Is water movement across cell membrane purely diffusive or it always requires channels?
Water movement across the cell membrane can occur through both diffusive and facilitated processes.
1. Diffusion: Water molecules can passively move across the cell membrane through a process called diffusion. This is driven by the concentration gradient, where water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). Diffusion does not require the presence of specific channels or proteins.
2. Facilitated Diffusion: In addition to simple diffusion, water can also move across the cell membrane through a process called facilitated diffusion. This process involves the assistance of specialized proteins called aquaporins. Aquaporins are membrane channels that are specifically designed to facilitate the movement of water molecules across the cell membrane. They provide a faster and more efficient pathway for water to cross the membrane, especially in cases where the concentration gradient is high or the rate of diffusion needs to be increased.
While diffusion can occur spontaneously, the presence of aquaporins increases the rate of water movement across the cell membrane. Therefore, while water movement can occur through diffusive processes alone, the presence of channels like aquaporins facilitates and enhances the movement of water in cells.
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