Casparian Strip Blocks Apoplast Pathway

Steps:

• Identify the structure: In dicot roots, the endodermis has a Casparian strip of suberin.
• Understand pathways: Water moves via apoplast (cell walls) or symplast (cytoplasm connections).
• Role of suberin: It waterproofs endodermal walls, blocking apoplast flow.
• Consequence: Forces solutes and water through cells, entering symplast for selective uptake.

Why A is correct:

• Suberin layer in endodermis precisely describes redirecting water/solutes from apoplast to symplast, ensuring regulated entry to xylem.

Why the others are wrong:

• B: Suberin blocks apoplast, not symplast; it doesn't switch from symplast to apoplast.
• C: Endodermis forces from apoplast to symplast, not symplast to apoplast.
• D: While endodermis is involved, it doesn't directly "move" solutes; suberin layer specifies the mechanism.

Final answer: A