Water relations — By which process does water primarily move from soil into the root cells of plants across the plasma membrane?

Introduction / Context:
Water uptake is fundamental to plant survival, supporting cell turgor, nutrient transport, and photosynthesis. Understanding the immediate mechanism by which water crosses membranes into root cells clarifies how plants maintain internal water status despite changing soil conditions.

Given Data / Assumptions:

• Root epidermal cells contact soil solution and have semi-permeable membranes.
• Solute accumulation in cells lowers water potential relative to soil water potential (under non-saline conditions).
• Aquaporins facilitate water flow but do not use ATP directly.

Concept / Approach:
Osmosis is the passive movement of water across a semi-permeable membrane from higher water potential (or lower solute concentration) to lower water potential (higher solute concentration). Ion pumps set up solute gradients; water follows those gradients osmotically into living cells of the root cortex and epidermis.

Step-by-Step Solution:

Verification / Alternative check:
Blocking aquaporins reduces hydraulic conductivity; however, water still moves passively down gradients, confirming an osmotic mechanism rather than direct ATP-driven pumping of water.

Why Other Options Are Wrong:

• Active transport: there are no ATP-driven water pumps in plants.
• Diffusion (for solutes): water’s membrane movement is termed osmosis.
• Pressure flow: a phloem mechanism, not for membrane-level water entry.
• Capillary action: acts in cell walls/xylem conduits, not across the lipid membrane.

Common Pitfalls:
Confusing xylem bulk flow (cohesion–tension) with membrane-level water entry; both are important but distinct processes.

Final Answer:
Osmosis