9.2 TRANSPORT IN THE PHLOEM

Translocation: The process of transporting organic compounds (sugars and amino acids) made during photosynthesis in the phloem sieve tubes

• Plant cells need organic compounds to function
  

• Many organic compounds are made in the chloroplasts

• The phloem transports the organic compounds around the plant

Phloem

• Coloumns of living cells
• Transports sugars, amino acids and plant hormones
• Companion cells carry out cell functions with many mitochondria that provide ATP for Active Transport
• Sieve tubes have sieve plates that controls the flow of sap

Xylem

• Coloumns of dead cells
• Transports water and minerals
• Thickened cell wall consisting of lignin
• Continuous tube which allows for an unbreakable coloumn of water

Phloem is made of living cells

Sieve elements: Elongated living cells that form from the phloem tissue

1. Phloem sieve tube: Develop from sieve tube elements (coloumns of cells) that break down nuclei and organelles. It has lumen for easy flow of sap

Sieve tube: Several sieve elements that are connected end to end

Sieve plate: Cross walls within the sieve tubes that are perforated (pores)

2. Sieve plate: Cross wall that strengthens the sieve tube, the plate has pores which allows sap to flow through

3. Cell membrane: Inside of wall that holds sap inside the sieve tubes and has pumps to load and unload sucrose

4. Cell wall: Resists high pressure within the sieve tubes

Translocation: Transport of organic compounds (sugars and amino acids) made during photosynthesis in the phloem sieve tubes

Plants do not transport Glucose it must be converted into Sucrose

Translocation of sap can occur in any direction always moving from a source to a sink

Sap: Mixture of water, carbohydrates, minerals, amino acids and plant hormones

Source: Photosynthesizing tissues and other plant organs that can export sugars to other parts of the plant

Sink: Plant organs that cannot produce sugars but need it for respiration or storage

• Translocation occurs from a source to a sink

1) Sugars and amino acids are loaded into phloem sieve tubes from sources [stems, leaves, storage organs]

2) Sugar and organic compounds are unloaded from phloem sieve tubes into sinks [roots, growing fruits, potato tubers]

• The loading and unloading are done through the process of Active Transport

Hydrostatic Pressure: Pressure in a liquid

Water is incompressible therefore it is able to be transported in phloem. Water is transported within the phloem because of hydrostatic pressure gradients 

As water is incompressible and sieve elements have rigid cell walls the inflow of water creates high hydrostatic pressure

• Sugars produced by photosynthesizing tissues or other sources are actively loaded using ATP into the sieve tubes by companion cells. This causes solute concentration to build up in the sieve tubes

• High concentration of solutes in sieve tubes at the source leads to water entering sieve tubes by osmosis from neighbouring xylem vessels this creates high hydrostatic pressure

• Hydrostatic pressure gradients is present between the sap at the source and the sink
• High hydrostatic pressure causes movement of water and carbohydrates through the pores of sieve plates down the tube towards the sink

• At the sink companion cells actively unload the sieve tube, some carbs converted starch or stored while some are used for respiration

• Concentration of solute decreases in phloem sieve tubes at the sink, water then exists sieve tubes by osmosis creating low hydrostatic pressure

• As sap goes from source to sink transported from area of high hydrostatic pressure to an area of low hydrostatic pressure

Pressure gradient makes the sap flow from source to sink in phloem sieve tubes

Radioactive isotopes can track sucrose movement during translocation. The radioactivity can be used to traced using photographic film

• Plant is grown in lab
• One leaf is exposed to carbon dioxide containing a radioactive isotope C14 for short period

What happens to 14CO2 within the leaf before any translocation occurs?

14CO2 taken up by photosynthesis and 14C incorporated into glucose and then sucrose

After 6 hours plants frozen in liquid nitrogen. Suggest why plant must be frozen quickly.

Freezing plants will prevent any further translocation of sucrose. Kills phloem cells

• Aphids have long piercing mouthparts called stylets which is inserted into leaves or stems and push into plant tissues until a sieve tube is pierced
• High pressure inside sieve tube pushes phloem sap out through stylet into gut of Aphid
• Aphid is cut from stylet leaving the stylet to act as a narrow tube for emerging sap

• If radioactively labelled carbon dioxide is supplied to leaf, radioactive sucrose is made and loaded into phloem 
• Time taken for radioactive sucrose to emerge from stylets at different distances from the leaf can be used to give a measure of the rate of movement of phloem sap

Tiny syringes cannot be used as the plant reacts and blocks them. Aphid saliva contains chemicals which stops this reaction

Measurements of phloem rates only possible after technique that radioactively labels carbon was developed

• Organic compounds can also be viewed using electrophoresis gel which shows the different compounds in the sap [carbs, amino acids, proteins]