A-LevelBiologyTransport in plants9700/12 · Oct/Nov 2017 · Q301 Mark
Measurements of the stem of a young tree showed that the diameter varied slightly. It decreased during the day and increased during the night. Which statement explains these observations?
ADuring the day, cohesion between water molecules and lignin in the walls of the xylem vessels pulls the walls of the vessels inwards.
BDuring the day, the tension developed in a moving column of water molecules reduces the pressure inside the xylem vessels, so that their walls move inwards.
CDuring the night, the adhesion of water molecules to the lignin in the walls of the xylem vessels pushes the walls of the vessels outwards.
DDuring the night, the pressure of the column of water molecules adhering to one another increases and pushes the walls of the xylem vessels outwards.
✓ Correct answer
B: During the day, the tension developed in a moving column of water molecules reduces the pressure inside the xylem vessels, so that their walls move inwards.
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