Is water movement across cell membrane purely diffusive or it always requires channels?
Water across cell membranes occurs due to osmosis( which is based on a concentration gradient). so water diffuses through the cell membrane. sometimes water channels called aquaporins can be found.
https://www.anaesthesiamcq.com/FluidBook/fl1_2.php
https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/aquaporin
In the loop of Henle, the water, and Na+ &amp; Cl- exchange occurs due to the counter-current principle takes place. the interstitial fluid has a certain sodium chloride concentration. the filtrate that is entering the descending limb of the loop of Henle, has a particularly low NaCl concentration.so because of that water moves from the loop of Henle to the interstitial fluid.simultaneously, at the ascending limb of the loop of Henle just adjacent to that particular point, water concentration is high in the interstitial fluid. so because of that NaCl moves to the interstitial fluid (Na+ actively, and for ion balance, Cl- passively.) this happens along the length of the loop of Henle. what restricts water movement from the ascending limb of the loop of Henle is that that limb has no aquaporins and the descending limb has numerous (with relative to other places in the nephron). anyway, here's a link for you.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468499/