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Ecological Glossary

C3 carbon fixation

C3 carbon fixation

C3 plants are plant species that use a photosynthetic pathway called the C3 cycle to fix carbon dioxide (CO2) during the photosynthesis process. This pathway is present in many plants and represents the main mechanism through which they transform sunlight into chemical energy.
During the C3 cycle, plants fix atmospheric CO2 using an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). CO2 is converted into a three carbon compound called 3-phosphoglyceric acid (3-PGA). This compound, in turn, is converted into sugars such as glucose or sucrose, which are the main form of energy that plants use for growth and development.
However, the C3 cycle has some limitations, especially under conditions of heat and high oxygen concentrations. When C3 plants are exposed to high temperatures, they can undergo a competitive reaction of the RuBisCO enzyme with oxygen, rather than CO2, in a process called photorespiration. This photorespiration can result in a loss of energy and reduce the overall efficiency of photosynthesis.
Specifically, they are called C3 because the first organic compound of photosynthesis is a carbon chain with 3 carbon atoms, 3-phosphoglyceraldehyde or glyceraldehyde 3-phosphate (G3P; 3-phosphoglycerate is abbreviated instead with the acronym 3PGA), which comes out of the Calvin cycle.
C3 plants are photosynthetically active during the day, while at night they close their stomata and become oxygen consumers. The process, unlike C4 cycle plants, takes place within a single cell and, unlike CAM plants, without the need for compartments.
C3 plants photosynthesize efficiently only at moderate temperatures (maximum efficiency occurs at 20°C) since, as the stomata are open during the day, an excessive temperature induces an increase in water transpiration from the leaves.
C3 carbon fixation is the most common of the three metabolic pathways for carbon fixation in photosynthesis, the other two being C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction: CO2 + H2O + RuBP → (2) 3-phosphoglycerate

This reaction was first discovered by Melvin Calvin, Andrew Benson and James Bassham in 1950. Fixation of carbon C3 occurs in all plants as the first step of the Calvin-Benson cycle. (In C4 and CAM plants, carbon dioxide is extracted from the malate and in this reaction rather than directly from the air.)
Furthermore, plants that survive exclusively on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are about 200 ppm or higher, and groundwater is abundant. C3 plants, native to the Mesozoic and Paleozoic eras, predate C4 plants and still account for about 95% of Earth’s plant biomass, including important food crops such as rice, wheat, soybeans, and barley.
For this reason C3 plants cannot grow in very hot areas at today’s atmospheric CO2 level (significantly reduced over hundreds of millions of years from over 5000ppm) because the RuBisCO enzyme incorporates more oxygen into RuBP (Ribulose-1,5-bisphosphate) as temperatures increase. This leads to photorespiration (also known as the photosynthetic oxidative carbon cycle, or C2 photosynthesis), which leads to a net loss of carbon and nitrogen from the plant and can therefore limit growth.
C3 plants lose up to 97% of the water absorbed by their roots through transpiration. In arid areas, C3 plants close their stomata to reduce water loss, but this prevents CO2 from entering the leaves and therefore reduces the CO2 concentration in the leaves. This lowers the CO2:O2 ratio and therefore also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot, dry areas and can therefore compete with C3 plants in these areas.
It should also be emphasized that not all C3 carbon fixation pathways work equally efficiently.

For example, bamboo and rice have an improved C3 efficiency. This improvement could be due to the ability to recover the CO2 produced during photorespiration; this feature is called: carbon refixation.
These plants achieve refixation by growing extensions of chloroplasts called “stromuli” around the stroma in mesophyll cells, so that any photorespired CO2 from the mitochondria must pass through the RuBisCO-filled chloroplast.
However refixation is also done by a wide variety of plants.
As mentioned, there are also other types of photosynthetic pathways, such as the C4 cycle and the CAM (crassulaceae acid metabolism), which have special adaptations to address photorespiration problems and make plants more efficient in certain environments. However, the C3 cycle is the most widespread and common type of photosynthesis among plants.
As far as growth environments are concerned, C3 plants are mainly found in environments with moderate temperature conditions and relatively high humidity. They are typical of temperate and subtropical regions, where temperatures do not reach extreme levels either in winter or in summer.
C3 plants include many herbaceous plants (such as wheat, oats, rice), shrub plants, and some woody plants. However, it is important to note that there are exceptions and some C3 plants may be adapted to drier or hotter climates, but in general, they prefer environments with moderate temperatures and adequate humidity for their optimal development.

Fonte foto:
https://it.wikipedia.org/wiki/Piante_C3#/media/File:Calvin-cycle4.svg
https://es.wikipedia.org/wiki/V%C3%ADa_de_3_carbonos#/media/Archivo:Cross_section_of_Arabidopsis_thaliana,_a_C3_plant..jpg




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