How aquaporins move water so fast
Aquaporins are membrane proteins that form dedicated channels for water, letting it cross cell membranes far faster than it could by simple diffusion. They typically assemble as tetramers, with each of the four subunits forming its own narrow pore. Estimates of throughput are very high, on the order of billions of water molecules per channel per second, which is why aquaporins are central wherever the body moves water quickly, such as in the kidney.
AQP2 in the kidney is a well-studied example: hormonal signals from antidiuretic hormone move AQP2 channels into the membrane to reabsorb water and concentrate urine.
Why they pass water but block ions and protons
The channel is engineered for selectivity. A constriction point and a pair of conserved asparagine-proline-alanine (NPA) motifs reorient water molecules as they thread through single file, while the local electric field prevents the relay of protons that would otherwise hop along a water chain. The pore is also too narrow for hydrated ions and larger solutes.
This combination lets water flow freely while keeping out charged particles, preserving the cell's electrical and chemical gradients. This is general educational information about molecular biology.