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Water moves across cell membranes most efficiently through specialized channel proteins that let water pass while excluding most solutes. These water-selective channels are called aquaporins. They form pores that allow rapid, facilitated diffusion of water in response to osmotic differences, which is crucial for maintaining cell turgor and body water balance in tissues like the kidney and plant roots. Other transport proteins have different roles: ion channels conduct charged ions, carrier proteins shuttle specific molecules by binding them and changing shape, and pumps use energy to move substances against their gradients. The key feature of aquaporins is their specific selectivity for water and their ability to move water quickly through the membrane.