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Cell Junctions (Tight and Gap Junctions)
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Cell Junctions (Tight and Gap Junctions)

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Questions and Answers

Which type of solutes have the highest permeabilities in lipid membranes?

  • Small hydrophobic solutes (correct)
  • Structurally related solutes
  • Ions
  • Hydrophilic solutes
  • Which of the following is a characteristic of carrier-mediated transport, but not of simple diffusion?

  • Stereospecificity (correct)
  • Steady-state flux
  • Proportional to concentration difference
  • No energy expenditure
  • What is the primary function of gap junctions between cells?

  • To permit intercellular communication and electrical coupling (correct)
  • To regulate the permeability of the membrane to solutes
  • To facilitate simple diffusion of solutes
  • To create a tight, impermeable barrier between cells
  • What is the term for the maximum transport rate achievable by a carrier-mediated transport system?

    <p>Transport maximum (Tm)</p> Signup and view all the answers

    What is the characteristic that distinguishes simple diffusion from other forms of transport?

    <p>It is not carrier-mediated</p> Signup and view all the answers

    What is the effect of a competitive inhibitor on carrier-mediated transport?

    <p>Decreases the transport rate</p> Signup and view all the answers

    Which of the following factors can affect the flux of an ionic solute across a cell membrane?

    <p>Both concentration and potential differences</p> Signup and view all the answers

    According to the diffusion equation, what is the unit of measurement for the flux (J)?

    <p>mmol/sec</p> Signup and view all the answers

    What type of transport is facilitated by water-filled channels or pores in cell membranes?

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

    What is the effect of increasing the oil/water partition coefficient of a solute on its permeability?

    <p>It increases the permeability of the solute</p> Signup and view all the answers

    What is the direction of the urea flux in the given sample calculation?

    <p>From blood to proximal tubular fluid</p> Signup and view all the answers

    What is the effect of decreasing the membrane thickness on the permeability of a solute?

    <p>It increases the permeability of the solute</p> Signup and view all the answers

    What is the characteristic of tight junctions that can vary between different tissues?

    <p>Their permeability to solutes</p> Signup and view all the answers

    What is the relationship between the radius of a solute and its diffusion coefficient?

    <p>A smaller radius increases the diffusion coefficient</p> Signup and view all the answers

    Study Notes

    Cell Attachments and Intercellular Communication

    • Tight junctions (zonula occludens) are attachments between cells, often epithelial cells, that can be either "tight" (impermeable) or "leaky" (permeable) depending on the size, charge, and characteristics of the junction.
    • Gap junctions are attachments between cells that permit intercellular communication, allowing current flow and electrical coupling between cells, such as myocardial cells.

    Simple Diffusion

    • Simple diffusion is the only form of transport that is not carrier-mediated.
    • It occurs down an electrochemical gradient ("downhill") and does not require metabolic energy, making it a passive process.
    • Diffusion can be measured using the equation: J = −PA(C1 − C2), where J is the flux, P is the permeability, A is the area, and C1 and C2 are the concentrations.
    • The direction of flux is from high to low concentration, indicated by the minus sign in the equation.

    Permeability

    • Permeability (P) describes the ease with which a solute diffuses through a membrane.
    • It depends on the characteristics of the solute and the membrane.
    • Factors that increase permeability include:
      • Increased oil/water partition coefficient of the solute, increasing solubility in the lipid of the membrane.
      • Smaller radius of the solute, increasing the diffusion coefficient and speed of diffusion.
      • Decreased membrane thickness, decreasing the diffusion distance.
    • Small hydrophobic solutes (e.g., O2, CO2) have the highest permeabilities in lipid membranes.
    • Hydrophilic solutes (e.g., Na+, K+) must cross cell membranes through water-filled channels or pores, or via transporters.

    Carrier-Mediated Transport

    • Carrier-mediated transport includes facilitated diffusion and primary and secondary active transport.
    • Characteristics of carrier-mediated transport include:
      • Stereospecificity, where the transport is specific to a particular isomer (e.g., D-glucose).
      • Saturation, where the transport rate increases as the concentration of the solute increases, until the carriers are saturated.
      • Competition, where structurally related solutes compete for transport sites on carrier molecules.

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    Description

    This quiz covers the different types of cell junctions, including tight junctions and gap junctions, their structures, and functions.

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