Prepare for NEET Biology Transport in Plants with MCQs & PYQs on NEET.GUIDE. Access free practice, previous year questions, and expert guidance to understand xylem, phloem, and movement of water.
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.
Which of the following factors does NOT directly influence the rate of diffusion of a solute in a solution?
The concentration gradient of the solute
The temperature of the solution
The size of the solute molecules
The presence of a catalyst
Diffusion is the net movement of particles from a region of:
Lower concentration to higher concentration
Higher concentration to lower concentration
Equal concentration to equal concentration
No specific direction
Which of the following processes is primarily driven by diffusion?
Active transport of ions across a cell membrane
Bulk flow of water in xylem
Gas exchange in alveoli
Muscle contraction
The diffusion of water across a selectively permeable membrane is called:
Active transport
Facilitated diffusion
Osmosis
Plasmolysis
