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 transport 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 of the Mass Flow Hypothesis

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.

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 (Pressure Flow)

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

• It is 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 Back to Xylem

• 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:

Source and Sink Concept

Source is any vegetative body part or organ of a plant from which the sugars are produced or mobilized for transport. For example, leaves, the photosynthetic organs where the sugars are produced.

Sink is any tissue or organ that consumes or stores sugars. For example, roots, fruits, seeds and organs like tubers and bulbs.

Differences Between Xylem and Phloem Transport

The differences between xylem and phloem transport are:

Significance of Mass Flow Hypothesis

The significance of mass flow hypothesis is:

• Explains how the products of the photosynthesis are distributed.

• Provides energy supply for the tissues which do not perform photosynthesis.

• Ensures growth between sources and sinks.

• Highlights the role of phloem as an active and living transport system.

Limitations of the Hypothesis

The significance of mass flow hypothesis is:

• Explains the unidirectional flow but in plants it is usually bidirectional.

• It does not account for an active role of cytoplasm and strongly holds that phloem translocation is a physical process. Phloem loading and translocation are active processes.

• It is generally held that sieve pores remain plugged with dense protoplasm preventing the mass flow of the solute molecules.

• According to this hypothesis high turgor pressure is needed to overcome the resistance at the cross wall plasmodesmata which is not possible.

Mass Flow Hypothesis NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Mechanism of Mass Flow Hypothesis

• Significance and limitation of Mass Flow Hypothesis

Practice Questions for NEET

Q1. Active absorption of mineral ions causes

1. High water potential in root

2. High water potential in soil

3. Low water potential in soil

4. None of these

Correct answer: 2) High water potential in soil

Explanation:

The water potential gradient in roots and, consequently, the uptake of water by osmosis are partially caused by the active uptake of ions. Maintaining cell turgor, aiding nutrient transfer, and sustaining physiological processes like transpiration and photosynthesis all depend on a steady supply of water, which is ensured by this process.

Hence, the correct answer is option 2) High water potential in soil.

Q2. Which of the following causes passive mineral absorption?

1. Pump proteins

2. Donnan equilibrium

3. Transfer proteins

4. All of these

Correct answer: 2) Donnan equilibrium

Explanation:

Mineral ions are absorbed passively without any use of energy. It is independent of metabolic inhibitors. This is explained by some theories, among which are the following:
Contact Exchange Theory: Direct exchange between soil and root surfaces by ions.
Carbonic Acid Exchange Theory: Involves carbonic acid in ion facilitation.
Donnan Equilibrium: A passive process that maintains charge equilibrium in the absence of active transporters but still enables the movement of mineral ions into plant roots across concentration gradients.

Hence, the correct answer is option 2) Donnan equilibrium.

Q3. What is the role played by the endodermal cells?

1. They facilitate passive absorption of mineral ions

2. They adjusts the quantity and types of solutes that reach the xylem

3. They actively transport ions in one direction only

4. Both b and c

Correct answer: 4) Both b and c

Explanation:

Active absorption of mineral ions occurs in root hair cells, where specific proteins are involved in pumping the ions from the soil into the cytoplasm of epidermal cells. The transport proteins within endodermal cells facilitate selective entry of only certain solutes through their plasma membranes, thus forming control points for the regulation of ions. The endodermis contains a suberin layer which must ensure active transport in only one direction; that is, minerals needed by the plant are being directed towards the xylem for transport upwards in the plant.

Hence the correct answer is option 4) Both b and c.

Also Read:

• MCQs on Uptake and Transport of Minerals

• Mass Flow Hypothesis

• Nitrogen Cycle

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