The Question is from neet-botany plant-physiology long-distance-transport, and it was created by the author Mr. NARESH KUMAR K

Question

Which of the following manipulations would MOST directly inhibit the loading of sucrose into the sieve tube elements at the source?

Mr. NARESH KUMAR K · Accepted Answer

## CORE INFORMATION
✓ **Answer**: [D]
## CONCEPT FRAMEWORK
	**Primary Concept**: Phloem Loading
	**Related Topics**: Active Transport, Proton Pumps, Sucrose Transport, Source-Sink Relationship
	**Prerequisites**: Understanding of phloem transport mechanisms, role of proton pumps, active transport, and the relationship between companion cells and sieve tube elements.
## SOLUTION PATHWAY
**Given Information**:
	The question asks about inhibiting sucrose loading into sieve tube elements at the source.
	Sucrose loading at the source requires active transport.
**Method & Steps**:
	1. Identify the key process: Sucrose loading into sieve tube elements at the source is an active process that relies on a proton gradient.
		Reasoning: Sucrose does not move by simple diffusion into the phloem. 
	2. Analyze each option in relation to its impact on the key process.
		Working: Proton pumps in companion cells are essential for generating a proton gradient that drives sucrose loading.
	3. Determine the manipulation that directly disrupts the key process.
		Result: Inhibiting the proton pump will stop sucrose loading
 ## OPTION ANALYSIS
[A] :x: Decreasing the hydrostatic pressure in the sieve tube elements would primarily affect the bulk flow of phloem sap, not the initial loading of sucrose. While reduced pressure might indirectly impact loading over time, it doesn't directly inhibit the loading mechanism itself.
[B] :x: Blocking plasmodesmata between companion cells and sieve tube elements would prevent the movement of sucrose once loaded into the companion cells. However, the question asks about the initial *loading* of sucrose into the sieve tube elements and does not directly affect the uptake of sucrose into companion cells. The bulk of sucrose is loaded into the sieve tube elements via symplastic or apoplastic pathways.
[C] :x: Increasing the concentration of sucrose in the sink would primarily affect the sink's ability to uptake sucrose. This would indirectly affect the source. However, it does not directly inhibit the loading of sucrose into the sieve tube elements at the source.
[D] ✓: Inhibition of apoplastic proton pumps in companion cells would directly inhibit the active transport of protons out of the cell. This directly impacts the proton gradient required to load sucrose into the sieve tube elements via a secondary active transport mechanism using a sucrose-proton symporter. This is the most direct method of inhibiting sucrose loading.
 ## LEARNING AIDS
	**Common Mistakes**:
		1. Confusing the effects of manipulations on loading vs. bulk flow: Loading is the process of getting sucrose into the phloem, whereas bulk flow is the movement of phloem sap through the phloem. Manipulations may affect one but not the other.
		2. Overlooking the role of proton pumps: Proton pumps in companion cells are critical for generating the proton gradient that drives sucrose loading. 
	**Quick Tips**:
		1. Remember that sucrose loading into phloem at the source is an active transport process. 
		2.  Think about the proton gradient and how it is generated and used in the process of loading.

Mr. NARESH KUMAR K · Answer

Decreasing the hydrostatic pressure in the sieve tube elements

Mr. NARESH KUMAR K · Answer

Blocking plasmodesmata between companion cells and sieve tube elements

Mr. NARESH KUMAR K · Answer

Increasing the concentration of sucrose in the sink

Mr. NARESH KUMAR K · Answer

The correct Option is "Inhibition of apoplastic proton pumps in companion cells"

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