Which of the following substances pass through the root cell membrane by osmosis?

Analyze the question: The question asks which substance moves across the root cell membrane specifically by osmosis. So we need to recall exactly what osmosis is.

Concept: Osmosis is the movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively (semi-) permeable membrane.

Other substances move differently:

• Oxygen and carbon dioxide move by diffusion (down their concentration gradients).
• Mineral salts (ions) move by active transport or diffusion.
• Cell sap is the fluid content of the vacuole. It is a solution, not something that enters the root cell by osmosis. Its concentration is what creates the water potential gradient that draws water in.

Solution:

1. Soil water is usually dilute, so it has a higher water potential.
2. The cell sap in root hair cells is more concentrated, so it has a lower water potential.
3. Water therefore moves from the soil, through the root hair's cell membrane, into the cell by osmosis.
4. Oxygen and carbon dioxide (options B and C) are gases that pass by diffusion, not osmosis.
5. Cell sap (option A) is not the substance absorbed. It is the solution inside the cell.

Answer: D. Water

Tip: Remember "Osmosis = water only." If the question says osmosis, look for water. If it mentions gases like O₂ or CO₂, think diffusion. If it mentions mineral ions going against a gradient, think active transport.