ch7 how cells harvest energy

• Cellular respiration


  Series of reactions where cells utilize enzyme-facilitated redox reactions to convert energy from food sources to ATP
• Nicotinamide adenosine dinucleotide (NAD+) is an electron carrier and enzymatic cofactor used in oxidation-reduction reactions
• Energy metabolism


  • Concerned with redox reactions where electrons are lost and accompanied by protons
• Most foods contain

  • A variety of carbohydrates, proteins, and fats, all rich in energy-laden chemical bonds (C—H, C—O)
• Final electron receptor in aerobic respiration

  Is oxygen (O2)
• NAD+ accepts 2 electrons and 1 proton to become NADH
• Final electron acceptor in anaerobic respiration

  Is an inorganic molecule (not O2)
• During redox reactions, electrons carry energy from one molecule to another
• Organisms can be classified based on how they obtain energy: Autotrophs (self-feeders) and Heterotrophs
• Redox reactions involve NAD+ oxidizing energy-rich molecules by acquiring electrons and then reducing other molecules by giving the electrons to them
• All organisms use cellular respiration to extract energy from organic molecules
• Dozens of redox reactions take place in overall cellular energy harvest
• Cellular respiration

  The oxidation of organic compounds to extract energy from chemical bonds
• Cellular respiration is the complete oxidation of glucose
• Final electron acceptor in fermentation

  Is an organic molecule
• ∆G (free energy) of hydrolyzing terminal phosphate = -7.3 kcal/mol
• Substrate-level phosphorylation


  Transfer phosphate group directly to ADP from another molecule
• When ATP is plentiful in animals, the reducing power of accumulated NADH is diverted to supplying fatty acid precursors with high-energy electrons to form fats for long-term energy storage
• Free energy of -686 kcal/mol of glucose in aerobic respiration
• In most organisms, Glycolysis and oxidative phosphorylation are combined to convert glucose and O2 to CO2 and H2O, making ATP. This involves a series of oxidation reactions passing energetic electrons to an electron transport chain (ETC) which transfers electrons through carriers
• Some carriers carry just electrons, some carry electrons and protons
• Final electron acceptor in fermentation is an organic molecule
• Electron transport

  ATP is generated when electrons transfer from one energy level to another. Electrons "fall" to lower and lower energy levels in steps, releasing stored energy with each fall as they tumble to the lowest (most electronegative) electron acceptor, O2
• Final electron receptor in aerobic respiration is oxygen (O2)
• Aerobic respiration


  C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (heat & ATP)
• Structure of NAD+ and NADH: NAD+ serves as an "electron shuttle" during cellular respiration, accepting a pair of electrons and a proton from catabolized macromolecules and being reduced to NADH
• NAD+ acquires two electrons and a proton to become NADH, which can supply high-energy electrons to other molecules and reduce them
• Ability of NADH to supply high-energy electrons is critical to energy metabolism and biosynthesis of organic molecules
• All carriers can be reversibly oxidized and reduced
• Some molecules like phosphoenolpyruvate (PEP) possess a high-energy phosphate bond similar to ATP. When PEP's phosphate group is transferred enzymatically to ADP, the energy in the bond is conserved, and ATP is created
• Final electron acceptor in anaerobic respiration is an inorganic molecule (not O2)
• Free energy can be even higher than -686 kcal/mol in a cell
• Oxidative phosphorylation

  ATP synthase uses energy from a proton gradient formed by high-energy electrons from the oxidation of glucose passing down an electron transport chain. ATP synthase catalyzes the reaction: ADP + Pi → ATP
• Electron carriers

  • Soluble carrier (NAD+), membrane-bound carrier, carrier that moves within membrane
• © McGraw-Hill Education

  2020
• Cells use ATP to drive endergonic reactions
• Two mechanisms for synthesis of ATP: Substrate-level phosphorylation and Oxidative phosphorylation
• Cells make ATP by two fundamentally different mechanisms
• Large amount of energy in aerobic respiration must be released in small steps rather than all at once
• Glycolysis is a 10-step biochemical pathway that occurs in the cytoplasm