Passage
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes. The transition from a hypoxic (low O 2 ) to an oxic atmosphere (21% O 2 ) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation. This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome. On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, there is a possibility of gene transfer from eukaryotes to prokaryotes. This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi. The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS). ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups, but can accumulate during times of biochemical and environmental stress. Superoxide (O 2 − ) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H 2 O 2 ) and O 2 gas.
Adapted from Bannister JV, Parker MW. The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proc Natl Acad Sci USA. 1985;82(1):149-52.
-In anaerobic environments, P. leiognathi can produce energy by using an inorganic final electron acceptor other than oxygen in the electron transport chain. Under anaerobic conditions, which of the following is NOT active in ponyfish cells but active in P. leiognathi?

Question

Passage
Eukaryotic organelles such as mitochondria and chloroplasts are believed to be relics of formerly free-living prokaryotes.  The transition from a hypoxic (low O 2 ) to an oxic atmosphere (21% O 2 ) is said to have enabled primitive eukaryotic anaerobes to engulf ancient aerobic prokaryotes and consequently acquire the ability to produce energy through oxidative phosphorylation.  This endosymbiotic theory of eukaryotic evolution also postulates that endosymbiosis resulted in larger eukaryotic genomes, which originated from the partial transfer of mitochondrial genes to the nuclear genome.  On integration into the host genome, mitochondria-derived genes became indistinguishable from the original nuclear genes.Researchers have alternatively proposed that after a prolonged period of symbiosis, there is a possibility of gene transfer from eukaryotes to prokaryotes.  This hypothesis was initially supported when copper/zinc (Cu/Zn) superoxide dismutase (SOD), a metalloprotein confined to the cytosol of eukaryotic cells, was found in Photobacterium leiognathi.  The free-living bacterium P. leiognathi is also a known symbiont of ponyfish, a small fish species native to the Indian and Pacific Oceans.SODs are antioxidant enzymes that serve as the cell's first line of defense against reactive oxygen species (ROS).  ROS produced by the electron transport chain damage proteins by oxidizing amino acid residues and metal ions on prosthetic groups, but can accumulate during times of biochemical and environmental stress.  Superoxide (O 2 − ) radicals, a form of ROS, are sequestered by SODs and converted into less toxic hydrogen peroxide (H 2 O 2 ) and O 2 gas.
Adapted from Bannister JV, Parker MW. The presence of a copper/zinc superoxide dismutase in the bacterium Photobacterium leiognathi: a likely case of gene transfer from eukaryotes to prokaryotes. Proc Natl Acad Sci USA. 1985;82(1):149-52.
-In anaerobic environments, P. leiognathi can produce energy by using an inorganic final electron acceptor other than oxygen in the electron transport chain.  Under anaerobic conditions, which of the following is NOT active in ponyfish cells but active in P. leiognathi?

Quizplus · Accepted Answer

11ecba2d_0f88_2ce3_a3bf_95d2195a7d6e_MD0008_00 Various organisms catabolize glucose, the primary fuel source for ATP production, via either cellular respiration or fermentation. Cellular respiration yields carbon dioxide and water whereas fermentation yields inorganic waste compounds. Both biochemical processes begin with glycolysis, a metabolic pathway that does not require O₂ and occurs under both aerobic and anaerobic conditions. In glycolysis, a single molecule of glucose is broken down into two pyruvate molecules in the cytosol of both eukaryotes and prokaryotes (eg, glycolysis is active in both ponyfish and P. leiognathi cells under anaerobic conditions) (Choice A). In eukaryotes, cellular respiration continues when pyruvate is imported into the mitochondria and converted into acetyl-CoA, which then enters the Krebs cycle (citric acid cycle) to generate the necessary electron carriers (NADH and FADH₂) for the electron transport chain (ETC). ETC protein complexes pump protons across the inner mitochondrial membrane, generating a proton gradient, with O₂ as the terminal electron acceptor. By comparison, prokaryotes have no mitochondria; instead, the ETC is located on the plasma membrane. The enzyme ATP synthase harnesses the potential energy of the ETC proton gradient to generate ATP from ADP and inorganic phosphate. It is stated in the question that P. leiognathi can utilize the ETC in anaerobic environments because it can use compounds other than O₂ as the final electron acceptor. Although ATP synthase does not directly need O₂ to generate ATP, the formation of the proton gradient by which it functions is oxygen-dependent in ponyfish cells but not in P. leiognathi. Consequently, under anaerobic conditions, ATP synthase is not active in ponyfish cells but is active in P. leiognathi. (Choices B and C) Fermentation metabolizes pyruvate under anaerobic conditions to oxidize NADH to NAD⁺. Fermentation yields lactic acid in the muscles of ponyfish and alcohol in bacteria. Gluconeogenesis is the process by which noncarbohydrate carbon sources are converted into glucose. Because both processes do not require O₂, fermentation and gluconeogenesis are active in both ponyfish and P. leiognathi cells. Educational objective: The Krebs cycle and the electron transport chain are only active in the presence of a final electron acceptor, such as oxygen (aerobic respiration) or inorganic ions (anaerobic respiration).

Passage Eukaryotic Organelles Such as Mitochondria and Chloroplasts Are Believed to to Be