Biology: The Unity and Diversity o...

15th Edition
Cecie Starr + 3 others
ISBN: 9781337408332



Biology: The Unity and Diversity o...

15th Edition
Cecie Starr + 3 others
ISBN: 9781337408332
Textbook Problem

Water transport from roots to leaves occurs by ______.

  1. a. a pressure gradient inside sieve tubes
  2. b. different solutes at source and sink regions
  3. c. the pumping force of xylem vessels
  4. d. evaporation, tension, and cohesion

Summary Introduction

Introduction: Water is the most crucial factor for a plant to perform its normal cellular functions. The water is absorbed by the roots. The movement of water from the roots to the leaves can be explained using cohesion tension theory of water transport.


Reason for the correct answer: The water movement from the roots to the vascular tube (xylem) is facilitated by osmosis. From the xylem, the molecules reach the leaves by adhering to one another (cohesion) and are pulled upward using the force called tension or transpiration pull. This force is applied by the water molecule that escapes out of the stomata (evaporation or transpiration).

Option d. is given as “evaporation, tension, and cohesion”. Water molecules escape to the atmosphere by evaporation. Evaporation of water facilitates the upward movement of cohesively adhered molecules by exerting a force called as tension.

Hence, the correct answer is option d.

Reasons for the incorrect answers:

Option a. is given as “a pressure gradient inside sieve tubes”. This is incorrect because the pressure gradient inside the sieve tubes facilitates the movement of sugar molecules in the phloem tube, but the water transport takes place in the xylem itself...

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