pressure flow hypothesis science definition

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Mass Flow Hypothesis

Table of Contents

Introduction

The Mass Flow Hypothesis was first proposed by German plant physiologist Ernst Munch in the year 1930. He theorised the movement of sap through the phloem tissue in plants. This theory is also known as the Pressure Flow Hypothesis. A highly concentrated organic sugar especially sugar in the cells of the phloem from a source like a leaf forms a diffusion gradient which draws water into the cells of phloem tissue from the adjacent xylem. This develops turgor pressure in the phloem, which is also called hydrostatic pressure.

Phloem movement occurs by mass flow from sources of sugar to sugar sinks. The phloem movement is bidirectional but unidirectional in xylem cells. Due to this multidirectional flow, it is not uncommon for sap in the sieve tubes besides to move in opposite directions based on the fact that sap cannot travel easily between adjacent sieve tubes.

When the movement of minerals and water via the xylem is driven mostly by negative pressure and movement via phloem is driven by hydrostatic pressure. This process is called translocation and is accompanied by a process known as phloem loading and unloading. Cells in sugar sources load a sieve tube by osmosis developing pressure that pushes the sap low. The cells deliver solutes out of the elements of sieve tube and produce opposite effects. The sugar gradient from the source creates pressure-flow via the sieve tube towards the sink.

• Glucose is formed by photosynthesis in the cells of mesophyll and some glucose is utilized in the cells during respiration. The leftover glucose is transformed into non-reducing sugar.
• Sucrose is delivered to the neighbour cells of minute veins of the leaves.
• Sucrose diffuses from neighbour cells to the elements of the sieve tube via plasmodesmata. Hence, the amount of sucrose rises in the elements of the sieve tube.
• Water travels from the close xylem to the leaf vein by osmosis and raises the hydrostatic pressure of the elements of the sieve tube.
• The Hydrostatic pressure shifts the sucrose along with other substances via the cell of the sieve tube towards the sink.
• In storage sinks, sucrose is eliminated into the apoplast before entering the sink’s symplast.
• The water travels out of the cells via osmosis and lowers the hydrostatic pressure in them. Hence, a gradient of pressure is developed as a result of the entry of sugar at the source and elimination of sucrose at the sink.
• The phloem sugar is eradicated by the cortex of the root and stem and utilized by cellular respiration. The starch is insoluble and does not exert any osmotic effect. Ultimately, pure water is left and drawn into xylem vessels by transpiration pull.

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what observations were not explained in the Munch’s mass flow

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The Pressure Flow or Mass Flow Hypothesis

In Plants, the system made sense of the movement of sugars from source to sink alludes to the strain stream speculation. Glucose produced by photosynthesis at the source is switched over completely to sucrose which is moved to the phloem sifter tube cells through the friend cell by dynamic vehicle. The excess vehicle in the phloem is done by the distinctions in the osmotic strain, which works with the vehicle from higher strain to bring down pressure areas of the sink. Again by dynamic vehicle, the sucrose is shipped from phloem to the cells of the sink where it will be put away for a really long time. Plant transport tissues – xylem and phloem. Plants have two vehicle frameworks – xylem and phloem. Xylem transports water and minerals. Phloem transports sugars and amino acids broken up in the water.

Mass Flow of Hypothesis

When the movement of minerals and water via xylem is driven mostly by negative pressure and movement via phloem is driven by hydrostatic pressure. This process is called translocation and is accompanied by a process known as phloem loading and unloading. Cells in sugar sources load a sieve tube by osmosis developing pressure that pushes the sap low. The cells deliver solutes out of the elements of the sieve tube and produce opposite effects. The sugar gradient from the source creates pressure-flow via the sieve tube towards the sink.

• 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. 
• Sucrose is delivered to the neighbor cells of minute veins of the 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
• Water travels from the close xylem to the leaf vein by osmosis and raises the hydrostatic pressure of the elements of the sieve tube
• This hydrostatic pressure turgor then shifts the sugars and other substances down the cells of the sieve tube towards the sink (roots).
• In storage sinks, sucrose is eliminated into the apoplast before entering the sink’s symplast.
• The phloem sugar is eradicated by the cortex of the root and stem and utilized by cellular respiration. The starch is insoluble and does not exert any osmotic effect. Ultimately, pure water is left and drawn into xylem vessels by transpiration pull.
• The xylem transports water and minerals from the roots up the plant stem and into the leaves.
• In a developed blossoming plant or tree, the majority of the phones that make up the xylem are particular cells called vessels. Vessels:
• Lose their end walls so the xylem shapes a consistent, empty cylinder. This permits water to handily stream.
• Become fortified by a compound called lignin. The cells are as of now not alive. Lignin invigorates and backs to the vessel.
• Transport in the xylem is an actual cycle. It doesn’t need energy.

