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Mass Flow Hypothesis

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What is the Mass Flow Hypothesis?

This theory was proposed by German plant physiologist Ernst Munch in 1930. It is stated by the mass flow hypothesis that the translocation of glucose and other sugars within phloem are 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 due to the high concentration of sugar and other organic substances by the phloem source cells. This results in water being drawn out from adjacent xylem and hydrostatic pressure which moves the sap. Above stated theory is the closest explanation available to describe mass flow in the phloem. 

Mechanism- How it Works

The mechanism of 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. 

Description of the Mass Flow Hypothesis 

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 the mass flow hypothesis in detail, 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 the 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 eliminate the phloem sugar and use 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 want to learn more, read high-level books, and converse with experts.

The Pressure Flow Hypothesis

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.

Criticisms of the Mass Flow Hypothesis

The majority of people that reject 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 the 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. 

Benefits of the Mass Flow Hypothesis Article

If you are preparing for the NEET exam and are looking forward to cracking it on the first attempt, you may come across various topics and concepts which will be very hard to comprehend. One such topic is the Mass Flow Hypothesis. If you want to understand the terminologies, concepts used in this topic, you can refer to this article. In this article, we have covered: 

  • Mass Flow Hypothesis states

  • who gave this hypothesis

  • what is the mechanism behind the translocation of sap

  • why was this theory criticized, etc

Via this article, you will be able to understand every term. This article will help you to clear your concept and if any question is asked from this topic, you will be easily able to answer that. Students who are unable to grasp some advanced terms should not worry, because Vedantu has got them covered. At Vedantu, they will be able to find different resources relevant to the Mass Flow Hypothesis. For example, they will find video lectures explaining this topic and in that video, everything is explained very deeply about the Mass Flow Hypothesis. 

Mass Flow Hypothesis Theory

The mass flow hypothesis is a theory that tells about how the sap flows through the phloem. This theory or hypothesis was first proposed in 1930 by German physiologist Ernst Munch. This theory tells us how simple sugar or a sugar that is highly concentrated organic sugar, is transported to other parts of a plant. It is based on the principle of hydrostatic pressure. If we take an example of a leaf, the phloem from that leaf creates a diffusion gradient with the help of which, it can draw water from the adjacent xylem. In phloem, the movement of sugar and other materials occurs by mass flow. This movement is bidirectional which means the mass can flow in both directions. This multi-directional flow doesn't let the sap flow in the opposite directions. It is usually seen that the movement of materials via the phloem is driven by positive pressure (hydrostatic). The same movement through the xylem is because of negative pressure or tension. This theory states that Glucose and other sugars are translocated in the phloem. This movement is caused because of the pressure created by the flow of water and nutrients. 

Mechanism Of Mass Flow Hypothesis 

In the xylem, the flow of nutrients, minerals, and water are caused because of the creation of negative pressure, as discussed earlier but the movement via the phloem is caused by the positive pressure (hydrostatic) this also results in the loading and unloading of phloem. 

Simple Explanation of the Munch Mass Flow Hypothesis

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. 

Fun Fact

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 yet. This is the reason why his name is taken synonymously with this hypothesis.

Last updated date: 26th Sep 2023
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FAQs on Mass Flow Hypothesis

1. Why was the Mass Flow Hypothesis criticized by most people?

The Mass Flow Hypothesis faced criticism from a majority of people. This happened because most people believed that the translocation of particles in plants is not due to the hydrostatic pressure, it is rather due to the metabolic processes. Critics also disapproved of the statement of the Mass Flow Hypothesis according to which, the rate of the transport of materials in plants is uniform throughout.

2. How does the process of transportation take place in plants?

There are two tissues in plants that are responsible for the transportation of materials in plants namely the xylem and phloem. Xylem is responsible for the transportation of water and minerals and phloem is responsible for the transportation of food. Roots absorb water and minerals from the soil which are transported via the xylem to the leaves where these are used in the process of photosynthesis. The food made with the help of photosynthesis is then transported to various parts of a plant via Phloem.

3. How will I be able to learn about the Mass Flow Hypothesis?

To learn about the Mass Flow Hypothesis, you can refer to the article above. In this article, we have provided all the details about this topic. You can refer to Vedantu's study notes and videos lectures as well, both of which will help you to get a strong grip on this topic. All these materials are available on the Vedantu website or its app and you can get them with a single click, once you sign up for Vedantu.

4. What are the other theories explaining the active transportation in Phloem?

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.

5. Briefly describe the mass flow hypothesis for the mechanism of translocation in Plants.

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.

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