9.2 Phloem transport

Translocation

The movement of organic compounds from sources to sinks

Source

Where organic compounds are synthesised (photosynthetic tissues)

Sink

Where the compounds are delivered to for use or storage - includes roots, fruits and seeds.

How are sugars transported?

As sucrose

Cells in the phloem

Sieve element cells, companion cells, parenchymal cells and schlerenchymal cells

Sieve element cells

Long and narrow cells that are connected to form the sieve tube.

Properties sieve element cells

* Connected by sieve plates at their transverse ends, which are porous to enable flow between cells
* Have no nuclei and reduced numbers of organelles to maximise space for translocation
* Have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow.

Companion cells

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink

Properties companion cells

* Possess and infolding plasma membrane which increases SA:vol ratio to allow for more material exchange
* Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
* Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube.

Phloem vs. xylem in roots of monocotyledon

Phloem vs. xylem in roots of dicotyledon

Xylem vs. Phloem in stem of monocotyledon

Xylem vs. Phloem in stem of dicotyledon

Plasmodesmata

small channels that directly connect the cytoplasm of neighboring plant cells to each other, establishing living bridges between cells

How are organic compounds loaded onto phloem?

By companion cells

* Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
* Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

Apoplastic loading

* Hydrogen ions are transported out of phloem by proton pumps (needs ATP)
* Hydrogen ions build up creating a conc. gradient
* Hydrogen ions passively diffuse back into the phloem cell via co-transport protein, which requires sucrose movement
* This results in the sucrose being loaded into the phloem

Mass flow in source

* The active transport of solutes makes the sap hypertonic
* Therefore water from the xylem flows into the phloem via osmosis
* This increases the hydrostatic pressure in the phloem
* This pressure forces sap to move towards areas of lower pressure
* Hence phloem transports solutes away from source and towards sink

Mass flow at sink

* Solutes are unloaded by companion cell into sink
* Causes sap to become hypotonic
* Consequently water moves back into xylem
* Therefore hydrostatic pressure at sink is always lower than pressure at source
* This will cause phloem sap to move from source to sink

How do Aphids drink sap from phloem?

* Have mouth piece that pierces the plant’s sieve tube to allow sap to be extracted
* Penetration of stylet is aided by digestive enzymes
* Even with a hole in stem, sap will continue to flow due to pressure

Measuring phloem transport

* a plant is grown in a lab with radioactive CO2
* The leaves will convert CO2 into radioactive sugars
* Aphids will feed from the sap
* The sap can then be analyzed for the presence of radioactive sugars
* The rate is calculated by finding the time taken for the radioisotope to be detected at different positions along the plant’s length.

Factors affecting translocation rate

* Rate of photosynthesis
* Rate of respiration
* Rate of transpiration
* Diameter of sieve tubes