Phloem Translocation and Mass Flow Hypothesis Simplified in 6 Steps

     Definition of Phloem Translocation: Phloem Translocation or Translocation of solute is the movement of organic food or organic solutes from site of synthesis (green parts) to site of utilization (non green parts) for consumption and storage.

Translocation of solute can be downward, upward or lateral or radial

Upward from leaves to buds, flowers and fruits, Downward from leaves to stem and roots, Lateral from Pith to cortex and epidermis

Two important terms for understanding Mass Flow Hypothesis

  • Source: site of synthesis or green parts of the plant  like leaves
  • Sink: Is the site of storage like fruits, seeds, tubers

Mass Flow Hypothesis (Munch, 1930):  

Phloem Translocation Mass Flow Hypothesis Simple

Most accepted theory on Phloem translocation

  1. Mass flow of solute is a passive process  occurs from source to sink along a hydrostatic pressure gradient or Turgor pressure gradient
  2. HPG exists between source and sink
  3. This difference drives the movement of solutes from  leaves to other parts through phloem
  4. The movement is always from the region high HPG to region of low HPG


Step 1: Sugar concentration in Mesophyll cells increases by photosynthesis. Water is absorbed from nearby cells increasing hydrostatic pressure in mesophyll cell

Hydrostatic pressure or turgor pressure is the pressure exerted on the wall due to entry of water by osmosis

Step 2: Phloem loading occurs.  Transfer of organic food or sugar from mesophyll cells to sieve tube of phloem due to turgor pressure or hydrostatic pressure

Step 3: Water entry from xylem to sieve elements by osmosis as the concentration inside sieve elements has increased by phloem loading

Step 4:This hydrostatic pressure forces the solute to the next sieve tube element. Thus  hydrostatic pressure difference (hydrostatic pressure gradient) drives solute movement from sieve tube to sink

Step 5: Phloem unloading

Transfer of organic food from sieve tube to sink, the site of storage

Step 6: This decreases the concentration in sieve elements and water moves back to xylem vessels by osmosis and transpiration pulls up water


1. Recent studies suggest that Phloem loading is an active process that requires ATP. Active phloem loading is the transport of sugars from the mesophyll cells into the phloem against a concentration gradient.

2. This hypothesis proposes the unidirectional flow of solutes but in plants bi directional flow is quite common.

3. The active role of cytoplasm in phloem translocation is not considered.

4. Turgor pressure may not always be higher at the supply end.

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