What is the role of hydrogen ions and companion cells in the mass flow theory?

  1. Hydrogen ions (H+) are actively transported out of companion cells into surrounding source cells.
  2. H+ is co-transported along its concentration gradient back into companion cells with sucrose.
  3. Sucrose can then diffuse along its concentration gradient through plasmodesmata from companion cells to sieve tube elements.

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What is the mass flow hypothesis?

The mass flow hypothesis proposes that translocation occurs due to pressure gradients:

  1. Solutes are actively transported into sieve tube elements, causing a high hydrostatic pressure at the source
  2. Solutes are moved by the phloem to respiring cells where they are removed from the sieve tube elements for use or storage, creating a low hydrostatic pressure at the sink
  3. This hydrostatic pressure difference results in the mass flow of solutes through the phloem

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Describe the processes in translocation that require energy.

  1. Active loading at source cells - actively transporting sucrose from source cells into companion cells, and subsequently into phloem sieve tube elements for translocation
  2. Active unloading at sink cells - actively transporting sucrose from the phloem sieve tube elements into companion cells, and subsequently into sink cells where it is used or stored

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What are sources and sinks in the context of plant translocation?

Sources are areas where sucrose is produced, like leaves.


Sinks are areas where sucrose is used or stored, like roots or meristems.


Other substances that are transported in the phloem, like amino acids, are also moved from sources to sinks. The locations of sources and sinks may change depending on the substance transported and the time of year.

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What is the role of water in the translocation of sugars?

Water provides the medium in which sugar, specifically sucrose, dissolves for transport in the phloem.

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What are assimilates in the context of plant translocation?

Assimilates are substances that have been manufactured or modified in the plant, such as sucrose and amino acids, that are transported in the phloem sieve tubes during translocation.

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Describe translocation in the context of plants.

Translocation is mass flow of assimilates, like sucrose and amino acids, from one part of a plant (the source) to another part of the same plant (the sink).

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