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Phloem and Sugar Transport

Phloem and Sugar Transport

Like many living organisms, plants have tissues that specialize in specific plant functions. For example, shoots are well adapted to rapid growth, while roots are structured to acquire resources efficiently. However, sugar production is primarily restricted to the photosynthetic cells that reside in the leaves of angiosperm plants. Sugar and other resources are transported from photosynthetic tissues to other specialized tissues by a process called translocation.

Xylem and Transpiration-driven Transport of Resources

Xylem and Transpiration-driven Transport of Resources

The xylem of vascular plants distributes water and dissolved minerals that are taken up by the roots to the rest of the plant. The cells that transport xylem sap are dead upon maturity, and the movement of xylem sap is a passive process.

Short-distance Transport of Resources

Short-distance Transport of Resources

Short-distance transport refers to transport that occurs over a distance of just 2-3 cells, crossing the plasma membrane in the process. Small uncharged molecules, such as oxygen, carbon dioxide, and water, can diffuse across the plasma membrane on their own. In contrast, ions and larger molecules require the assistance of transport proteins due to their charge or size. Transport across membranes also occurs within individual cells, playing a variety of essential roles for the plant as a whole.

The Apoplast and Symplast

The Apoplast and Symplast

Plant growth depends on its ability to take up water and dissolved minerals from the soil. The root system of every plant is equipped with the necessary tissues to facilitate the entry of water and solutes. The plant tissues involved in the transport of water and minerals have two major compartments - the apoplast and the symplast. The apoplast includes everything outside the plasma membrane of living cells and consists of cell walls, extracellular spaces, xylem, phloem, and tracheids. The...

Water and Mineral Acquisition

Water and Mineral Acquisition

Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.

Pharmacodynamic Models: Link Model and Systems Pharmacodynamic Model

Pharmacodynamic Models: Link Model and Systems Pharmacodynamic Model

The link model is a fundamental pharmacokinetic-pharmacodynamic (PK–PD) approach to account for delayed drug responses when the observed effect does not immediately correlate with the drug's plasma concentration peak. This delay is mathematically addressed by introducing an effect compartment concentration, Ce, which is kinetically linked to the plasma concentration, Cp, via a first-order rate constant, ke0. The linkage allows for a more accurate prediction of drug effects over time. A higher...

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Related Experiment Video

Updated: Jun 24, 2026

Using High Resolution Computed Tomography to Visualize the Three Dimensional Structure and Function of Plant Vasculature

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Linking phloem function to structure: analysis with a coupled xylem-phloem transport model.

T Hölttä¹, M Mencuccini, E Nikinmaa

¹School of GeoSciences, University of Edinburgh, Crew Building, West Mains Road, EH9 3JN Edinburgh, UK. teemu.holtta@helsinki.fi

Journal of Theoretical Biology

|April 14, 2009

Summary

This study models plant sugar transport, revealing that phloem transport rates depend on solution viscosity and xylem water potential. Adding solute relays optimizes phloem function and reduces conduit requirements.

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Area of Science:

Plant Physiology
Biophysics

Background:

Phloem transport is crucial for distributing sugars throughout plants.
The Münch hypothesis provides a theoretical basis for understanding this process.

Purpose of the Study:

To theoretically analyze phloem transport using a coupled xylem-phloem model.
To determine factors limiting sugar transport and conduit requirements.

Main Methods:

Developed a coupled xylem-phloem transport model.
Performed theoretical analysis based on the Münch hypothesis.

Main Results:

Maximum phloem sugar transport is limited by solution viscosity.
Xylem water potential significantly affects transport requirements.
Calculated minimum xylem and phloem conduits for transpiration and assimilation.
Solute relays reduce parallel sieve tubes needed, uniformize turgor pressure, and speed information transmission.

Conclusions:

Xylem and phloem transport conductances are coupled.
Structural investments in xylem can reduce phloem investment, and vice versa.