Lesson 3: Transport in Plants: Phloem

The function of phloem is to move organic molecules throughout the plant. This is known as translocation. The organic molecules (sugars, amino acids, hormones) are dissolved in water producing a substance known as phloem sap. Phloem consists of two types of living cells: Sieve tube cells – stacked end to end, these cells are perforated to allow for the movement of solutes. Companion cells – connected to sieve tube cells by plasmodesmata, they produce and transport proteins to the sieve tube cells in order to keep them alive.

Xylem moves water and dissolved minerals in only one direction, from roots to leaves. Phloem moves organic compounds in any direction throughout the plant. Translocation always moves organic material from a source (where the material is stored or made) to sink (where the material is used) This type of movement occurs through a pressure flow mechanism. Phloem sap moves passively throughout the plant as a result of a hydrostatic pressure gradient.

The products of photosynthesis (sugars and amino acids) move from leaf cells (source) to phloem by active transport. This sugar loading raises the solute concentration at the source end of the phloem tube High solute concentration draws water into the tube from the xylem by osmosis This inflow of water raises water pressure at the source end of the phloem tube.

Sugars and amino acids leaves the phloem by active transport. This reduces the solute concentration at the sink end of the phloem tube. Low solute concentration causes water to return to the xylem by osmosis. The outflow of water lowers the water pressure at the sink end of the phloem tube

Phloem sap moves around plants at a faster rate than can be explained by diffusion. The pressure flow mechanism can be difficult to test as mechanisms for testing disrupt the structure and function of the phloem tubes. Natural phenomena have been observed that prove this mechanism, most notably the feeding of aphids.Aphids insert a stylet into a sieve tube cell in the phloem in order to feed off the sugars produced by the plant. If the aphid stylet is inserted closer to the source, the flow of sap has been observed to be faster than if the stylet is inserted closer to the sink. This increase in flow rate seems to indicate that the sap is under higher pressure when closer to the source.

Vascular bundles in the roots, stems and leaves of plants are made up of both xylem and phloem.

Composed of a column of dead cells when mature Continuous tube of cells enables an unbroken column of water (cohesive forces) Thickened with lignin to withstand negative pressure as water vapour is lost Transport water and minerals from root to leaves

Composed of a column of living cells with perforated cell walls Living cells enable substances to be loaded by active transport. Companion cells carryout cell functions and supply energy for active transport Transport sugars, amino acids and hormones to all parts of the plant by mass flow.