Phloem moves sugar that the plant has delivered by photosynthesis to where it is required for cycles, for example, developing pieces of the plant for guaranteed use capacity organs like bulbs and tubers

creating seeds breath. 

• Transport in the phloem is consequently both all over the stem. 
• The transport of substances in the phloem is called movement.
• Strainer tubes – are particular for transport and have no cores. Each sifter tube has a punctured end so its cytoplasm interfaces cell to cell.
• Sidekick cells – transport of substances in the phloem requires energy. At least one friend’s cell appended to each sifter tube gives this energy. A sifter tube is totally reliant upon its buddy cell(s).

The xylem and phloem are dispersed diversely in roots and stems. In the root, the xylem shapes a focal section, framing a strong help. The phloem is towards the middle, outside the xylem. In the stem, the vehicle tissues of the xylem and phloem are assembled into vascular groups. Correlation of transport in the xylem and phloem

Conceptual Questions

Question 1: Make sense of the mass stream speculation of transport in the phloem?

The Pressure Flow Hypothesis is otherwise called the Mass Flow Hypothesis . It is the most acknowledged hypothesis of the development of food through the phloem. it was proposed by Ernst Munch in the 1930s. A high centralization of glucose in the cells of phloem at the source sets up the osmotic slope. This outcome in the development of water from the xylem into the phloem. After this, the phloem sap moves sugar from source to sink as a result of turgor pressure. Since pressure is involved subsequently it Is called pressure stream speculation. The development of substance happens In mass and consequently, this is additionally called mass stream speculation.

Question 2: What do you mean by Xylem?

Xylem alludes to the extremely durable and dead tissue that conveys water and fundamental substances in the plant. The essential capability of xylem is the transportation of water, dissolvable supplements, inorganic particles, and minerals. Moreover, this transportation occurs in an upwards way from the roots to different pieces of the plants.

Question 3:  What do you mean by Phloem?

Phloem alludes to a complicated tissue that transports solvent natural mixtures all through the vascular plants. Phloem utilizes energy and turgor strain to ship sugars and different substances to different organs of the plant. One distinction between xylem and phloem is that the previous is a non-living tissue while the last option is living.

Question 4: Write two differences between xylem and phloem.

Xylem is known as the permanent and dead tissue which tends to carry both water and essential substances inside the plant. The primary function of the xylem is known to be to transport water, minerals, inorganic ions, and soluble nutrients. Also, this transportation occurs in an upward movement from the roots to the different parts of the plant.  Phloem , on the other hand, is known as a complex tissue that helps in the transportation of the soluble organic compounds, all throughout the different parts of the vascular plants. Phloem tends to use the energy and turgor pressure for transporting the sugars and many other substances to several parts of the plant.

Question 5: What is the function of the xylem in ferns?

The function of the xylem in any plant is to transport water and minerals from the root to all aerial parts of the plant body.

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The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the best-supported theory to explain the movement of sap through the phloem. It was proposed by Ernst Munch, a Germany plant physiologist in 1930. A high concentration of organic substances, particularly sugar, inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem. This creates turgor pressure, also known as hydrostatic pressure, in the phloem. Movement of phloem sap occurs by bulk flow (mass flow) from sugar sources to sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

1. Sources and Sinks

A sugar source is any part of the plant that is producing or releasing sugar.

During the plant's growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks.

After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks.

2. Mechanisms

While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressure. This process is termed translocation , and is accomplished by a process called phloem loading and unloading . Cells in a sugar source "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect. The gradient of sugar from source to sink causes pressure flow through the sieve tube toward the sink.

The mechanisms are as follows:

