An evolutionary approach to learning energy metabolism How do cells obtain energy to make ATP? oxidation-reduction How do cells make ATP? oxidative phosphorylation Images in this video are from Wikipedia, unless otherwise indicated.

Oxidation-reduction is the core of energy metabolism Oxidation is the loss of electrons Organic molecules (food) and inorganic chemical electron donors are oxidized Reduction is the gain of electrons The amount of free energy released depends on the reduction potential difference of the redox pair Catabolic pathways feature a series of redox reactions (electron-transfer reactions) NAD+/NADH is the primary electron carrier

Summary of respiration of glucose C6H12O6 + 6O2  6CO2 + 6H2O C6H12O6 + 6O2 + 6H2O  6CO2 + 12H2O C is oxidized to CO2 O2 is reduced to H2O Electrons are transferred from glucose to O2 via NADH First equation is balanced, but doesn’t tell the full story of how oxygen is reduced.

Oxidized and reduced forms of NAD NAD = nicotinamide adenine dinucleotide. For NADH + H+ +1/2 O2 ↔ NAD+ + H2O, ΔGo = -52.4 kcal/mol.

Hydrogen powers ATP synthesis

Respiration: transfer of electrons from electron donors to electron acceptors to charge a membrane proton gradient H+ electrochemical gradient H+ Electron transport chain membrane NADH Terminal electron acceptors O2, NO3-, SO42-, Mn4+, Fe3+, CO2, etc. NAD+ Electron donors {[CH2O], H2, H2S, CH4, Fe2+, etc.} Diagram by J. Choi

Organisms may be classified by the source of energy and the source of organic carbon: Heterotrophs – both energy and carbon come from organic molecules (food) Autotrophs – make their own organic carbon from CO2 Photoautotrophs – energy from sunlight Chemoautotrophs – energy from reduced inorganic molecules

Terminal Electron Acceptors Different e- acceptors are used sequentially in microbial ecosystems. O2 ∆G = -479 kJ mol-1 NO3- ∆G = -453 kJ mol-1 Mn4+ ∆G = -349 kJ mol-1 Fe3+ ∆G = -114 kJ mol-1 SO42- ∆G = -77 kJ mol-1 Use of other terminal electron acceptors than oxygen = anaerobic respiration

Chemiosmosis in prokaryotes Electron transport chain generates proton gradient across membrane. Resulting proton motive force drives ATP synthesis and active transport. Fenchel, Origin & Early Evolution of Life, Oxford U Press 2002, Fig 6.2 Biology 1510

Electron transport chain in mitochondria

Proton gradient powers ATP synthase during respiration (oxidative phosphorylation) F0 F0 portion in membrane resembles flagellar motor F1 portion (ATP synthase) -resembles DNA helicase www.youtube.com/watch?v=KU-B7G6anqw F1