What mechanism allows water absorption in the colon?

• A. Osmosis following absorption of solutes (Na+, Cl-); aquaporins
• B. Active transport of water against gradient
• C. Simple diffusion of water
• D. Endocytosis of water droplets

Answer: (A)

Water movement in the colon is driven by osmosis after solute uptake. As ions like Na+ and other solutes are absorbed from the lumen into the mucosa, the lumen becomes less osmotic relative to the tissue, so water follows that gradient into the body. This water passes through epithelial cells mainly via aquaporin water channels and also migrates through the spaces between cells as needed. Because it’s a passive process riding down the osmotic gradient, water isn’t pumped actively against its gradient. Endocytosis of water droplets isn’t a physiological mechanism here, and while some diffusion of water can occur, it’s the osmotic pull from solute absorption, facilitated by aquaporins, that best explains colon water absorption.