Active Transport against an Electrochemical Potential Gradient Requires Energy

Active Transport against an Electrochemical Potential Gradient Requires Energy 

Active transport of a solute against a concentration gradient also can be driven by a flow of an ion down its concentration gradient. Table 17.6 lists some of the active-transport systems that operate in this way. In some cases, the ion moves across the membrane in the opposite direction to the primary substrate (antiport) in others, the two species move in the same direction (symport). Many eukaryotic cells take up neutral amino acids by coupling this uptake to the inward movement of Na+ (see fig. 17.26c). As we discussed previously, Na+ influx is downhill thermodynamically because the Na+-K+ pump keeps the intracellular concentration of Na+ lower than the extracellular concentration and sets up a favorable electric potential difference across the membrane. Another example is the /3-galactosidc transport system of E. coli, which couples uptake of lactose to the inward flow of protons (see fig. 17.26 i). Proton influx is downhill because electron-transfer reactions (or, 

Some Systems of Active Transport Driven by ATP Hydrolysis 

Eukaryotic cell plasma membranes Muscle sarcoplasmic reticulum Eukaryotic cell plasma membranes Stomach epithelial cell plasma membranes 

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