Final answer:
Secondary active transport involves carrier proteins moving a substrate against its concentration gradient by harnessing energy from primary transport. This occurs through mechanisms such as symporters and antiporters, for example, during glucose reabsorption in the kidneys facilitated by Na+/glucose symport proteins.
Step-by-step explanation:
In secondary active transport, carrier proteins have the ability to move another substrate simultaneously, irrespective of its concentration gradient, by utilizing the energy derived from the primary transport mechanism. These carrier proteins, including symporters and antiporters, facilitate the movement of molecules across the cell membrane.
For example, Na+/glucose symport proteins allow both Na+ and glucose to enter the cell during secondary active transport in the kidneys. Na+ moves down its electrochemical gradient, which has been established by the Na+/K+ ATPase on the basal membrane, while glucose is simultaneously moved into the cell against its own gradient.
For example, in the kidney, a Na+/glucose symport protein assists in the movement of both Na+ and glucose into the cell against their concentration gradients. The Na+/K+ ATPase pump on the basal membrane maintains a strong electrochemical gradient, allowing Na+ to move into the cell from the tubular lumen. The cotransporter then moves glucose into the cell as Na+ moves down the electrochemical gradient created by the ATPase pump.