Mineral nutrition — The uptake of most mineral ions, including potassium (K+), into root cells occurs primarily by which mechanism?

Introduction / Context:
Plants require mineral ions for metabolism and structure. Many essential cations and anions occur at low concentrations in soil solution. Cells must therefore accumulate them against steep electrochemical gradients, a task that hinges on membrane transport proteins and energy input.

Given Data / Assumptions:

• Root epidermal and cortical cells express specific ion transporters and channels.
• Proton pumps (H+-ATPases) establish membrane potential and pH gradients.
• Potassium is a key macronutrient with tightly regulated uptake.

Concept / Approach:
Active transport uses metabolic energy (ATP) to drive ions into the symplast against gradients, often via H+-coupled co-transporters or channels energized by the proton motive force. Passive diffusion alone cannot achieve the high intracellular concentrations commonly observed for K+ and other nutrients.

Step-by-Step Solution:

Verification / Alternative check:
Pharmacological inhibitors of H+-ATPase reduce ion uptake; electrophysiology shows voltage-dependent K+ channels facilitating uptake consistent with active mechanisms.

Why Other Options Are Wrong:

• Osmosis: water movement, not ion accumulation.
• Apoplastic diffusion alone: cannot explain uphill transport into cells.
• Bulk flow: moves ions in xylem long-distance, not into living root cells across membranes.
• Endocytosis: not the primary route for inorganic ion uptake.

Common Pitfalls:
Assuming that because ions are small they enter passively; physiological concentrations demand active, regulated uptake systems.

Final Answer:
Active transport across cell membranes