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

Sugar transport from leaves to roots 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: Vascular system is a system in plants where conducting tubes such as xylem and phloem are present. Phloem vessels are involved in transporting the prepared food (glucose) from leaves to all parts of the plants. Specialized types of elongated and slender tube found in the phloem tube are called as sieve tube. Sieve tubes are characteristics of flowering plants. Sieve tubes transport carbohydrates to different parts of the plant.


Reason for correct answer: The sugar molecules that are produced in the leaves (source) get transported to different parts of the plants (sink). The translocation of sugar from source to the sink region can be explained using the pressure flow model. At the source, the sugar concentration is higher, which creates a lower solute potential and the water molecules are drawn from the xylem to the phloem tube. Increased concentration of the solute and water increases the turgor pressure in the phloem near the source region. This drives the bulk movement of sugar and water molecules to the sink region. Removal of sugar to sink increases the solute potential and thereby decreases the turgor pressure. A decrease in the turgor pressure forces the water to return to the xylem vessel. Again this process continues.

Option a. is given as “a pressure gradient inside sieve tubes”. A pressure potential or the difference in the turgor pressure inside the phloem tube drives the movement of the sugar molecules from the source to the sink region.

Hence, the correct answer is option a.

Reasons for incorrect answers:

Option b. is given as “different solutes at source and sink regions”. Source and sink regions are different regions in a plant named for the transport of sugar molecules...

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