statement correctly describes the process of simple diffusion across a membrane?
Which statement correctly describes the process of simple diffusion across a membrane?
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How does the partition coefficient (K) relate to a substance's ability to diffuse through a lipid bilayer?
How does the partition coefficient (K) relate to a substance's ability to diffuse through a lipid bilayer?
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Which of the following factors contributes least to the rate of diffusion across a membrane?
Which of the following factors contributes least to the rate of diffusion across a membrane?
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Which mechanism allows the transport of small molecules against their concentration gradient?
Which mechanism allows the transport of small molecules against their concentration gradient?
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What is the main feature of facilitated diffusion?
What is the main feature of facilitated diffusion?
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Which type of transport mechanism uses pumps to move substances?
Which type of transport mechanism uses pumps to move substances?
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What distinguishes secondary active transport from primary active transport?
What distinguishes secondary active transport from primary active transport?
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What role do gated ion channels play in cellular transport?
What role do gated ion channels play in cellular transport?
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Which statement accurately describes the function of uniport transporters?
Which statement accurately describes the function of uniport transporters?
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Which ion plays the most significant role in maintaining the membrane potential?
Which ion plays the most significant role in maintaining the membrane potential?
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How do Na+/H+ and Cl-/HCO3- antiporters affect cytosolic pH under specific conditions?
How do Na+/H+ and Cl-/HCO3- antiporters affect cytosolic pH under specific conditions?
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What process generates a negative membrane potential across the plasma membrane?
What process generates a negative membrane potential across the plasma membrane?
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What type of transport mechanism is represented by the Na+/glucose symporter?
What type of transport mechanism is represented by the Na+/glucose symporter?
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Which statement regarding membrane transport proteins is correct?
Which statement regarding membrane transport proteins is correct?
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What is the role of the Na+/glucose symporter in the uptake of glucose from the intestinal lumen?
What is the role of the Na+/glucose symporter in the uptake of glucose from the intestinal lumen?
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Which substances are used in rehydration therapy to stimulate the Na+/glucose symporter?
Which substances are used in rehydration therapy to stimulate the Na+/glucose symporter?
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How do proton pump inhibitors affect stomach acid production?
How do proton pump inhibitors affect stomach acid production?
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What effect does ouabain have on muscle contraction?
What effect does ouabain have on muscle contraction?
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What is the full name of the system inhibited by proton pump inhibitors?
What is the full name of the system inhibited by proton pump inhibitors?
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What is the potential consequence of inhibiting the Na+/K+ pump with ouabain?
What is the potential consequence of inhibiting the Na+/K+ pump with ouabain?
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Why is the Na+/glucose symporter essential in managing dehydration?
Why is the Na+/glucose symporter essential in managing dehydration?
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What effect does sodium have in the function of the Na+/Ca2+ antiporter?
What effect does sodium have in the function of the Na+/Ca2+ antiporter?
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What is a key aspect of GLUT transporters in humans?
What is a key aspect of GLUT transporters in humans?
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What does Vmax represent in the context of uniport transport?
What does Vmax represent in the context of uniport transport?
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Which statement accurately describes osmotic pressure?
Which statement accurately describes osmotic pressure?
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How do aquaporins influence cell membranes?
How do aquaporins influence cell membranes?
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What is the primary function of ATP-powered ion pumps?
What is the primary function of ATP-powered ion pumps?
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What distinguishes gated channels from ungated channels?
What distinguishes gated channels from ungated channels?
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Which statement best describes the flippase mechanism?
Which statement best describes the flippase mechanism?
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Which of the following accurately describes the function of MDR1 in the ABC superfamily?
Which of the following accurately describes the function of MDR1 in the ABC superfamily?
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Study Notes
Transmembrane Transport of Ions & Small Molecules
- Biomembranes are selectively permeable due to their lipid bilayer structure.
- This means only certain types of substances can move through.
- Gases (CO2, N2, O2), small uncharged polar molecules (ethanol), and water are permeable.
- Urea and glucose/fructose are slightly permeable.
- Ions (K+, Mg2+, Ca2+, Cl-, HCO3-, HPO42-), charged polar molecules (amino acids, ATP, proteins, and nucleic acids) are impermeable.
