What is a symport?

A symport is a specific type of membrane transport mechanism where two molecules are transported simultaneously in the same direction across a cell membrane. This process typically involves the use of a transport protein that facilitates the movement of both substances together. Symport systems are often seen in the transport of ions and nutrients, where one molecule may move along its concentration gradient while simultaneously driving the transport of another molecule against its concentration gradient, sharing the same path.

In contrast, the other transport modes, such as those described in the incorrect options, define different mechanisms. For example, the transport of two molecules in opposite directions (as stated in one option) refers to an antiport system, while the transport of only one molecule at a time would describe a uniport system. Additionally, the notion of transporting two molecules without using energy typically applies to passive transport, but that does not capture the unique characteristic of a symport, which often relies on the coupling of two molecules. Therefore, the defining feature of a symport is the simultaneous movement of two molecules in the same direction.