Water movement in cells

Is water movement across cell membrane purely diffusive or it always requires channels?

Water movement across the cell membrane can occur through two main mechanisms: diffusion and facilitated transport.

1. Diffusion: Water molecules can move across the cell membrane by simple diffusion. This process occurs when water molecules move from an area of higher concentration (lower solute concentration) to an area of lower concentration (higher solute concentration) until equilibrium is reached. This movement of water across the membrane is driven by the concentration gradient of solute particles, including ions, inside and outside the cell. Diffusion does not require the presence of channels or transport proteins in the cell membrane.

2. Facilitated Transport: Water movement can also occur through facilitated transport, which requires the presence of specialized protein channels called aquaporins. Aquaporins are integral membrane proteins that form pores in the cell membrane to facilitate the movement of water molecules across the membrane. These channels allow water to pass through the hydrophobic core of the membrane more rapidly than would be possible by simple diffusion. Aquaporins help regulate water balance, especially in tissues where rapid water transport is required, such as the kidneys and the cells lining the small intestine.

In summary, while water can move across the cell membrane by simple diffusion, facilitated transport through aquaporin channels significantly enhances the rate of water movement in certain tissues and cells.

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