Question

Downward flow of organic and even some inorganic solutes takes place from the leaves through the stem by way of

• A. Phloem by protoplasmic streaming
• B. All parenchymatous cells of the stem by diffusion of solute downward
• C. Phloem by mass flow of solutes from higher turgor pressure of leaf mesophyll cells along a turgor pressure gradient in phloem cells
• D. Phloem by diffusion

Answer 1

Answer: (C)

Phloem by mass flow of solutes from higher turgor pressure of leaf mesophyll cells along a turgor pressure gradient in phloem cells

• The pressure-flow hypothesis explains the movement of sap through the phloem. 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 the 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.

Therefore, the correct answer is option C.