This document discusses the transport of substances across epithelial cells that line various tissues in the body. It explains that epithelial cells are polarized, with transporters located specifically in the apical or basolateral membranes. Glucose absorption in the small intestine is used as an example, where glucose enters cells via the sodium-glucose cotransporter on the apical surface and exits into the blood via the glucose transporter on the basolateral surface. Intestinal fluid secretion is also summarized, involving the transport of chloride ions into and out of cells.