Cell Junctions (Tight and Gap Junctions)

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?

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

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

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

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

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

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

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

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

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

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

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.

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

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Cell Junctions (Tight and Gap Junctions)

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