Question

We know that in xylems, the water move through the lumen of the vessel elements and move to the other vessel element through a pit. As the vessel elements are dead , there is no cytoplasm to block their way. The water can move through the cell walls(apoplastic pathway) or through the cytoplasm (symplastic pathway).

Phloems are living tissue and made up of sieve elements and companion cells. I have a doubt regarding the movement of sap in phloems.

In phloems, does the sap move through the sieve element and move to the other seive element through sieve plate pore? If yes, does the sap move through the sieve element's cytoplasm (like symplastic pathway) ? If no, then how does the sap move?

I guess there is no function of companion cells in the transportation. Their only function is to carry metabolic activities in sieve elements.

Answer 1

In phloems, the sap primarily moves through the sieve elements and traverses from one sieve element to another through sieve plate pores. The movement is predominantly facilitated by mass flow, where pressure gradients generated by active loading and unloading of solutes drive sap movement through the sieve tubes. This process is known as the pressure flow mechanism, and it does not involve the cytoplasm of sieve elements.

Step-by-step explanation:

The movement of sap in phloem is a vital process orchestrated by sieve elements, specifically sieve tubes. Unlike xylem vessels, phloem cells are living and possess a highly specialized structure for sap transport. The primary route for sap movement is through sieve elements, and it occurs via the pressure flow mechanism. This mechanism relies on the active transport of sugars into the sieve tubes at source regions, creating a higher osmotic pressure.

As a result, water moves into the sieve tubes, generating positive pressure. This pressure gradient propels the sap to areas of lower pressure, such as sink regions, where sugars are actively unloaded. The sap traverses through the sieve plate pores, which are perforated regions between adjacent sieve elements, allowing for efficient flow.

Companion cells play a crucial role in supporting the metabolic needs of sieve elements. Although they are not directly involved in sap transport, their metabolic activities are essential for maintaining the functionality of sieve elements. The companion cells supply ATP and other substances necessary for the loading and unloading of solutes in sieve tubes, ensuring the success of the pressure flow mechanism.

Therefore, while the primary responsibility for sap movement lies with sieve elements and the pressure flow mechanism, companion cells contribute significantly to the overall efficiency and functionality of phloem transport.