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- Mass Flow Hypothesis MCQ - Practice Questions with Answers
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6 Questions around this concept.
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MEDIUMWhich one of the following statements describes the process of phloem loading?
The loading at the source leads to the accumulation of sugar and other osmotically active solutes in the sieve tube elements. This causes:
Which of the following statements aboutthe mass flow hypothesis is wrong?
What is the direction of movement of sugars in phloem?
Concepts Covered - 0
Phloem Transport: The Pressure Flow or Mass Flow Hypothesis
- The accepted mechanism used for translocation of sugars from source to sink is called the pressure-flow hypothesis.
- As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a disaccharide).
- The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
- This process of loading at the source produces a hypertonic condition in the phloem.
- Water in the adjacent xylem moves into the phloem by osmosis.
- As osmotic pressure builds up, the phloem sap will move to areas of lower pressure.
- At the sink osmotic pressure must be reduced.
- Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar – converting it into energy, starch, or cellulose.
- As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.
- Hence, the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube.
- Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
- As hydrostatic pressure in the sieve tubes of phloem increases, pressure flow begins, and the sap moves through the phloem.
- Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates.
- The loss of solute produces a high water potential in the phloem, and water passes out, returning eventually to xylem.
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