AP BIOLOGY

Detailed explanation-1: -The main function of the PCT is to reabsorb water and solutes like sodium, which is continuously pumped into the interstitium to create a gradient that allows many other solutes and even water to be reabsorbed by tubule cells.

Detailed explanation-2: -The PCT reabsorbs solute isosmotically. Water is reabsorbed through the aquaporin-1 (AQP1) water channel (Nielsen et al. 1993b; Preston et al. 1992) located in both the apical and basolateral plasma membranes.

Detailed explanation-3: -Water reabsorption is by osmosis through water channels in the membrane. These water channels consist of a family of proteins called aquaporin. At least seven different aquaporin isoforms are expressed in the kidney.

Detailed explanation-4: -Most of the solute reabsorbed in the proximal tubule is in the form of sodium bicarbonate and sodium chloride, and about 70% of the sodium reabsorption occurs here. Sodium reabsorption is tightly coupled to passive water reabsorption, meaning when sodium moves, water follows.

Detailed explanation-5: -Cotransport-this process is particularly important for the reabsorption of water. Water can follow other molecules that are actively transported, particularly glucose and sodium ions in the nephron.