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    which statement about the pressure flow model of fluid flow in phloem is true?

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    Pressure flow hypothesis

    Pressure flow hypothesis

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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.[1][2] It was proposed by Ernst Münch, a German plant physiologist in 1930.[3] 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 to . 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.

    Contents

    1 Sources and sinks 2 Mechanisms 3 Evidence 4 Criticisms 5 Other theories 6 References

    Sources and sinks[edit]

    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.

    Mechanisms[edit]

    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 , and is accomplished by a process called and . 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 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.

    Evidence[edit]

    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 Stylet 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.

    Criticisms[edit]

    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.

    Source : en.wikipedia.org

    Which of the following is not correct with respect to mass flow hypothesis?

    Click here👆to get an answer to your question ✍️ Which of the following is not correct with respect to mass flow hypothesis?

    Question

    Which of the following is not correct with respect to mass flow hypothesis?

    A

    Loading of phloem sets up a water potential gradient that facilitates the mass movement in phloem.

    B

    The sugar which is transported in phloem sieve.

    C

    The sugar is moved bidirectionally

    D

    As hydrostatic pressure in phloem sieve tube increases, pressure flow stops and sap accumulates in phloem.

    Medium Open in App Solution Verified by Toppr

    Correct option is D)

    According to the mass flow hypothesis, the loading of phloem or accumulation of sugars sets up a water potential which causes the water from nearby xylem cells to enter the phloem tissue and a turgor pressure is developed which causes mass movement.

    The flow of sugar is through sieve tubes and the flow is many a time bidirectional.

    Due to hydrostatic pressure, pressure flow increases and the phloem sap starts moving.

    So the correct answer is 'As hydrostatic pressure in phloem sieve tubes increases, pressure flow stops and sap accumulates in phloem'.

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    BCOR 11 Launchpad Flashcards

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    The water potential of pure water

    is positive.

    depends upon the solution it is compared to.

    is zero. is negative.

    Click card to see definition 👆

    is zero.

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    Water flows from regions of _______ water potential to regions of _______ water potential.

    zero; negative negative; positive zero; positive positive; zero

    Click card to see definition 👆

    zero; negative

    Click again to see term 👆

    1/12 Created by Aeio626

    Terms in this set (12)

    The water potential of pure water

    is positive.

    depends upon the solution it is compared to.

    is zero. is negative. is zero.

    Water flows from regions of _______ water potential to regions of _______ water potential.

    zero; negative negative; positive zero; positive positive; zero zero; negative

    When considering the effects of water potential on a plant cell, what factor or factors of the cell itself contribute to the direction and extent of water movement into or out of the cell?

    Type of solutes dissolved in the cell

    Both b and c

    Concentrations of solutes dissolved in the cell

    Pressure exerted on liquids in the cell by the cell wall

    Both b and c

    Refer to the table below.

    Four U-tubes similar to the one shown in the tutorial are set up with initial conditions as described in the table. Left and right sides are separated by a membrane permeable to water but impermeable to the solute. After allowing enough time for each of these setups to reach equilibrium, which will have the highest pressure potential?

    D

    What structures allow water vapor to escape the leaves of a plant?

    Leaf veins Mesophyll cells Cell walls Stomata Stomata

    According to the current model of fluid flow in xylem, what creates the force that moves water from the roots to the leaves?

    Differences in water potential between roots and leaves

    Cellular pumps in leaves

    An increase in root pressure

    An increase in water tension in leaves

    An increase in water tension in leaves

    Where does the energy come from that drives water transport in plants?

    Membrane pumps Water The Sun ATP The Sun

    Which would you expect to increase the rate of water transport in a plant?

    An increase in humidity

    The removal of leaves and a seal of the excision sites

    Blockage of the stomata

    A rise in temperature

    A rise in temperature

    Movement of sucrose and other carbohydrates through the phloem in a plant is called

    transdirection. redirection. relocation. translocation. Translocation

    Which of the following statements about the pressure flow model of fluid flow in phloem is true?

    Fluid always flows in a downward direction from high pressure to low pressure.

    Pressure is exerted on fluid in phloem by changes in water volume due to evaporation from stomata.

    Sucrose concentrations determine the direction of the fluid flow.

    Fluid can flow in any direction from an area of low pressure to an area of high pressure.

    Sucrose concentrations determine the direction of the fluid flow.

    Refer to the partially completed flowchart below.

    The flowchart describes the mechanism underlying fluid flow in phloem. Which phrases best fill in the blanks?

    A = Water movement by active transport; B = Pressure decrease

    A = Water movement by osmosis; B = Pressure increase

    A = Water movement by osmosis; B = Pressure decrease

    A = Water movement by active transport; B = Pressure increase

    A = Water movement by osmosis; B = Pressure increase

    Refer to the table below.

    In the spring, a plant grows from a root system that had been dormant throughout the winter. The plant produces a stem, several branches, leaves, and begins to flower. The relative concentrations of sucrose in cells of various organs of the plant are shown in the table. How would you describe the fluid pressure in the plant phloem at this time?

    Highest in photosynthetic tissues and lowest in non-photosynthetic tissues

    Highest in root and lowest in flower bud

    Highest at the top of the plant and lowest at the bottom

    Highest in leaf and lowest in flower bud

    Highest in root and lowest in flower bud

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