What is the outline sequence by which `CO_(2)` may be incorporated into starch by photosynthesis?

Options

A. PGA(+) `CO_(2)` rightarrow RuBP rightarrowTriose (P) rightarrowHexose (P)rightarrow Starch
B. PGA(+) `CO_(2)` rightarrow RuDP rightarrowHexose (P) rightarrowTriose (P)rightarrow Starch
C. RuBP(+) `CO_(2)` rightarrow PGA rightarrowHexose (P) rightarrow Triose (P) rightarrow Starch
D. RuBP(+) `CO_(2)`rightarrow PGA rightarrow Triose (P) rightarrow Hexose (P) rightarrow Starch

Answer

To outline the sequence by which \( CO_2 \) may be incorporated into starch by photosynthesis, we can follow these steps:

### Step-by-Step Solution:

1. **Carbon Fixation**:
- The process begins with the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyzing the reaction between \( CO_2 \) and ribulose bisphosphate (RuBP). This produces 3-phosphoglycerate (3-PGA).
- **Hint**: Remember that RuBP is the initial five-carbon sugar that reacts with \( CO_2 \).

2. **Reduction Phase**:
- The 3-PGA is then phosphorylated and reduced to glyceraldehyde-3-phosphate (G3P or triose phosphate) using ATP and NADPH produced in the light reactions.
- **Hint**: Think about the energy carriers (ATP and NADPH) that are essential for this reduction step.

3. **Formation of Hexose Phosphate**:
- Some of the G3P molecules are used to regenerate RuBP, while others are used to form glucose and other carbohydrates. Two G3P molecules combine to form glucose, which can then be converted into hexose phosphate.
- **Hint**: Remember that G3P is a three-carbon molecule that can combine to form six-carbon sugars.

4. **Polymerization into Starch**:
- The hexose phosphate can be further polymerized to form starch, which serves as an energy storage molecule in plants.
- **Hint**: Consider how plants store energy and the role of starch in that process.

### Summary of the Sequence:
- \( CO_2 + \text{RuBP} \rightarrow \text{3-PGA} \rightarrow \text{G3P} \rightarrow \text{Hexose Phosphate} \rightarrow \text{Starch} \)

### Final Answer:
The correct outline sequence for the incorporation of \( CO_2 \) into starch by photosynthesis is:
1. \( \text{RuBP} + CO_2 \) → 3-PGA
2. 3-PGA → G3P (triose phosphate)
3. G3P → Hexose phosphate
4. Hexose phosphate → Starch