Editing Photosynthesis (section)

==Factors==

[[File:Leaf 1 web.jpg|thumb|The [[leaf]] is the primary site of photosynthesis in plants.]]
There are four main factors influencing photosynthesis and several corollary factors. The four main are:<ref>{{cite journal |vauthors= Stirbet A, Lazár D, Guo Y, Govindjee G |date= September 2020 |title= Photosynthesis: basics, history and modelling |journal= Annals of Botany |volume= 126 |issue= 4 |pages= 511–537 |doi= 10.1093/aob/mcz171 |pmc= 7489092 |pmid= 31641747 |url= https://academic.oup.com/aob/article/126/4/511/5602694?login=true |access-date= 2023-02-09 }}</ref>
* Light [[irradiance]] and [[wavelength]]
* Water absorption
* [[Carbon dioxide]] [[concentration]]
* [[Temperature]].

Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of [[leaf]] area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to the [[chloroplast]]s to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.<ref>{{cite book |vauthors= Chapin FS, Matson PA, Mooney HA |year= 2002 |title= Principles of Terrestrial Ecosystem Ecology |publisher= Springer |pages= 97–104 |isbn= 978-0-387-95443-1 |url= https://books.google.com/books?id=shsBCAAAQBAJ&pg=PA97 |location= New York |access-date= 2019-04-17 |archive-date= 2023-01-19 |archive-url= https://web.archive.org/web/20230119181855/https://books.google.com/books?id=shsBCAAAQBAJ&pg=PA97 |url-status= live }}</ref>

===Light intensity (irradiance), wavelength and temperature===
{{See also|PI curve|label 1=PI (photosynthesis-irradiance) curve}}
[[File:Chlorophyll ab spectra-en.svg|thumb|[[Absorbance]] spectra of free chlorophyll ''a'' (<span style="color:blue;">blue</span>) and ''b'' (<span style="color:red;">red</span>) in a solvent. The action spectra of chlorophyll molecules are slightly modified ''in vivo'' depending on specific pigment–protein interactions.]]
The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life.<ref>{{cite book |vauthors= Jones HG |date= 2014 |title= Plants and Microclimate: a Quantitative Approach to Environmental Plant Physiology |edition= Third |location= Cambridge |publisher= Cambridge University Press |isbn= 978-0-521-27959-8 |url= https://books.google.com/books?id=BYALAgAAQBAJ |access-date= 2019-04-17 |archive-date= 2023-01-19 |archive-url= https://web.archive.org/web/20230119181859/https://books.google.com/books?id=BYALAgAAQBAJ |url-status= live }}</ref>

The radiation climate within plant communities is extremely variable, in both time and space.

In the early 20th century, [[Frederick Blackman]] and [[Gabrielle Matthaei]] investigated the effects of light intensity ([[irradiance]]) and temperature on the rate of carbon assimilation.
* At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance.
* At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased.

<!--[[File:carbon a t.jpg|thumb|Carbon assimilation at a constant temperature.]] unsourced graph with swapped axis titles -->
These two experiments illustrate several important points: First, it is known that, in general, [[photochemical]] reactions are not affected by [[temperature]]. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are the light-dependent 'photochemical' temperature-independent stage, and the light-independent, temperature-dependent stage. Second, Blackman's experiments illustrate the concept of [[limiting factor]]s. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, Cyanobacteria have a light-harvesting complex called [[Phycobilisome]].<ref>{{cite journal |vauthors= Adir N, Bar-Zvi S, Harris D |date= April 2020 |title= The amazing phycobilisome |journal= Biochimica et Biophysica Acta (BBA) - Bioenergetics |series= Light harvesting |volume= 1861 |issue= 4 |pages= 148047 |doi= 10.1016/j.bbabio.2019.07.002 |doi-access= free |pmid= 31306623 |s2cid= 196810874 }}</ref> This complex is made up of a series of proteins with different pigments which surround the reaction center.

===Carbon dioxide levels and photorespiration===
[[File:Photorespiration.svg|thumb|350px|Photorespiration]]
As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. [[RuBisCO]], the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, called [[photorespiration]], uses energy, but does not produce sugars.

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
# One product of oxygenase activity is phosphoglycolate (2 carbon) instead of [[3-phosphoglycerate]] (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the [[Calvin-Benson cycle]].
# Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
# Salvaging glycolate is an energetically expensive process that uses the glycolate pathway, and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produce [[ammonia]] (NH<sub>3</sub>), which is able to [[Molecular diffusion|diffuse]] out of the plant, leading to a loss of nitrogen.

::A highly simplified summary is:

:::2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP + NH<sub>3</sub>

The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.