Transport In Plants

Phloem translocation is explained by the pressure-flow or mass-flow hypothesis, as set out in NCERT Class 11, Chapter 11 (Transport in Plants). At the source, such as a photosynthesising leaf, sugars are actively loaded into the sieve tube elements, raising their solute concentration and lowering their water potential. Water therefore enters the sieve tubes from the adjacent xylem by osmosis, building up high turgor pressure. At the sink, sugars are unloaded and used or stored, water leaves, and turgor pressure falls. This difference in turgor pressure between source and sink drives the bulk flow of the sugary sap from high-pressure to low-pressure regions. An electrical potential gradient is not the driving force of mass flow, though membrane transport is involved in loading. A temperature gradient does not power translocation, and movement is bidirectional rather than purely gravitational, so a gravitational gradient is incorrect. A plausibility check supports this: since phloem can transport sugars upward to growing shoot tips as well as downward to roots, only a pressure-driven mechanism, not gravity, fits the observed multidirectional flow, which is why girdling experiments that interrupt the phloem cause sugars to accumulate above the cut and confirm source-to-sink transport.