Mechanisms of Transmembrane Transport
- Simple (passive) diffusion: Movement of a substance across a membrane from a region of higher concentration to one of lower concentration, without energy expenditure.
- Facilitated diffusion: Movement of a substance across a membrane from a region of higher concentration to one of lower concentration with the help of a transport protein; still no energy expenditure.
- Primary active transport: Movement of a substance from a region of lower to higher concentration across a membrane, requiring energy expenditure and using proteins called pumps (ATPases).
- Secondary active transport (cotransport): Energetically unfavorable transport of a substance against its concentration gradient is coupled to the energetically favorable movement of an ion down its electrochemical gradient (symport or antiport).
Simple Diffusion
- Diffusion is spontaneous due to increasing entropy as a substance moves from high to low concentration.
- The change in free energy (ΔG) is negative because products have more entropy and less free energy than reactants.
- ΔG = ΔH – TΔS (where ΔH is enthalpy and ΔS is entropy, and T is temperature in Kelvin).
What Determines Diffusion Rate Across a Membrane?
- Concentration gradient across the membrane.
- Size of the diffusing particles.
- Electric potential across the membrane (for charged particles).
- Hydrophobicity of the substance.
Hydrophobicity
- Hydrophobicity measures the degree to which a substance is hydrophobic.
- Measured by the substance's partition coefficient (K).
- K is the equilibrium constant for its partition between oil and water.
- Higher K = more lipid-soluble and faster rate of movement across the bilayer.
- Hydrophobicity is influenced by the proportion of uncharged regions in a substance (e.g., long-chain fatty acids are more hydrophobic than shorter-chained ones).
How do Impermeable Substances Move Through Membranes?
- Impermeable substances require transport proteins: pumps, channels or transporters
- These protein-mediated processes may or may not require energy.
- Multiple transport proteins can work together
Transport Protein Mediated Processes
- Facilitated Diffusion: Passive movement across membranes, no energy needed. typically with channel or uniporters.
- Primary Active Transport: Active movement from a low concentration to high, requiring ATP hydrolysis through pump proteins.
- Secondary Active Transport (Cotransport): Coupled transport of substances, one substance (e.g ion) going down its electrochemical gradient drives the movement of another substance against its gradient.
Membrane Transport Proteins
- ATP-powered pumps: 100-10^3 ions per second, move substances against their concentration gradient, requiring energy.
- Ion channels: 10^7-10^8 ions per second, allow passive transport of ions based on electrochemical gradient.
- Transporters (uniporter, symporter, antiporter): 10^2-10^4 molecules per second, facilitate passive or active transport of substances based on concentration.
Mechanisms for Transporting Ions and Small Molecules across Cell Membranes
- Simple diffusion: no protein required, high to low concentration gradient.
- Facilitated transport: protein required, high to low concentration gradient.
- Active transport: protein required, low to high concentration gradient.
- Cotransport: coupled to energetically favorable movement of another substance.
Comparing Simple Diffusion & Uniport Transport
- Uniport transport is a form of facilitated diffusion involving small hydrophilic molecules through a membrane and is faster than simple diffusion.
- K is irrelevant as the molecule doesn't enter the hydrophobic membrane core.
- Uniport transport has a maximal rate (Vmax).
- It is reversible (direction based on concentration gradient).
- It is specific.
Glucose Uniport Transporters (GLUT)
- Humans have 12 GLUT isoforms.
- GLUTs are tissue-specific: GLUT1 in erythrocytes, GLUT2 in liver cells.
- GLUT transport is a key mechanism for glucose entry.
Osmotic Pressure and Water Movement
- Osmotic pressure causes water to move across membranes from a region of lower solute concentration to a region of higher solute concentration.
- Water moves to equalize solute concentration across a semipermeable membrane.
- Hydrostatic pressure is required to prevent net water flow.
Aquaporins
- Aquaporins increase water permeability of cell membranes.
- Aquaporins are water channel proteins.
4 Classes of ATP-Powered Pumps
- P-class: transport ions
- V-class: acidify organelles
- F-class: used in ATP synthesis
- ABC: transport large molecules, lipids, etc
Model of Ca2+ ATPase
- Ca2+ ATPase is a protein that pumps calcium ions across membranes.
