Biology : Transportation & Structure of Plants

Transportation happens in Plants . Further explanation within examples and pictures . Enjoy :)

1. TRANSPORT IN ANIMALS AND PLANTS

2. THE PRESSURE–FLOW HYPOTHESIS EXPLAINS TRANSLOCATION IN PHLOEM Current experimental evidence supports the translocation of dissolved sugar in phloem by the pressure–flow hypothesis, which was first proposed in 1926 by the German scientist Ernst Münch .

3. The pressure–flow hypothesis states that solutes (such as dissolved sugars) move in phloem by means of a pressure gradient—that is, a difference in pressure The pressure gradient exists between the source, where the sugar is loaded into phloem, and the sink, where the sugar is removed from phloem. According to the pressure flow hypothesis, translocation which takes place in three stages which involves a combination of active transport and mass flow.

4. First stage occur at the leaves (source) Occurrence at leaves where photosynthesis is carried out by mesophyll cells to produce organic substances. Then, glucose is produced during photosynthesis and condenses into sucrose which is a more suitable soluble substance for transport. The leave ,which sucrose and other organic solutes are actively loaded into sieve tubes. The loading is carried out by modified companion cells called transfer cells.

5. These modified companion cells contains numerous mitochondria to provide Adenosine Triphosphate (ATP) energy for the active loading and numerous ingrowths (internal projections) of their cell wall to increase surface area for more efficient loading. These transfer cells are used in active mechanism to load into sieve tubes by cotransport

6. The Route of sugar transport from mesophyll cells to sieve tube cells. At companion cells , sugars in cell walls are actively loaded into cytoplasm as known Symplast route . The sugar then moves into the adjoining sieve tubes elements through plasmodesmata . In some species, sugar transport involves a combination of symplast and apoplast routes and needs ATP energy to actively load sugar into sieve tube members.

7. SUGAR LOADING Sugar loading involves a chemiosmostic mechanism that uses a cotransport. Chemiosmostic mechanism is involved in the transport of sucrose from transfer cells into sieve tube. Proton pumps hydrogen ions (H+) out of the cell membrane resulting in a proton gradient across the membrane. A cotransport protein then carries the hydrogen ions (H+) down its concentration gradient back into the cell and sugar is transported into the cell as well.

8. The ATP supplies energy to pump protons out of the sieve tube elements, producing a proton gradient that drives the uptake of sugar through specific channels by the cotransport of protons back into the sieve tube elements. The sugar therefore accumulates in the sieve tube element. The increase in dissolved sugars in the sieve tube element at the source-a concentration that is 2 to 3 times as great as in surrounding cells-decreases (makes more negative) the water potential of that cell. As a result, water moves by osmosis from the xylem cells into the sieve tubes, increasing the turgor pressure (hydrostatic pressure) inside them.

9. Thus, phloem loading at the source occurs as follows:

10. At its destination (the sink), sugar is unloaded by various mechanisms , both active and passive, from the sieve tube elements. With the loss of sugar, the water potential in the sieve tube elements at the sink increases (becomes less negative). Therefore, water moves out of the sieve tubes by osmosis and into surrounding cells where the water potential is more negative . Most of this water diffuses back to the xylem to be transported upward . This water movement decreases the turgor pressure inside the sieve tubes at the sink.

11. Thus, phloem unloading at the sink proceeds as follows:

12. The Pressure Flow- Hypothesis Sugar is actively loaded into the sieve tube element at the source . As a result, water diffuses from the xylem into the sieve tube element . At the sink, the sugar is actively or passively unloaded, and water diffuses from the sieve tube element into the xylem . The pressure gradient within the sieve tube, from source to sink, causes translocation from the area of higher turgor pressure (the source) to the area of lower turgor pressure (the sink).

13. b) Second stage ( Translocated in the stem from source to the sink by Mass Flow) Sink- is a area where organic substances translocate from the source are used or stored.E.g .: stem tubes, tap roots, fruits and seeds. The concentration of sucrose in the sink is lower than that found in the source and sieve tubes as sugar is continuously being consumed or converted into starch to be stored . This allows sucrose to diffuse out of sieve tubes down the concentration gradient and its continuous flow from the source to the sink.

14. Mass Flow – Ernst Munch Mass flow is a physical process demonstrated by Ernst Munch in his famous osmometer experiment or model in 1930.

15. In mass flow, Munch’s model demonstrates that fluid flows from region of high hydrostatic pressure to region of low hydrostatic pressure. As fluid flow, it carried the whole mass of different substance. In osmometer A, concentrated sucrose solution (leaf) has lower water potential. Water flows into it from a high water potential region (xylem vessel) to a low water potential region (leaf cells) by osmosis. This create high hydrostatic pressure in A and forces sucrose solution to enter into the connecting tube (sieve tube) and pass to B (root cell) As the flow of mass from osmometer A to osmometer B continues, the sucrose solution is pushed along and finally appears in B.

16. In B, contain water / dilute sugar solution, water moves out from a higher water potential region by the hydrostatic pressure gradient produced and redistributed through connecting tube (xylem vessels) between the two container. Mass flow continues until the concentration of sugar solution in A and B are equal (balanced). In nature, equilibrium is not reached because solutes are constantly synthesized at source A and utilized at the sink B.

17. C) The third stage of Pressure Flow. The sucrose and other organic substances are actively unloaded at the sink (root cells) involving companion cells and energy. Here, sucrose converted into insoluble starch, used for cellular respiration and synthesis the cellulose of cell wall. Now, the water potential in the cell sap of the root cells is reduced. Water follows the organic solutes from the sieve tube into the root cells by osmosis. This reduces the hydrostatic pressure at the sink.

