15–20 — ELECTRON DISPOSITION
Capturing electrons is only half the redox problem. The organism must also provide a destination for them.
Operational definition
Layer type: Electron-fate layer Governs: Items 15–20
Formal definition
The Electron Disposition and Redox Closure layer identifies where acquired electrons ultimately go, how oxidized electron carriers are regenerated, and how the focal metabolic pathway achieves a sustainable redox balance.
Governing question
After electrons are obtained from a donor, what accepts or retains them?
Every continuing metabolism must solve both sides of the redox problem:
Where do the electrons come from?
and:
Where do the electrons go?
The electron-source axis answers the first question. This layer answers the second.
Included electron fates
- oxygen reduction in aerobic respiration;
- nonoxygen external acceptors in anaerobic respiration;
- extracellular terminal acceptors;
- internally generated acceptors in fermentation;
- return to a reaction center in cyclic electron flow;
- retention in biomass;
- retention in reduced storage or excreted products.
Inclusion rule
A pathway belongs here when it identifies the functional endpoint of electron flow within the specified metabolic boundary.
The boundary is essential. A product such as lactate may be terminal for one organism but become the electron donor of another.
Multiple-pathway rule
A single organism may simultaneously or conditionally use:
- aerobic respiration;
- anaerobic respiration;
- fermentation;
- reductive biosynthesis;
- reduced-product export.
Therefore this layer usually requires a profile rather than one permanent label.
Distinction from energy conservation
Electron disposition describes redox closure.
Energy conservation describes how part of the reaction’s free-energy change is retained for biological work.
Two organisms may use the same terminal acceptor while conserving different amounts of energy through different respiratory architectures.
15. Aerobic respiration
Terminal external electron acceptor:
O_2
Typical products:
CO_2 + H_2O
depending on the donor.
16. Anaerobic respiration
External terminal electron acceptors other than oxygen may include:
- nitrate
- nitrite
- sulfate
- sulfur
- ferric iron
- manganese compounds
- carbon dioxide
- fumarate
- chlorinated compounds
- other oxidized substrates
17. Extracellular respiration
Electrons terminate on:
- insoluble minerals
- extracellular conductive matrices
- electrodes
- extracellular partner organisms
18. Fermentation
The original substrate supplies both:
- the oxidized portion that yields energy
- the internally generated electron acceptor needed for redox balance
ATP is generally obtained primarily through substrate-level phosphorylation rather than a conventional respiratory chain using an external terminal acceptor. (PubMed Central (PMC))
19. Cyclic electron flow
Electrons return to the original reaction system after driving ion-gradient formation.
Primary result:
light → ion motive force → ATP
without net production of reducing equivalents from the cyclic pathway itself.
20. Reductive assimilation or product formation
Electrons are retained in:
- newly synthesized biomass
- reduced storage compounds
- methane
- acetate
- alcohols
- hydrogen
- other reduced secreted products
This is distinct from merely discarding electrons to a terminal acceptor.