Metabolism and Energy Pathways



Metabolism is the biological process that builds or breaks molecules to provide cells with the capacity to acquire and utilize energy. Metabolic pathways are the sets of chemical reactions that enable cells to live, grow, and reproduce, and can be classified as anabolic or catabolic. Catabolic pathways break down large molecules into smaller ones and produce energy. Anabolic pathways build complex molecules from simpler molecules and require energy. Catabolic and anabolic pathways are controlled and regulated by enzymes. Adenosine triphosphate (ATP) is the most common source of energy used by cells. Electron carriers are molecules that gain electrons in order to carry them to other compounds. Electron carriers facilitate energy transfer between metabolic pathways. Microbes are classified according to their use of energy sources. Photoautotrophs use light to create organic carbon. Chemoautotrophs use chemicals to create organic carbon. Heterotrophs obtain organic carbon from other biological sources.

At A Glance

  • Catabolic reactions break the bonds of larger molecules into smaller molecules, while anabolic reactions take smaller molecules and put them together into larger macromolecules that the cell needs.
  • Enzymes are specific protein catalysts that increase the rate of a chemical reaction without becoming part of the product.
  • Electron carriers are transporters that are continually accept and release electrons and hydrogen atoms to transfer redox energy.
  • Adenosine triphosphate (ATP) is the most important form of biological energy. The chemical bonds between each phosphate group store a large amount of energy that can be used for cellular activities.
  • Microbes that produce their own food are autotrophs, those that obtain energy from light are called photoautotrophs, and those that obtain energy from chemicals are chemoautotrophs. Heterotrophs must obtain carbon from biological sources.
  • Glycolysis is the metabolic pathway that breaks down glucose in the presence (aerobic respiration) or absence (anaerobic respiration) of oxygen to generate energy in the form of energy carriers (ATP and NADH) and pyruvate.
  • The citric acid cycle is a series of steps that transfers stored energy by reducing electron carriers to drive the synthesis of ATP.
  • Electron transport and oxidative phosphorylation serve as the final processing for electrons and hydrogen ions, yielding about 30 ATP per glucose molecule.
  • Fermentation is the process in which pyruvate, the end product of glycolysis, is further processed to release energy in the absence of oxygen.
  • Photosynthesis is the process of converting light energy from the sun into chemical energy and then using that energy to synthesize glucose through the Calvin cycle.
  • Chemoautotrophic bacteria can oxidize inorganic compounds to liberate energy that is used to reduce carbon dioxide or methane.
  • Fatty acids can be formed from acetyl-CoA and NADPH.