•During photosynthesis, organic compounds are synthesised in a plant’s leaves –The leaves act as a source •Most of these organic compounds are moved to parts of the plant that are actively growing, to storage organs or to seeds –These regions act as sinks •Transport is from source to sink •Transport occurs in the phloem tissue
Evidence that transport of organic compounds occur
•14C used to label organic compounds – subsequent autoradiography shows labelled compounds are in the phloem •The bark of a tree contains the active phloem – remove a ring of bark and downward movement of organic compounds stops where bark removed •Sap-sucking insect – piercing mouthpart found to be in phloem and sap removed via piercing mouthpart found to contain organic compounds
Structure of Phloem
•Two types of cell •Sieve tube elements –Stacked one on top of another –No nucleus, thin layer of cytoplasm around large central ‘vacuole’, few organelles, P-protein in cytoplasm –Large pores in end walls (sieve plates) with cytoplasm passing from element to element –Smaller pores in side walls with strands of cytoplasm (plasmodesmata) running through to adjacent: •Companion cells –Smaller cells with nucleus, cytoplasm and organelles –Each controls activity of adjacent sieve tube element
Mass flow Model
Two ‘cells’, containing solutions of different concentration, are connected by tube C. Each ‘cell’ has a partially permeable membrane and is surrounded by distilled water.
The two containers are themselves connected. Water enters ‘cell’ A by osmosis because it has the more concentrated solution (lower/more negative water potential). This increases the volume within ‘cell’ A and, hence, the hydrostatic pressure within it.
This increase in pressure pushes the solution from ‘cell’ A to ‘cell’ B through tube C. As the pressure increases in ‘cell’ B, water (and other small molecules) is forced through the partially permeable membrane surrounding it.
Water flows back to the container holding ‘cell’ A as water finds its own level. Can students identify which parts of the plant are represented by the components of this model? [Cell A = source; cell B = sink; tube C = phloem; water = xylem]
Transport in the Phloem
Loading from source
In the leaves, solutes are passed from photosynthesising cells into the companion cells of the phloem. The companion cells then load the solutes into the sieve tube elements via plasmodesmata. ATP hydrolysis is involved in this transfer
Pressure flow from source to sink
Entry of solutes into sieve tube elements lowers their water potential (makes the water potential more negative). As a result, water enters the sieve tube elements from the surrounding xylem vessels. This increases the pressure within the sieve tube elements, pushing their contents through the pores in the sieve plates
Unloading to a sink
As solutes are pushed down a pressure gradient, they are surrounded by cells with a lower solute concentration. Solutes leave the phloem into these cells, lowering their water potential •Water leaves the phloem by osmosis, returned to the surrounding xylem and is re-circulated •Cells in the sink use, or store, the solutes
Contents of the Phloem
•Non-reducing sugars / ~80–120 –Very little reducing sugar transported •Amino acids / ~ 5 •Proteins / ~ 2 •Organic acids / ~ 3 •Inorganic ions / 5