Related Experiment Video
Updated: May 29, 2026

Collection and Analysis of Arabidopsis Phloem Exudates Using the EDTA-facilitated Method
Published on: October 23, 2013
(Questions)n on phloem biology. 2. Mass flow, molecular hopping, distribution patterns and macromolecular signalling
Aart J E van Bel1, Alexandra C U Furch, Jens B Hafke
1Plant Cell Biology Research Group, Institute of General Botany, Justus Liebig University, Senckenbergstrasse 17, Giessen, Germany. Aart.v.Bel@bot1.bio.uni-giessen.de
Abstract:
This review speculates on correlations between mass flow in sieve tubes and the distribution of photoassimilates and macromolecular signals. Since micro- (low-molecular compounds) and macromolecules are withdrawn from, and released into, the sieve-tube sap at various rates, distribution patterns of these compounds do not strictly obey mass-flow predictions. Due to serial release and retrieval transport steps executed by sieve tube plasma membranes, micromolecules are proposed to "hop" between sieve element/companion cell complexes and phloem parenchyma cells under source-limiting conditions (apoplasmic hopping). Under sink-limiting conditions, micromolecules escape from sieve tubes via pore-plasmodesma units and are temporarily stored. It is speculated that macromolecules "hop" between sieve elements and companion cells using plasmodesmal trafficking mechanisms (symplasmic hopping). We explore how differential tagging may influence distribution patterns of macromolecules and how their bidirectional movement could arise. Effects of exudation techniques on the macromolecular composition of sieve-tube sap are discussed.
Related Concept Videos
Phloem and Sugar Transport
Short-distance Transport of Resources
The Apoplast and Symplast
Cellular Membranes and Drug Transport
Phospholipids arrange themselves into a bilayer, with hydrophilic heads oriented outward and hydrophobic tails facing inward.
Facilitated Diffusion
In this process, substrates such as organic compounds and ions interact with a transporter on one side, triggering conformational changes in proteins that enable...
The Significance of Membrane Transport
Transporters facilitate either an active or passive movement of solutes. They can allow a single-molecule transport down its...

