S hape of cells in the basal layer

N umber of layers of epithelial cells

P resence of a basal lamina

S ize of the nuclei

P resence of cytokeratin

1440

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1515

2000

The gut epithelial cells transport glucose from the gut lumen across the gut epithelium and, ultimately, into the blood. T he gut epithelial cells possess a glucose– Na + symport, which they can use to take up glucose from the gut lumen, even when the concentration of glucose is higher in the epithelial cell’s cytosol than it is inside the gut.

As the electrochemical gradient for Na + is so steep, when Na + moves into the cell down its gradient, glucose is, in a sense, “dragged” into the cell along with it. Because the binding of Na + and glucose is cooperative—the binding of one enhances the binding of the other—if one of the two solutes is missing, the other fails to bind .

The gut epithelial cells have two types of glucose transporters located at opposite ends of the cell. In the apical domain of the plasma membrane, which faces the gut lumen, they have the glucose–Na + symports. In the basal and lateral domains of the plasma membrane, the cells have passive glucose uniports, which release the glucose down its concentration gradient for use by other tissues.

The two types of glucose transporters are kept segregated in their proper domains of the plasma membrane by a diffusion barrier formed by a tight junction around the apex of the cell. This prs mixing of membrane components between the two domains.

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The apical and basal faces of an epithelium are different: each contains a different set of molecules that reflect the polarized organization of the individual epithelial cells: each has a top and a bottom, with different properties and functions. This polarity is crucial for epithelial function.

For example, the columnar epithelium that lines the small intestine of a mammal. It mainly consists of two intermingled cell types: absorptive cells, which take up nutrients, and goblet cells (so called because of their shape), which secrete the mucus that protects and lubricates the gut lining. Both cell types are polarized.

The absorptive cells import food molecules from the gut lumen through their apical suce and export these molecules from their basal suce into the underlying tissues. To do this, absorptive cells require different sets of membrane transport proteins in their apical and basal plasma membranes. The goblet cells also have to be polarized, but in a different way: they have to synthesize mucus and then discharge it from their apical end only; their Golgi apparatus, secretory vesicles, and cytoskeleton are all polarized so as to bring this about.

For both types of epithelial cells, polarity depends on the junctions that the cells form with one another and with the basal lamina. These cell junctions in turn control the arrangement of an elaborate system of membrane-associated intracellular proteins that create the polarized organization of the cytoplasm.

0

0

1000

540

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1000

1450

1350

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