Final answer:
Secondary active transport, such as Na+/glucose symport, transports glucose against its concentration gradient using energy indirectly from an ion gradient created by Na+/K+ ATPase. This allows glucose to be absorbed efficiently into cells despite higher internal concentrations.
Step-by-step explanation:
The transport of glucose across a cell's plasma membrane by a secondary active transport mechanism involves glucose moving against its concentration gradient, which requires energy indirectly obtained from an ion gradient. A common example of secondary active transport in cells is the Na+/glucose symport mechanism found in intestinal and kidney tubular cells.
In this process, sodium ions (Na+) move down their electrochemical gradient into the cell, which is maintained by the Na+/K+ ATPase, and glucose is simultaneously transported into the cell against its own gradient. This symport mechanism ensures that glucose is reabsorbed efficiently even when its concentration is higher inside the cell than outside.
If the concentration of glucose were equal inside and outside of the cell, there would typically be no net flow of glucose due to lack of a concentration gradient to drive the diffusion. However, in the case of secondary active transport, the movement of another substance (like Na+) down its gradient can still facilitate the transport of glucose into the cell, independent of its own concentration gradient.