![]() Ionized weak organic acids and bases, however, slowly permeate the plasma membrane through pores. Weak organic acids and bases may cross plasma membranes by simple diffusion when they are nonionized. In the kidney glomeruli, however, the pores may be as large as 70–80 Å in diameter, which permits more efficient renal elimination of potentially toxic compounds. The pores in most cells are approximately 4 Å in diameter. Water readily traverses the plasma membrane through pores and may carry with it small hydrophilic solutes. ![]() The products of this mixing are large-surface-area micelles (hydrophobic interior) that deliver the lipids to the brush border of the intestine for diffusion across the membrane. Ingestion of very lipid soluble pesticides, which are not miscible in the aqueous intestinal fluid, can be presented as emulsions and brought into solution through the action of detergent-like bile acids. These scenarios can prolong absorption half-life across the skin, which can also prolong the body burden of the penetrant. Some compounds can also form a reservoir in the dermis. If lipid solubility increases, the penetrant may remain in the stratum corneum of the skin and form a reservoir. Theoretically, the determinants of flux or diffusion rates across the skin or gastrointestinal (GI) tract may be altered clinically or experimentally through manipulation of pesticide formulations. The low brain glucose causes a devastating neurological syndrome ( Box 10.3).įlux = diffusion coefficient × surface area × partition coefficient × conc. Individuals with a mutant GLUT-1 gene have a cerebrospinal fluid glucose concentration that is approximately 50% of normal despite a normal blood glucose level. GLUT-1 also is expressed in the brain, where it mediates glucose transport across the blood-brain barrier and glucose uptake by glial cells. The need for carrier-mediated glucose transport is exemplified by the consequences of mutations in the human GLUT-1 gene. This is the general structure of many transporter molecules. In contrast, the hydrophilic surfaces of the five helices face one another and form a central, water-filled transmembrane channel or pore ( Fig. The hydrophobic regions interact with the surrounding lipids in the bilayer. GLUT-1 has 12 membrane-spanning helices, 5 of which are amphiphilic: each has a hydrophobic and a hydrophilic surface. This protein, GLUT-1, belongs to a family of sugar transporters ( Box 10.2). A good example is the glucose carrier that mediates glucose transport across the human red blood cell (RBC) PM. Several solutes are transported by solute-selective simple carriers. ![]() Blaustein MD, in Cellular Physiology and Neurophysiology, 2020 Sugars are transported by carriers In almost all cases of carrier-mediated transport in the kidney, two or more ions or molecules share the carrier: one moiety moves down its electrochemical gradient, and the other(s) moves against the gradient and is transported “uphill.”ĭiffusion through a membrane channel (or pore) formed by specific integral membrane proteins is also a form of facilitated diffusion, because it allows charged, polar, and lipophobic molecules to pass through the membrane at a high rate. Simple diffusion does not require a direct energy source, although active transport is usually necessary to establish the initial concentration and electrical gradients.įacilitated diffusion (coupled or carrier-mediated diffusion) depends on an interaction of the molecule or ion with a specific membrane carrier protein that facilitates its passage across the cell membrane's lipid bilayer. With an undissociated molecule, only the concentration gradient is relevant for a charged ion the electrical gradient also must be considered. Simple diffusion always occurs down an electrochemical gradient, which is a composite of the concentration and electrical gradients (electrochemical gradient). John Feehally DM, FRCP, in Comprehensive Clinical Nephrology, 2019 Passive Transport
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