6.2 Metabolism: Transformations of Energy and Matter

Now that we know more about energy in general (Chapter 6.1), we can talk about more concrete ways in which energy is transformed as it flows through living systems, such as cells. The energy source of cells allows us to categorize them as either autotrophic or heterotrophic (Figure 6.7). Autotrophic organisms (noun = autotrophs) synthesize energy-storing organic molecules themselves, while heterotrophic organisms (noun = heterotroph) acquire energy-storing organic molecules by feeding. Chemical reactions transforming this energy are broken down into multiple steps. All the chemical reactions that transpire inside cells, including those that use and release energy, make up the cell’s metabolism. To understand how cells transform energy, we must discuss some key concepts associated with metabolism.

Anabolic and Catabolic Pathways

Cellular metabolism takes place via metabolic pathways, which involve multiple steps and multiple enzymes to convert reactants to products (Figure 6.8). There are two major classifications of metabolic pathways. Cells can break down larger molecules into smaller ones through a process called catabolism (Figure 6.9). Alternatively, larger molecules can be synthesized from smaller subunits through the process of anabolism (Figure 6.9).

Anabolic reactions and pathways are generally endergonic and require a source of free energy. This is because anabolic pathways involve a decrease in entropy (combining many disordered small molecules into a larger structurally-ordered large molecule), which usually results in a positive change in free energy. An example of an anabolic pathway is when a cell has an excess of glucose that is not immediately needed for cellular respiration. In animals, glucose can be stored in the large, branched polysaccharide glycogen. A series of anabolic chemical reactions in liver and muscle cells link individual glucose molecules through α-1,4 and α-1,6 linkages (Figure 6.10). The source of free energy for these anabolic reactions come from exergonic hydrolysis reactions of molecules like ATP. Another example of an anabolic reaction is photosynthesis. During photosynthesis, light energy is used to convert carbon dioxide and water to glucose and oxygen.

Catabolic reactions and pathways are generally exergonic, as the chemical energy stored in bonds is released as Gibbs free energy. This is because catabolic pathways involve an increase in entropy (breaking a larger structurally-ordered large molecule down into many disordered small molecules), which usually results in a negative change in free energy. An example of a catabolic pathway is glycogenolysis, in which cells break down glycogen into individual glucose molecules that can then be used for cellular respiration (which is also a catabolic process) (Figure 6.10). The free energy released by catabolic pathways is often stored as chemical potential energy in molecules like ATP. Cellular respiration is another example of catabolism. During cellular respiration, glucose is broken down in the presence of oxygen to generate carbon dioxide and water; during this process free energy is released and stored in ATP.

The examples we’ve discussed in this chapter have focused on carbohydrates (sugars, glycogen, etc.) but the same principles can be applied to any group of macromolecules discussed in Chapter 3, including proteins, lipids, and nucleic acids. Each time a cell builds one of these macromolecules, it is using anabolic pathways that are endergonic. Breaking down these macromolecules involves catabolic pathways that are exergonic. The metabolic pathways of different types of molecules can also “intersect.” For example, some amino acids can be broken down into chemicals that participate in cellular respiration (Figure 6.11). In eukaryotic cells, these metabolic processes often occur in different parts of the cell, including the cytosol (e.g., glycolysis), chloroplasts (e.g., photosynthesis), mitochondria (e.g., parts of cellular respiration like the citric acid cycle), peroxisomes (e.g., some fatty acid metabolism), the nucleus (e.g., RNA and DNA synthesis).