<prompt> Transport mechanisms requiring carrier protein are all exceptSimple diffusionPassive diffusionActive transportFacilitated diffusion  <response>: Ans. a Simple diffusion. Ref. Ganong, Physiology, 21st ed., 31TRANSPORT 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. 