In the context of principles of transmembrane transport, which process does NOT require direct energy input from the cell (e.g., ATP hydrolysis)?

• A. Transport of protons ($H_{+}$) against their electrochemical gradient by a proton pump
• B. Secondary active transport of glucose into intestinal epithelial cells via the ${\mathrm{Na}}_{+}$-glucose symporter (SGLT)
• C. Facilitated diffusion of glucose through a GLUT transporter down its concentration gradient
• D. Primary active transport of ${\mathrm{Na}}_{+}$ and $K_{+}$ by the ${\mathrm{Na}}_{+}$/$K_{+}$-ATPase

Answer: (C)