Phloem Tubes

1. Phloem Tubes

2. Function of Phloem • transports organic substances (sucrose) made by plant during photosynthesis • mostly away from leaves to storage organs • transport referred to astranslocation • Solutes move up and down phloem unlike movement in xylem

3. Structure of Phloem

4. Structure of Phloem

5. Structure of Phloem • Made from cells called sieve tube elements • Elements arranged end to end and fused together to form sieve tubes giving it its tubular appearance and conducting capacity • end walls of sieve tube elements are partially perforated due to enlarged plasmodesmataforming sieve plates • Sieve plates separate one sieve tube element from the next

6. Structure of Phloem • sieve plates allow for the flow of solution from one element to the next • sieve elements are living cells with active cytoplasm and their walls contain no lignin unlike xylem vessels.

7. Companion Cells

8. Companion Cells

9. Companion Cells • cells closely associated with sieve tube elements. • formed from cell division of a parent cell which formed sieve tube element • Many plasmodesmata connect the cytoplasm of the sieve tube element and the companion cell. • have all the cellular organelles • very metabolically active (contain many mitochondria) • have a very intimate relationship with the sieve tube elements assisting them in metabolic processes

10. Phloem Loading • Photosynthesis produces sugars • Sugars converted into sucrose for transport in phloem • Sucrose-relatively inactive, highly soluble, unused in transit • Easily converted back into glucose and fructose

11. Phloem Loading • Sucrose dissolves into water of mesophyll cell, moves across leaf by apoplast or symplast pathways • Loading-substances made in the leaves by photosynthesising cells (source) must get to the phloem tissue before it is translocated to where they are needed (sinks)

12. Phloem Loading • sugars produced in the leaves are of a lower concentration (0.5%) compared to the higher concentration (30%) found in companion cells and sieve tube elements • Thus, sucrose moves against a concentration to get into the sieve tube elements • Movement occurs by active transport • active loading of amino acids, sucrose, phosphates, potassium and ammonium ions into the companion cells are thought to be carried out by specific carrier proteins in the cell membranes of the companion cells

13. Phloem Loading • Uses energy produced in mitochondrion of companion cell • H+ ions pumped out of companion cell by active transport-excess H+ ions outside cell • H+ ions diffuse rapidly back into cell down conc gradient through carrier protein along with sucrose • Sucrose mol carried through this co-transporter into companion cell against conc gradient

14. Phloem Loading • Active transport into companion cells result in a very negative solute potential in them • Hence, water enters companion cells by osmosis • Sucrose moves from companion cell into sieve tube through plasmodesmata connecting them

15. Phloem Unloading • Sucrose unloaded into any tissue where needed • Sucrose leaves sieve tubes thru plasmodesmata by facilitated diffusion • Solute potential inside sieve tubes becomes less negative, maintaining the pressure gradient between source (where sugar is loaded) and sink (sugar unloaded • Unloading may also occur thru cell surface membrane of sieve tube into the cell walls

16. Phloem Unloading • Sucrose then converted into glucose and fructose by the tissue • This decreases its conc in the phloem thus maintaining a conc gradient from the phloem into the tissue

17. Mass Flow • phloem tubes transport organic substances made in the leaves of the plant • substances move from where they are made (leaves)-source • substances are moved to the roots, storage organs and other parts of the plant to where they are needed/used-sink

18. Radioactive Tracers • There are three basic pieces of experimental evidence which suggest this: • Using radioactive tracers: by labelling CO2 with 14C (radioactive isotope) to be used by the plant for the production of organic substances by photosynthesis. It is seen that the organic substances produced are transported in the phloem tissue because the radioactive organic carbon compounds blacken photographic film in the areas that are thought to be phloem in the stem of that plant

19. Radioactive Tracers

20. Aphid • Using sap-sucking insects- these insects eg. aphids, insert their tubular mouthparts into phloem tissue for a replenishing supply of sugars and amino acids. However, if we were to put the insect on anaesthetics and remove it but leaving its mouth part immersed in the phloem tissue it would show that fluid oozes out due to the pressure in the sieve tubes.

21. AN APHID STIKING ITS STYLET INTO A PHLOEM TISSUE TO OBTAIN NUTRITIOUS SAP

22. Ringing Experiment • Using ringing experiments- phloem tissues are usually just found under the bark of a tree, thus if the tree is ringed, ie, tissue is removed then the phloem would be as well. However, the plant continues to survive for some time because the xylem lies much deeper in side but over time the plant dies because where the ring is made, the area just above swells as substances needed by the root accumulates there as they are not translocated and roots are starved of nutrients and die.

23. Ringing Experiment

24. Mass Flow • The proposed mechanism of transport of organic substances down the phloem tubes is called the MÜNCH’S MASS FLOW HYPOTHESIS. • Firstly, organic substance (sucrose) is actively uploaded into the phloem tubes by active transport via the companion cells. • This creates a very negative solute potential/low water potential in the sieve tube elements of the phloem tissue. • water from the xylem vessels rushes into the sieve tube elements to create a mass flow of solution or a large hydrostatic pressure there.

25. Mass Flow • This pushes the organic substances in solution down the phloem tissues to where it is needed in the sinks eg roots where there is a high water potential/less negative solute potential. • This maintains the pressure gradient which allows for the flow of solution down the phloem tissues into the sinks from the sources.

26. Evidence supporting the Mass Flow Hypothesis • There is a flow of sol’n from phloem when it is cut or punctured by the stylet of an aphid • There is some evidence of conc gradients of sucrose- with high conc in the leaves and lower conc in roots • Some reasearchers have observed mass flow in microscopic sections of living sieve elements • Viruses or growth chemicals applied to leaves are only translocated downwards to roots when the leaf is illuminated.

27. Evidence Against the Mass Flow Hypothesis • Sugars have been observed to move at different speeds and in different directions in the same vascular bundles. The theory suggests that all materials being transported travel at the same speed • theory offers no explanation for the existence of sieve plates which act as a series of barriers impeding flow • Theory does not explain why sieve tubes should be living as opposed to the dead xylem