Anabolic and Catabolic Pathways


Anabolic and catabolic pathways are shown. In the anabolic pathway (top), four small molecules have energy added to them to make one large molecule. In the catabolic pathway (bottom), one large molecule is broken down into two components: four small molecules plus energy.
Anabolic pathways are those that require energy to synthesize larger molecules. Catabolic pathways are those that generate energy by breaking down larger molecules. Both types of pathways are required for maintaining the cell’s energy balance.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are synthesizing large proteins from amino acid building blocks, and synthesizing new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the cell’s life, take place constantly, and demand energy that ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH provide.

– What is a chemical reaction in which the standard change in free energy is positive, and an additional driving force is needed to perform this reaction?

ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single glucose molecule can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve degrading (or breaking down) complex molecules into simpler ones. Molecular energy stored in complex molecule bonds release in catabolic pathways and harvest in such a way that it can produce ATP. Other energy-storing molecules, such as fats, also break down through similar catabolic reactions to release energy and make ATP.

– What is a chemical reaction where the change in the free energy is negative indicating a spontaneous reaction?

It is important to know that metabolic pathway chemical reactions do not take place spontaneously. A protein called an enzyme facilitates or catalyzes each reaction step. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Cells are incredibly adept at creating complex molecules, like therapeutics, and can do so much better than many of our best factories. Synthetic biologists look to re-engineer cells to make these molecules for specific needs, including pharmaceuticals and energy applications. But the trial-and-error process is difficult and time-consuming, and often competes with the cell’s other goals and processes, like growth and survival.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:


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