Other Free Encyclopedias » Science Encyclopedia » Science & Philosophy: Philosophy of Mind - Early Ideas to Planck length » Photosynthesis - History Of Research, Location Of Light Reactions, Cam Photosynthesis, Photorespiration, Cyanobacteria, Anaerobic Photosynthetic Bacteria - Light reactions, Dark reactions, Photosynthesis in lower organisms, Chloroxybacteria

Photosynthesis - Photorespiration

oxygen dioxide plants rubisco

In the 1920s, the German biochemist Otto Warburg (1883-1970) discovered that plants consumed oxygen at a higher rate when they were illuminated. He also found that this increased rate of oxygen consumption inhibited photosynthesis. Stimulation of oxygen consumption by light is now referred to as photorespiration. Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy molecules made by the light reactions. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.

RuBISCO, the enzyme which fixes carbon dioxide during the Calvin cycle, is also responsible for oxygen fixation during photorespiration. In particular, carbon dioxide and oxygen compete for access to RuBISCO. RuBISCO's affinity for carbon dioxide is much higher than its affinity for oxygen. Thus, fixation of carbon dioxide typically exceeds fixation of oxygen, even though atmospheric carbon dioxide levels are about 0.035% whereas oxygen is about 21%.

If photorespiration is so wasteful, why does it occur at all? Many plant physiologists believe that photorespiration is an artifact of the ancient evolutionary history of photosynthesis. In particular, RuBISCO originated in bacteria several billion years ago when there was very little atmospheric oxygen present. Thus, there was little selection pressure for the ancient RuBISCO to discriminate between carbon dioxide and oxygen and RuBISCO originated with a structure that reacts with both. Even though most modern plants are under great selection pressure to reduce photorespiration, evolution cannot easily alter RuBISCO's structure so that it fixes less oxygen yet still efficiently fixes carbon dioxide.

Interestingly, photorespiration has been observed in all C-3 plants which have been examined, but is virtually nonexistent in C-4 plants. This is because C-4 plants segregate their RuBISCO enzyme in bundle sheath cells deep within the leaf and the carbon dioxide concentration in these cells is maintained at very high levels. C-4 plants generally have higher growth rates than C-3 plants simply because they do not waste their ATP and NADPH in photorespiration.


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over 3 years ago

The helped me a lot in understanding the photosynthesis as a relatively wasteful process.

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almost 2 years ago

tnx



www.parrniane3ard.blogfa.com

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over 2 years ago

There is a new paper about the role of Rubisco and photorespiration on the regulation of atmosphere and, also, on the nice climate on the earth:

Nisbet et al., 2011 at:
www.solid-earth-discuss.net/3/769/2011/sed-3-769-2011.pdf

my site:
http://aprovenc.perso.neuf.fr/oxygen/

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over 2 years ago

There is a new paper about the role of Rubisco and photorespiration on the regulation of atmosphere and, also, on the nice climate on the earth:

Nisbet et al., 2011 at:
www.solid-earth-discuss.net/3/769/2011/sed-3-769-2011.pdf

my site:
http://aprovenc.perso.neuf.fr/oxygen/

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almost 3 years ago

Rubisco evolution is a fascinating subject.
Yes, that enzyme “Rubisco is painfully ineffective” However, it succeed to feed all the biosphere by grabbing traces of carbon dioxide as low as 0.04 % of the present atmosphere!

Yes, Oxygen disturbs that grabbing but it is 600 times more concentrated than Carbon dioxide.

The energetic cost of the negative oxygen effect by photorespiration is not 27 % but more than 50 % at the beginning of the industrial era. See two papers online at Science Direct:

Article titles:
Modelling 18O2 and 16O2 unidirectional fluxes in plants - I. Regulation
of pre-industrial atmosphere



Modelling 18O2 and 16O2 unidirectional fluxes in plants - II. Analysis
of Rubisco evolution

http://dx.doi.org/10.1016/j.biosystems.2010.10.003

The last one shows that, if plant evolution (and, of course, the Rubisco evolution) was directed by the gain of carbon, the gain was not as large as possible, because the negative effect of oxygen was used and developped as protective mechanism against the lag of carbon dioxide. For that, we demonstrate that the maximum velocity of oxygenase reaction of Rubisco was increased faster than the maximum velocity of carboxylase, the most useful for carbon gain. That is an example of compromise between safety and richness. Photorespiration could be considred as a costly insurance.

Another aspect of Rubisco evolution is developped in
Nisbet et al., 2011. The regulation of the air: a hypothesis. Solid Earth Discussion, 3, 769-788.
with discussions en open access:
http://www.solid-earth-discuss.net/3/769/2011/sed-3-769-2011.html
where it is question of the role of the tail of dog…

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over 3 years ago

I have published a paper showing that photorespiration was not repressed by evolution but used in the co-evolution palnts-atmosphere-biosphere:
Article titles:
Modelling 18O2 and 16O2 unidirectional fluxes in plants - I. Regulation
of pre-industrial atmosphere



The second paper show how Rubisco was selected to increase the Oxygenation Capacity of plants as tool to face the high light and water stress of terrestrial environments:
Modelling 18O2 and 16O2 unidirectional fluxes in plants - II. Analysis
of Rubisco evolution