The Question is from neet-botany anatomy-of-flowering-plants , and it was created by the author D VINOD

Question

Which of the following BEST explains why the ascent of sap in tall trees is primarily attributed to the cohesion-tension theory rather than root pressure?

D VINOD · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Ascent of sap in plants
 **Related Topics**: Cohesion-tension theory, root pressure, transpiration, water potential
 **Prerequisites**: Understanding of water properties (cohesion, adhesion), pressure gradients, and plant anatomy.
## SOLUTION PATHWAY
**Given Information**:
The question asks why the cohesion-tension theory is the primary explanation for sap ascent in tall trees, rather than root pressure.
**Method & Steps**:
1. Understanding Cohesion-Tension Theory: This theory explains that water molecules are drawn upwards in a continuous column due to the cohesive forces between water molecules (hydrogen bonds) and adhesive forces between water and the xylem walls. Transpiration (water loss from leaves) creates a negative pressure (tension) at the top of the column, pulling the water upwards.
2. Understanding Root Pressure: Root pressure is the positive pressure generated in the roots due to active ion uptake and osmosis. This pressure pushes water upwards, but it's limited in its ability to lift water to significant heights.
3. Comparing the two theories in the context of tall trees: Root pressure alone is insufficient to overcome the gravitational force in tall trees.  The height of the water column would exceed the pressure generated by the roots.  The cohesion-tension theory, however, can generate the necessary pulling force to lift water to great heights.
4. Evaluating the options: Option A is partially true but doesn't fully explain why the cohesion-tension theory is dominant. Options B and D are true statements but don't directly answer the question of why root pressure is insufficient for tall trees.
 **Result**: Option C accurately explains why the cohesion-tension theory is the primary mechanism for sap ascent in tall trees. It directly addresses the limitations of root pressure in overcoming gravity in tall plants.
 **Verification**: The fact that tall trees exist and successfully transport water to their tops validates the cohesion-tension theory as the primary mechanism.  Root pressure plays a minor role, especially in shorter plants.
## OPTION ANALYSIS
A :x: Root pressure does primarily operate at night due to reduced transpiration, but this doesn't explain why it's insufficient for tall trees.
B :x: While root pressure is involved in nutrient transport, it also contributes to water transport, albeit to a lesser extent in tall trees.
C ✓: This option correctly points out the limitation of root pressure in overcoming gravity in tall trees.
D :x: While the cohesion-tension theory might be more efficient, the primary reason for its dominance is its ability to overcome gravity in tall trees.
## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing root pressure with the primary mechanism of sap ascent in tall trees: Remember root pressure is a contributing factor, but its magnitude is insufficient for tall trees.
  2.Overlooking the role of transpiration in the cohesion-tension theory: Transpiration is crucial for creating the negative pressure that pulls water upwards.
 **Quick Tips**:
  1.Remember that cohesion and adhesion are key properties of water that enable the cohesion-tension mechanism.
  2.Visualize the water column as a continuous chain of water molecules being pulled upwards by transpiration.

D VINOD · Answer

Root pressure only operates during the night

D VINOD · Answer

Root pressure is primarily involved in nutrient transport, not water transport

D VINOD · Answer

The correct Option is "Root pressure alone cannot generate sufficient pressure to overcome the gravitational force in tall trees"

D VINOD · Answer

The cohesion-tension theory is more energy-efficient than root pressure

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