The two phases of photosynthesis are usually called light and dark. The presence of light is required for the light phase to occur. Both the dark and the light phases can be observed.

During the light phase light energy is converted into energy by the molecules ATP or NADPH.

Thanks to NADPH and ATP generated in the light phase during the dark phase CO2 can be converted into carbohydrates.

The thylakoid membrane is the light phase, while the chloroplast matrix is the dark phase. ADP and NADPH will be produced by the dark phase’s use of ATP and NADPH. These ADP- and NADP+ molecules can be used in the light phase to synthesize ATP or NADPH.

1. Phase Light

• The light phase is when light energy is absorbed into and converted to energy in the chemical bonds of ATP or NADPH. This phase is also called the light energy conversion phase.
• Photosynthetic pigment molecules are responsible for absorbing light energy.
• After being absorbed into the photosynthetic colors, the energy is converted to a series of redox reactions in the photosynthetic electronic transport chain. NADPH, ATP are synthesized through the actions of the photosynthetic electronic transport chain.
• Both photosynthetic pigments as well as components of the photosynthetic electronic transport chain are located in the thylakoid cell membrane of the chloroplast. They are organized in complexes to ensure efficient light energy absorption and conversion.
• Water molecules are the source of O2 in the light phase.

2. Dark phase

• The CO2 will be converted to carbohydrates. This process is known as CO2 fixation. The carbohydrate molecules “fix” the CO2 molecules that are free.
• There are currently several CO2 fixation routes. The most popular of all these pathways is however the C3 cycle (Figure 17.2). The C3 cycle, also known as the Calvin Cycle, is also known. This cycle is made up of several chemical reactions that are catalyzed using different enzymes.