The Redox Business of Photosynthesis & Cellular Respiration

Written By Vaishnavi Kolluru

Photosynthesis is the process by which autotrophs (plants, some protists/bacteria/fungi) utilize sunlight to convert inorganic carbon into bio-usable forms, such as glucose. Cellular respiration harvests energy in the form of ATP from the products of photosynthetic carbon fixation, and re-releases inorganic forms into the atmosphere. This results in a cyclic exchange of carbon between organisms and their environments, as carbon switches between its organic and inorganic forms.

Photosynthesis is the anabolic reductive method for plants to fix carbon into an organic substance. Carbon dioxide (CO2) is collected from the atmosphere through stomata, or pores in the leaf surface. In most plants (C3), CO2 is used in photosynthesis immediately after collection; however, in C4 and CAM plants, CO2 is stored in an organic form to better control when photosynthesis occurs.

The first phase of photosynthesis, the light-dependent reactions, utilize photons from sunlight to kickstart an electron transport chain, which ultimately produces ATP and NADPH (among other products). The high-energy products are utilized in the light-independent reactions, or the Calvin cycle. In the reduction phase of the Calvin cycle, intermediates are phosphorylated (using the phosphate groups from the ATP) and reduced (using the electrons from the NADPH), which allows for G3P and eventual glucose production in photosynthesis.

The Calvin Cycle uses reductases to reduce the intermediates. Since enzymes can catalyze both directions of a chemical reaction, the reductases also catalyze oxidation of electron carriers.

Cellular respiration utilizes oxidation to catabolize glucose and create ATP. In the Krebs cycle, intermediates are oxidized, and the resulting free electrons are added to empty electron carriers such as FAD and NAD+. The catabolism of glucose results in the release of inorganic carbon (CO2) back into the atmosphere.

The Krebs Cycle uses oxidases and dehydrogenases to oxidize the intermediates. Since enzymes can catalyze both directions of a chemical reaction, the dehydrogenases/oxidases also catalyze reduction of electron carriers.

In conclusion, every reduction reaction must have an oxidative partner. Every oxidative reaction must have a reductive partner. Although photosynthesis and cellular respiration have individually reductive and oxidative steps that complement each other, the processes themselves can also be thought of as complements: photosynthesis is the reduction to cellular respiration’s oxidation.

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Vaishnavi Kolluru
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