Photosynthesis: Light & Dark Phases Simplified

Photosynthesis: Anabolic Process Converting Inorganic to Organic

Photosynthesis converts inorganic matter into organic using solar light. Sunlight's energy is converted into chemical energy within organic compounds. During photosynthesis, oxygen (O2) is released into the atmosphere. The overall process can be summarized as: CO2 + H2O + Light -> Organic Matter + O2.

However, this simplified equation doesn't reflect the actual process. The oxygen released comes from water, not carbon dioxide. Glucose formation and oxygen release are independent processes.

Light Phase (Hill Reaction)

The light phase occurs in the thylakoids, where photosystems are located. It requires light and produces:

• Reducing power: NADPH + H+
• Energy: ATP (produced using sunlight)

Chlorophyll and other pigments within photosystems capture light at different wavelengths (680nm and 700nm). Light excites chlorophyll, causing electrons to jump to a higher energy level. These electrons are passed through a series of carriers, including pheophytin, cytochrome B6, and plastocyanin, ultimately reaching Photosystem I.

Photosystem II loses electrons, which are replaced by splitting water (photolysis). This process releases oxygen. As electrons move through the carriers, energy is released and used to produce ATP.

Photosystem I also releases electrons, which are passed to ferredoxin and then to NADP+, forming NADPH + H+. This provides the reducing power for the dark phase.

Dark Phase (Light-Independent Reactions)

The dark phase occurs in the stroma of the chloroplast and doesn't require direct sunlight. It uses ATP and NADPH produced in the light phase to synthesize organic molecules.

The key process is the reduction of carbon dioxide (CO2). Ribulose 5-phosphate, a sugar within the chloroplast, is phosphorylated using ATP. The enzyme rubisco catalyzes the fixation of CO2 to ribulose 5-phosphate, forming an unstable intermediate that breaks down into two 3-carbon molecules.

These molecules are then reduced using NADPH, ultimately forming an aldehyde. This process consumes the reducing power and regenerates NADP+.