- Leaves are 'sugar source'. Sugar diffuse from mesophyll cells into the sieve tubes of the phloem through symplast route/cytoplasm.
- Concentration of sugar in sieve tube is high, causing water potential in leaves to decrease/low.
- H2O enters sieve tube from the xylem, creating high hydrostatic pressure.
- Growing roots, shoot tips, stems & other parts of the plants are 'sugar sink'.
- Sugar diffuse out of sieve tube/phloem into 'sink tissues'. Water potential in 'sink tissues' decreases.
- As a result, H2O moves into other cells by osmosis, creating low hydrostatic pressure in sieve tubes.
- The hydrostatic pressure gradient formed between 'sugar source' & 'sugar sink' drives mass flow of H2O.
Sunday, November 25, 2012
Munch's Pressure (Mass Flow) Hypothesis
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The sugar does not "diffuse" from mesophyll cells into the sieve tube. Diffusion is a process that occurs in a contained area. The sugar moves by osmosis, which is the movement of a substance through a semi-permeable membrane from an area of greater concentration to lesser concentration.
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