International A Level Biology / Control of the internal environment
Selective reabsorption in the proximal tubule
Edexcel International A Level Biology (YBI11), chapter 22: Control of the internal environment, specification point 7.20. Free lesson.
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The proximal tubule selectively reabsorbs useful substances from the filtrate back into the blood. Its cell adaptations support glucose and amino acid co-transport, while water follows the solutes by osmosis.
How proximal tubule cells are adapted
The lumen is the space inside the tubule, and it contains the filtrate. Each proximal tubule cell lies between the filtrate and a nearby blood capillary. Its structure helps substances cross from one side to the other. Microvilli face the lumen. They form a brush border and give the cell a large surface area for reabsorption. Co-transporter proteins in the membrane facing the lumen bring sodium ions and glucose into the cell together. Many mitochondria release energy as ATP by respiration. Sodium-potassium pumps in the membrane next to the blood use ATP to pump sodium ions out…
How glucose is reabsorbed
Glucose reabsorption depends on a low concentration of sodium ions inside the cell. Pumps first create this difference, then sodium ions move in from the filtrate and carry glucose with them. The sodium-potassium pumps actively transport sodium ions out of the cell and into the blood, using ATP. This keeps the sodium ion concentration inside the cell low, creating a concentration gradient from the filtrate into the cell. Sodium ions move down this gradient through co-transporter proteins. Glucose enters with them, even against its own concentration gradient. This is co-transport.…
What is reabsorbed, and how
The proximal tubule takes back several useful substances, but not all in the same amount. Moving solutes into the blood also makes water follow from the filtrate. Glucose is reabsorbed by co-transport with sodium ions. Normally, all of it is reabsorbed. Amino acids are reabsorbed by the same method. Normally, all of them are reabsorbed. Sodium ions are co-transported into the cell and then pumped into the blood. About two thirds are reabsorbed here. As solutes move into the blood, its water potential falls. Water moves out of the filtrate into the blood by osmosis. About two…
Why glucose can appear in urine
There are only a certain number of co-transporter proteins. If the filtrate contains much more glucose than usual, the transporters can all be occupied before all the glucose has been reabsorbed. In untreated diabetes, blood glucose concentration is very high, so the filtrate contains much more glucose. The co-transporter proteins become saturated, meaning they are all in use. Some glucose stays in the filtrate and passes out of the body in urine.
Use the right terms
Keep the different transport processes clear when you describe reabsorption. Osmosis moves water only, not glucose. Write sodium ions, not just sodium. Glucose enters the cell by co-transport with sodium ions, then moves into the blood by facilitated diffusion. Do not say that glucose is reabsorbed by osmosis.