The Question is from neet-botany plant-physiology mass-flow-hypothesis, and it was created by the author Mr. NARESH KUMAR K

Question

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?

Mr. NARESH KUMAR K · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Mass flow hypothesis of phloem transport
 **Related Topics**:  Phloem structure, sucrose transport, pressure gradient, source-sink relationship
 **Prerequisites**: Understanding of the mass flow hypothesis, which posits that phloem transport is driven by a pressure gradient generated by the loading and unloading of sugars.
## SOLUTION PATHWAY
**Given Information**:
  Sucrose transporters in companion cells actively transport sucrose *out* of sieve tubes near the source.
  All other aspects of phloem structure and function remain the same.
**Method & Steps**:
 1. Understand the mass flow hypothesis:  Sucrose is actively loaded into sieve tubes at the source (leaves), creating a high solute concentration and thus high water potential. Water enters via osmosis, generating positive pressure. At the sink (e.g., roots, fruits), sucrose is unloaded, reducing solute concentration and pressure. This pressure gradient drives the bulk flow of phloem sap.
  Reasoning: The pressure gradient is crucial for mass flow.  Active loading at the source is essential for establishing this gradient. 
  Formula/Rule:  No specific formula, but the concept relies on the relationship between solute concentration, water potential, and pressure potential.
 2. Analyze the effect of the modification: If sucrose is actively transported *out* of the sieve tubes near the source, the solute concentration in the sieve tubes will decrease.  This will reduce the water potential, leading to less water entering the sieve tubes via osmosis.  Consequently, the pressure gradient driving mass flow will be significantly reduced or even reversed.
  Working:  The pressure gradient ($ΔP$) is directly related to the difference in solute concentration between the source and sink.  Reducing the concentration at the source reduces $ΔP$.
  Key Point: The active loading of sucrose at the source is the driving force for the pressure gradient in the mass flow hypothesis. Removing this force will stop or severely reduce transport.
 3. Determine the most likely effect: The reduction or elimination of the pressure gradient will significantly reduce or stop translocation.
  Result: Translocation would be significantly reduced or stopped, as the pressure gradient would be reversed or eliminated.
  Verification: The mass flow hypothesis directly links sucrose loading to the pressure gradient.  Removing sucrose from the sieve tubes near the source directly counters this mechanism.
 ## OPTION ANALYSIS
A :x:  The pressure gradient is dependent on companion cell activity (sucrose loading). 
B :x: Actively removing sucrose from the sieve tubes at the source would not create a stronger sink; it would weaken the source.
C ✓: Correctly identifies that the pressure gradient would be disrupted, leading to reduced or stopped translocation.
D :x: Reversing the direction of translocation is not the most likely effect; the pressure gradient would be significantly reduced, not simply reversed.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the roles of source and sink:  Understanding the direction of sucrose movement is crucial.
  2.Overlooking the importance of the pressure gradient: The mass flow hypothesis relies on this gradient.
 **Quick Tips**:
  1.Visualize the pressure gradient as a water column; reducing pressure at the top reduces flow.
  2.Remember that active loading at the source is the key to the mass flow hypothesis.

Mr. NARESH KUMAR K · Answer

Translocation would proceed normally, as the pressure gradient is independent of companion cell activity.

Mr. NARESH KUMAR K · Answer

Translocation would be enhanced, as sucrose would be actively removed from the sieve tubes, creating a stronger sink.

Mr. NARESH KUMAR K · Answer

The correct Option is "Translocation would be significantly reduced or stopped, as the pressure gradient would be reversed or eliminated."

Mr. NARESH KUMAR K · Answer

The direction of translocation would be reversed, with flow occurring from sink to source.

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