Explain the information about the discovery of $C_3$ and $C_4$ pathways.

(N/A) The products of the light reaction are $ATP$,$NADPH$,and $O_2$. $O_2$ diffuses out of the chloroplast,while $ATP$ and $NADPH$ are used to drive the synthesis of food.
- This is the biosynthetic phase of photosynthesis. It does not directly depend on light but depends on the products of the light reaction ($ATP$ and $NADPH$),along with $CO_2$ and $H_2O$.
- Research of $C_3$ reaction: Scientists were interested in identifying the first product formed when $CO_2$ is fixed. After World War $II$,the use of radioactive $^{14}C$ by Melvin Calvin in algal photosynthesis studies led to the discovery that the first $CO_2$ fixation product was a $3$-carbon organic acid,$3$-phosphoglyceric acid $(PGA)$. This pathway is known as the Calvin cycle.
- Research of $C_4$ reaction: Further experiments on a wide range of plants led to the discovery of another group of plants where the first stable product was a $4$-carbon organic acid,identified as oxaloacetic acid $(OAA)$.
- Consequently,$CO_2$ assimilation is classified into two types:
$(1)$ $C_3$ pathway: Plants where the first product is a $3$-carbon acid $(PGA)$.
$(2)$ $C_4$ pathway: Plants where the first product is a $4$-carbon acid $(OAA)$.