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8.7: Oxidation, Reduction, and Living Things

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  • Learning Objective

    • Know the overall reactions involved in respiration and photosynthesis.

    Oxidation and reduction reactions are especially crucial in biological processes such as cellular respiration and photosynthesis.

    Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions.

    Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and the most common oxidizing agent (electron acceptor) is molecular oxygen (O2).


    Figure \(\PageIndex{1}\) Aerobic respiration (red arrows) is the main means by which both fungi and animals utilize chemical energy in the form of organic compounds that were previously created through photosynthesis (green arrow). (CC-BY-SA 2.0,  Mikael Häggström, Wikimedia)

    The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as:

    \[ C_6H_{12}O_6 + 6 O_2 → 6 CO_2 + 6 H_2O + 2880 kJ/mol \]

    Green plants carry out the redox reaction that makes possible almost all life on Earth. They do this through a process called photosynthesis, in which carbon dioxide and water are converted to glucose (\(\ce{C6H12O6}\)). The synthesis of glucose requires a variety of proteins called enzymes and a green pigment called chlorophyll that converts sunlight into chemical energy

    A simplified overall formula for photosynthesis is:[68]

    \[ 6 CO_2 + 6 H_2O + photons → C_6H_{12}O_6 + 6 O_2 \]

    or simply

    carbon dioxide + water + sunlight → glucose + oxygen gas

    In this reaction, carbon dioxide is reduced to glucose, and water is oxidized to oxygen gas. Other reactions convert the glucose to more complex carbohydrates, plant proteins, and oils.


    • Redox reactions are common in organic and biological chemistry, including aerobic respiration, and photosynthesis.

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