NEET 2025 Botany Question 125 Solution

PCT is lined by simple cuboidal brush border epithelium which greatly increases the surface area for reabsorption:

• • Nearly all essential nutrients (glucose, amino acids) and 70–80% of electrolytes and water are reabsorbed here (indicated by outward arrows from lumen to interstitium: $HCO_3^-$, $NaCl$, $H_2O$).
• • PCT maintains pH and ionic balance of body fluids by selective secretion of hydrogen ions ($H^+$) and ammonia ($NH_3$) into the filtrate (indicated by inward arrows into the tubule lumen).

DCT performs conditional, hormonally regulated reabsorption and fine-tuning of acid-base balance:

• • Conditional reabsorption of $Na^+$ and water takes place in DCT under the influence of aldosterone and ADH (arrows pointing outwards: $NaCl$, $H_2O$).
• • DCT is also capable of reabsorbing bicarbonate ions ($HCO_3^-$) to preserve buffer capacity (arrow pointing outwards: $HCO_3^-$).
• • DCT selectively secretes potassium ($K^+$) and hydrogen ($H^+$) ions into the tubular fluid to maintain blood pH and $Na^+$-$K^+$ balance (arrows pointing inwards into lumen: $K^+$, $H^+$).

Evaluating the arrow schematics in Option (2):

• • Diagram P has upward (outward) arrows for $HCO_3^-$, $NaCl$, and $H_2O$, and upward (inward) arrows entering from the bottom for $H^+$ and $NH_3$.
• • Diagram D has upward (outward) arrows for $NaCl$, $H_2O$, and $HCO_3^-$, and upward (inward) arrows entering from the bottom for $K^+$ and $H^+$.
• • This is a literal, 1:1 reproduction of NCERT Figure 19.5.

Diagram (2) represents the true transport vectors of the nephron.