What are the differences between the transport of xylem and phloem?


Xylem and phloem are both transport vessels that combine to form a vascular bundle in higher order plants

  • The vascular bundle functions to connect tissues in the roots, stem and leaves as well as providing structural support

Xylem

  • Moves materials via the process of transpiration
  • Transports water and minerals from the roots to aerial parts of the plant [unidirectional transport]
  • Xylem occupy the inner portion or centre of the vascular bundle and is composed of vessel elements and tracheids
  • Vessel wall consists of fused cells that create a continuous tube for the unimpeded flow of materials
  • Vessels are composed of dead tissue at maturity, such that vessels are hollow with no cell contents

Phloem

  • Moves materials via the process of active translocation
  • Transports food and nutrients to storage organs and growing parts of the plant [bidirectional transport]
  • Phloem occupy the outer portion of the vascular bundle and are composed of sieve tube elements and companion cells
  • Vessel wall consists of cells that are connected at their transverse ends to form porous sieve plates [function as cross walls]
  • Vessels are composed of living tissue, however sieve tube elements lack nuclei and have few organelles

Comparison of Xylem and Phloem

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Transport in plants

Xylem moves water from roots to the leaves, and phloem moves food from the leaves to the rest of the plant. During transpiration water evaporates from the leaves and draws water from the roots.

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Xylem and phloem

Plants have tissues to transport water, nutrients and minerals.

Xylem transports water and mineral salts from the roots up to other parts of the plant, while phloem transports sucrose and amino acids between the leaves and other parts of the plant.

Xylem and phloem in the centre of the plant root

This table explains what is transported by the xylem and phloem:

TissueWhat is movedProcess
Xylem Water and minerals Transpiration stream
Phloem Sucrose and amino acids Translocation

Xylem

Mature xylem consists of elongated dead cells, arranged end to end to form continuous vessels [tubes].

Mature xylem vessels:

  • contain no cytoplasm
  • are impermeable to water
  • have tough walls containing a woody material called lignin

Phloem

Phloem consists of living cells arranged end to end. Unlike xylem, phloem vessels contain cytoplasm, and this goes through holes from one cell to the next.

Phloem transports sucrose and amino acids up and down the plant. This is called translocation. In general, this happens between where these substances are made [the sources] and where they are used or stored [the sinks].

This means, for example, that sucrose is transported:

  • from sources in the root to sinks in the leaves in spring time
  • from sources in the leaves to sinks in the root in the summer

Applied chemicals, such as pesticides, also move through the plant by translocation.

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