Summary
Highlights
The sodium ion-potassium ion pump on the basal membrane actively transports sodium ions from the PCT cell into the capillaries, consuming ATP. This creates a sodium ion concentration gradient, with a higher concentration in the PCT lumen and a lower concentration within the PCT cell.
The video focuses on selective reabsorption in the PCT, which is responsible for reabsorbing all glucose and amino acids, most water, and some salts and urea. This is a significant topic often tested in exams.
The PCT is a tube with a one-cell-thick wall, similar to capillaries, but transporting filtrate. The PCT wall consists of specialized cells with microvilli facing the lumen and basal membranes facing the capillaries. Tight junctions between PCT cells prevent leakage, ensuring reabsorption happens through the cell membrane, which is partially permeable and allows for selective control.
PCT cells have microvilli with co-transport proteins (sodium ion-glucose and sodium ion-amino acid co-transporters) and basal membranes with sodium ion-potassium ion pumps (active transport) and glucose/amino acid transport proteins (facilitated diffusion). Mitochondria are abundant in PCT cells to provide ATP for active transport.
Due to the sodium ion concentration gradient, sodium ions diffuse from the PCT lumen into the cell via co-transporters. These co-transporters simultaneously pull in glucose or amino acids, ensuring their reabsorption from the lumen into the cell. Subsequently, glucose and amino acids move from the PCT cells into the capillaries via facilitated diffusion through transport proteins on the basal membrane.
The reabsorption of solutes like salts and glucose causes the solute concentration in the PCT lumen to decrease, increasing its water potential. This creates a water potential gradient, leading to the reabsorption of water from the PCT lumen into the capillaries via osmosis. Some urea is also passively reabsorbed due to its ability to diffuse through the phospholipid bilayer, an unavoidable process.
The PCT has several adaptations: tight junctions prevent fluid leakage; microvilli increase surface area for co-transporter proteins; folded basal membranes increase surface area for sodium ion-potassium ion pumps and facilitated diffusion proteins; and numerous mitochondria ensure high ATP production for active transport.