18. There exists a hydrostatic pressure gradient in the sieve tube which causes the passive mass flow of water and dissolved solutes from source to the sink region due to the differences in water potential between the leaves and roots. If sugar continues to be produces in the source and converted to starch or to be oxidized at the sink , the gradient will be maintained and the mass flow continues. The return of excess water from the sink (root) to the leaves (source) through xylem vessels is brought by transpiration pull.

19. Supporting The Pressure Flow Hypothesis There are different pieces of evidences that support the hypothesis. Firstly , there is an exudation of solution from the phloem when the stem is cut or punctured by the mouthparts of an aphid - A classical experiment demonstrating the translocation function of phloem, indicating that the phloem sap is under pressure  Phloem translocation is difficult to study in plants. Because phloem cells are under pressure, cutting into phloem to observe it releases the pressure and causes the contents of the sieve tube elements (the phloem sap) to exude and mix with the contents of other severed cells that are also unavoidably cut .

20. In the 1950s, scientists developed a unique research tool to avoid contaminating the phloem sap: aphids, which are small insects that insert their mouthparts into phloem sieve tubes for feeding The pressure in the punctured phloem drives the sugar solution through the aphid’s mouthpart into its digestive system . When the aphid’s mouthpart is severed from its body by a laser beam, the sugar solution continues to flow through the mouthpart at a rate proportional to the pressure in phloem . This rate can be measured, and the effects on phloem transport.

21. Secondly , concentration gradients of organic solutes are proved to be present between the sink and the source . Thirdly, when viruses or growth chemicals are applied to a well-illuminated (actively photosynthesizing) leaf, they are translocated downwards to the roots. Yet, when applied to shaded leaves, such downward translocation of chemicals does not occur, hence showing that diffusion is not a possible process involved in translocation.

22. Against The Pressure Flow Hypothesis  Some argue that mass flow is a passive process while sieve tube vessels are supported by companion cells. Hence, the hypothesis neglects the living nature of phloem. It is difficult to make measurements of transporting in the phloem due to disruptions caused to the phloem. In this hypothesis, substances cannot flow opposite directions in the same sieve tubes. In actual fact, phloem contains many sieve tubes and different solutes could travel in opposite directions at the same time in different sieve tube with different sources and sinks.

23. The Electro-osmosis Hypothesis This mechanism is proposed by Spanner Electroosmosis is the movement of ions in an electrical field through a fixed porous which is electrically charged by carrying water and any dissolved solutes.

24. The sieve plates and phloem protein are normally negatively charged, thus forming a fixed porous surface with an electrical charge As mass flow occur through the negatively charged sieve plates, anions will be repelled but cations will be able to pass through. When mass flow occur downwards through the phloem, the repulsed anions will accumulates above the sieve plates so that the cell above the sieve plate will become negative. The sieve plate will now be a fixed porous surface within a electrical field, such as is needed for electroosmosis to occur. When a critical potential difference across the sieve plate is reached , protons (H+ ions) surge from the wall of upper cell into its cytoplasm by lowering its pH and making the cytoplasm above the sieve tube to positively charged.

25. The increased positive charged generated by the H+ surge pushes other cations mainly ( K+) by electrical repulsion, through the sieve plate from the upper to the lower cell and therefore, electroosmosis occurs. The Potassium ions are then secreted on the other side of the sieve plates. The presence of Positive K+ ions on the other side of the sieve plate induces negative charges on the other side of the sieve creating a potential different across the sieve plates. This surge the hydrated Potassium ions carries water molecules and dissolved solutes like sucrose across the sieve plates. With this energy causes an electro-osmosis flow of polar water molecules and dissolved solutes through the sieve pores to the adjacent sieve tube element.

26. Supporting Electro-Osmosis The energy derived from potential difference shows that translocation is an active transport. The presence of porous sieve plates in all sieve tubes of plants. High concentration of K+ ions have been found in phloem sap.

27. Against Electro-Osmosis There is a lack of evidence and no detailed mechanism of how substance or solutes move between the sieve plates.

28. The Cytoplasmic Streaming Hypothesis Was proposed by Thaine in 1962. The cytoplasm of plant cells is often observed to move around within the cell, a process called Streaming It has been proposed that solutes might be carried from one end of a sieve tube element to the other by streaming and then transferred across sieve plates by active transport. Both streaming and transfer through the sieve plates would be energy-dependent, explaining high turnover of ATP in phloem cells Different solutes move through the sieve pore at different rates due to their different molecular characteristics and level of impermeability of sieve plates to different solutes.

29. Supporting C.S.H Upwards and downwards movement of solutes that occur within the confines of the sieve tube explains the two way flow substance. Low temperature and metabolic poison affect cytoplasmic streaming indicating an active process is involved.

30. Against C.S.H Cytoplasmic streaming is not enough to account for the rate of translocation observed in phloem. Cytoplasmic streaming has been observed in immature sieve tube only.

31. Peristalsis Waves Transcellular strands contain contractile protein which are present in some phloem sieve tubes . sieve tube  is filled with  fine cytoplasmic filaments continuous  from  sieve tube  to the next thru pores  of  sieve plate . contain phloem sap tube  constrict + relax alternately pushing sap  from one sieve tube to the  next. constriction + relaxation/peristaltic movement form a pattern of wave = peristaltic wave can be at  diff speed + in opposite  direction (in sieve tube) depends on metabolic energy/ATP . Their rhythmic contraction produces peristalsis waves that facilitate the long-distance transport of solutes in phloem.