Which of the following defines co-transporters?

Co-transporters are defined as membrane transport proteins that facilitate the simultaneous movement of two substances across a membrane. This process often involves the transport of one substance down its concentration gradient, which releases energy that is then utilized to transport the second substance against its concentration gradient. This mechanism is vital for various cellular processes, including nutrient absorption in the intestines and reabsorption of ions in the kidneys.

Understanding that co-transporters work by coupling the movements of two different substances is key. For instance, in the case of sodium-glucose co-transporters, sodium ions move into the cell along with glucose molecules. The energy released from the sodium ions moving down their gradient allows glucose to be transported into the cell, even if it's going against its concentration gradient.

This clear definition helps differentiate co-transporters from options that either pertain to individual substances or refer to passive processes where no energy coupling occurs. Additionally, digestive nutrient transfer does not specify the dual-substance mechanism essential to co-transporters. Thus, option C accurately captures the essence of co-transporters and their role in cellular transport systems.