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Questions and Answers
Which type of solutes have the highest permeabilities in lipid membranes?
Which of the following is a characteristic of carrier-mediated transport, but not of simple diffusion?
What is the primary function of gap junctions between cells?
What is the term for the maximum transport rate achievable by a carrier-mediated transport system?
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What is the characteristic that distinguishes simple diffusion from other forms of transport?
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What is the effect of a competitive inhibitor on carrier-mediated transport?
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Which of the following factors can affect the flux of an ionic solute across a cell membrane?
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According to the diffusion equation, what is the unit of measurement for the flux (J)?
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What type of transport is facilitated by water-filled channels or pores in cell membranes?
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What is the effect of increasing the oil/water partition coefficient of a solute on its permeability?
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What is the direction of the urea flux in the given sample calculation?
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What is the effect of decreasing the membrane thickness on the permeability of a solute?
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What is the characteristic of tight junctions that can vary between different tissues?
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What is the relationship between the radius of a solute and its diffusion coefficient?
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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.
