Can occur as long as there is a sufficient supply of NAD+ to drive the redox reactions required for activity since it does not require oxygen
Options for energy generation in the absence of oxygen
Anaerobic Cell Respiration
Fermentation
Anaerobic Cell Respiration
Other molecules such as sulfur or nitrogen as final electron acceptors; mostly occurs with bacteria; does use the ETC
Fermentation
Inefficient breakdown of sugars without oxygen; does not use the ETC or Krebs cycle; all events remain in the cytoplasm
Neither anaerobic respiration nor fermentation is as efficient as cellular respiration and not all organisms are capable of anaerobic respiration. Often fermentation is the only alternative.
Fermentation
A metabolic pathway which converts pyruvic acid in order to regenerate low energy electron carrier precursors (NAD+)
Fermentation does not require oxygen and occurs in the cytoplasm in all cells
ATP generated in fermentation is by substrate-level phosphorylation ONLY
Types of fermentation
Lactic Acid Fermentation
Ethanol Fermentation
Mixed Acid Fermentation
Butanediol Fermentation
Butanol Fermentation
Propionic Acid Fermentation
Lactic Acid Fermentation
Pyruvic acid (generated during glycolysis) is converted into lactic acid to regenerate the NAD+ molecules needed to continue glycolysis
Types of Lactic Acid Fermentation
Homolactic Fermentation
Heterolactic Fermentation
Ethanol Fermentation
Pyruvic acid is converted into acetaldehyde by pyruvate decarboxylase with CO2 produced as a byproduct, then subsequently ethanol via alcohol dehydrogenase
Ethanol buildup can become toxic, with most yeasts becoming toxic around 16-17% v/v. Higher alcohol content must be distilled.
Other Alternatives to Lactic Acid and Ethanol Fermentation
Mixed Acid Fermentation
Butanediol Fermentation
Butanol Fermentation
Propionic Acid Fermentation
Photosynthesis
Consists of two main sets of reactions: Light Reactions and Light-Independent (Calvin Cycle) Reactions
Photosynthesis Reactions
Light Reactions
Light-Independent (Calvin Cycle) Reactions
Light Reactions
Reactions which take place only in the presence of light and involve the capture of light energy by photosynthetic pigments in the thylakoids which is used to split water and generate ATP and high energy molecules (NADPH) for Calvin cycle; O2 waste
Light-Independent (Calvin Cycle) Reactions
Reactions which take place in the stroma and do not require light; result in the production of sugars from CO2, a process called carbon fixation
Photosystem
A reaction center complex (a type of protein complex with a special pair of chlorophyll a molecules) surrounded by light-harvesting complexes
Photosystems
Light-harvesting complexes (which contain chlorophyll bound to proteins) transfer the energy of light photons to the reaction center
Excited electrons are accepted by a primary electron acceptor in the reaction center complex which initiates the light reactions cascade
Photosystems in the Light Reactions
Photosystem II (PSII)
Photosystem I (PSI)
Microbes employ a wide array of light-capturing pigments including chlorophyll, bacteriochlorophyll, carotenoids, phycoerythrin, and phycocyanin
Electron Transport Chain
The 3 proteins in the membrane that pass electrons via paired oxidation-reduction reactions are plastoquinone (Pq), a cytochrome complex, and plastocyanin (PC). Energy released in the e- transfer powers formation of H+ gradient.
Calvin Cycle / Light-Independent Reactions
Take carbon dioxide (CO2) gas and convert it into glucose (C6H12O6) using ATP and NADPH
Steps of theCalvinCycle
Carbon Fixation / Carboxylation
Reduction
Regeneration
Several alternative CO2 fixation pathways are known to exist including the Reductive (Reverse) TCA Cycle and the Reductive Acetyl-CoA Pathway
Types of Photosynthesis
Oxygenic Photoautotrophy
Anoxygenic Photoautotrophy
Anoxygenic Photoheterotrophy
Rhodopsin-based Phototrophy
Oxygenic Photoautotrophy
Photosynthesis involving oxygen production which occurs in oxygen-rich conditions. Uses both photosystems. "Classic" photosynthesis.
Cyanobacteria
The only known oxygenic photosynthetic bacteria which perform photosynthesis on flattened sacs of membrane folds called thylakoids
Employ photoactive molecules including bacteriochlorophyll, phycoerythrin, and phycocyanin for light reactions and undergo the Calvin cycle
Contain polyhedral bodies called carboxysomes rich in RuBisCO and carbonic anhydrase to concentrate and sequester carbon for the Calvin cycle
Anoxygenic Photoautotrophy
Photosynthesis driving CO2 fixation processes through the use of either water or often sulfur compounds as an electron donor. These organisms use 1 photosystem to generate ATP and do NOT produce oxygen as a byproduct.
AnoxygenicPhotoheterotrophy
Photosynthesis involving non-CO2 fixation processes to convert organic compounds (such as acetate) into useable carbon sources. These pathways also only utilize 1 photosystem for light harvesting. Does NOT produce oxygen gas.
Rhodopsin-based Phototrophy
The simplest type of phototrophy which uses a light-driven proton pump to fuel ATP synthesis. The proton pump contains rhodopsin proteins containing the photoactive pigment, retinal.