Q.62 Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Sugars translocated in phloem are in non-reducing fonn.
Reason R: The non-reducing sugars are less reactive than reducing sugars.
In the light of the above statements, choose the most appropriate answer from the options given below:
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is NOT the correct explanation of A
3. A is true but R is false
4. A is not false but R is true
Both A and R are true but R is NOT the correct explanation of A.
Detailed Explanation
Assertion A: True—Sugars translocated in phloem are primarily sucrose (non-reducing disaccharide: glucose-α1,2-fructose). Reducing sugars (glucose, fructose with free anomeric carbon) are converted to sucrose at sources before mass flow via pressure-flow mechanism.
Reason R: True—Non-reducing sugars lack reactive aldehyde/ketone groups, making them chemically less reactive than reducing sugars (which participate in Maillard reactions, oxidation).
Key Issue: R explains sucrose’s chemical stability but NOT the primary reason for phloem transport preference:
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Main reason: Osmotic stability—sucrose (MW 342) creates higher osmotic pressure per carbon than hexoses (MW 180), preventing back-diffusion from sieve tubes
-
Sucrose is osmotically more efficient for loading/unloading
Options Explained
1. Both true, R explains A: Wrong—R true but doesn’t explain primary transport mechanism.
2. Both true, R NOT explanation: Correct—sucrose chosen for osmotic efficiency, not just reactivity.
3. A true, R false: Wrong—R factually correct about reactivity.
4. A false, R true: Wrong—A definitively true (sucrose = phloem standard).
Introduction
Sugars translocated in phloem are in non-reducing form (sucrose) due to its superior osmotic properties for pressure-flow, though non-reducing sugars are indeed less chemically reactive. Both statements true, but reactivity doesn’t explain transport preference.
Why Sucrose Dominates Phloem Transport
| Property | Reducing Sugars (Glucose) | Non-Reducing Sucrose | Advantage |
|---|---|---|---|
| Molecular Weight | 180 Da | 342 Da | Higher osmolarity/mass |
| Reactivity | High (aldehyde free) | Low | Secondary benefit |
| Solubility | High | High | Equal |
| Phloem Concentration | <1% | 10-25% | Loading efficiency |
Pressure-Flow Requirements
Source (leaf) → High sucrose → High turgor → Mass flow → Sink (root)
↓
2 Glucose → **Sucrose** (invertase + sucrose synthase)
Osmotically stable → No back-diffusion
Primary vs Secondary Reasons
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Primary: Osmotic efficiency (sucrose = 2 hexoses, half osmotic particles)
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Secondary: Chemical stability (less Maillard reaction in phloem sap)
Exam Key Distinction
R explains sucrose STABILITY, not SELECTION for phloem. Option 2 tests understanding beyond surface facts.
Memory Aid: “Sucrose = Stronger Osmosis, Less Reactive (but osmosis first)”.
