C4 carbon fixation
C4 carbon fixation
C4 plants are a group of plants characterized by a particular mode of photosynthesis, which differentiates them from another category of plants called C3 plants. The nomenclature comes from the number of carbon atoms in the first chemical compound that is produced during the photosynthetic process.
C4 plants have been classified as an evolutionary adaptation that allows them to grow in hot, dry environments where water availability may be limited and where C3 plants may have difficulty surviving. This type of photosynthesis is often associated with tropical and herbaceous plants.
The main difference between C4 and C3 plants is in their anatomical and biochemical structure, which allows C4 plants to be more efficient at absorbing and utilizing atmospheric CO2 during photosynthesis.
In C4 plant tissues, atmospheric CO2 is captured and converted into the four-carbon compound, phosphoenolpyruvate (PEP), through a series of initial chemical reactions that occur in the mesophyll cells. This compound is then transported to cells in the bundle-sheath, a specialized tissue, where the Calvin cycle takes place, a step in photosynthesis involving the fixation of CO2 into usable sugars.
This mechanism allows C4 plants to concentrate CO2 near photosynthetic enzymes, thus reducing water loss by evaporation (transpiration) and increasing photosynthetic efficiency in hot and dry conditions.
Examples of C4 plants are: corn, sugar cane, sorghum and several species of tropical grasses. C4 plants have been the subject of intense research and scientific interest to understand their adaptation strategies to the environment and to evaluate their role in the context of climate change and sustainable agriculture.
In C4 type plants, biochemistry covers a fundamental aspect of their physiology.
As mentioned, C4 plants are a group of plants that have evolved an additional photosynthesis mechanism to adapt to hot and dry climatic conditions, allowing them to maximize carbon assimilation and reduce water loss during photosynthesis.
Below are some key points about the biochemistry of C4 plants:
1. Carbon Fixation: C4 plants possess a carbon fixation system which is spatially separated in different cells. Atmospheric carbon is first fixed into a 4-carbon compound, malic acid, in the mesophyll cells.
2. Hatch-Slack Cycle (or C4 Cycle): Malic acid is then transported from the mesophyll cells to the sheath cells of the vascular bundles, where decarboxylation occurs, liberating carbon dioxide (CO2) within the sheath cells.
3. C4 Photosynthesis: At this point, CO2 is used in the Calvin cycle in the sheath cells of the vascular bundles for actual photosynthesis. This spatial separation of carbon fixation (in mesophyll cells) and the Calvin cycle (in sheath cells of vascular bundles) allows for higher efficiency in carbon capture and a reduction in the photorespiratory effect compared to C3 plants.
4. Adaptation to environment: The C4 mechanism is particularly beneficial for plants growing in hot, dry environments, where evaporation of water through stomata (small openings on the surface of leaves) can be a challenge. C4 plants reduce water loss during photosynthesis and can tolerate drought conditions better than C3 plants.
5. Photosynthetic efficiency: Thanks to the C4 mechanism, these plants have a higher photosynthetic efficiency and therefore a higher productivity than C3 plants under certain environmental conditions.
Therefore, C4 plants are defined as some species of plants in warm climates but with reduced water availability, such as corn, sorghum and sugar cane, which use a different pathway for CO2 fixation (one of the steps necessary to complete the photosynthetic process). These plants have developed an alternative pathway to the Calvin-Benson cycle, organized on the presence of two functionally and morphologically different cell types, the mesophyll cells and the bundle sheath cells. The C4 photosynthesis is therefore, together with the CAM photosynthesis, an adaptation adopted by some species of plants, living in arid climates, to save water in the carbon fixation phase. This biosynthetic pathway was discovered in 1966 by two Australian researchers, M. D. Hatch and C. R. Slack, and is in fact also referred to as the Hatch-Slack biosynthetic pathway.
Photo source:
– https://en.wikipedia.org/wiki/C4_carbon_fixation#/media/File:C4_Plant_Anatomy.svg
– https://it.wikipedia.org/wiki/Piante_C4#/media/File:HatchSlackpathway2.svg