The mass flow hypothesis (proposed by Ernst Münch) explains how phloem sap moves from source to sink. Mass flow is the bulk movement of a liquid, with everything dissolved in it, down a pressure gradient.
2. This lowers the water potential inside the sieve tubes.
3. Water enters the sieve tubes from the nearby xylem by osmosis.
4. The extra water raises the hydrostatic pressure in the sieve tubes at the source end.At the sink5. Sucrose is removed from the sieve tubes and used or stored.
6. This raises the water potential inside the sieve tubes, so water leaves by osmosis.
7. The hydrostatic pressure at the sink end falls.Along the phloem8. There is now a hydrostatic pressure gradient, high at the source and low at the sink.
9. Phloem sap flows down this gradient, from source to sink, through the sieve tubes and the pores in the sieve plates.The water that left the phloem at the sink returns to the xylem and is carried back up the plant, so water circulates between the two tissues.Because the sap moves as a whole, the hypothesis predicts that all the dissolved solutes should travel at the same speed. The evidence for and against the hypothesis is covered in the next lesson.
2. This lowers the water potential inside the sieve tubes.
3. Water enters the sieve tubes from the nearby xylem by osmosis.
4. The extra water raises the hydrostatic pressure in the sieve tubes at the source end.At the sink5. Sucrose is removed from the sieve tubes and used or stored.
6. This raises the water potential inside the sieve tubes, so water leaves by osmosis.
7. The hydrostatic pressure at the sink end falls.Along the phloem8. There is now a hydrostatic pressure gradient, high at the source and low at the sink.
9. Phloem sap flows down this gradient, from source to sink, through the sieve tubes and the pores in the sieve plates.The water that left the phloem at the sink returns to the xylem and is carried back up the plant, so water circulates between the two tissues.Because the sap moves as a whole, the hypothesis predicts that all the dissolved solutes should travel at the same speed. The evidence for and against the hypothesis is covered in the next lesson.