Magnesium ions enter root cells via active transport against the gradient

Steps:

• Plant metabolism depletes magnesium ions inside the root cell, but cell concentration remains higher than in soil water.
• For plant health, magnesium ions must continuously enter the cell to replenish supply.
• Concentration gradient favors outward diffusion, so passive movement cannot sustain uptake.
• Active transport uses energy to move ions inward against the gradient.

Why A is correct:

• Active transport enables movement of ions from low to high concentration, as required for essential mineral absorption in plant roots per biological transport laws.

Why the others are wrong:

• B: Diffusion follows the gradient, moving ions out of the higher-concentration cell.
• C: Active transport outward would deplete ions further, harming the plant.
• D: Diffusion outward loses ions when the plant needs influx.

Final answer: A