The Question is from neet-botany photosynthesis-in-higher-plants , and it was created by the author NISHANTH

Question

If a plant is exposed to $^{14}CO_2$ in the dark and then transferred to light, the $^{14}C$ label will appear first in which of the following carbohydrates?

NISHANTH · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Photosynthesis and Carbon Fixation
 **Related Topics**: Calvin Cycle, Carbon Metabolism in Plants
 **Prerequisites**: Understanding of the light-dependent and light-independent reactions of photosynthesis.
## SOLUTION PATHWAY
**Given Information**:
  * Plant exposed to $^{14}CO_2$ in the dark.
  * Plant then transferred to light.
  * We need to identify the first carbohydrate to incorporate the $^{14}C$ label.
**Method & Steps**:
 1. **Carbon Fixation in the Dark:** In the dark, the plant can't perform the light-dependent reactions of photosynthesis.  Therefore, no ATP or NADPH are generated, which are necessary for the Calvin cycle (light-independent reactions).
 2. **Carbon Fixation in the Light:** When the plant is transferred to light, the light-dependent reactions occur, producing ATP and NADPH.  These molecules are essential for the Calvin cycle to proceed.
 3. **The Calvin Cycle:** The first step of the Calvin cycle involves the enzyme RuBisCO carboxylating RuBP (ribulose-1,5-bisphosphate) with $CO_2$.  This forms an unstable six-carbon compound that quickly breaks down into two molecules of 3-phosphoglycerate (3-PGA).
 4. **$^{14}C$ Incorporation:** The $^{14}CO_2$ will be incorporated into 3-PGA.  This 3-PGA is then reduced using ATP and NADPH to form glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), which are triose phosphates.
 5. **First Appearance:**  Therefore, the $^{14}C$ label will first appear in the triose phosphates (G3P and DHAP).
 6. **Subsequent Carbohydrate Formation:**  G3P and DHAP are then used to synthesize other carbohydrates like fructose-1,6-bisphosphate, sucrose, and starch. However, these are formed after the initial incorporation of $^{14}C$ into triose phosphates.
 Result: Triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate)
 Verification: The order of carbohydrate synthesis in the Calvin cycle supports this conclusion.
 ## OPTION ANALYSIS
[A] :x: Starch is a storage carbohydrate formed later in the process.
[B] :x: Sucrose is a transport sugar also formed later in the process.
[C] ✓: Triose phosphates are the first carbohydrates formed in the Calvin cycle after carbon fixation.
[D] :x: Fructose-1,6-bisphosphate is an intermediate in the synthesis of other carbohydrates, formed after triose phosphates.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the order of carbohydrate synthesis in the Calvin cycle: Remembering that triose phosphates are the initial products is crucial.
  2.Ignoring the role of light-dependent reactions: The light-dependent reactions provide the energy needed for the Calvin cycle.
 **Quick Tips**:
  1.Focus on the first step of the Calvin cycle: Carbon fixation and the subsequent formation of 3-PGA and then triose phosphates.
  2.Remember that the Calvin cycle is the site of carbohydrate synthesis in photosynthesis.

NISHANTH · Answer

Starch

NISHANTH · Answer

Sucrose

NISHANTH · Answer

The correct Option is "Triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate)"

NISHANTH · Answer

Fructose-1,6-bisphosphate

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