Active transport saturation in ion uptake

Steps:

• Nitrate absorption by root hairs uses active transport, which depends on carrier proteins.
• Rate increases with soil nitrate concentration as more ions bind carriers.
• At high concentrations, carriers saturate, so rate plateaus (Michaelis-Menten kinetics).
• Point X (equal concentrations) shows positive rate due to active pumping against gradient.

Why A is correct:

• A shows hyperbolic curve leveling off, matching saturation of transport proteins per Michaelis-Menten equation: rate = V_max [S] / (K_m + [S]).

Why the others are wrong:

• B: Linear rise ignores carrier saturation at high concentrations.
• C: Exponential growth overpredicts rate without biological limits.
• D: Initial drop contradicts increasing uptake with substrate availability.

Final answer: A