What describes the primary function of xylem and the mechanism by which water moves from roots to leaves?

What describes the primary function of xylem and the mechanism by which water moves from roots to leaves?

• A. Xylem transports sugars from leaves to roots by active transport.
• B. Xylem transports water from roots to leaves; movement is via cohesion-tension theory driven by transpiration.
• C. Water moves through xylem by osmosis alone.
• D. Xylem transports minerals but not water.

Answer: (B)

The main idea is that xylem carries water (and dissolved minerals) from the roots up to the shoots, and this upward movement is driven by the cohesion-tension mechanism created by transpiration in the leaves. As water evaporates from stomata, it generates negative pressure (tension) in the xylem. The strong hydrogen bonds between water molecules (cohesion) transmit this tension upward through the continuous xylem column, while adhesion helps water cling to the xylem walls. This pull moves water against gravity from roots to leaves. Root pressure can contribute a little, but it isn’t the primary force behind long-distance water transport. The statement about osmosis alone or about xylem transporting only minerals without water doesn’t capture the main process.

The main idea is that xylem carries water (and dissolved minerals) from the roots up to the shoots, and this upward movement is driven by the cohesion-tension mechanism created by transpiration in the leaves. As water evaporates from stomata, it generates negative pressure (tension) in the xylem. The strong hydrogen bonds between water molecules (cohesion) transmit this tension upward through the continuous xylem column, while adhesion helps water cling to the xylem walls. This pull moves water against gravity from roots to leaves. Root pressure can contribute a little, but it isn’t the primary force behind long-distance water transport. The statement about osmosis alone or about xylem transporting only minerals without water doesn’t capture the main process.