Prepare for NEET Biology Transport in Plants (Mass Flow Hypothesis) with MCQs & PYQs on NEET.GUIDE. Access free practice, previous year questions, and expert solutions to understand the pressure-flow model of phloem transport.
NEET Questions / Botany / Transport in Plants / Mass Flow Hypothesis
A plant is exposed to for a short period. After 5 minutes, the radioactive carbon is detected in the phloem sieve tubes near the source. After 20 minutes, it's found further down the stem in the phloem. Which observation MOST directly contradicts the mass flow hypothesis?
Sucrose moves bidirectionally in the phloem, sometimes against the concentration gradient.
ATP is required for active loading of sucrose into the sieve tubes.
The concentration of sucrose is higher in the source than in the sink.
Water moves from xylem to phloem at the source and from phloem to xylem at the sink.
If the mass flow hypothesis were entirely accurate, which scenario would be LEAST likely to occur in a plant?
Movement of sucrose from a leaf to a developing fruit.
Higher turgor pressure in phloem sieve tubes near a photosynthesizing leaf compared to those near a root tip.
Simultaneous bidirectional transport of different organic molecules within the same sieve tube element.
Changes in phloem sap composition based on the source and sink tissues involved.
Researchers manipulate a plant's phloem by selectively blocking plasmodesmata connecting companion cells to sieve tube elements near a leaf. Which outcome would provide the STRONGEST evidence AGAINST the mass flow hypothesis?
Accumulation of sucrose in the leaf and reduced transport to the roots.
Increased turgor pressure in the sieve tubes near the leaf.
Uninterrupted flow of sucrose from the leaf to the roots.
Decreased water potential in the sieve tubes near the leaf.
Which experimental observation would be MOST difficult to reconcile with the mass flow hypothesis?
A higher concentration of sucrose in source tissues compared to sink tissues.
A positive correlation between the rate of transpiration and the rate of translocation.
Bidirectional movement of different solutes within the same sieve tube element.
The presence of a pressure gradient between source and sink tissues in the phloem.
A researcher observes that applying a metabolic inhibitor to the sink tissues of a plant reduces the rate of translocation. How does this observation relate to the mass flow hypothesis?
It refutes the hypothesis, as metabolic energy should not be required for passive flow.
It supports the hypothesis, as reduced sink activity would decrease the pressure gradient driving flow.
It is irrelevant to the hypothesis, as the inhibitor's effect is localized to the sink.
It partially supports the hypothesis, suggesting an additional active component to the predominantly passive flow.
Imagine a hypothetical plant where sucrose transporters in companion cells are genetically modified to actively transport sucrose out of sieve tubes near the source. Assuming all other aspects of phloem structure and function remain the same, what would be the MOST likely effect on translocation according to the mass flow hypothesis?
Translocation would proceed normally, as the pressure gradient is independent of companion cell activity.
Translocation would be enhanced, as sucrose would be actively removed from the sieve tubes, creating a stronger sink.
Translocation would be significantly reduced or stopped, as the pressure gradient would be reversed or eliminated.
The direction of translocation would be reversed, with flow occurring from sink to source.
According to the mass flow hypothesis, what is the main driving force for the translocation of organic solutes in phloem?
Transpiration pull
Root pressure
Capillary action
Osmotic pressure gradient between source and sink
Which of the following is NOT a requirement for the mass flow hypothesis to function?
Presence of living phloem cells
A pressure gradient between source and sink
Active transport of sugars into phloem at the source
Active transport of water into phloem at the sink
In the mass flow hypothesis, what happens to the water potential in the phloem at the source?
Decreases
Increases
Remains the same
Fluctuates randomly
Which experimental evidence supports the mass flow hypothesis?
Radioactive tracers show bidirectional movement in phloem
Blocking phloem transport near the sink does not affect source activity
Aphid stylets exude sap with higher pressure at the source than at the sink
Cutting the phloem stops transpiration
