Plant Bioenergetics


Cellular energy metabolism drives the biochemical processes essential for the growth and survival of all organisms. Nearly all forms of life depend on the photosynthetic activity of plants to provide electron‐rich substrates to fuel metabolism. Despite an overall low efficiency of conversion, the trapping of solar energy by photosynthesis is the only source of free energy used to sustain life.

Keywords: bioenergetics; thermodynamics; gibbs free energy; oxidation–reduction; photosynthesis; respiration; chemiosmosis

Figure 1.

Energy level diagram depicting the Z scheme of photosynthetic electron transport. Light absorbed by the antennae leads to excitation of the reaction centre chlorophylls of PS II (P680) and PS I (P700). The primary photochemical event in PS II is the transfer of an electron from the excited state of P680* to the primary acceptor pheophytin (Pheo). Reduced pheophytin transfers electrons to the primary quinone acceptor QA, which transfers electrons to the secondary quinone acceptor QB, which picks up two protons from the stromal side of the thylakoid membrane and merges into the plastoquinone (PQ) pool. The positively charged hole created by excitation of P680 is filled by an electron from the secondary donor, YZ, which, in turn, is reduced by electrons supplied by water. The oxygen evolving complex on the donor side of PS II contains a cluster of four manganese (Mn) ions that catalyse the oxidation of water. Excitation of PS I facilitates electron transport beyond plastoquinone. Photochemistry from the excited state of P700* results in the reduction of ferredoxin (Fd) via a series of bound PS I acceptors. Electron donation from the plastoquinone pool, via the cytochrome b/f complex and plastocyanin (PC) restores P700. Beyond PS I, the re‐oxidation of ferredoxin leads to the reduction of NADP+ and, finally, of CO2 to carbohydrate. Electron transport through the two photosystems generates a transthylakoid proton gradient (ΔpH), which is used to drive the synthesis of ATP from ADP and Pi. The transfer of electrons from water to NADPH requires 109 kJ mol−1 per electron transferred, which explains why light, with its relatively large amount of energy (Table ), is needed to drive photosynthesis.

Figure 2.

The chemiosmotic mechanism of ATP formation in chloroplasts and mitochondria. In chloroplasts, electron transport results in translocation of protons from the stroma to the thylakoid lumen, while, in mitochondria, protons are translocated from the matrix to the intermembrane space. In both cases, a pH gradient results that is used to drive the formation of ATP by a membrane‐bound ATP synthase. cyt, cytochrome.


Further Reading

Alberts B, Johnson A, Lewis J et al. (2002) Molecular Biology of the Cell, 4th edn. New York: Garland Science.

Boyer PD (1993) The binding change mechanism for ATP synthase – some probabilities and possibilities. Biochimica et Biophysica Acta 1140: 215–250.

Hall DO and Rao KK (1994) Photosynthesis, 5th edn. Cambridge: Cambridge University Press.

Haynie DT (2001) Biological Thermodynamics. Cambridge: Cambridge University Press.

Loomis RS and Amthor JS (1999) Yield potential, plant assimilatory capacity, and metabolic efficiencies. Crop Science 39: 1584–1596.

Nobel PS (1991) Physicochemical and Environmental Plant Physiology. San Diego, CA: Academic Press.

Siedow JN and Umbach AL (1995) Plant mitochondrial electron transfer and molecular biology. Plant Cell 7: 821–831.

Taiz L and Zeiger E (1998) Plant Physiology. Sunderland, MA: Sinauer Associates.

Walker DA (1992) Energy, Plants and Man. Brighton, UK: Oxygraphics Limited.

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Grace, Stephen C(Mar 2004) Plant Bioenergetics. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001461]