AP BIOLOGY

Detailed explanation-1: -Water is the most abundant molecule in a cell. Although the plasma membrane separates the interior of the cell from its extracellular environment, specialized membrane channels facilitate water transport across these biomembranes. Such water channels, which are in the form of proteins, are called aquaporins.

Detailed explanation-2: -Aquaporins (AQP) are integral membrane proteins that serve as channels in the transfer of water, and in some cases, small solutes across the membrane. They are conserved in bacteria, plants, and animals. Structural analyses of the molecules have revealed the presence of a pore in the center of each aquaporin molecule.

Detailed explanation-3: -The AQP2 gene provides instructions for making a protein called aquaporin 2. This protein forms a channel that carries water molecules across cell membranes. It is found in the kidneys in structures called collecting ducts, which are a series of small tubes that reabsorb water from the kidneys into the bloodstream.

Detailed explanation-4: -Antidiuretic hormone stimulates water reabsorbtion by stimulating insertion of “water channels” or aquaporins into the membranes of kidney tubules.

Detailed explanation-5: -Aquaporin-1 is extremely abundant in the proximal convoluted tubule and descending thin limb, but is notably absent in highly water-impermeable segments such as the ascending thin limb, thick ascending limb, and distal tubule.