Sugar transport cotransport

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

An oral formula suitable for the correction of a variety of diarrheal fluid losses has been recommended by the World Health Organization (WHO). It consists of 90 mM Na, 20 mA1 K, 00 mM Cl, 30 mM HCOj, and HO mM glucose. The glucose aids in intestinal absorption of sodium ions, because glucose transport is mediated by the Ma-glucosc cotransporter. Only low levels of sugar are used in oral replace-... 

Other cotransporters facilitate the transport of other sugars, osmolytes, and amino acids. In humans, a disorder of intestinal glucose and galactose absorption is due to a defective sodium-glucose transporter. 

Organic solutes such as nutrients (amino acids, sugars, vitamins, and bile acids), neurotransmitters, and drugs are transferred across cellular membranes by specialized transport systems. These systems encompass integral membrane proteins that shuttle substrates across the membrane by either a passive process (channels, facilitated transporters) or an active process (carriers), the latter energized directly by the hydrolysis of ATP or indirectly by coupling to the cotransport of a counterion down its electrochemical gradient (e.g., Na, ... 

With the exception of the phosphotransferase system that is responsible for uptake of several sugars by bacteria, the active uptake of organic solutes is secondary active and coupled via cotransport to the downhill transport of a cation, Na in animal cells and H ions in microorganisms. In transcellular transport in epithelia, such as small intestine and proximal tubule of the kidney, the solutes are accumulated inside the cell via a cotransport mechanism at the luminal membrane, and leave the cell passively presumably by facilitated diffusion at the eontraluminal side. 

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