When we eat plants or eat other animals that eat plants, we’re ingesting chemicals such as carbohydrates that plants have created and turned into rich stores of Gibbs free energy. In a precise reversal of the dark reactions in photosynthesis, animal cells release the free energy stored in carbohydrates and use it to make their own ATP molecules. These fuel many of the chemical processes that take place in animal cells, enabling them to live. At the end of this process, the carbon that the plants fixed from atmospheric carbon dioxide is reunited with oxygen and breathed out again as carbon
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