Question 2b (ii)
Explain the reason for the high abundance of mitochondria in the cells of the proximal convoluted tubule. [4]
Answer:
The proximal convoluted tubule cells contain many mitochondria because:
Function of the Proximal Convoluted Tubule
- Reabsorption: The PCT is responsible for reclaiming approximately 65-70% of the filtrate produced in the nephron.
- Ions: Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), calcium (Ca²⁺), and bicarbonate (HCO₃⁻) are actively and passively reabsorbed.
- Nutrients: Essential molecules like glucose and amino acids are reabsorbed via secondary active transport mechanisms.
- Water: Reabsorption of water follows the osmotic gradients established by ion transport, ensuring fluid balance.
Energy Requirements for Reabsorption
- Active Transport Mechanisms: Many substances are reabsorbed against their concentration gradients, necessitating energy input.
- Primary Active Transport: Direct use of ATP to transport molecules.
- Na⁺/K⁺-ATPase Pump: Located on the basolateral membrane, it actively pumps Na⁺ out of PCT cells into the interstitial fluid and K⁺ into the cells, maintaining low intracellular Na⁺ levels.
- Secondary Active Transport: Utilizes the Na⁺ gradient created by the Na⁺/K⁺-ATPase to co-transport other substances.
- Symporters and Antiporters: For example, glucose and amino acids are reabsorbed via symport with Na⁺.
- Primary Active Transport: Direct use of ATP to transport molecules.
Role of Mitochondria in PCT Cells
- ATP Production:
- Oxidative Phosphorylation: Mitochondria generate ATP through the electron transport chain and ATP synthase, providing the necessary energy for active transport.
- High Demand: Continuous reabsorption processes require a constant supply of ATP, necessitating abundant mitochondria.
- Metabolic Functions:
- Citric Acid Cycle (Krebs Cycle): Provides intermediates for biosynthetic processes and generates electron donors for ATP production.
- Ammoniagenesis: Mitochondria are involved in the production of ammonia, which helps in acid-base balance.
Cellular Adaptations in PCT Cells
High Mitochondrial Density:
- Energy Supply: Ensures that ATP production meets the high energy demands of active transport.
- Spatial Distribution: Mitochondria are strategically located near areas of high metabolic activity, such as the basolateral membrane where the Na⁺/K⁺-ATPase is active.
Abundant Microvilli:
- Increased Surface Area: Enhances the capacity for reabsorption by providing more membrane surface for transporters and channels.
- Energy Demand: More microvilli mean more active transport sites, further increasing the need for ATP and, consequently, mitochondria.