Active transport saturation kinetics for nitrate uptake

Steps:

• Nitrate ions are absorbed by root hair cells via active transport, which requires energy and uses carrier proteins.
• The rate increases with soil nitrate concentration as more ions bind to carriers, but saturates when all carriers are occupied.
• At point X (equal soil and cell concentrations), net diffusion is zero, but active transport maintains uptake, so rate remains positive and plateaus at high concentrations.
• The graph must show a hyperbolic curve rising to an asymptote, not linear or declining trends.

Why A is correct:

• A depicts a hyperbolic saturation curve, matching Michaelis-Menten kinetics where rate = V_max [S] / (K_m + [S]), with V_max as maximum transport rate.

Why the others are wrong:

• B shows linear increase, ignoring carrier saturation at high concentrations.
• C shows a sigmoidal curve, typical for cooperative binding, not simple active transport.
• D shows initial rise then decline, contradicting continuous uptake even at equal concentrations.

Final answer: A