• Glucose is produced by photosynthesis in the mesophyll cells of green leaves. Some glucose is used within the cells during respiration. The rest of the glucose is converted into non-reducing sugar i.e. sucrose. It has been shown that the sucrose concentration in sieve tubes in leaves is commonly between 10 and 30 percent whereas it forms only 0.5% solution in the photosynthesis cells.
• The sucrose is actively transported to the companion cells of the smallest veins in the leaves.
• The sucrose diffuses through the plasmodesmata from the companion cells to the sieve tube elements. As a result, concentration of sucrose increases in the sieve tube elements.
• Water moves by osmosis from the nearby xylem in the same leaf vein. This increases the hydrostatic pressure of the sieve tube elements.
• Hydrostatic pressure moves the sucrose and other substances through the sieve tube cells, towards a sink.
• In the storage sinks, such as sugar beet root and sugar cane stem, sucrose is removed into apoplast prior to entering the symplast of the sink.
• Water moves out of the sieve tube cells by osmosis, lowering the hydrostatic pressure within them. Thus the pressure gradient is established as a consequence of entry of sugars in sieve elements at the source and removal of sucrose at the sink. The presence of sieve plates greatly increases the resistance along the pathway and results in the generation and maintenance of substantial pressure gradients in the sieve elements between source and sink.
• The phloem sugar is removed by the cortex of both stem and root, and is consumed by cellular respiration or else converted into starch. Starch is insoluble and exerts no osmotic effect. Consequently, the osmotic pressure of the contents of phloem decreases. Finally relatively pure water is left in the phloem and this is thought to leave by osmosis or be drawn back into nearby xylem vessels by suction of the transpiration pull.

The pressure flow mechanism depends upon:

• Turgor pressure
• Difference of osmotic pressure gradient along the direction of flow between the source and the sink.

3. Evidence

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. 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 photosynthesising) 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.

4. Criticisms

Opposition or criticisms against the hypothesis are often voiced. 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. Moreover, it is found that amino acids and sugars (examples of organic solutes) are translocated at different rates, which is contrary to the assumption in the hypothesis that all materials being transported would travel at uniform speed. Bi-directional movements of solutes in translocation process as well as the fact that translocation is heavily affected by changes in environmental conditions like temperature and metabolic inhibitors are two defects of the hypothesis.

An objection leveled against the pressure flow mechanism is that it does not explain the phenomenon of bidirectional movement i.e. movement of different substances in opponent directions at the same time. The phenomenon of bidirectional movement can be demonstrated by applying two different substances at the same time to the phloem of a stem at two different points, and following their longitudinal movement along the stem. If the mechanism of translocation operates according to pressure flow hypothesis, bidirectional movement in a single sieve tube is not possible. Experiments to demonstrate bidirectional movement in a single sieve tube are technically very difficult to perform. Some experiments indicate that bidirectional movement may occur in a single sieve tube, whereas others do not.

5. Other Theories

Some plants appear not to load phloem by active transport. In these cases a mechanism known as the polymer trap mechanism was proposed by Robert Turgeon. [ 1 ] In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinose and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.

The symplastic phloem loading is confined mostly to plants in tropical rain forests and is seen as more primitive. The actively transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, and particularly in those in temperate and arid conditions. This mechanism may therefore have allowed plants to colonise the cooler locations.

Organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs are transported in the phloem through sieve tube elements.

• Turgeon, R (1991). "Symplastic phloem loading and the sink-source transition in leaves: a model". Recent Advances Phloem Transport and Assimilate Compartmentation. 

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Mass Flow Hypothesis: Definition, Diagram, Mechanism Of Transportation

What is the mass flow hypothesis.

The Mass Flow Hypothesis is also known as the Pressure Flow Hypothesis. It is one of the major hypotheses of plant biology, explaining how nutrient transportation, mainly sugars, takes place through the phloem tissue in plants. It was put forward by German plant physiologist Ernst Munch in 1930 and elaborates on how the sap moves from areas of high concentration, called sources, to areas of low concentration, called sinks. The basis of the process is the diffusion gradient that draws water into the phloem, thus generating the needed hydrostatic pressure that drives the flow of sap. Knowing how this works is most important to understanding how plants distribute essential nutrients for growth and development.

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Compared to the negative pressure that drives water through the xylem, hydrostatic pressure drives nutrient movement through the phloem. This nutrient transport process is called translocation, and it involves the following major steps:

Phloem Loading

Glucose produced through photosynthesis in the source tissues, primarily the leaves, is changed into sucrose.

This sucrose is actively transported into the sieve tube elements of the phloem.

When sucrose starts to collect, high solute concentrations build up in the sieve tubes and decrease their water potential.

The Influx Of Water

Because of this high concentration of sucrose, water from the nearby xylem moves into the phloem through osmosis, creating an osmotic gradient.

This influx of water raises the turgor pressure inside the sieve tubes, developing hydrostatic pressure.

The pressure that is developed in the phloem is hydrostatic and squeezes the sap from the source to the sink.

It's a two-way flow, which enables the nutrition transport to all plant parts according to needs.

Phloem Unloading

At the sink, sucrose is actively pumped out of the sieve tubes into the surrounding cells where it can be used for growth or stored.

Such unloading reduces the concentration of solutes in the sieve tubes, which then reduces the turgor pressure.

