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The Pressure Flow or Mass Flow Hypothesis

Last Updated : 03 Jul, 2022
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In Plants, the system made sense of the movement of sugars from source to sink alludes to the strain stream speculation. Glucose produced by photosynthesis at the source is switched over completely to sucrose which is moved to the phloem sifter tube cells through the friend cell by dynamic vehicle. The excess vehicle in the phloem is done by the distinctions in the osmotic strain, which works with the vehicle from higher strain to bring down pressure areas of the sink. Again by dynamic vehicle, the sucrose is shipped from phloem to the cells of the sink where it will be put away for a really long time. Plant transport tissues – xylem and phloem. Plants have two vehicle frameworks – xylem and phloem. Xylem transports water and minerals. Phloem transports sugars and amino acids broken up in the water.


Mass Flow of Hypothesis

When the movement of minerals and water via xylem is driven mostly by negative pressure and movement via phloem is driven by hydrostatic pressure. This process is called translocation and is accompanied by a process known as phloem loading and unloading. Cells in sugar sources load a sieve tube by osmosis developing pressure that pushes the sap low. The cells deliver solutes out of the elements of the sieve tube and produce opposite effects. The sugar gradient from the source creates pressure-flow via the sieve tube towards the sink.

  • Plants respire through the process of photosynthesis, which involves the formation of glucose in mesophyll cells. Not all of the sugars are utilized; leftover glucose becomes non-reducing sugar. 
  • Sucrose is delivered to the neighbor cells of minute veins of the leaves.
  • Through the plasmodesmata, the sugars diffuse from neighbor cells and enter the sieve tube. The number of sugars within the sieve tube and its elements increase. This is the beginning of the mass flow of the hypothesis
  • Water travels from the close xylem to the leaf vein by osmosis and raises the hydrostatic pressure of the elements of the sieve tube
  • This hydrostatic pressure turgor then shifts the sugars and other substances down the cells of the sieve tube towards the sink (roots).
  • In storage sinks, sucrose is eliminated into the apoplast before entering the sink’s symplast.
  • The phloem sugar is eradicated by the cortex of the root and stem and utilized by cellular respiration. The starch is insoluble and does not exert any osmotic effect. Ultimately, pure water is left and drawn into xylem vessels by transpiration pull.


  • The xylem transports water and minerals from the roots up the plant stem and into the leaves.
  • In a developed blossoming plant or tree, the majority of the phones that make up the xylem are particular cells called vessels. Vessels:
  • Lose their end walls so the xylem shapes a consistent, empty cylinder. This permits water to handily stream.
  • Become fortified by a compound called lignin. The cells are as of now not alive. Lignin invigorates and backs to the vessel.
  • Transport in the xylem is an actual cycle. It doesn’t need energy.


Phloem moves sugar that the plant has delivered by photosynthesis to where it is required for cycles, for example, developing pieces of the plant for guaranteed use capacity organs like bulbs and tubers

creating seeds breath. 

  • Transport in the phloem is consequently both all over the stem. 
  • The transport of substances in the phloem is called movement.
  • Phloem comprises living cells. The cells that make up the phloem are adjusted to their capability:
    • Strainer tubes – are particular for transport and have no cores. Each sifter tube has a punctured end so its cytoplasm interfaces cell to cell.
    • Sidekick cells – transport of substances in the phloem requires energy. At least one friend’s cell appended to each sifter tube gives this energy. A sifter tube is totally reliant upon its buddy cell(s).

The xylem and phloem are dispersed diversely in roots and stems. In the root, the xylem shapes a focal section, framing a strong help. The phloem is towards the middle, outside the xylem. In the stem, the vehicle tissues of the xylem and phloem are assembled into vascular groups. Correlation of transport in the xylem and phloem

  Sort of transport 

Substances transported

Direction of transport
Xylem Physical process Water and minerals Upwards
Phloem Requires energy Products of photosynthesis, including sugars and amino acids disintegrated in water Upwards and downwards.

Conceptual Questions

Question 1: Make sense of the mass stream speculation of transport in the phloem?


The Pressure Flow Hypothesis is otherwise called the Mass Flow Hypothesis. It is the most acknowledged hypothesis of the development of food through the phloem. it was proposed by Ernst Munch in the 1930s. A high centralization of glucose in the cells of phloem at the source sets up the osmotic slope. This outcome in the development of water from the xylem into the phloem. After this, the phloem sap moves sugar from source to sink as a result of turgor pressure. Since pressure is involved subsequently it Is called pressure stream speculation. The development of substance happens In mass and consequently, this is additionally called mass stream speculation.

Question 2: What do you mean by Xylem?


Xylem alludes to the extremely durable and dead tissue that conveys water and fundamental substances in the plant. The essential capability of xylem is the transportation of water, dissolvable supplements, inorganic particles, and minerals. Moreover, this transportation occurs in an upwards way from the roots to different pieces of the plants.

Question 3:  What do you mean by Phloem?


Phloem alludes to a complicated tissue that transports solvent natural mixtures all through the vascular plants. Phloem utilizes energy and turgor strain to ship sugars and different substances to different organs of the plant. One distinction between xylem and phloem is that the previous is a non-living tissue while the last option is living.

Question 4: Write two differences between xylem and phloem.


  • Xylem is known as the permanent and dead tissue which tends to carry both water and essential substances inside the plant. The primary function of the xylem is known to be to transport water, minerals, inorganic ions, and soluble nutrients. Also, this transportation occurs in an upward movement from the roots to the different parts of the plant. 
  • Phloem, on the other hand, is known as a complex tissue that helps in the transportation of the soluble organic compounds, all throughout the different parts of the vascular plants. Phloem tends to use the energy and turgor pressure for transporting the sugars and many other substances to several parts of the plant.

Question 5: What is the function of the xylem in ferns?


The function of the xylem in any plant is to transport water and minerals from the root to all aerial parts of the plant body.

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