Photosynthesis
The many reactions that occur during photosynthesis in plants can be grouped into two broad categories:

1.   Electron-transfer reactions or the light reactions: In the choloroplast, energy derived from sunlight energizes an electron of chlorophyll, enabling the electron to move along an electron-transport chain in the thylakoid membrane in much the same way that an electron moves along the respiratory chain in mitochondria. The chlorophyll obtains its electrons from water (H₂O), producing O₂ as a by-product. During the electron-transport process, H⁺ is pumped across the thylakoid membrane, and the resulting electrochemical proton gradient drives the synthesis of ATP in the stroma. As the final step in this series of reactions, high-energy electrons are loaded onto NADP⁺, converting it to NADPH. All of these reactions are confined to the chloroplast.

2. Carbon-fixation reactions or the dark reactions wherein the ATP and the NADPH produced by the photosynthetic electron-transfer reactions serve as the source of energy and reducing power, respectively, to drive the conversion of CO₂ to carbohydrate. The carbon-fixation reactions, which begin in the chloroplast stroma and continue in the cytosol, produce sucrose and many other organic molecules in the leaves of the plant. The sucrose is exported to other tissues as a source of both organic molecules and energy for growth.

Thus, the formation of ATP, NADPH, and O₂ and the conversion of CO₂ to carbohydrate are separate processes, although elaborate feedback mechanisms interconnect the two. Several of the chloroplast enzymes required for carbon fixation, for example, are inactivated in the dark and reactivated by light-stimulated electron-transport processes.

The chloroplast genome
It is believed that evolved from bacteria that were engulfed by nucleated ancestral cells and this theory is known as the endosymbiotic theory. All angiosperms and land plants have chloroplast DNAs (cp DNA) which range in size from 120-160 kb. They are circular possessing very few repeat elements and other short sequences of less than 100 bp. The notable exception is a large inverted repeat (10-76 kb) section, which when present, always contains the rRNA genes. For the majority of species, this repeat region is 22-26 kb in size. More than 20 chloroplast genomes have now been sequenced. The genomes of even distantly related plants are nearly identical, and even those of green algae are closely related.