The phloem transports dissolved products of photosynthesis in various directions around the plant

Phloem Structure

  • The individual sieve tube elements that make up the phloem are made of cells; however, they do not have a nucleus and have few organelles. They are more just strands of cytoplasm.
  • The ends of the cell walls form sieve plates through which cytoplasm can pass through. This differs to the xylem as it has cell walls that disappear at the ends
  • As the sieve tubes have minimal elements to keep it alive, it is aided by the companion cells which respire, excrete etc on the cells behalf.
  • The cytoplasm of the companion cells and their sieve tube elements is joined through pores on the side walls.

Translocation

  • Translocation is the transport of soluble organic substances (assimilate) within a plant

Mass Flow Hypothesis

  • Solute transport occurs in plants. Any part that produces sucrose is produced in plants is known as the source, and any area the consumes the sucrose is the sink. Sucrose is actively transported into the sieve tubes at the source (e.g. leaves/roots) lowering the water potential inside the sieve and so water enters the tubes through osmosis, thus creating a higher pressure inside the sieve tubes at the source At the sink sugar leaves the phloem to be used up increasing the water potential inside the sieve tubes. Therefore, water leaves via osmosis lowering the pressure inside the sieve tubes. A pressure gradient from the source to the think pushing sugar to where they are required.
For Against
Sap oozes out, showing there is a pressure gradient inside the plant Sugar travels to many different sinks
Equal water potential throughout the plant Sieve plates are barriers to mass flow
ATP is present in the phloem since it is an element for active transport The sieve plates are living, whereas there is no reason for them to be according to mass flow.
Phloem sap has a high pH, due to H+ ions being actively transported