The Mehler reaction is named after Alan H. Mehler, who, in 1951, presented data to the effect that isolated

chloroplasts A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, ...

reduce oxygen to form

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

(). Mehler observed that the formed in this way does not present an active intermediate in photosynthesis; rather, as a reactive oxygen species, it can be toxic to surrounding biological processes as an

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...

. In scientific literature, the Mehler reaction often is used interchangeably with the Water-Water Cycle to refer to the formation of by photosynthesis. ''Sensu stricto'', the Water Water Cycle encompasses the

Hill reaction The Hill reaction is the light-driven transfer of electrons from water to Hill reagents (non-physiological oxidants) in a direction against the chemical potential gradient as part of photosynthesis. Robin Hill discovered the reaction in 1937. H ...

, in which water is split to form oxygen, as well as the Mehler Reaction, in which oxygen is reduced to form and, finally, the scavenging of this by antioxidants to form water. Beginning in the 1970s, Professor Kozi Asada elucidated that oxygen can be reduced by electrons emerging from

ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...

photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses ...

, to form

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the ...

, which is then reduced by

superoxide dismutase Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide () radical into ordinary molecular oxygen (O2) and hydrogen peroxide (). Superoxide is produced as a by-product of oxygen me ...

to form . This photochemical is then reduced by the action of

ascorbate peroxidase Ascorbate peroxidase (or L-ascorbate peroxidase, APX) () is an enzyme that catalyzes the chemical reaction :L-ascorbate + H2O2 \rightleftharpoons dehydroascorbate + 2 H2O It is a member of the family of heme-containing peroxidases. Heme peroxi ...

to form water and oxidized

ascorbate Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) and ...

. Asada argued that oxygen presents an important sink for excess excitation energy acquired during plant exposure to bright light. He would often begin seminars by asking: 'Why aren't plants sunburnt despite being exposed to light?'. How much of a photoprotective role the Water Water Cycle plays has been occasion for some debate. In terrestrial plants, transfer of electrons to oxygen from ferredoxin at PSI accounts for easily less than 10% of total photosynthetic electron transport. In algae and other uni-cellular photosynthetic organisms, however, this amount can account for 20 to 30% of total electron transport. It is possible that the reduction of oxygen by free electrons emerging from PSI prevents components of the electron transport chain from becoming over-reduced. The Water Water Cycle is not related to

photorespiration Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction i ...

, as it comprises different reactions and results in no net oxygen consumption.

References

{{Reflist Photosynthesis