Cell Biology and Signalling Block: Membrane Transport
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Questions and Answers

Which of the following transporters has a low affinity for glucose?

  • GLUT1
  • GLUT2 (correct)
  • GLUT4
  • GLUT3
  • In which tissues is GLUT1 abundant and GLUT4 scarce?

  • Neurones and adipocytes
  • Liver and pancreatic ß-cells
  • Erythrocytes and skeletal muscle (correct)
  • Muscle and erythrocytes
  • What is the primary function of GLUT2 in pancreatic ß-cells?

  • Regulate blood glucose levels
  • Transport glucose into pancreatic ß-cells when blood glucose is high (correct)
  • Transport glucose out of pancreatic ß-cells
  • Increase glucose uptake in response to insulin
  • What is the effect of insulin on GLUT4?

    <p>Increase the amount of GLUT4 in the plasma membrane</p> Signup and view all the answers

    What is the primary mechanism of ion channel gating?

    <p>Change in membrane potential</p> Signup and view all the answers

    What is the effect of insulin on glucose uptake in muscle and adipose tissue?

    <p>Increase glucose uptake</p> Signup and view all the answers

    What is the primary distinguishing feature of small-molecule transfer across membranes by active transport?

    <p>Requires energy input to pump molecules against their concentration gradient</p> Signup and view all the answers

    Which of the following molecules can freely cross the membrane by simple diffusion?

    <p>O2</p> Signup and view all the answers

    What is the role of the Na+/K+-ATPase membrane pump?

    <p>To maintain the electrochemical gradient across the membrane</p> Signup and view all the answers

    Which type of transport requires a membrane protein, such as an ion channel?

    <p>Facilitated diffusion</p> Signup and view all the answers

    What is the relationship between the Partition Coefficient of a solute and its rate of diffusion?

    <p>Solute with a higher Partition Coefficient equilibrate more quickly</p> Signup and view all the answers

    What is the function of the GLUT family of glucose transporters?

    <p>To transport glucose into the cell</p> Signup and view all the answers

    What is the significance of the Km in transporter affinity?

    <p>It represents the external concentration at which the transport rate is half-maximal</p> Signup and view all the answers

    What is the primary difference between facilitated diffusion and active transport?

    <p>Facilitated diffusion moves solutes down their concentration gradient, while active transport moves solutes against their concentration gradient</p> Signup and view all the answers

    Which of the following is an example of secondary active transport?

    <p>Na+/glucose transporter</p> Signup and view all the answers

    What is the term for the selective permeability of membranes to certain molecules?

    <p>Selective permeability</p> Signup and view all the answers

    What is the primary function of the Na+/K+ pump in nerve cells?

    <p>To maintain the ion gradient of sodium and potassium across the plasma membrane</p> Signup and view all the answers

    What type of transport occurs when two solutes move in the same direction across a membrane?

    <p>Symport</p> Signup and view all the answers

    What is the result of inhibiting the Na+/K+ pump in heart muscle cells?

    <p>Increased contraction of heart muscle cells</p> Signup and view all the answers

    What is the function of the CFTR protein in cells responsible for producing mucus, sweat, saliva, and tears?

    <p>To facilitate the movement of chloride ions out of the cell</p> Signup and view all the answers

    What is the purpose of oral rehydration therapy in treating cholera?

    <p>To drive the uptake of glucose into cells via SGLUT</p> Signup and view all the answers

    What type of transport is facilitated by the Na+/K+ pump?

    <p>Primary active transport</p> Signup and view all the answers

    What is the result of the Na+/Ca2+ cotransporter in muscle cells?

    <p>Export of calcium ions from muscle cells</p> Signup and view all the answers

    What is the role of the phosphorylation step in the Na+/K+ pump?

    <p>To cause a conformational change in the pump</p> Signup and view all the answers

    What is the effect of digitoxin on the Na+/K+ pump?

    <p>It inhibits the dephosphorylation step of the pump</p> Signup and view all the answers

    What is the mechanism by which SGLUT transports glucose into cells?

    <p>Symport with sodium ions down a concentration gradient</p> Signup and view all the answers

    Study Notes

    Membrane Transport

    • Membranes are selective permeability barriers, blocking the passage of almost all hydrophilic molecules into cells and organelles.
    • Small uncharged or hydrophobic molecules can freely cross the membrane by simple diffusion along their concentration gradients.
    • Charged polar molecules require specialist proteins (pumps, transporters, pores) to allow them to cross the membrane.

