Question 8b
Explain the mechanisms involved in the reabsorption process within the proximal convoluted tubule, and describe how the epithelial cells lining this tubule are specially adapted to facilitate this process. (9)
Mechanisms of Reabsorption in the Proximal Convoluted Tubule:
- Active Transport:
- Sodium ions (Na⁺) are actively transported out of the tubule cells into the interstitial fluid surrounding the tubule. This creates a low concentration of Na⁺ within the cells, facilitating further Na⁺ reabsorption from the tubular fluid by diffusion.
- Glucose and amino acids are reabsorbed from the filtrate via co-transport with Na⁺ ions through specific carrier proteins in the cell membrane.
- Osmosis:
- As Na⁺ and other solutes are reabsorbed, the osmotic gradient within the tubule cells increases, leading to the passive reabsorption of water. Water moves from the tubular lumen into the tubule cells and eventually into the blood capillaries.
- Facilitated Diffusion:
- Once inside the tubule cells, glucose, amino acids, and other nutrients move into the bloodstream through facilitated diffusion, ensuring essential nutrients are retained.
- Passive Transport:
- Urea and certain ions are reabsorbed by passive transport down their concentration gradients. This process helps maintain a balance of ions and other molecules within the blood.
Adaptations of Epithelial Cells in the Proximal Convoluted Tubule:
The cell membranes contain specific carrier proteins and co-transporters that facilitate the selective reabsorption of glucose, amino acids, and ions, ensuring efficient recovery of these molecules from the filtrate.
Microvilli:
The epithelial cells have microvilli on their surface, forming a brush border. This significantly increases the surface area for reabsorption, allowing more molecules to be absorbed efficiently.
Mitochondria:
These cells are packed with mitochondria, which supply ATP necessary for the active transport of ions, especially Na⁺. This energy is crucial for maintaining concentration gradients.
Tight Junctions:
Tight junctions between cells prevent the leakage of reabsorbed substances back into the tubular lumen, ensuring that valuable molecules remain in the blood.
Carrier Proteins and Co-transporters:
The cell membranes contain specific carrier proteins and co-transporters that facilitate the selective reabsorption of glucose, amino acids, and ions, ensuring efficient recovery of these molecules from the filtrate.