What is "photorespiration" and why it is needed by plants?

Photorespiration occurs when carbon dioxide levels within the leaf tissue dwindle to about 50 parts per million, usually on a hot, dry day where a plant has closed its stomates to prevent water loss. At this concentration of CO2, oxygen reacts with ribulose bisphosphate (RuBP) in the presence of RubisCO, an enzyme used in the Calvin cycle of photosynthesis, and creates 2 molecules of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate. Under normal oxygen and carbon dioxide concentrations, the reaction is between CO2, RuBP, and RubisCO to produce 12 molecules of 3-phosphoglycerate. To top it all off, 2-phosphoglycolate is toxic, so the plant has to convert it into a non-detrimental compound through a number of steps, each of which costs energy and CO2. Photorespiration is wasteful, as well, in that it does not produce any 3-phosphoglyceraldehyde (PGAL), which is used by the plant for a large number of functions. If you take a look at the Calvin cycle (http://en.wikipedia.org/wiki/Calvin_cycle) you can see where things go wrong with photorespiration. It is interesting to note that photorespiration only occurs in C3 plants, which thrive where temperature and light intensity are moderate, water is abundant, and carbon dioxide concentrations are 200 ppm or higher. Something like 95% of all plants on Earth use this form of metabolism. Plants that have developed a way to circumvent the problems of photorespiration are called C4 and CAM (crassulacean acid metabolism) plants. The website http://www.park.edu/bhoffman/courses/bi225/recaps/C3C4.htm has an excellent, brief overview of the differences between C3 and C4 plants.

First of all pop93star, you should probobly just stick to your media, because obviously you know nothing about biology. And Erin, don't just copy and paste things you have no prior knowledge or understanding about. In order to give the truth about this matter, perhaps you people should be looking for an answer from someone who know's what they are talking about. Like myself, for example who is a Biology Major. For starts, reading the article on Wikipedia wouldn't hurt, it seems to have a somewhat decent article on the subject which could give anyone a good general overview of photorespiration.

But in order to make matters regarding RuBP and RuBisCo actually TRUTHFUL, here's what you really need to know. First of all, they are not the same thing. RuBP stands for Ribulose Bisphosphate which is a 5-Carbon organic compound already contained within the chloroplasts of plant cells. When CO2 (Carbon Dioxide) molecules go through the Calvin Cycle, RuBisCo (Ribulose Bisphosphate Carboxylase) which is an ENZYME, adds the CO2 molecules (one at a time) to RuBP, thus forming a 6-Carbon Compound which is highly unstable. As a result, this compound immediately splits into 2 SEPERATE 3-Carbon molecules called 3-Phosphoglycerates, which will then travel through the rest of the cycle ultimately yielding Glyceraldehyde-3-Phosphate (G3P).

However, in Photorespiration, instead of CO2 being added by RuBisCo to RuBP, O2 (Oxygen) is instead. This is a result of hot & dry weather in which a plant closes certain pores called stomata so that water doesn't evaporate from its cells. The outcome caused by photorespiration is thus considered wasteful because when O2 travels through the Calvin Cycle rather than CO2, the end result is a release of CO2 into the surrounding atmosphere and NO production of sugar which the plant needs as a source of energy. Not to mention, the light reactions of photosynthesis which normally produce products needed BY THE CALVIN CYCLE to convert CO2 to sugar (stored energy), are simply wasted in photorespiration because nothing essential to the cell is produced.

In Photosynthesis:
1 turn of Calvin Cycle produces 1 G3P, but Calvin Cycle most go through 2 turns and make 2 G3P because 2 turns uses 1 CO2, 2 NADH, 3 ATP (you can't use 1/2 CO2)

In Photo-respiration:
2 turns in Calvin Cycle creates 1 G3P and 1 phosphoglycolate (which is toxic to plants and takes a detour to break down). Photo-respiration is used so that plants don't lose water, but still able to produce G3P, but inefficient compare to photosynthesis.
Courtesy of Biology-Online.org
http://www.biology-online.org/dictionary/Photorespiration