What does cotransport (symport) involve?

• A. Transporting substances in opposite directions
• B. Transporting two substances in the same direction
• C. Transporting only one substance
• D. Transport solely dependent on electrical gradients

Answer: (B)

Cotransport, specifically symport, involves the movement of two different substances in the same direction across a cell membrane. This process is a type of active transport, where the energy from the movement of one substance down its concentration gradient is used to drive the movement of another substance against its concentration gradient.

In many cellular processes, symport is crucial for the uptake of nutrients or ions that are otherwise not able to cross the membrane effectively. For instance, in the intestinal epithelium, glucose is often transported into cells along with sodium ions. The sodium ion moves down its gradient, which provides the energy needed to import glucose, even though glucose is moving against its gradient.

Understanding symport is important because it highlights how cells efficiently utilize energy to transport essential substances, enabling various physiological functions. Other options incorrectly suggest mechanisms that do not pertain to the simultaneous transport of two substances in the same direction, which is the defining characteristic of cotransport through symport.