We saw in Chapter 6 that several types of specialized regions of the plasma membrane, called cell junctions, connect epithelial cells forming the sheetlike lining of the intestines and other body surfaces (see Figure 6-5). Of concern to us here are tight junctions, which prevent water-soluble materials on one side of an epithelium from moving across to the other side through the extracellular space between cells. For this reason, absorption of nutrients from the intestinal lumen occurs by the import of molecules on the luminal side of intestinal epithelial cells and their export on the blood-
facing (serosal) side, a two-stage process called transcellu-lar transport.
An intestinal epithelial cell, like all epithelial cells, is said to be polarized because the apical and basolateral domains of the plasma membrane contain different sets of proteins. These two plasma-membrane domains are separated by the tight junctions between cells. The apical portion of the plasma membrane, which faces the intestinal lumen, is specialized for absorption of sugars, amino acids, and other molecules that are produced from food by various digestive enzymes. Numerous fingerlike projections (100 nm in diameter) called microvilli greatly increase the area of the apical surface and thus the number of transport proteins it can contain, enhancing the cell's absorptive capacity.
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