- It involves binding and conformational changes involving ATP.
- It is an important regulatory protein in muscle contraction and other cellular processes.
Structure of Catalytic a-subunit of Muscle ATPase
- Shows how a-subunit is conformational involved with calcium and phosphorylation.
Na+/K+ ATPase
- A pump that moves Na+ out of and K+ into the cell, maintained by ATP hydrolysis.
Effects of V-class H+ pumps on H+ concentration and electric potential gradients
- V-class pumps change the pH gradient across a membrane.
- This gradient creates an electric potential across the membrane.
- The effects depend on whether or not other membrane channels are also present.
ATP-powered ion pumps generate and maintain ion gradients.
- Illustrates the concentration differences of ions inside and outside a mammalian cell (e.g. K+, Na+, CI-).
- Provides context for the importance of ion gradients in cellular function and homeostasis.
ABC Superfamily
- About 50 ABC superfamily pumps in mammals.
- Transport of lipids, sugars, other molecules.
- e.g., MDR1 to expel drugs, CFTR for Cl- transport and other substances.
- Present in bacteria, too
Selected Human ABC Proteins
- Table including protein, tissue expression, function and disease caused by defective protein
Flippase Mechanism
- Mechanism used by ABC superfamily pumps to transport lipid-soluble substances across membranes involving ATP binding.
Gated & Ungated Channels
- Gated channels open and close in response to stimuli (chemical or voltage).
- Non-gated (leaky) channels are always open, allowing ion flow to maintain membrane resting potential.
- e.g. Na+, K+, Cl-, Ca2+ channels.
Generation of Transmembrane Electric Potential
- Diagram showing the role of selective permeable membranes.
- Na+ and K+ permeability differences create membrane potential.
- Illustrates how differences in ion concentrations and membrane permeability generate electric potential across a cell membrane.
Structure of Resting Bacterial K+ Channel
- Illustrates the structure of a potassium channel, highlighting the selectivity filter, P segment, and membrane helices.
K+ and Na+ ions in the pore of a K+ channel
- The figure illustrates why potassium channel selectively transports K+ over Na+.
K+ ions in the pore of a K+ channel (side view).
Ion concentration gradient and Membrane electric potential
- Illustrates the free-energy change during Na+ transport across a membrane based on the ion concentration gradient and the membrane electric potential.
Cotransport
- Na+/H+ antiporter, Cl-/HCO3- antiporter, 2Na+/glucose symporter, 3Na+/Ca2+ antiporter are examples
- Cotransport mechanisms that affect cytosolic pH of mammalian cells.
Figure 11-26: 2 Na+/1 Leucine Symporter
- Shows structural details of the leucine symporter.
Co-transporters and pH
- The activity of membrane transport proteins regulating cytosolic pH in mammalian cells depends on pH.
- Na+/H+ and Na⁺HCO₃⁻/Cl⁻ antiporters increase cytosolic pH when it decreases.
- CI⁻/HCO₃⁻ antiporter reduces cytosolic pH at high pH.
Transport Across Epithelial Membranes: Transcellular Glucose Transport
- Illustrates transport of glucose across epithelial cells in the small intestine.
- Importance of Na+/K+ ATPase and transport proteins.
Rehydration Therapy
- Ingestion of water, sugar, and salt can stimulate the Na+/glucose symporter, facilitating rehydration.
- Principle for drinks like Gatorade.
Transport Across Epithelial Membranes: Acidification of Stomach Lumen
- Illustrates the acidification of the stomach using the H+/K+ ATPase and CI-/HCO3- antiporter.
Proton Pump Inhibitors
- Block stomach acid production through inhibition of the proton pump (hydrogen-potassium adenosine triphosphate enzyme system).
Ouabain, A Cardiac Glycoside
- Endogenous steroid hormone.
- Blocks Na+/K+ pump.
- Increased cytosolic Ca2+ levels and stronger muscle contraction.
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Description
Explore the mechanisms of transmembrane transport including simple diffusion, facilitated diffusion, and active transport. This quiz covers the selectively permeable nature of biomembranes and the types of substances that pass through them. Test your knowledge on the permeability of ions and small molecules.