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When cells conserve energy, what form does it take?
- 1) proton gradient across cytoplasmic membrane
- 2) high-energy compounds that are used to power unfavorable chemical reactions
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electron carriers
- 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
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How are electron carriers used in the cell?
during respiration, they can also donate electrons to electron transport chains, which create a proton gradient across the IM
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How do bacteria conserve energy derived from nurients?
- 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
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chemoorganotrophs
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
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chemolithotrophs
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
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phototrophs
get their energy from light and their carbon from either organic compounds or CO2
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mechanism of ATP synthesis in fermentation
- 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
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mechanism of ATP synthesis in respiration
ATP is produced by oxidative phosphorylation at the expense of proton motive force
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fermentation
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
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glycolysis
- 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+
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balance sheet for energy production from glycolysis
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.
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respiration
- 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
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energetics balance sheet for aerobic respiration
- 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
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How do electron transport chains help to generate ATP?
- 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
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How does an electron tower work?
electrons pass down the electron tower, from compounds with lower reduction potential to those with higher reduction potential
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