The proximal convoluted tubule (PCT) is the first, coiled part of the nephron after the Bowman's capsule, in the cortex. It reabsorbs most of the filtrate: all the glucose and amino acids, and about 65% of the water and sodium ions.The cells of the PCT wall are adapted for reabsorption:• microvilli on the surface facing the lumen give a large surface area
• many mitochondria provide ATP for active transport
• many carrier and co-transporter proteins in their membranes
• tight junctions between neighbouring cells, so substances pass through the cells rather than leaking between them
• a folded basal membrane next to the capillaries, giving a large surface area for transport into the blood
2. Na+ diffuses from the filtrate into the cell, down its concentration gradient, through co-transporter proteins in the luminal membrane. Each carries a glucose molecule (or an amino acid) with it, against the glucose concentration gradient.
3. Glucose builds up in the cell and moves into the blood by facilitated diffusion through carrier proteins in the basal membrane.Removing solutes from the filtrate raises its water potential and lowers the water potential of the cells and blood, so water follows by osmosis. Chloride ions follow the sodium ions, and about half the urea diffuses back into the blood.By the end of the PCT the filtrate volume has fallen greatly, but its water potential is about the same as that of blood.
• many mitochondria provide ATP for active transport
• many carrier and co-transporter proteins in their membranes
• tight junctions between neighbouring cells, so substances pass through the cells rather than leaking between them
• a folded basal membrane next to the capillaries, giving a large surface area for transport into the blood
2. Na+ diffuses from the filtrate into the cell, down its concentration gradient, through co-transporter proteins in the luminal membrane. Each carries a glucose molecule (or an amino acid) with it, against the glucose concentration gradient.
3. Glucose builds up in the cell and moves into the blood by facilitated diffusion through carrier proteins in the basal membrane.Removing solutes from the filtrate raises its water potential and lowers the water potential of the cells and blood, so water follows by osmosis. Chloride ions follow the sodium ions, and about half the urea diffuses back into the blood.By the end of the PCT the filtrate volume has fallen greatly, but its water potential is about the same as that of blood.