Light-Dependent Reactions of Photosynthesis
The first major set of processes in photosynthesis, in which light energy is initially converted into chemical energy as ATP and NADPH, takes place across the chloroplast thylakoid membranes, between the chloroplast stroma and the thylakoid space. It is often represented by what is known as the Z-scheme, which is a diagrammatic representation of the electron transfer chain (below) against a y-axis representing the free energy of the components. This need not worry us too much, and the below representation is sufficient illustration.
The major components of the Z-scheme are:
Two linked photosystems (PSII and PSI): these are transmembrane protein complexes, housing the light-harvesting chlorophyll pigment proteins. Resonance energy transfer (RET) occurs between the peripheral chlorophyll molecules to transfer the absorbed light energy to a central pair of chlorophyll molecules called the reaction centre (RC). Here, the energetic excitation causes the loss of an electron from the photosystem, and its entry into an electron transfer chain (ETC).
- An oxygen-evolving complex (OEC): the electron that is excited and lost from PSII must be replaced, and this is achieved via the splitting of water molecules in the OEC. The splitting of two water (H2O) molecules releases 4 electrons (e–), 4 protons (H+) and one molecule of oxygen gas (O2): 2H2O –> O2 + 4H+ + 4e–. As one photon of light absorbed by PSII results in the excitation of one electron, the absorption by PSII of 4 photons results in the splitting of two water molecules and the release of one oxygen molecule. However, as shall become clear below, PSII and PSI are closely linked, with every electron excited from PSI being ultimately replaced by the excited electron from PSII. Therefore, the total number of photons absorbed per O2 released is 8. Structurally, the OEC possesses a metalloenzyme core of manganese and calcium.
- Plastoquinone (PQ): an electron acceptor which accepts 2 excited e– from PSII and 2H+ from the stroma to become plastoquinol (PQH2). It has the dual functions of transferring excited electrons from PSII to the cytochrome b6f complex (below) as part of the ETC, and of shuttling protons from the stroma to the thylakoid lumen as part of the generation of a protonmotive force (pmf). This latter function is important for the generation of ATP (see ATP synthase, below).
- Cytochrome b6f complex: mediates transfer of e– from PQ to PC (below) as well as of H+ from the stroma to the thylakoid lumen via PQ, making a substantial contribution to the pmf. The b6f complex is…well…complex! But the exact details of its constituent parts and of its function need not concern us here. If, however, you are interested and seek further information about it, please refer to the wikipedia article.
- Plastocyanin (PC): much like PQ, PC acts as an electron carrier. Unlike PQ, it accepts only one e– from the cytochrome b6f complex (and is thus reduced), before reducing PSI (and is thus oxidised). In this step, the electron that is passed to PSI replaces that which has been lost by excitation by a photon of light, thus this may be considered the step that directly links the two photosystems.
- Ferredoxin: the final electron carrier in the light-dependent reactions, which accepts an electron from PSI to become reduced ferredoxin, before reducing NADP+ in the reaction catalysed by ferredoxin NADP+ reductase (below).
- Ferredoxin NADP+ reductase (FNR): the culmination of the ETC is the reduction of NADP+ to NADPH. This entails a convergence of the ETC with the protons in the stroma, as FNR catalyses the oxidation of 2 reduced ferredoxin molecules to reduce one NADP+. This involves the transfer of two electrons (one per ferredoxin) and one proton from the stroma.
- ATP synthase: a final crucial component of the light-dependent reactions is the phosphorylation of ADP to produce ATP. This is catalysed by ATP synthase, powered by the passive diffusion of H+ from the thylakoid lumen to the stroma, via ATP synthase itself. The inputs of energy that we have seen above, driving the ETC and the active transport of protons from the stroma to the thylakoid lumen, generating the pmf, provide the energy behind this diffusion of protons and the concurrent synthesis of ATP.
Thus, the light-dependent reactions can be summarised as the harnessing of light energy to drive electron transport and proton pumping, in order to convert the light energy into a biologically usable form (ATP) and to produce a biologically usable source of reducing power (NADPH).