15. Microbial Energetics I

• 1) proton gradient across cytoplasmic membrane
• 2) high-energy compounds that are used to power unfavorable chemical reactions

• in catabolism electrons are extracted from organic or inorganic molecules and transferred to electron carriers NAD+, NADP+ and FAD
• the reduced forms can then donate electrons to electron transport chains or to biosynthetic reactions

during respiration, they can also donate electrons to electron transport chains, which create a proton gradient across the IM

• it depends on:
• 1) energy source or electron donor and
• 2) whether an exogenous electron acceptor is available

• 1) energy (e- or photons) can come from:
• organic carbon source
• light
• inorganic electron donors

2) when an exogenous electron acceptor is present, electrons can be passed down an electron transport chain to a final acceptor

• regardless of the electron acceptor this process is called respiration
• if no exogenous electron acceptor is present, cells must obtain energy by fermentation

get their carbon and electrons from the same source

• some of the carbs are used for synthesis of macromolecules, while others are oxidized to CO₂ and the electrons go to an electron transport chain
• animals and many bacteria are aerobic chemoorganotrophs, but many bacteria can use organic carbon sources in combination with alternative electron acceptors

get their carbon and electrons from two different sources, both chemical

these organisms are typically autotrophs, meaning they obtain C for macromolecules from CO₂ - known as carbon fixation

get their energy from light and their carbon from either organic compounds or CO₂

• ATP is produced by substrate-level phosphorylation
• a phosphate group is added to a substrate, becoming a high-energy group that is finally transferred to ADP to generate ATP

ATP is produced by oxidative phosphorylation at the expense of proton motive force

an internally balanced oxidation-reduction reaction in which some atoms of the electron donor become oxidized while others become reduced

in a typical fermentation, most of the carbon is excreted as a partially reduced end product of energy metabolism and only a small amount is used in biosynthesis

• the breakdown of glucose to pyruvate, which can then be used for fermentation reactions or the TCA cycle
• Stage I - uses 2 ATP and has no redox reactions
• Stage II - generates 4 ATP by substrate-level phosphorylation and converts 2NAD+ to NADH, continues generating pyruvate
• if respiration is possible can enter the TCA cycle
• if not, Stage III - reduction reactions make fermentation products and regenerate NAD+

inefficient energy generation because glucose is fully oxidized to CO₂ in the absence of an exogenous electron acceptor

NAD+ is regenerated during formation of fermentation products rather than having NADH donate electrons to an electron transport chain. NAD+/NADH is a limiting reagent in the cell, so NAD+ must be regenerated by making fermentation products.

• pyruvate is completely oxidized to CO₂ using the TCA cycle, rather than being converted to fermentation products
• for every glucose this cycle produces 8 NADH, 2 FADH₂, and 2 GTP or ATP
• the electrons on NADH or FADH₂ must go somewhere to regenerate NAD+ and FAD+
• this cycle also generates intermediates of many metabolic pathways

• respiration generates much more energy because glucose can be fully oxidized to CO₂, electrons are fed into electron transport chains to regenerate NAD+ and H+ gradient is generated to conserve energy
• 38 ATP can be produced by complete oxidation of glucose to CO₂, via the TCA cycle and oxidative phosphorylation

• an electron transport chain is a series of electron donor/acceptor molecules that ends in a terminal electron acceptor
• some reactions in the chain generate enough energy to pump protons across the cytoplasmic membrane
• compounds have a standard propensity to accept electrons and be reduced
• the amount of energy released by a redox reaction is a function of the difference in the reduction potential of the electron donor and acceptor

electrons pass down the electron tower, from compounds with lower reduction potential to those with higher reduction potential