This theory was proposed in 1930 by German plant physiologist, Ernst Munch. The mass flow hypothesis states that the translocation of glucose and other sugars within the phloem is caused by a never-ending flow of water and dissolved nutrients between the source (a place where sugars are made) and sink (where sugars are utilized). A diffusion gradient or osmotic gradient is caused by the phloem source cells due to the high concentration of sugar and other organic substances. This results in water being drawn out from adjacent xylem and hydrostatic pressure which moves the sap. This theory is the closest explanation available to describe mass flow in phloem.
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Translocation is the movement of minerals and water through the xylem as a result of negative pressure and movement through the phloem as the result of hydrostatic pressure. This movement is accompanied by the process of phloem loading and unloading.
The cells present in food sources load a sieve tube through osmotic potentials, pushing the sap lower. These cells then deliver the solutes out of the sieve-tube producing the unloading effect.
When asked to explain mass flow hypothesis in details, this is the answer:
Plants respire through the process of photosynthesis, which involves the formation of glucose in mesophyll cells. Not all of the sugars are utilized; leftover glucose becomes non-reducing sugar.
Sugars like sucrose are then delivered to the neighbor cells present in the veins of leaves.
Through the plasmodesmata, the sugars diffuse from neighbor cells and enter the sieve tube. The number of sugars within the sieve tube and its elements increase. This is the beginning of the mass flow of the hypothesis mentioned.
At the same time, water travels from nearby xylem to the source cells via osmosis to increase hydrostatic pressure in the elements of the sieve tube. This is the mass flow hypothesis for the mechanism of translocation in plants.
This hydrostatic pressure turgor then shifts the sugars and other substances down the cells of the sieve tube towards the sink (roots).
At the sink, the sugars are destroyed to become apoplast and then enter the symplast.
The water that traveled out of the cells reduces the hydrostatic pressure in them. The gradient created is the result of the pressure developed by the entry of sugar and the source and the elimination of it at the sink. Learning mass flow hypothesis at a level is more complex.
The root cortex and stem eliminates the phloem sugar and uses it for cellular respiration. Starch does cause any pressure as it is insoluble. At the end through transpiration pull the pure water left is absorbed.
For mass flow hypothesis a level biology the concepts are more advanced and detailed. If you are interested in learning more, read high-level books, and talk to experts!
To explain the Pressure Flow Hypothesis in Brief: The transport begins in sieve tubes at source regions (the place where photosynthesis takes place) after photosynthesis. The creation of food raises osmotic pressure within the sieve tube. This removes the sugars from sieve tubes towards the sink regions (the place where utilization of materials takes place) to lower the pressure. A potential is created from the source to the sink within the sieve tubes, in which the solution flows.
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The majority of people rejecting this hypothesis believe that mechanisms such as the mechanism of translocation in plants take place due to metabolic processes, not hydrostatic pressure. Critics believe that mass flow is mainly a passive process and companion cells support sieve tube vessels. This indicates that the hypothesis negates phloem's living nature.
The hypothesis also states that the rate of transport of materials is uniform which has been disproved. Amino acids and other nutrients like sugars are translocated at different rates throughout.
Also, the Munch mass flow hypothesis only accounts for the unidirectional movement of pressure flow. It doesn't explain the bilateral movement phenomena of materials moving at the same time in opposite directions. If the hypothesis is accepted, then that would mean that the bidirectional movement isn't possible, which is false.
The mass flow hypothesis mechanism was sourced from Ernst Munch’s book Die Stoffbewegungen in der Pflanze (translated to The Movements of Matter in Plants), with 230 pages dedicated to just fluid circulation within plants. Interestingly, Munch claimed that his concept was nothing novel; his famed colleagues had understood it before he did. No other botanist has formulated a hypothesis like Munch’s, however. This is why his name is taken synonymously with this hypothesis.
Q1. What are the other theories explaining the active transportation in Phloem?
Ans. It has been found that not every plant utilizes the phloem for active transportation. The most popular theory proposed besides the mass flow hypothesis in plants is Robert Turgeon's polymer trap mechanism theory. According to this, small sugars like sucrose flow through the narrow plasmodesmata in intermediate cells and get polymerized into raffinose and other oligosaccharides.
From here they move through the sieve tube vessels, much like in the mass flow hypothesis. This mechanism is found to take place in tropical regions, particularly more primitive plants. The active transportation mechanism is found in more evolved plants in temperate and arid regions.
Q2. Briefly describe the mass flow hypothesis for the mechanism of translocation in Plants.
Ans. To define mass flow hypothesis: It is a theory explaining the transportation of food through the phloem as a result of gradients from high concentration to low concentration places. The force used to push these materials down is the result of fluctuating osmotic potential through the phloem. This creates a turgor or hydrostatic pressure within the source and sinks tissues.
In a simple manner, this is how the Munch mass flow hypothesis explains the phenomena: The leaves are the end, that is, the place where the food is synthesized and produced. The roots are the sink end, where the food is absorbed and utilized. In an experiment with two osmometers with different concentrations, it was seen that the theory is right about the movement of food from the leaves to the roots.