Physiological Ecology of Respiration in Animals

The bulk of ATP in animals is generated aerobically in mitochondria by the ETSs and oxidative phosphoryla-tion. As a consequence, tissues both constantly require O2 and constantly produce CO2 during most ATP synthesis. These gases must be exchanged with the environment, O2 delivered to each cell and CO2 removed, a process that in this section we call 'respiration'. Smaller organisms can exchange these gases by simple diffusion, but more complex metazoans often possess respiratory structures that come in contact with either the air or water in which they live. All such respiratory structures have a thin membrane across which gas exchange occurs by passive diffusion, driven by differences in partial pressures of O2 and CO2.

Release (accidental) to cytoplasm (or

.. . . . . retention of vesicle as outer membrane) Uptake as food '

Release (accidental) to cytoplasm (or

.. . . . . retention of vesicle as outer membrane) Uptake as food '

Proto-endosymbiont in phagolysosome

Low virulence results in long-term preservation of intracellular symbiosis

Uptake as replication-suppressed, temporary endosymbiont (here shown initially in phagosome) results in selection on bacterium to maintain more permanent relationship

Figure 10 Speculation on possible routes toward eukaryote acquisition of mitochondria. Shown are three routes: endosymbiont acquisition initially as food, endosymbiont acquisition initially as an intracellular pathogen, and endosymbiont acquisition initially as a 'temporary' endosymbiont with division inhibited by the eukaryote host both to better divert ATP output to the host and to keep the bacterium from over-replicating (i.e., keeping it 'well-behaved'). This latter state could have been an extension of consortia (close-linked, mutualistic associations different microbial species) between pre-mitochondrial eukaryotes (or proto-eukaryotes) and aerobic bacteria, each providing the other with valuable metabolic by-products.

Proto-endosymbiont in phagolysosome

Low virulence results in long-term preservation of intracellular symbiosis

Uptake as replication-suppressed, temporary endosymbiont (here shown initially in phagosome) results in selection on bacterium to maintain more permanent relationship

Figure 10 Speculation on possible routes toward eukaryote acquisition of mitochondria. Shown are three routes: endosymbiont acquisition initially as food, endosymbiont acquisition initially as an intracellular pathogen, and endosymbiont acquisition initially as a 'temporary' endosymbiont with division inhibited by the eukaryote host both to better divert ATP output to the host and to keep the bacterium from over-replicating (i.e., keeping it 'well-behaved'). This latter state could have been an extension of consortia (close-linked, mutualistic associations different microbial species) between pre-mitochondrial eukaryotes (or proto-eukaryotes) and aerobic bacteria, each providing the other with valuable metabolic by-products.

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