In phloems, does the sap move through the sieve element cytoplasm?

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 phloem, the sieve tube elements are not lignified so no concept of 'pits' and since cell wall is permeable (cellulose) and also has plasmodesmata, substances can easily transfer through diffusion and/or osmosis. Sieve tube elements are living cells having reduced cytoplasm restricted to the sides and the sieve pores in the sieve plates are modified versions of plasmodesmata. This allows a nearly empty lumen for continuous bulk transport of organic food solution.

Companion cells keep the sieve tube elements alive and help them in their transport act.

For further details visit the following website:

https://academic.oup.com/jxb/article/64/16/4839/593231

You can also find many books explaining the whole process.

Hope it helps.