Photosynthesis

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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

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 26-09-2021]