Water Movement

As sucrose is unloaded, water leaves the sieve tubes to re-enter the xylem by osmosis, further reducing the hydrostatic pressure in the phloem.

This produces a continuous pressure gradient that allows for the continual flow of sap from the source to the sink.

Overview Of Mechanism

The mechanism involved in the mass flow hypothesis is:

Photosynthesis

Glucose is produced in mesophyll leaf cells.

Conversion into Sucrose

Excess glucose is converted into transportable sucrose.

Sucrose Transport

Due to the presence of plasmodesmata, sucrose diffuses into sieve tubes from the adjacent cells.

Water enters the phloem from the xylem, adding to hydrostatic pressure.

Sap Movement

The pressure pushes the flow of the sap toward the sinks. There, the sucrose is unloaded.

Water leaves the phloem; as a result, the pressure drops, thus keeping the gradient in the flow.

The Mass Flow Hypothesis provides a comprehensive framework for understanding the transport of nutrients in plants. This hypothesis is an explanation of mechanisms for nutrient distribution, wherein a pressure gradient and osmotic movements are identified as factors involved in phloem functioning. Even with some criticisms at times due to the oversimplification of the transport process, the Mass Flow Hypothesis remains one of the important concepts in plant physiology, explaining how plants efficiently manage the distribution of essential nutrients.

Recommended video on Mass Flow Hypothesis

Frequently Asked Questions (FAQs)

The flow of nutrients and water in the phloem of plants due to hydrostatic pressure caused by osmotic gradients is explained by the Mass Flow Hypothesis.

A German plant physiologist, Ernst Munch, had proposed the hypothesis in the year 1930.

The source is the term given to the parts of the plant where sugars are produced, while sink refers to areas where sugars are either being used or stored, root for example.

The critics consider that it is an oversimplification of the transport process and ignores the active role of companion cells and also the differences in the rates of nutrient transport.

This would go a long way in explaining how the plants distribute their nutrients while maintaining, at the same time, physiological functions very essential to them for growth and survival.

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The Pressure Flow or Mass Flow Hypothesis

It is the hypothesis by which transport of sap by phloem can be best described. This hypothesis was given by Ernst Munch in 1930 who was an German plant physiologist. According to this hypothesis a high concentration of organic substances like sugar, inside the phloem of a leaf creates an osmotic gradient or diffusion gradient that draws water from the adjacent xylem to the cells. This is called a turgor pressure or also called hydrostatic pressure in the phloem. Movement of the phloem saps take place from the mass flow of sugar source to the sugar sinks. Movement of food by the phloem is bi directional and xylem is upward always means uni directional.

Mechanism of the Pressure Flow Hypothesis:

Movement of water and minerals are driven by negative pressure force in the xylem most of the time and movement of the substances by the phloem is by the positive hydrostatic pressure. This process is called translocation and it takes place by the loading and unloading of the phloem. Cells of the sieve elements are loaded by the sugar due to the active transport- of the solute and the movement through the sieve tube is osmosis. This movements create pressure that pushes the sap down the tube. In sugar sinks cells actively transported solutes out of the sieve tube elements that creates opposite effect.

Mechanism of the Pressure Flow Hypotheses -

Steps are -

1. Some of the glucose that produce during respiration are used by the plants whereas other glucose converted into starch and stored at different parts of the plants.

2. The sucrose is then actively transported to the companion cells of the smallest veins in the leaves.

3. The sucrose is then transported through the plasmodesmata from the companion cells to the sieve tubes which increases sucrose concentration. Thus sucrose concentration increases in sieve tubes.

4. Hydrostatic pressure of sieve elements are increased as water flows from the near by xylem to the veins of the same leaf.

5. Hydrostatic pressure causes movement of this from the sieve towards the sink.

6. In those storage sink large amount of sugar is converted into apoplast prior to the entering of the symplast. Examples are Beet root, sugar cane stem etc.

7. Water then moves out from that area which causes lowering of hydrostatic pressure. This pressure gradient is created between entry of sugar in the sieve elements at the source and removal of sucrose at the sink. This thing creates a continuous pressure gradient throughout the pathway by maintaining resistance along the pathway.

8. Phloem sugar is removed from the cortex of both stem and root and is digested by cellular respiration, converting into starch. As starch is insoluble in water this it has no effect on osmotic pressure. At last the pressure is decreases and the comparatively pure water is left in the phloem.

Dependency of the pressure flow mechanism- Pressure flow mechanism depends upon the –

1. Turgor pressure.

2. Difference between the osmotic pressure gradient created that flow between the source and the sink.

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