Transport in Plants Question 22
22. What is the direction of movement of sugars in phloem?
[2019]
(a) Non-multidirectional
(b) Upward
(c) Downward
(d) Bi-directional
Show Answer
Answer : d
Hints & Solutions
(d) The direction of movement of sugar in phloem is bi-directional. The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the bestsupported theory to explain the movement of sap through the phloem. It was proposed by Ernst Munch, a German plant physiologist in 1930. A high concentration of organic substances, particularly sugar, inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem. This creates turgor pressure, also known as hydrostatic pressure, in the phloem. Movement of phloem sap occurs by bulk flow (mass flow) from sugar sources to sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.
