Absorption (biology)
Absorption (biology): The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin, and in each location a specific cell layer called an epithelium regulates the passage of materials.
Absorption across epithelia may occur by several different passive and active processes. Simple diffusion is the net movement of molecules from the apical to basolateral surfaces of an epithelium down chemical and electrical gradients without the requirement of cellular energy sources. Facilitated diffusion across the epithelium is similar to simple diffusion in that energy is not required, but in this process, molecular interaction with protein binding sites (carriers) in one or both membranes must occur to facilitate the transfer. Active molecular transport involves the use of membrane protein carriers as well as cellular energy supplies to move a transported molecule up an electrochemical gradient across the epithelium. Endocytosis and phagocytosis are also examples of active transport because metabolic energy is required, but in these processes whole regions of the cell membrane are used to engulf fluid or particles, rather than to bring about molecular transfer using single-membrane proteins. [Related topics: CELL MEMBRANES; ENDOCYTOSIS; OSMOREGULATORY MECHANISMS; PHAGOCYTOSIS.]
Although a wide variety of ions are absorbed by different types of epithelial cells, the mechanisms of Na+ and Cl- transport in mammalian small intestine are perhaps best known in detail. Transepithelial transport of these two ions occurs in this tissue by three independent processes: active Na+ absorption, not coupled directly to the flow of other solutes but accompanied indirectly by the diffusional absorption of Cl-; coupled NaCl absorption; and cotransport of Na+ with a wide variety of nutrient molecules. [Related topics: ION TRANSPORT]
Net water transport across the epithelium is coupled to net ion transport in the same direction. Pump sites for Na+ are believed to be located along the lateral borders of epithelial cells. Energy dependent Na+ efflux from the cells to the intercellular spaces creates a local increase in osmotic pressure within these small compartments. An osmotic pressure gradient becomes established here, with greatest solute concentrations located nearest the tight junctions. Water flows into the cell across the brush border membrane and out the lateral membranes in response to the increased osmotic pressure in the paracellular spaces. Once water is in the intercellular compartment, a buildup of hydrostatic pressure forces the transported fluid to the capillary network.