22.1 The Energy Transformations that Sustain Life

Plants, animals, fungi, and all other living organisms must take in energy from the environment and convert it into a form that their cells can use. Matter and its stored energy enter an organism’s body in one form and are converted into another form that can fuel the organism’s life functions.

In the process of photosynthesis, plants and other photosynthetic producers take in energy in the form of light (solar energy) and convert it into chemical energy in the form of glucose, which stores this energy in its chemical bonds. Then, a series of metabolic pathways, collectively called cellular respiration, extracts the energy from the bonds in glucose and converts it into ATP, a form that all living things can use.

Relationship between photosynthesis and cellular respiration
Eukaryote organelles that accomplish energy transformations. The products of photosynthesis are the reactants for cellular respiration, and vice versa. Note that prokaryotes also perform photosynthesis and cellular respiration. (Figure by Melissa Hardy is used under a Creative Commons Attribution-NonCommercial license).

In some ways, the processes of photosynthesis and cellular respiration are opposite from one another. The products of photosynthesis are the starting materials for cellular respiration, and vice versa. We can summarize the reactions as chemical formulae.

Photosynthesis: 6H2O + 6CO2 → C6H12O6 + 6O2

Cellular Respiration: C6H12O6 + 6O2 → 6H2O + 6CO2

Energetically, photosynthesis is anabolic and endergonic, while cellular respiration is catabolic and exergonic.

 

Comparison of cellular respiration and photosynthesis
A summary of some key differences between photosynthesis and respiration. (Table by Melissa Hardy is in the public domain).

There are similarities between the two processes as well. One of the most important is that ATP is produced by the same mechanism — chemiosmosis — in both of these metabolic pathways.


Text adapted from OpenStax Biology 2e and used under a Creative Commons Attribution License 4.0.
Access for free at https://openstax.org/books/biology-2e/pages/1-introduction
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