Water is delivered to cells primarily through two mechanisms: simple diffusion and facilitated diffusion via aquaporins.
Water is essential for cell function, and its transport across the cell membrane is crucial. The cell membrane, composed of a lipid bilayer, presents a barrier to many molecules. However, water can still cross this barrier.
Simple Diffusion
Because water is a small molecule, it can move directly across the cell membrane through simple diffusion. This movement follows the concentration gradient, meaning water moves from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration). This passive transport doesn't require any energy from the cell.
Facilitated Diffusion via Aquaporins
While simple diffusion occurs, the primary and most efficient way water enters and exits cells is through channel proteins called aquaporins. Aquaporins are integral membrane proteins that form pores in the cell membrane, allowing water to pass through rapidly. This is a form of facilitated diffusion, which means it still follows the concentration gradient and does not require cellular energy, but it utilizes a protein channel to speed up the process. Different cells have varying numbers of aquaporins depending on their need for water transport.
In summary, while water can passively diffuse across the cell membrane, the majority of water transport into and out of cells is facilitated by aquaporins.
