The first stable product of CO2 fixation in C3 plants is a three-carbon organic acid. The enzyme ribulose-1,5-bisphosphate carboxylase–oxygenase (RUBISCO) combines CO2 with the phosphosugar ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglyceric acid. In contrast, carbon fixation in C4 plants results in the formation of four-carbon organic acids, aspartate and malate, as the first stable product of carbon fixation

The first stable product of photosynthesis in C-3 plants is 3-phosphoglyceric acid.

Photosynthesis is the process by which plants convert sunlight, carbon dioxide, and water into glucose and oxygen. C-3 plants are a type of photosynthetic pathway in which the first stable product formed during the process is a three-carbon compound called 3-phosphoglyceric acid (3-PGA).

Here's a detailed explanation of why 3-phosphoglyceric acid is the first stable product of photosynthesis in C-3 plants:

1. Initial steps of photosynthesis:
- Photosynthesis starts with the absorption of light by chlorophyll molecules in the thylakoid membranes of chloroplasts.
- The absorbed light energy is used to split water molecules into oxygen and hydrogen ions, releasing oxygen as a byproduct.
- The hydrogen ions are transferred to carrier molecules, such as nicotinamide adenine dinucleotide phosphate (NADP+), which becomes reduced to NADPH.

2. Calvin cycle (Dark reactions):
- The Calvin cycle, also known as the dark reactions or the light-independent reactions, occurs in the stroma of chloroplasts.
- The key enzyme involved in this cycle is ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).
- Rubisco combines carbon dioxide (CO2) with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon compound.
- This unstable compound immediately breaks down into two molecules of 3-phosphoglyceric acid (3-PGA), which is a three-carbon compound.

3. 3-phosphoglyceric acid:
- 3-phosphoglyceric acid (3-PGA) is the first stable product of photosynthesis in C-3 plants.
- It is an important intermediate in the Calvin cycle.
- 3-PGA can be further converted into other compounds, such as glucose and other sugars, through a series of enzymatic reactions.
- The formation of 3-PGA marks the beginning of carbon fixation, where CO2 is converted into organic molecules.

In conclusion, the first stable product of photosynthesis in C-3 plants is 3-phosphoglyceric acid (3-PGA). This three-carbon compound is formed during the Calvin cycle and serves as an important building block for the synthesis of glucose and other sugars.