Resumé: active and passive transport Luminal contents face two possible pathways across the epithelium, either around the enterocytes (paracellular) or through the cells (transcellular). Paracellular transport is always passive; transcellular transport may be either passive or active. Whether movement of a particular solute is active or passive depends on chemical and electrical gradients. Chemical gradients describe the differences in concentration between two compartments (ie, lumen and cell). Electrical gradients refer to the potential difference across membranes. An ion such as sodium or chloride will respond to both electrical and chemical gradients whereas for uncharged particles, only a chemical gradient is relevant.Because electrical and chemical gradients for a particular ion may be either in similar or opposite directions, the sum of these forces can be calculated as an electrochemical gradient. Passive transport proceeds in the direction of the electrochemical gradient for a specific solute or ion whereas active transport involves movement against a gradient and requires the expenditure of energy. Water transport is always passive in response to osmotic forces due to the shift of solute from lumen to subepithelial space, or vice versa. |
source; AstroZeneca's GastroAtlas-the online version of Mark Feldman's Textbook of Gastroenterology