IMG_8838.jpeg

# Concept 10.1: Catabolic pathways yield energy by oxidizing organic fuels Living cells require transfusions of energy from outside sources to perform their many tasks, for example, assembling polymers, pumping substances across membranes, moving, and reproducing. The outside source of energy is fo...

# Concept 10.1: Catabolic pathways yield energy by oxidizing organic fuels Living cells require transfusions of energy from outside sources to perform their many tasks, for example, assembling polymers, pumping substances across membranes, moving, and reproducing. The outside source of energy is food, and the energy stored in the organic molecules of food ultimately comes from the sun. * **Energy flow into an ecosystem** * Energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical elements essential to life are recycled. * **Photosynthesis**: Photosynthesis generates oxygen, as well as organic molecules used by the mitochondria of eukaryotes as fuel for cellular respiration. * **Respiration**: Respiration breaks this fuel down, using oxygen (O2) and generating ATP. * **Waste products**: 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**: Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. * **Compounds**: Compounds that can participate in exergonic reactions can act as fuels. * **Activity**: 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. * **Energy**: Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat. ### Catabolic Pathways * **Fermentation**: One catabolic process, fermentation, is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen. * **Aerobic Respiration**: However, the most efficient catabolic pathway is aerobic respiration, in which oxygen is consumed as a reactant along with the organic fuel. * **Aerobic**: Aerobic is from the Greek aer, air, and bios, life. * **Cells**: The cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration. * **Prokaryotes**: 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. * **Anaerobic**: The prefix an- means "without". * **Cellular Respiration**: Technically, the term cellular respiration includes both aerobic and anaerobic processes. ## Redox Reactions: Oxidation and Reduction * **Catabolic Pathways**: How do the catabolic pathways that decompose glucose and other organic fuels yield energy? * **Energy**: The answer is based on the transfer of electrons during the chemical reactions. * **Electron**: The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP. ### The Principle of Redox * **Chemical Reactions**: In many chemical reactions, there is a transfer of one or more electrons (e¯) from one reactant to another. * **Electron Transfers**: These electron transfers are called oxidation-reduction reactions, or redox reactions for short. * **Redox Reactions**: In a redox reaction, the loss of electrons is called oxidation, and the gain of electrons is called reduction.