Experiments were conducted on three species studied by van Bel and Hess with especially high hexose: The sugar is actively transported out of mesophyll cells into the cell walls of the minor veins apoplast.
This suggests that the sugars in the EDTA solution represent compounds extracted from the petioles, rather than sugars in transit in the phloem.
Active phloem loading may not be universal as there are many indications of passive loading at least in tree species Rennie and Turgeon, ; Turgeon, b. Water moves down this gradient.
This process is termed translocation, and is accomplished by a process called phloem loading and unloading. This is achieved by maintaining high solute concentrations in the mesophyll cells of such species.
However, active theories suggesting involvement of energy are proposed to explain translocation in phloem.
The leaves are referred to as the source, and the shoot and root tips - sink. Non-living vessel elements and tracheids Phloem: The different data are discussed in relation to the phloem transport pathways. However, almost identical results were obtained when the leaf blades were removed and only petiole stubs were immersed.
Since the analyses focused specifically on radiolabelled compounds, it was clear that these substances had been exported from the leaf blade and were in transit down the petiole in the phloem.
In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells.
Results 14CO2 pulse—chase experiments on attached leaves To identify sugars that are mobile in the sieve tubes, blades of mature, attached leaves were exposed to 14CO2 and radiolabelled transport compounds were given time during a chase period to migrate into the petioles see Materials and methods.
This video provides a concise overview of sugar sources, sinks, and the pressure flow hypothesis: Experiments to demonstrate bidirectional movement in a single sieve tube are technically very difficult to perform. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap.
This is achieved by maintaining high solute concentrations in the mesophyll cells of such species.
This mechanism may therefore have allowed plants to colonise the cooler locations. The first osmometer contains a solution more concentrated than the solution surrounding it, and the, second osmometer contains a solution loss concentrated than the solution surrounding it: As a result, concentration of sucrose increases in the sieve tube elements.
The Problems with Pressure Flow The pressure flow hypothesis visualizes a steady, unidirectional flow of sugar solution through living sieve elements. Unloading at the sink end of the phloem tube can occur either by diffusion, if the concentration of sucrose is lower at the sink than in the phloem, or by active transport, if the concentration of sucrose is higher at the sink than in the phloem.
If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the. From s to the mids, the mechanism of phloem translocation was a subject of research.
Now, one theory is generally accepted as the correct explanation for translocation. This theory called the pressure flow hypothesis is favoured by most plant physiologists and was proposed by E. Munch in Germany in Translocation of soluble products of photosynthesis occurs in the part of the vascular tissue known as the phloem.
The phloem is the principle food conducting tissue associated with xylem in the vascular system.
Jul 24, · Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem.
However, sugar transport through the phloem can be. 02 II (2) | (a) The figure below shows the initial set-up of a model of the mass flow hypothesis for translocation in the lookbeyondthelook.com of mass flow hypothesis (i) It was observed that the dye moved from osmometer X to osmometer Y during the course of the experiment.
Explain this observation according to the mass flow hypothesis.
Translocation Assimilates (sucrose and amino acids) move between sources (leaves and storage organs) and sinks (buds, flowers, fruits, roots and storage organs) in phloem sieve tubes in a process called translocation.
The products from the source are usually translocated to the nearest sink through the phloem.Explain translocation sucrose occurs phloem proposed mass