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Hill reaction

hill reaction in botany, hill reaction
The Hill reaction was discovered by the biochemist Robin Hill from the University of Cambridge in 1937 The Hill reaction is the light-driven transfer of electrons from water to Hill reagents non-physiological oxidants against a chemical potential gradient The Hill reaction demonstrated that oxygen O2 is produced in plants in a process that is separate from the process that converts carbon dioxide CO2 to sugars Hill's observation made a basic underlying refinement to our modern understandings of the process of photosynthesis

Contents

  • 1 History
  • 2 Attributions
  • 3 Biochemistry
    • 31 Natural electron acceptor of photosynthesis
    • 32 Using chloroplasts in vitro
    • 33 Relation to phosphorylation
  • 4 Hill reagent
  • 5 See also
  • 6 References

History

The evolution of oxygen during the light-dependent steps in photosynthesis Hill reaction was proposed and proven by the British biochemist Robin Hill He demonstrated that isolated chloroplasts would make oxygen O2 but not fix carbon dioxide CO2 This is evidence that the light and dark reactions happen in different places within the cell

Attributions

Plant cells with visible chloroplasts from a moss, Plagiomnium affine

The Hill reaction of photosynthesis was discovered by Robin Hill 1937 He found that isolated chloroplasts from plants can release oxygen when they are illuminated by sunlight in the presence of a suitable electron acceptor such as ferricyanide To demonstrate the Hill reaction in the laboratory dichlorophenolindophenol DCPIP was used as the terminal electron acceptor, replacing NADP which was not available due to the extraction of the chloroplasts from the plant cellular environment The ferrocyanide is reduced just as NADP is when the chloroplast is in vivo while water H2O is oxidized into oxygen O2 and hydrogen cations during the reaction By using this technique to observe the reaction, Hill revealed a variety of valuable facts about photosynthesis The Hill reaction confirms that oxygen O2 is produced by a reaction that is separate from carbon dioxide CO2 fixation The reaction in which oxygen is released requires light; therefore the Hill reaction can be described as the light dependent reaction of photosynthesis The reaction in which oxygen is released takes place within the chloroplast of plants and the Hill reaction demonstrated that the release of oxygen O2 is only a partial reaction which requires one step of photosynthesis

Isolated chloroplasts from spinach leaves, viewed under light microscope

The Hill reaction implies that the light dependent reaction of photosynthesis is a result of a series of redox reactions and a suitable terminal electron acceptor is required for that reaction to occur Plants have natural electron acceptors such as NADP, that play a major role in the oxidation of water The Hill reaction also shows that the natural electron acceptors of the reaction can be substituted by an artificial electron acceptor such as DCPIP, again allowing for the discharge of oxygen The technique of replacing the natural electron acceptors with artificial electron acceptors can be used in the laboratory to provide a means to measure phosphorylation in chloroplasts and observe its relation to the discharge of oxygen in this reaction

Hill's finding was that the origin of oxygen in photosynthesis is water H2O not carbon dioxide CO2 as previously believed Hill's reaction also demonstrates that the light dependent redox reaction is the first reaction to take place in photosynthesis The electrons thus freed by the splitting of water provides the electrons needed by the antenna system of photosystem II where they are boosted in energy and then sent along the electron transport system ETS Those high energy electrons are able to move against a chemical potential gradient Hence, solar energy is converted to chemical energy by the reduction of NADP to NADPH

Biochemistry

Noncyclic photophosphorylation through light-dependent reactions of photosynthesis at the thylakoid membrane

Natural electron acceptor of photosynthesis

Photosynthesis is the process in which light energy is absorbed and converted to chemical energy This chemical energy is eventually used in the conversion of carbon dioxide CO2 to sugar CH2O in plants During the process of photosynthesis, a natural electron acceptor, nicotinamide adenine dinucleotide phosphate NADP, is reduced in chloroplasts to NADPH Overall within a chloroplast, the following equilibrium reaction takes place

A reduction reaction that stores energy as NADPH:

NADP+ + 2H+ + 2e− → NADPH + H+ Reduction

An oxidation reaction as NADPH's energy is used elsewhere:

NADP+ + 2H+ + 2e− ← NADPH + H+ Oxidation

Ferredoxin, also known as a NADH+ reductase, is an enzyme that catalyzes the reduction reaction It is easy to oxidize NADPH but difficult to reduce NADP+, hence a catalyst is involved Cytochromes are conjugate proteins that contain a haem group The iron atom from this haem group undergoes redox reactions:

Fe3+ + e− → Fe2+ Reduction

Fe3+ + e− ← Fe2+ Oxidation

Using chloroplasts in vitro

A diagram of the Hill reaction which shows with the usage of an artificial electron acceptor such as DCPIP, and the chloroplast is subjected to light there is a release of oxygen, Also with the absence of CO2 there is no sugar production A diagram of the Hill reaction taking place under dark conditions there is no oxygen emitted and the no reduction of the electron acceptors occur

