Question

Explain the Mechanisms of transport across apical and basolateral membranes in the kidney, stomach, and intestine.

Answer 1

• The transport mechanism in kidney: In kidney the transport of ions and water is occured in the nephron ( renal tubule). In the renal tubule the reabsorption and secretion of ions is occured by transcellular and paracellular pathway across the apical and basolateral membrane of the tubular cells. Transcellular pathway helps to transport ions through the cells by different channels. These channels can be operated via energy (active transport). Paracellular pathway helps to transport molecules in between the cells. In proximal tubule water is reabsorbed through this paracellular pathway from apical side to basolateral side. Apical side is the side where the tubular fluid is present and basolateral side is the side where blood capillaries are present. In the proximal tubule sodium is transported from apical side to basolateral sidd via Na+-H+ antiporter in the apical side and that sodium is transported in the blood in the basolateral side through Na+-K+ ATPase channel. Tubular secretion is also a important matter in the renal tubule where the molecules from the blood is transported to the tubular fluid from basolateral side to apical side of the cells. There are different types if channels in the limb of loop of henle for the transcellular transport of the ions like Na+-K+-2Cl- symporter which helps to transport sodium ion from apical to basolateral side. This way the transport of molecules across apical and basolateral membrane is done in the kidney.
• The transport mechanism in stomach: In stomach the the transport of H+ ions is the main function of the parietal cells of the stomach in human body. This parietal cells also have a apical side beside gastric lumen and another basolateral side where the bloodstream is present. In the basolateral side the ions can be transported to the blood and in the apical side the molecules can transport to gastric lumen via transcellular pathway. The carbonic acid (H2CO3) in the parietal cells produce bicarbonate ion and H+ ion by carbonic anhydrase enzyme . This H+ is transported via apical side to the gastric lumen via H+-K+ antiporter in the apical membrane. The bi-carbonate ion is transported to the blood from the basolateral side by HCO3- /Cl- antiporter present in thr basolateral side of the membrane. In this way the acid(H+) is produced in the gastric lumen through this transcellular transport mechanism.
• Transport mechanism in the intestine: The main transport in the intestinal epithelial cell line is the glucose absorption. The glucose is absorbed from the gut lumen in the apical side of the epithelial cell of intestine. This glucose is transported by sodium glucose co transporter (SGLT1) in the apical side of the cell. This is a active transport mechanism, so this transport is coupled with sodium-potassiumATPase transpoter. Then this glucose molecules are absorbed in the blood through basolateral membrane by GLUT-2 transporter (facilitated diffusion). Intestine is the another side for the absorption of amino acids, dipeptides and tripeptides. The amino acids and peptides are absorbed through apical side of the epithelial cells from gut lumen via active transport mechanism. Then those dipeptides and tripeptides are simplified into amino acids inthe epithelial cells by peptidase enzyme and transported to the bloodstream from the basolateral side of the membrane.