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.

Mechanism

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.

Mass Flow

• 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 Movement

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.

Osmosis

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

Conclusion

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.

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