Question

The ability of phloem to distribute sugar depends on the companion cell and sieve element working as a complex. Consider the following statements with regard to possible effects of reduction in companion cell plasmodesmata.
(i) Sugar loading into sieve tubes reduces due to collapse in osmotic pressure gradient.
(ii) Water moves from companion cells into the xylem to maintain osmotic balance.
(iii) The sieve tubes collapse structurally due to loss of turgor pressure.
(iv) Reverse loading of sugar in phloem begins immediately due to pressure inversion.
Under the given condition, the correct statements are:

• A. (i) and (iii)
• B. (i) and (ii)
• C. (iii) and (iv)
• D. (ii) and (iv)

Hint:

Sieve elements lack nuclei and many organelles, making them dependent on companion cells. Turgor pressure is the "skeleton" of the sieve tubes; without sugar loading to drive osmosis, they lose turgor and structurally cave in.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the correct statements regarding the effects of reducing companion cell plasmodesmata on phloem sugar distribution.

Step 2: Detailed Explanation:

• Phloem transport is driven by a pressure-flow mechanism, where sugars are actively loaded into the sieve tubes at the source (leaves).

• This loading is facilitated by the companion cells, which are linked to the sieve elements via numerous specialized cytoplasmic channels called plasmodesmata, forming a functional sieve element-companion cell (SE-CC) complex.

• If there is a significant reduction in companion cell plasmodesmata, symplastic sugar transport into the sieve tubes is severely impaired.

• Consequently, sugar loading into the sieve tubes reduces drastically. This leads to a drop in solute concentration inside the sieve tube, preventing the establishment of the osmotic gradient needed to draw water from adjacent xylem.

• Thus, the osmotic pressure gradient collapses, confirming statement (i).

• Sieve tubes are structurally thin-walled living cells that rely heavily on internal turgor pressure to maintain their shape and open channel structure.

• When turgor pressure is lost due to the lack of solute accumulation and water entry, the high external physical pressures in surrounding tissues cause the sieve tubes to collapse structurally, confirming statement (iii).

• Statement (ii) is incorrect because water would not move from companion cells to xylem to maintain balance under these circumstances.

• Statement (iv) is incorrect because reverse loading does not begin immediately; rather, the transport system simply breaks down due to structural collapse.


Step 3: Final Answer:
Therefore, the correct statements are (i) and (iii).