Difference between revisions of "Photosynthesis"

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This article gives an overview of the process called photosynthesis.  
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{{Definition|title=Photosynthesis
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|definition= the process by which carbon dioxide, water, and certain inorganic salts are converted into carbohydrates by green plants, algae and certain bacteria. }}
  
  
 
==Introduction==
 
==Introduction==
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Photosynthesis is the process of using light energy to synthesize carbohydrate molecules (<math> (CH_2O) _n </math>) from carbon dioxide and water through a complex series of reactions. The overall process can be described as:
  
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Photosynthesis is the transformation of light energy into chemical energy for use in living cells. The overall process can be described as:
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<math>n \; CO_2 + n \, H_2O + \mathrm{light} \rightarrow (CH_2O)_n + n\, O_2 , \qquad (1)</math>
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<math>CO_2 + 2 H_2O  \rightarrow (CH_2O)_n + H_2O + O_2</math>
 
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With the aid of light and plants, water is split into hydrogen and oxygen. Carbohydrates (<math>CH_2O</math>) such as sugars and starches are produced.
 
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In plants, photosynthesis occurs in the '''chloroplasts'''. These are cell organelles bounded by a double membrane. Within these chloroplasts is a membrane system where light is captured and transformed into chemical energy, through the reaction of carbon dioxide (<math>CO_2</math>) and water (<math>H_2O</math>). The membranes are surrounded by spaces where this energy is used to produce the carbohydrates.
 
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<gallery>
 
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image:photosynthesis-overview.gif|Photosynthesis in chloroplast
 
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image:Chloroplast-new.jpg|Chloroplasts <ref>http://en.wikipedia.org/wiki/Chloroplast</ref>
 
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image:chloroplast.jpg|Chloroplast in cells
 
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</gallery>
 
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Light energy is captured by '''photosynthetic pigments'''. All plants have '''chlorophyll a''', which absorbs light energy most efficiently in the blue and green regions of the visible light spectrum.
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where the carbohydrate molecules <math>(CH_2O) _n </math> stand for monosaccharides, for example: glyceraldehyde if n=3, glucose or fructose if n=6.  
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Other pigments also absorb light, but in the parts of the light spectrum where chlorophyll a is relatively inefficient. These kind of pigments are called '''accessory pigments'''.
 
  
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The carbohydrate molecules are used in the metabolism of the photosynthesizing organisms. The photosynthesis process takes place in two steps: the first step, the light-dependent step, is the conversion of light energy into chemical energy. The second step is the use of this chemical energy to fix carbon for the production of carbohydrate molecules; this light-independent step is called the "Calvin cycle".
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The first step uses pigments such as chlorophyll to absorb light, which allows the stripping of electrons from water molecules. This process yields oxygen gas as a byproduct. The electrons serve to reduce two molecules, called ADP and NADP, into ATP and NADPH respectively. ATP and NADPH provide chemical energy that is used  in the second step, the Calvin cycle. In this second step an enzyme called RuBisCO captures CO<sub>2</sub> for the  production of carbon sugars.
  
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The whole process of photosynthesis is divided into the '''light reactions''' and the '''Calvin cycle'''. The light reactions capture the light energy. This is done by two reactions. The first one is that light energy is used in conjunction with a complex series of reactions to oxidize water and to split it. This produces molecular oxygen, protons (<math>H^+</math>) and electrons (<math>e^-</math>).
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In plants, the chlorophyll pigment is held inside organelles called chloroplasts, where the photosynthesis process takes place, see Fig. 1. In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes.  
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The electrons are transported by a chain of reactions and the in two distinct biochemical '''photosystems'''. The electrons then help ADP to convert into ATP, the major source of energy for the Calvin cycle. Next to this, electrons reduce NADP to NADPH, a major source of chemical reducing power in the Calvin cycle.  
 
  
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The Calvin cycle involves the molecules NADPH (nicotinamide adenine dinucleotide phosphate), ATP (adenosine triphosphate) and RuBP (ribulose-1,5-bisphosphate). The enzyme Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCo) catalyzes the carboxylation of RuBP.  The Calvin cycle can be expressed by the equation
  
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In the Calvin cycle, carbon is fixed to carbohydrates.
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<math>3 \, CO_2 + 9 \, ATP + 6 \, NADPH + 6 \, H^+ \rightarrow  C_3H_6O_3 phosphate + 9 \, ADP + 8 \, P_i + 6 \, NADP^+ + 3 \, H_2O  , \qquad (2) </math>
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The overall reaction is:
 
  
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[[image:Calvin_cycle.jpg|thumb|center|300px|caption|The Calvin cycle]]
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where <math>P_i</math> stands for inorganic phosphate. The Calvin cycle is schematically depicted in Fig. 2.
  
