Which statement about water movement across the cell membrane is true?

Which statement about water movement across the cell membrane is true?

• A. Water cannot cross the cell membrane at all
• B. Water usually flows freely
• C. Water moves only through active transport
• D. Water is always blocked by the membrane

Answer: (B)

Water movement across the membrane happens mainly by osmosis, the diffusion of water across a semi-permeable membrane in response to solute differences. Water is small and can cross the lipid bilayer, especially via water channels called aquaporins, so it moves without directly using energy. Because of this, water typically flows across membranes to balance solute concentrations rather than being blocked or requiring active transport. The idea that water cannot cross at all or that it moves only by active transport is not accurate, and the notion that water is always blocked by the membrane is also incorrect.

Water movement across the membrane happens mainly by osmosis, the diffusion of water across a semi-permeable membrane in response to solute differences. Water is small and can cross the lipid bilayer, especially via water channels called aquaporins, so it moves without directly using energy. Because of this, water typically flows across membranes to balance solute concentrations rather than being blocked or requiring active transport. The idea that water cannot cross at all or that it moves only by active transport is not accurate, and the notion that water is always blocked by the membrane is also incorrect.