Putting It Together: Metabolic Pathways


Now you’ve learned about the important energy exchanges that occur in living systems. In particular we covered these topics:

  • The movement of electrons and energy in the partnered redox reactions
  • Cellular respiration, which converts stored energy in chemical bonds (glucose) to cellular energy (ATP)
  • Fermentation creates ATP in the absence of oxygen
  • Photosynthesis is the ultimate source of energy on this planet, as it converts solar energy to the energy of chemical bonds in sugar.

Whether the organism is a bacterium, plant, or animal, all living things access energy by breaking down carbohydrate molecules. But if plants make carbohydrate molecules, why would they need to break them down, especially when it has been shown that the gas organisms release as a “waste product” (CO2) acts as a substrate for the formation of more food in photosynthesis? Remember, living things need energy to perform life functions. In addition, an organism can either make its own food or eat another organism—either way, the food still needs to be broken down. Finally, in the process of breaking down food, called cellular respiration, heterotrophs release needed energy and produce “waste” in the form of CO2 gas.

In nature, there is no such thing as waste. Every single atom of matter and energy is conserved, recycling over and over infinitely. Substances change form or move from one type of molecule to another, but their constituent atoms never disappear (Figure 1).

CO2 is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria. Both processes use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in biological, cyclical harmony to allow organisms to access life-sustaining energy that originates millions of miles away in a burning star humans call the sun.

This photograph shows a giraffe eating leaves from a tree. Labels indicate that the giraffe consumes oxygen and releases carbon dioxide, whereas the tree consumes carbon dioxide and releases oxygen.

Figure 1. Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide. These two processes play an important role in the carbon cycle. (credit: modification of work by Stuart Bassil)


Let’s return to the important process of photosynthesis. Obviously its important for providing energy for living organisms to power themselves. But is that the only power that photosynthesis provides? What about biofuels? Watch this 14 minute video for an amazing discussion of a proposed biofuel source that doesn’t use arable land, doesn’t take away food crops, and utilizes wastewater from cities.