  
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The enzyme '''ribulose biphosphate carboxylase''' or '''Rubisco''' enables the reaction of carbon dioxide, water and the sugar ribulose biphosphate (<math>C_5</math>) to produce a <math>C_3</math> molecule. This <math>C_3</math> molecule is the precursor of the <math>C_6</math> sugars. ATP and NADPH are both required to make the reaction possible. Rubisco is generally regenerated from most of the reaction series. <ref>Levinton J.S. 2001. Marine biology: function, biodiversity, ecology. Oxford University Press. p. 515</ref>
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{| border="0" align="center"
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|-
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| valign="top"|
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[[File:ChloroplastScheme.jpg|thumb|left|400px|Fig. 1. Schematic representation of the chloropast with the two-stage photosynthesis process. Credit: Mariana Ruiz Villarreal for CK-12 Foundation.]]
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| valign="top"|
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[[File:CalvinCycleScheme.jpg|thumb|left|400px|Fig. 2. Schematic representation of the Calvin cycle in which glycerate 3-phosphate (C<sub>3</sub>H<sub>6</sub>O<sub>3</sub>-phosphate) is produced as a result of CO<sub>2</sub> fixation by the enzyme RuBisCo. Credit Mike Jones. https://commons.wikimedia.org/w/index.php?curid=9504547]]
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|}
  
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Organisms that carry out photosynthesis are called [[photoautotroph]]s.
  
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A more detailed description of the photosynthesis process can be found in the wikipedia https://en.wikipedia.org/wiki/Photosynthesis and in many textbooks, for example
  
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Levinton J.S. 2001. Marine biology: function, biodiversity, ecology. Oxford University Press. pp. 515.
  
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==References==
 
  
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<references/>
 
  
  
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{{2Authors
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|AuthorID1=16323
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|AuthorFullName1=Katrien Töpke
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|AuthorName1=Topke K
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|AuthorID2=120
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|AuthorFullName2=Job Dronkers
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|AuthorName2=Dronkers J
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}}
  
  
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{{author
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[[Category:Coastal and marine ecosystems]]
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|AuthorID=16323
 
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|AuthorFullName=TÖPKE, Katrien
 
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|AuthorName=Ktopke}}
 

Latest revision as of 11:30, 4 October 2021

Definition of Photosynthesis:
the process by which carbon dioxide, water, and certain inorganic salts are converted into carbohydrates by green plants, algae and certain bacteria.
This is the common definition for Photosynthesis, other definitions can be discussed in the article


Introduction

Photosynthesis is the process of using light energy to synthesize carbohydrate molecules ([math] (CH_2O) _n [/math]) from carbon dioxide and water through a complex series of reactions. The overall process can be described as:

[math]n \; CO_2 + n \, H_2O + \mathrm{light} \rightarrow (CH_2O)_n + n\, O_2 , \qquad (1)[/math]

where the carbohydrate molecules [math](CH_2O) _n [/math] stand for monosaccharides, for example: glyceraldehyde if n=3, glucose or fructose if n=6.

The carbohydrate molecules are used in the metabolism of the photosynthesizing organisms. The photosynthesis process takes place in two steps: the first step, the light-dependent step, is the conversion of light energy into chemical energy. The second step is the use of this chemical energy to fix carbon for the production of carbohydrate molecules; this light-independent step is called the "Calvin cycle".

The first step uses pigments such as chlorophyll to absorb light, which allows the stripping of electrons from water molecules. This process yields oxygen gas as a byproduct. The electrons serve to reduce two molecules, called ADP and NADP, into ATP and NADPH respectively. ATP and NADPH provide chemical energy that is used in the second step, the Calvin cycle. In this second step an enzyme called RuBisCO captures CO2 for the production of carbon sugars.

In plants, the chlorophyll pigment is held inside organelles called chloroplasts, where the photosynthesis process takes place, see Fig. 1. In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes.

The Calvin cycle involves the molecules NADPH (nicotinamide adenine dinucleotide phosphate), ATP (adenosine triphosphate) and RuBP (ribulose-1,5-bisphosphate). The enzyme Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCo) catalyzes the carboxylation of RuBP. The Calvin cycle can be expressed by the equation

[math]3 \, CO_2 + 9 \, ATP + 6 \, NADPH + 6 \, H^+ \rightarrow C_3H_6O_3 phosphate + 9 \, ADP + 8 \, P_i + 6 \, NADP^+ + 3 \, H_2O , \qquad (2) [/math]

where [math]P_i[/math] stands for inorganic phosphate. The Calvin cycle is schematically depicted in Fig. 2.


Fig. 1. Schematic representation of the chloropast with the two-stage photosynthesis process. Credit: Mariana Ruiz Villarreal for CK-12 Foundation.
Fig. 2. Schematic representation of the Calvin cycle in which glycerate 3-phosphate (C3H6O3-phosphate) is produced as a result of CO2 fixation by the enzyme RuBisCo. Credit Mike Jones. https://commons.wikimedia.org/w/index.php?curid=9504547

Organisms that carry out photosynthesis are called photoautotrophs.

A more detailed description of the photosynthesis process can be found in the wikipedia https://en.wikipedia.org/wiki/Photosynthesis and in many textbooks, for example

Levinton J.S. 2001. Marine biology: function, biodiversity, ecology. Oxford University Press. pp. 515.



The main authors of this article are Katrien Töpke and Job Dronkers
Please note that others may also have edited the contents of this article.
Citation: Katrien Töpke; Job Dronkers; (2021): Photosynthesis. Available from http://www.coastalwiki.org/wiki/Photosynthesis [accessed on 25-11-2024]
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