Definition for Active Transport
From Biology Forums Dictionary
Active transport is the movement of a solute across a membrane against its gradient—that is, from a region of low concentration to higher concentration. Active transport is energetically unfavourable and requires the input of energy. Primary active transport involves the functioning of pumps that directly use energy to transport a solute against a gradient.
Secondary active transport involves the use of an existing gradient to drive the active transport of a solute. For example, an H+/sucrose symporter (proton-sucrose symporter) can use an H+ electrochemical gradient, established by an ion pump, to move sucrose against its concentration gradient. For the H+/sucrose symporter, only sucrose is actively transported. Hydrogen ions move down (with) their electrochemical gradient. H+/solute symporters are more common in bacteria, fungi, algae, and plant cells, because H+ pumps are found in their plasma membranes. In animal cells, a pump that exports Na+ maintains the Na+ gradient across the plasma membrane. Na+/solute symporters are prevalent in animal cells.