Homeostasis : JUNE 2012 / VARIANT 41 / Q10 B

HOMEOSTASIS

JUNE 2012 / VARIANT 41 / Q10 B 

Describe the mechanisms involved in reabsorption in the proximal convoluted tubule and describe how the epithelial cells of the proximal convoluted tubule are adapted to carry out this process. [9]

This questions demands a great detail to be written. A good approach of writing the answer would be:

The proximal convoluted tubule is the main site of selective reabsorption in the kidney. It reabsorbs approximately 100 % small proteins, glucose and amino acids. The mechanism of reabsorption of glucose is as follows. The Na+- K+ pump on the basolateral membrane actively pumps 3Na+ outside and 2K+ into the cells against the concentration gradient. This creates a concentration gradient to allow Na+ to move into the cell from the luminal fluid.  Na+ are co-transported into the PCT cells together with glucose via the Na+- glucose co-transporter. This mechanism of glucose reabsorption into the PCT cells is known as secondary active transport. Glucose thereafter moves via facilitated diffusion from the proximal convoluted tubule cells into the neighbouring peritubular capillaries via GLUTs ( glucose transporters).
The amino acids are reabsorbed into the peritubular capillaries via a similar mechanism to glucose but with  different proteins to glucose. The examples are the Na+ - specific amino acid co-transporters in place of Na+ - glucose co-transporters and specific amino acid transporters in place of GLUTs. The mechanism of reabsorption of water begins with the entry of Na+ into the proximal convoluted tubule cells lowering their water potential. This causes the water to move from a region of higher water potential in the luminal fluid to a region of lower water potential within the proximal convoluted tubule cells via osmosis. These are the basic processes of reabsorption in the proximal convoluted tubule and are brought about by special features of the proximal convoluted tubule cells. The cells have numerous microvilli on the luminal membrane and invaginations on the basolateral membrane which collectively provide a large surface area for transport proteins required for selective reabsorption. The presence of numerous mitochondria within the cells provide energy in the form of ATP , since selective reabsorption is an active process. The presence of a great number of transport proteins (eg. Na+ - K+ pump, channel proteins, etc) enable reabsorption of different solutes such as glucose, amino acids etc.
Therefore, all these features as a group provide an excellent site for selective reabsorption.

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