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An iron–sulfur electron-transfer protein. Photosynthetic ferredoxin receives electrons downstream of photosystem I and supplies reduction pathways including NADP reduction.

An enzyme coupling electrons from reduced ferredoxin to NADP reduction, producing NADPH on the stromal side of the chloroplast electron-transfer system.

Glutamate synthase transfers the amide nitrogen of glutamine to 2-oxoglutarate with coupled reduction, producing two glutamate molecules. It works with glutamine synthetase in nitrogen assimilation: the first enzyme captures ammonium into glutamine and the second distributes that nitrogen into glutamate. Different forms use different electron donors, so the enzyme name alone does not specify whether NADPH, NADH or reduced ferredoxin supplies reducing power. ^[ref]

Antennae collect photons; reaction centres convert excitation into charge separation; electron flow then generates reducing power and a proton gradient for carbon assimilation. Chlorophyll and accessory pigments in light-harvesting complexes broaden light absorption and transfer excitation towards a photosynthetic reaction centre. Excitation-energy transfer between pigments is distinct from transferring an electron along the electron-transport chain. The reaction centre couples excitation to separation of an electron from its donor, creating an oxidizing partner and a reduced acceptor before the excitation is lost as heat or fluorescence.

In the thylakoid membrane, photosystem II uses light excitation of its reaction centre to drive electron transfer to acceptors. Its oxidized donor is regenerated using electrons extracted from water by the oxygen-evolving complex, which contains a manganese–calcium cluster. Four oxidizing equivalents permit the net reactionThe protons are released on the lumenal side. Electrons pass through plastoquinone, the cytochrome b6f complex and plastocyanin to photosystem I. Plastoquinone takes up protons on reduction and contributes to their net delivery into the lumen through the cytochrome-b6f pathway. The two photosystems operate in series because excitation at one centre alone does not bridge the full energetic span from water oxidation to efficient nicotinamide adenine dinucleotide phosphate reduction.

Excitation in photosystem I enables electron transfer to ferredoxin; ferredoxin-NADP reductase generates NADPH on the stromal side. For the four electrons corresponding to one oxygen molecule, the ideal minimum is eight photochemical excitations, four at each photosystem, yielding two NADPH. Real light requirements are higher because not every absorbed photon produces productive chemistry. The proton-motive force drives ATP synthase as protons return from the lumen to the chloroplast stroma. Counterion movements often allow a large pH component to accumulate without an equally large membrane voltage. In a classical test, an imposed acid-to-base transition in isolated chloroplast material supported ATP formation in darkness, separating the gradient's role from the immediate presence of light.^[ref] Appropriate controls compare transitions that produce a transmembrane gradient with treatments that dissipate it.

In the stroma, the Calvin cycle uses ATP and NADPH to reduce fixed carbon and regenerate the carbon acceptor. RuBisCO adds carbon dioxide to ribulose bisphosphate; subsequent phosphorylation and reduction produce triose phosphate. Fixation of three carbon dioxide molecules to one net triose phosphate requires nine ATP and six NADPH in the standard accounting, with most intermediate carbon recycled to regenerate the acceptor. “Light-independent” chemistry is not physiologically independent of light: energy supply, stromal conditions and enzyme regulation coordinate it with photochemistry.

Linear electron flow need not exactly match the ATP/NADPH demand. Cyclic electron flow around photosystem I increases proton-gradient-driven ATP production without net NADPH generation or water oxidation. Excess excitation is dangerous; non-photochemical quenching, changes in antenna allocation and photosystem repair limit damage. The chloroplast combines efficient collection with regulated conversion and protection, rather than maximizing photon capture regardless of metabolic demand.

A Cytochrome P450 contains an iron–haem centre coordinated by a protein cysteine thiolate. The characteristic spectrum of its reduced carbon-monoxide complex gives the family its name. A common monooxygenation can be writtenOne oxygen atom enters the product and the other is reduced to water. Electron transfer and protonation activate bound oxygen, generating a powerful oxidant such as P450 compound I. Hydrogen abstraction followed by oxygen rebound can hydroxylate a C–H bond, but the family also supports other reactions, including epoxidation and successive oxidation steps. The exact chemistry is enzyme- and substrate-dependent; not every P450 simply performs a single hydroxylation.

This selectivity is valuable for complex natural products. Closely related substrates can differ in the position and stereochemistry of oxygenation, and an enzyme can distinguish sites that are difficult to separate by ordinary chemical synthesis. Oxygenated terpenoid intermediates, drug precursors and other specialized metabolites are therefore attractive targets. The opportunity is selective chemistry, not a guarantee that every plant pathway will be cheap to reproduce in a microbial host.

In an oxygenic photosynthetic chassis, light absorption by photosystem II supports extraction of electrons from water. Transfer through the photosynthetic chain and photosystem I generates reduced ferredoxin, which can feed a compatible P450 directly or contribute to NADPH production for an appropriate reductase system. The distinction matters: native microsomal P450s often use a flavoprotein reductase, whereas engineered direct ferredoxin delivery can bypass that partner. Electrons must still reach the P450 at adequate rates and with correct coupling to substrate oxidation.

There is a clear experimental proof of principle. A plant CYP79A1 catalytic domain was fused to the photosystem-associated PsaM anchor in a cyanobacterium, positioning it near the thylakoid electron supply. Activity was observed in light-driven experiments. This demonstrates functional rerouting of photosynthetic reducing power into a plant P450 reaction, rather than establishing commercial production of a finished pharmaceutical.^[ref]

A production design starts from the entire pathway. Supply a suitable scaffold from endogenous metabolism or introduced enzymes, express the required P450s with compatible membrane anchors and redox partners, and include any downstream transferases or other finishing enzymes. CYP79A1, for example, participates in the pathway to dhurrin; expressing it alone establishes one activity, not the completed multienzyme pathway. For a new high-value oxygenated terpenoid, one must similarly verify every intermediate rather than equating scaffold accumulation with final-product synthesis.

