The following processes are involved in water movement in the endodermis except

Analyze the question: The question asks which process is NOT involved in water movement through the endodermis. The endodermis is the innermost layer of the root cortex. It surrounds the vascular tissue (stele) and has the Casparian strip.

Concept: Water moves from the root hair cells across the cortex to the xylem by three routes:

1. Apoplast pathway: through cell walls and intercellular spaces. It is blocked at the endodermis by the Casparian strip, which is waterproof (suberin).
2. Symplast pathway: through the cytoplasm via plasmodesmata.
3. Vacuolar pathway: through the cytoplasm and vacuoles of successive cells.

At the endodermis, water is forced to pass through the cell membranes and cytoplasm. This movement happens by:

• Osmosis, down a water potential gradient.
• Diffusion, a passive movement of water and dissolved substances.
• The vacuolar (and symplastic) pathway, through the living cell contents.

Active transport is the movement of substances against a concentration gradient using ATP. Water itself is never moved by active transport. It always moves passively by osmosis. Active transport of mineral salts into the stele does occur, and it helps lower the water potential, which draws in water. Even so, the movement of the water is still osmosis, not active transport.

Solution:

• A. Osmosis: involved.
• B. Vacuolar pathway: involved.
• C. Diffusion: involved.
• D. Active transport: not involved in the movement of water itself.

Answer: D

Tip: Remember that water never moves by active transport. It follows water potential gradients by osmosis. When you see "except" questions about water movement, look for "active transport" as the likely odd one out. Also remember that the Casparian strip blocks the apoplast route at the endodermis.