using chemical energy - only autotrophs can use energy of sunlight through photosynthesis 

• heterotrophs - use autotrophs for food; accounts for 95% of earth's organisms
• digestion - 1st step of harvesting energy; breaks down large molecules into smaller ones
• catabolism - harvesting energy from C-H and other chemical bonds

cellular respiration - harvests energy by shifting electrons from 1 molecule to the next 

• energy from electrons used for ATP, or lost as heat
• electrons at end of process lose most of their energy, get transferred to final electron acceptor
• aerobic respiration - where final acceptor is oxygen
• anaerobic respiration - where final acceptor is nonorganic molecule other than oxygen
• fermentation - where final acceptor is organic molecule
• C6H12O6 + 6O2 >> 6CO2 + 6H2O
• -720 kilocalorie change per mole in free energy

ATP synthase - enzyme that creates most of ATP 

• uses energy in gradient of protons produced by pumping protons across the membrane
• energy used for reactions come from catabolism or light striking chlorophyll
• spins due to mov't of protons
• mechanical energy from spin used to attach 3rd phosphate to ADP

glucose catabolism - ATP from catabolism forms in 2 ways 

• substrate-level phosphorylation - additional phosphate directly transferred
  • glycolysis - glucose chemical bonds shifted to provide energy for ATP
• aerobic respiration - uses electrons from organic molecules to power ATP synthase
  • electrons donated to oxygen gas in final stage
  • used by eukaryotes, aerobic prokaryotes for majority of ATP
• organisms combine the 2 processes
• glycolysis - stage 1
  • 10-reaction biochemical pathway that produces ATP through substrate-level phosphorylation
  • catalyzed by free floating enzymes
  • uses 2 ATP, produces 4 ATP, 4 electrons for NAD+, 2 pyruvate molecules
• aerobic respiration - stages 2-4
• pyruvate oxidation - stage 2
  • pyruvate gets converted into CO2 and acetyl-CoA
  • NADH forms for every pyruvate molecule that gets converted
• Krebs cycle - stage 3
  • aka citric acid cycle or tricarboxylic acid cycle
  • cycle of 9 reactions that produce 2 ATP from substrate-level phosphorylation
  • lots of electrons removed to NADH
• electron transport chain - stage 4
  • uses electrons from NADH to pump protons across the membrane
  • ATP synthase uses proton gradient to make ATP
• procedure occurs in prokaryotes and mitochondria of eukaryotes

anaerobic respiration - occurs w/o O; replaced by S, NO3, other inorganic molecules 

• methanogens - use CO2 as final electron acceptor, reducing it to CH4
• sulfur bacteria - reduces SO4 to H2S; set the stage for photosynthesis evolution