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Plants C3:examples, Calvin-Banson cycle, photorespiration and differences with C4 plants (Kranz anatomy). Useful explanations and plant physiology.
CAM, C3 and C4 plants have many differences both from a morphological point of view and from a functional point of view. What they have in common is the activity of an enzyme, the most abundant in the plant kingdom, Rubisco. Let's start here.
Plants C3, photorespiration
Rubisco is the enzyme that can fix both carbon and molecular oxygen. When Rubisco reacts with carbon dioxide it produces two molecules3-phosphoglycerate, essential to kick off the Calvin-Banson cycle (cycle C3). The cycle C3 it is used by plants for growth, to produce sugars and biomass.
When the Rubisco reacts with oxygen, it starts thephotorespirationwhich produces, instead, one molecule of 2-phosphoglycolate and only one molecule of 3-phosphoglycerate. This reaction is considered to be limiting for the growth and development of the plant.
It is no coincidence that theplants C3, thephotorespirationoccurs when the environmental conditions are not optimal. Photorespiration, although it is described as a limiting process for the growth ofplants C3, is an important phenomenon. It occurs when the stomata are closed, in this situation CO2 cannot enter and the cell tends to accumulate oxygen.
Oxygen is dangerous because the cascade of electrons from the Photosystems (photosynthesis) can produce superoxide, which is very unstable and harmful.
So although many authors will tell you that the photorespiration it is a mechanism that slows down plant growth, it is actually safeguarding cellular life. Photorespiration:
- manages to dispose of oxygen that is dangerous for the cell
- produces useful intermediates for fixing nitrogen and sulfur
- produces CO2 that cannot enter the cell with closed stomata
- produces a molecule of 3-phosphoglycerate useful for maintaining basal levels of the Calvin-Banson cycle
In the articledifferences between C3 and C4 plants, I have already explained a lot about the activity of this enzyme. Unlike C4 and CAM plants, in plants C3 the Rubisco does not enjoy a system ofconcentration and accumulationof CO2, therefore depending on the conditions that occur in the cell, it can bind both CO2 and O2.
Plant Anatomy C3
In C3 plants we can describe four different morphological compartments. Observing the leaf blade, we can report:
- Upper epidermis
It is used to waterproof the leaves, is free of photosynthetic cells and rich in substances such as waxes and suberin.
- Palisade parenchyma
Nicknamed for its appearance, it is characterized by elongated cells with intense photosynthetic activity.
- Gaps in the parenchyma
It is characterized by cells that are organized more loosely, the spaces are called "gaps" and are used to promote gas exchange.
- Lower epidermis
It is characterized by the presence ofstomata, photosynthetic cells that allow the phenomenon of leaf transpiration but also the release of oxygen and the entry of CO2.
Differences with Anatomia Kranz, cycle C3 and cycle C4
Theplants C4they do not share the same anatomy as the C3s. In C4 we speak ofKranz anatomybecause the two parenchyma are not distinguished (palisade and gaps) but a single compartment defined, in a generic way,foliar mesophyll. In theKranz anatomythe two epidermis are the same as those described for C3, the only difference is that the cells are larger. In the Kranz anatomy the structural difference between the chloroplasts of the mesophyll cells and the chloroplasts of the sheath cells of the bundle is fundamental.
The chloroplasts of the mesophyll cells are canonical but lack Rubiscus and primary starch (in fact the Calvin Banson cycle or the c3 cycle does not occur here). What is called occurs in the chloroplasts of mesophyll cellscycle C4. It is producedSickstarting fromPhosphenolpyruvate. Thesickit is used as a source of CO2 in the sheath cells.
In the chloroplasts of the sheath cells, on the other hand, photosystem II is missing (so no oxygen is produced) but rubiscus and starch are also present. It is in these organelles that the C4 cycle continues with the Calvin Banson cycle. The diseased is transported from the mesophyll cells to the chloroplasts of the sheath cells. In the chloroplasts of the sheath cells, the patient produces CO2 which is essential for carbon fixation and the beginning of the Calvin-Banson cycle.
What are the C3 plants? All plants do the “Calvin-Banson cycle”, even CAMs and C4s. Plants that do only the C3 and C2 cycle (understood as the Calvin Banson cycle and photorespiration) are plants that generally have their habitat in temperate climates and a strong growth. One of the many examples of the C3 plant is Arabidopsis thaliana, undoubtedly the most studied species of all time! The anatomy of the C3 is shown in the photo above.
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