    Molecules Crossing Membranes

    • Hydrophobic molecules that can freely cross the membrane include O2, N2, CO2, benzene, and short-chain fatty acids.
    • Small uncharged polar molecules that can cross the membrane include H2O, urea, and glycerol.
    • Large uncharged polar molecules that cannot cross the membrane include glucose, sucrose, and ions like H+, Na+, Mg2+, HCO3-, K+, Ca2+, and Cl-.
    • Charged polar molecules that cannot cross the membrane include amino acids and ATP.

    Mechanisms of Transport

    • Passive transport: solutes move down a concentration gradient, crossing the membrane.
    • Facilitated diffusion: solutes move down a concentration gradient, crossing the membrane with the help of a transport protein.
    • Gated ion channels: ion channels that allow facilitated diffusion, selective for different ions, open or close in response to a stimulus.
    • Primary active transport: solutes move against a concentration gradient, requiring energy from ATP hydrolysis.
    • Secondary active transport: solutes move against a concentration gradient, using energy from the electrochemical gradient established by primary active transport.

    Passive Transport

    • Solutes move down a concentration gradient, crossing the membrane.
    • At equilibrium, the concentration inside the cell equals the concentration outside the cell.
    • The rate of diffusion depends on the partition coefficient of the solute.
    • Solutes that are more hydrophobic have a higher partition coefficient and equilibrate more quickly.

    Facilitated Diffusion

    • Solutes move down a concentration gradient, crossing the membrane with the help of a transport protein.
    • Examples include Cl-/HCO3- channel in erythrocytes, aquaporin (water channel), and GLUT glucose transporters.
    • The kinetics of transport for passive transport and facilitated diffusion show that Jmax (rate of uptake) is dependent on the external concentration.

    Transporter Affinity

    • Transporter affinity for solutes is given by the Km (Michaelis constant).
    • A lower Km indicates a higher affinity for the solute.

    Glucose Transporters (GLUT)

    • A family of related glucose transporters with different locations and functions.
    • GLUT1: ubiquitous, low Km (high affinity), mediates constitutive glucose uptake in many tissues.
    • GLUT2: liver, pancreatic β-cells, high Km (low affinity), large Jmax (high capacity), transports glucose into hepatocytes and β-cells.
    • GLUT3: neurons, low Km (high affinity).
    • GLUT4: muscle, adipocytes, Km similar to fed-state blood glucose concentration, regulated by insulin.

    Insulin-Stimulated Glucose Uptake

    • Insulin stimulates glucose uptake in muscle and adipose tissue by increasing the amount of GLUT4 in the plasma membrane.
    • Insulin triggers the movement of vesicles containing GLUT4 to the plasma membrane, increasing the uptake of glucose into the cell.

    Gated Ion Channels

    • Ion channels that allow facilitated diffusion, selective for different ions, open or close in response to a stimulus.
    • Examples include ligand-gated channels (e.g., acetylcholine receptor) and voltage-gated channels (e.g., Na+ and K+ channels in axons involved in nerve transmission).

    Active Transport

    • Solutes move against a concentration gradient, requiring energy from ATP hydrolysis.
    • Primary active transport: ATP hydrolysis directly causes the movement of solutes (uniport).
    • Example: Na+/K+ pump (Na+/K+ ATPase) in plasma membrane.

    Na+/K+ Pump (Na+/K+ ATPase)

    • A tetramer (α2β2) that pumps Na+ out of the cell and K+ into the cell, establishing an electrochemical gradient.
    • The pump consists of an open cytoplasmic access, phosphorylation from ATP at the cytoplasmic site, conformational change, and hydrolysis of the phosphate group.

    Co-Transport Systems

    • A pre-established gradient is used to drive transport of solutes across the membrane against a gradient.
    • ATP hydrolysis is used to establish the primary gradient.
    • Symport: transport of two solutes in the same direction.
    • Antiport: transport of two solutes in opposite directions.

    Na+/Glucose Cotransporter (SGLUT)

    • A symport that uses the Na+ gradient established by the Na+/K+ pump to drive the uptake of glucose into cells.
    • Secondary active transport.

    Clinical Considerations

    • Digitoxin: inhibits Na+/K+ pump, leading to increased [Na+] and [Ca2+] inside heart muscle, and increased contraction.
    • Ouabain: inhibits Na+/K+ pump, leading to increased [Na+] and [Ca2+] inside heart muscle, and increased contraction.
    • Cystic fibrosis: caused by mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR), leading to reduced chloride transport and thick mucus production.
    • Cholera treatment: oral rehydration therapy includes high glucose concentration to drive Na+ uptake into cells via SGLUT, reducing electrolyte and fluid secretion.

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    Description

    Learn about the mechanisms of small-molecule transport across membranes, including passive diffusion, facilitated transport, and active transport. Understand the structure and function of the Na+/K+-ATPase membrane pump.

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