Transport mechanisms requiring carrier protein are all except

Ans. a (Simple diffusion). (Ref. Ganong, Physiology, 21st ed., 31)TRANSPORT ACROSS CELL# A]. When carrier proteins move substances in the direction of their chemical or electrical gradients, no energy input is required and the process is called facilitated diffusion.- A typical E.g. is glucose transport by the glucose transporter, which moves glucose down its concentration gradient from the ECF to the cytoplasm of the cell.# B] Other carriers transport substances against their electrical and chemical gradients. This form of transport requires energy and is called active transport.- E.g. 1. ATPases is Na+-K+ ATPase pump.- 2. H+-K+ ATPases in the gastric mucosa and the renal tubules.- 3. Ca2+ ATPase pumps Ca2+ out of cells.- 4. Proton ATPases acidify many intracellular organelles, including Golgi complex and lysosomes.# Some of the transport proteins are called uniports, because they transport only one substance.# Others are called symports, because transport requires the binding of more than one substance to the transport protein and the substances are transported across the membrane together.- E.g. symport in the intestinal mucosa responsible for the cotransport by facilitated diffusion of Na+ and glucose from the intestinal lumen into mucosal cells.# Other transporters are called antiports because they exchange one substance for another.# C]. Some transport proteins are simple aqueous ion channels; many of these have special features that make them effective for a given substance such as Ca2+ or, in the case of aquaporins, for water. Many transport proteins are continuously open, and some are gated. Some are gated by alterations in membrane potential (voltage-gated), whereas others are opened or closed when they bind a ligand (ligand-gated).- A typical voltage-gated channel is the Na+ channel, and a typical ligand-gated channel is the acetylcholine receptor.- A ligand is often external (eg, a neurotransmitter or a hormone).- However, it can also be internal; intracellular Ca2+, cAMP, lipids, or one of the G proteins produced in cells can bind directly to channels and activate them.