Figure 8.  Epithelial transport. Transport across the epithelium of the intestine will be discussed briefly merely to demonstrate the similarity of the basic principles.  The topic will be discussed in more detail by Professor Wright.  At the lumenal or brush-border side of this polarized intestinal epithelial cell (top), sodium and glucose are symported via a sodium-coupled symport protein (SGLT1), leading to accumulation of glucose against a large concentration gradient in the cytosol.  The accumulated glucose is then allowed to flow down its chemical gradient via a glucose uniporter (a.k.a., facilitated diffusion carrier) on the basolateral surface (bottom) into the serosal fluid, and sodium is pumped out on the basolateral surface by the Na⁺/K⁺-ATPase.  The tight junctions keep the membrane proteins on the brush border from mixing with the proteins on the basolateral membrane and vice versa.