Several constraints can dominate performance. Plant membrane enzymes may fold poorly, insert in the wrong orientation or fail to acquire haem. An abundant P450 may remain inactive because its substrate or electron donor is limiting. Competition with carbon fixation and essential redox metabolism can make maximal electron diversion harmful to growth. Uncoupled oxygen activation can generate reactive oxygen species instead of product. Excess light may increase oxidative damage, while high cell density limits light penetration. Oxygen supply is required for ordinary P450 monooxygenation even though the host produces oxygen photosynthetically.

Optimization should therefore measure carbon flux, enzyme abundance, redox state and products together. Adjust expression ratios, redox-partner choice, subcellular localization and induction timing; relieve precursor bottlenecks without starving essential pathways; and consider export or product capture to limit toxicity. A bioreactor must balance mixing, illuminated surface area, gas exchange and biomass density.

Useful controls include dark incubation, a catalytic-site mutant, pathway-minus strains and perturbation of photosynthetic electron supply while monitoring cell viability. Mass spectrometry and authentic standards establish product identity; oxygen uptake or a fluorescent reporter alone does not. Finally report titre, productivity, carbon yield and stability over repeated cultivation. Light-powered enzyme activity is the biochemical entry point; reliable, selective and economically recoverable product formation is the engineering goal.

In a Gram-negative heterotroph, oxidation of organic substrates supplies electrons to intracellular carriers and the inner-membrane quinone pool. An insoluble iron oxide or an electrode cannot enter the cytoplasm or simply accept an electron through the lipid phase. Extracellular electron transfer must bridge the inner membrane, periplasm and outer membrane before reaching the mineral or electrode surface. This is a sequence of chemically coupled steps, not unrestricted electronic conductivity through the whole cell.

Shewanella oneidensis provides a useful mechanistic example. Quinol oxidation connects through CymA and periplasmic carriers to the outer-membrane MtrCAB assembly. Its cytochromes place redox-active haem groups on the inward and outward sides of a porin-associated structure; surface MtrC can exchange electrons with external acceptors. Reconstitution of oriented MtrCAB in proteoliposomes demonstrated transfer from an internal electron-carrier pool to external Fe(III) oxides, directly testing the membrane-crossing function rather than inferring it from a mutant phenotype alone.^[ref]

Direct contact is one strategy. Surface cytochromes can reduce a mineral that is close enough, and extracellular conductive structures can extend electron transfer beyond the cell body. Some microbial nanowires are cytochrome-rich filaments or membrane-associated structures; the mechanism should be assigned to the organism and structure actually characterized. Calling every extracellular appendage a conducting pilus is unwarranted. Contact area, attachment and the conductivity of the surrounding biofilm can limit current even when the intracellular respiratory route is functional.

A second strategy uses a redox mediator. The cell reduces a soluble compound, which diffuses to a distant acceptor, transfers its electrons and returns in an oxidized form. Flavin-mediated transfer in Shewanella and phenazine-mediated transfer in some other bacteria illustrate how diffusible chemistry can complement direct contact. Some mediators also associate with proteins rather than remaining entirely free in solution. Mediator availability, diffusion, side reactions and loss into the environment therefore affect efficiency. A poised electrode and an insoluble mineral are not interchangeable acceptors: their accessible potentials, surface chemistry and spatial distribution differ.

For oxygenic phototrophs, light supplies a different upstream route. Photosystem II extracts electrons from water, and photosystem I generates reduced ferredoxin and downstream reducing power. In many cyanobacteria, these reactions occur on internal thylakoid membranes; an electron collected outside must therefore pass through additional metabolic and envelope steps. In eukaryotic algae, the chloroplast envelope and plasma membrane add further compartmental barriers. One should not transplant a Shewanella Mtr diagram into these organisms merely because both produce an electrode current.

The evidence for cyanobacterial export includes mediator-dependent reduction and sensitivity to envelope permeability. In one Synechocystis study, weakening outer-membrane attachment increased photocurrent, while replacing the culture supernatant with fresh medium removed that current under the tested conditions. Ferricyanide reduction provided a complementary readout. These results support a role for extracellular redox-active compounds and an envelope barrier, not identification of one universal cyanobacterial electron-export protein.^[ref]

The energetic roles also differ. Mineral-reducing heterotrophs can use an external acceptor in respiratory energy metabolism. A phototroph must retain enough reducing power for carbon fixation, nutrient assimilation and repair; maximal export is not automatically advantageous. Current can persist in darkness through metabolism of stored carbon, and secreted organic material can support heterotrophs that generate part of a measured community current. Oxygen evolution also changes local redox chemistry and can divert electrons away from the electrode.

Mechanistic experiments should separate these possibilities. Compare axenic cultures with mixed communities, light and dark conditions, intact and damaged cells, and cell-free supernatants. Use electrode-potential controls, viability assays, electron-transfer mutants and rescue, together with oxygen and carbon balances. Ferricyanide access must be interpreted relative to its membrane permeability. Most importantly, “no added mediator” does not prove direct transfer: cells can secrete their own mediator. The comparison is therefore between a relatively well-defined family of respiratory conduits and several phototrophic routes requiring organism-specific evidence, with the same general requirements of redox compatibility, envelope crossing and an accessible external sink.

Photosystem I is a light-driven reaction-centre complex in oxygenic photosynthesis. Excitation supports electron transfer to acceptors including ferredoxin; the balance between linear and cyclic routes affects reductant and ATP supply.