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## Concept 10.1: Catabolic pathways yield energy by oxidizing organic fuels Living cells require energy from outside sources to perform tasks such as assembling polymers, pumping substances across membranes, moving, and reproducing. The outside source of energy is food, and the energy stored in org...

## Concept 10.1: Catabolic pathways yield energy by oxidizing organic fuels Living cells require energy from outside sources to perform tasks such as assembling polymers, pumping substances across membranes, moving, and reproducing. The outside source of energy is food, and the energy stored in organic molecules of food ultimately comes from the sun. **Energy Flow and Chemical Recycling** As shown in Figure 10.1, energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical elements essential to life are recycled. Photosynthesis generates oxygen, as well as organic molecules used by the mitochondria of eukaryotes as fuel for cellular respiration. Respiration breaks this fuel down, using oxygen (O2) and generating ATP. The waste products of this type of respiration, carbon dioxide (CO2) and water (H2O), are the raw materials for photosynthesis. **Catabolic Pathways and Production of ATP** Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. Compounds that can participate in exergonic reactions can act as fuels. Through the activity of enzymes (see Concept 6.4), a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy. Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat. * **Fermentation** is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen. * **Aerobic respiration**, in which oxygen is consumed as a reactant along with the organic fuel (aerobic is from the Greek *aer*, air, and *bios*, life) is the most efficient catabolic pathway. Both eukaryotes and many prokaryotes can carry out aerobic respiration and some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without oxygen; this process is called anaerobic respiration (the prefix *an-* means "without"). Technically, the term cellular respiration includes both aerobic and anaerobic processes. **Redox Reactions: Oxidation and Reduction** How do the catabolic pathways that decompose glucose and other organic fuels yield energy? The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP. **The Principle of Redox** In many chemical reactions, there is a transfer of one or more electrons (e-) from one reactant to another. These electron transfers are called oxidation-reduction reactions, or redox reactions for short. In a redox reaction, the loss of electrons is called oxidation, and the gain of electrons is called reduction.