Robin Hill 1937 studied the redox reactions in photosynthesis using artificial electron acceptors He studied the reaction with the absence of CO2 by placing isolated active chloroplasts under light and dark conditions During his observations of isolated chloroplasts placed under light conditions but with the absence of CO2, the artificial electron acceptors were reduced and then oxidized, completing the cycle and allowing the process to proceed Oxygen O2 was released as a byproduct, but there was no production of a sugar CH2O under those conditions On the other hand, during Hill's observation of chloroplasts placed under dark conditions and in the absence of CO2 the artificial electron acceptor was oxidized but not reduced, terminating the process, with the result that there was also no production of oxygen and sugar This observation allowed Hill to conclude that oxygen is released during the light-dependent steps Hill reaction of photosynthesis

A diagram of the hill reaction under light conditions and the use of a natural electron acceptor A diagram of the light independent reaction as found by Arnon1954 which shows the formation of sugar without the presence of light

Further studies of the Hill reaction were made in 1957 by the American plant physiologist Daniel I Arnon Arnon studied the Hill reaction using a natural electron acceptor, NADP This natural electron acceptor inhibits the formation of ATP, NADPH, and H+ which are used in the light independent reaction He demonstrated that the isolated active chloroplasts which are subjected to light but without CO2 will release oxygen but no sugar will be produced He then demonstrated the light independent reaction, observing the reaction under dark conditions with an abundance of carbon dioxide He found carbon fixation was independent of the light reaction Arnon had effectively separated the light dependent reaction, which produces ATP, NADPH, H+ and oxygen, from the light independent reaction, which produces sugars He was able to conclude that that part of the reaction happens without the presence of light

Relation to phosphorylation

The association of phosphorylation and the reduction of an electron acceptor such as ferricyanide increase similarly with the addition of phosphate, magnesium Mg, and ADP The existence of these three components is important for maximal reductive and phosphorylative activity Similar increases in the rate of ferricyanide reduction can also be stimulated by a dilution technique The dilution technique will not cause a further increase in the rate in which ferricyanide is reduced with the accumulation of ADP, phosphate, and Mg to a treated chloroplast suspension ATP has been shown to inhibit the rate of ferricyanide reduction By the studies of light intensities, it is concluded that the effect is largely on the light-independent steps of the Hill reaction These observations are explained in terms of a proposed method in which phosphate esterfies during electron transport reactions, leading to the reduction of ferricyanide, and the rate of electron transport is limited by the rate of phosphorylation If there is an increase in the rate of phosphorylation, then there will be an increase in the rate by which electrons are transported in the electron transport system

Hill reagent

The addition of DCPIP experimentally to a chlorophyll molecule containing solution which shows a change in color due to the reduction of DCPIP

It is possible to introduce into the light reaction an artificial electron acceptor, such as a dye that changes color when it is reduced These are known as Hill reagents The Hill reagent was discovered in 1937 by Robin Hill These dyes permitted the finding of electron transport chains during photosynthesis dichlorophenolindophenol DCPIP, an example of these dyes, is widely used by experimenters DCPIP is a dark blue solution that becomes lighter as it is reduced It gives experimenters a simple visual test and observation of the light reaction In another approach to studying photosynthesis, light-absorbing pigments such as chlorophyll can be extracted from the chloroplasts of plant leaves or other photosynthetic living organisms Like so many important biological systems in the cell, the photosynthetic system is ordered and compartmentalized in a system of membranes

See also

  • Cell biology
  • Photophosphorylation
  • Daniel I Arnon

References

  1. ^ Hill, R 1937 "Oxygen Evolved by Isolated Chloroplasts" Nature 139 3525: 881 doi:101038/139881a0 
  2. ^ Hill, R; Scarisbrick, R 1940 "Production of Oxygen by Illuminated Chloroplasts" Nature 146 3689: 61 doi:101038/146061a0 
  3. ^ Hill, R 1939 "Oxygen Produced by Isolated Chloroplasts" Proceedings of the Royal Society B: Biological Sciences 127 847: 192 doi:101098/rspb19390017 
  4. ^ Hall, David Oakley 1981 Photosynthesis 3rd ed University of London: Edward Arnold pp 14, 79, 84 
  5. ^ a b Barber, James 1976 The intact chloroplast 1st ed Imperial college of Science and Technology p 476 
  6. ^ Dilley, Richard 1989 Photosynthesis molecular biology and biochemistry Norosa p 441 
  7. ^ Avron, M "Photosynthetic Phosphorylation relation to the Hill reaction" Research Gate Johns Hopkins University of  Missing or empty |url= help
  8. ^ Stiban, Johnny 2015 Cell Biology lab manual 6th ed Birzeit University: Dr Stiban 
  9. ^ Pentz, Lundy 1989 The biolab book 2nd ed The Johns Hopkins University press: Lundy 

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