3. translocation of sucrose

TRANSPORT IN PHLOEM

• movement of substances in phloem tissue is called translocation

• main substances that are moved are sucrose and AAs, which are in solution in water

• these substances have been made by plant and are called assimilates

• sucrose is the main assimilate

• glucose made by photosynthesis is converted into sucrose for transport because sucrose has less of an osmotic effect

SOURCES AND SINKS

• vascular plants produce sucrose in their leaves

• then transported to rest of the shoot or to the root tips, where growth occurs

• leaves are referred to as the source, and shoot and root tips- sink

• source- organ that produces more sugar than it requires-where assimilates enter the phloem

• sink- organ that consumes sugar for its own growth and storage- where assimilates leave phloem

PHLOEM LOADING

• sucrose is loaded into phloem sieve elements from surrounding cell

• very effective process

• 2 main loading routes:

• symplast pathway

• apoplast pathway

PHLOEM LOADING- SYMPLAST PATHWAY

• sucrose moves by diffusion down a conc gradient from source cells through cytoplasm of other cells into companion cells then into phloem sieve tubes

• lowers the water potential in tube so water moves in by osmosis

• generates a pos hydrostatic pressure which moves the phloem sap towards the sinks so a constant conc gradient is maintained

PHLOEM LOADING- APOPLAST PATHWAY

• sucrose moves by diffusion through cellulose cell walls and cell spaces to the companion cells

• in companion cells sucrose is moved from wall spaces across membrane into cytoplasm by active transport using ATP

• produces a high conc of sucrose in companion cells and so it moves into sieve tubes through many plasmodesmata

• water moves into companion cells by osmosis producing a high hydrostatic pressure

• moves assimilates and water into and through sieve tubes

• hydrostatic pressure moves sucrose towards sinks where pressure is lower

PHLOEM UNLOADING

• phloem sap is unloaded into sink cells by diffusion from sieve tubes

• sucrose then moves into other cells by diffusion and may be converted into other compounds (e.g. starch)

• maintains conc gradient between phloem and sink cells

• as sucrose moves out of sieve tubes water potential of phloem sap rises so water moves out by osmosis

• some moves into xylem

SUMMARY- dont bother but yeah

1. Glucose is made by photosynthesis in source cells and converted into sucrose.

2. The sucrose moves down the symplast pathway by diffusion though plasmodesmata to companion cells and into the phloem.

3. Some sucrose also moves via the apoplast pathway by diffusion into the cell walls of the companion cells. This is then actively pumped into the cytoplasm of the companion cells and moves into the phloem.

4. The movement of sucrose into the phloem lowers the water potential beside the source cells.

5. Water moves into the phloem from the nearby xylem vessels down the water potential gradient.

6. This creates a higher hydrostatic pressure here so the phloem sap moves down the phloem towards sink cells where pressure is lower.

7. at the sink cells sucrose moves into cells by diffusion and is converted into starch. This maintains the concentration gradient.

8. The removal of sucrose increases the water potential in the phloem so water moves back into the xylem down a water potential gradient.

EVIDENCE FOR TRANSLOCATION

• radioactive isotopes (carbon-14) can be made available to leaves of plants so they are used to make glucose in photosynthesis

• sucrose made from this glucose is also labelled and path it takes through a plant can be traced using autoradiography

• labelled sucrose is found in phloem

• if steam is used to kill a full outer ring of bark in a young shoot, just below a leaf, movement in phloem stops

• movement of water in xylem is not affected

• aphids (green fly) penetrate phloem tubes with their mouth parts to access sucrose

• sometimes pressure of phloem sap is so great it moves though the aphids digestive system and appears as a drop at the end of its body- if aphid is removes sap oozes out

MASS FLOW HYPOTHESIS- WHO

• mass transport= transport of materials from one point to another in a transport system using a transport medium and pressure or a force to bring about movement

• translocation fulfils all of these

• 1930- Ernst Munch developed a simple mass transport hypothesis to movement of solutes in phloem of plants

MASS FLOW HYPOTHESIS

• water moves into both containers by osmosis

• as A has a much higher conc of sugar than C water moves in more rapidly so there is a flow of solution from A to C

• hydrostatic pressure this makes forces water out of C 

• flow continues until the concentrations in A and C are equal

  

MASS FLOW HYPOTHESIS- LIMITATIONS

• doesn’t take into account active loading of sucrose into phloem by companion cells at the source- changes conc gradient and can even change direction of flow

• translocation is continuous (doesn’t end when concentrations are equal) model doesn’t take into account continuous loading of sucrose from sources and removal at sinks that make it possible

• water can move into sieve tubes at any point by osmosis

• return route for water to the cells is xylem- not represented in model