Secondary Active Transport
Facilitated diffusion can occur between the bloodstream and cells as the concentration gradient between the extracellular and intracellular environments is such that no ATP hydrolysis is required.
However in the kidney, glucose is reabsorbed from the filtrate in the tubule lumen, where it is at a relatively low concentration, passes through the simple cuboidal epithelia lining the kidney tubule, and into the bloodstream where glucose is at a comparatively high concentration. Therefore the concentration gradient of glucose opposes its reabsorption, and energy is required for its transport.
The secondary active transport of glucose in the kidney is Na+ linked; therefore an Na+ gradient must be established. This is achieved through the action of the Na+/K+ pump, the energy for which is provided through the hydrolysis of ATP. Three Na+ ions are extruded from the cell in exchange for two K+ ions entering through the intramembrane enzyme Na+/K+-ATPase; this leaves a relative deficiency of Na+ in the intracellular compartment. Na+ ions diffuse down their concentration gradient into the columnar epithelia, co-transporting glucose. Once inside the epithelial cells, glucose reenters the bloodstream through facilitated diffusion through GLUT2 transporters.
Hence reabsorption of glucose is dependent upon the existing sodium gradient which is generated through the active functioning of the NaKATPase. As the cotransport of glucose with sodium from the lumen does not directly require ATP hydrolysis but depends upon the action of the ATPase, this is described as secondary active transport.
There are two types of secondary active transporter found within the kidney tubule; close to the glomerulus, where glucose levels are high, SGLT2 has a low affinity yet high capacity for glucose transport. Close to the loop of Henle and in the distal convoluted tubule of the nephron where much glucose has been reabsorbed into the bloodstream, SGLT1 transporters are found. These have a high affinity for glucose and a low capacity. Functioning in conjunction, these two secondary active transporters ensure that only negligible amounts of glucose are wasted through excretion in the urine.
Read more about this topic: Glucose Uptake
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