Difference Between C3 C4 and CAM Pathway

Many a time you may have read in the newspaper that there is a 20 per cent reduction in yielding wheat, soybean or any other crop. But, what is the factor behind reduced yield? The answer is photorespiration. This process starts when the carbon-fixing enzyme – RuBisCO encounters oxygen instead of carbon dioxide and affects the plants.

C₃, C₄ and Cam pathways are the adaptations that permit several plant species to lessen photorespiration in them. These pathways function in a way such that RuBisCO grabs large concentrations of carbon dioxide and prevents it from binding with oxygen. Before studying the difference between C₃, C₄ and Cam pathways, you must know them individually.

Notably, this is one of the vital chapters for your NEET exam and consequently, understanding their differences is of paramount importance.

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C₃ Pathway

In the process of photosynthesis, C₃ is the most typical of the three pathways used for carbon fixation. In other words, the C₃ Pathway is related to a normal plant that lacks photosynthetic adaptations to minimise photorespiration. For a 3-PGA (three-carbon molecule), produced by the reaction, C3 plants use a standard mechanism for carbon fixation by rubisco (which is the first step of the Calvin Cycle). C₃ plants account for around 85 percent of all plant species on the earth, including rice, wheat, soybeans, and all trees.

Here, ribulose bisphosphate (RuBP), carbon dioxide, and water react to form two molecules of 3-phosphoglycerate by the below-mentioned reaction.

CO₂ + H₂O + RuBP -> (2) 3 – phosphoglycerate

This is the first step that takes place in the Calvin-Benson cycle. Moreover, plants that completely live on the C₃ pathway (also called C₃ plants) tend to thrive in areas with:

•     Moderate sunlight,

•     Medium temperatures,

•     Concentrations of CO₂ are about 200 ppm or more,

•     Ample groundwater.

Note: C₃ plants have their origin from the Paleozoic and Mesozoic eras and precede C4 plants. They constitute nearly 95 per cent of the plant biomass of Earth which includes essential food crops like rice, soybeans, wheat and barley.

C₄ Pathway

Also known as the Hatch-Slack pathway, this one is another photosynthetic process. It involves the primary step in removing carbon from CO₂, to utilise it in sucrose (sugar) and various other biomolecules. The term C₄ signifies the four-carbon molecule, which is the initial product of this pathway.

Plants which follow C₄ carbon fixation are more advantageous than plants possessing C₃ pathway under extreme conditions of temperature, drought and low carbon dioxide and nitrogen. For instance, consider C₃ and C₄ grasses growing in a similar environment of 30 degrees Celsius. It is observed that C₄ grasses lose only about 277 molecules of H₂O per carbon dioxide molecule whereas C₃ grasses lose 833 molecules, which is quite a high number.

There are approximately 8,100 species of plants that utilise the C₄ pathway, and all of them are angiosperms. Poaceae (a grass species) uses C₄ carbon fixation the most. Some crops which fall under C₄ plants are:

•     Sugar cane

•     Maise

•     Millet

•     Sorghum

CAM Pathway

Crassulacean acid metabolism or CAM is the third carbon fixation procedure used by plants where the stomata remain closed during the daytime to lessen evapotranspiration. But, it opens during the night to gather carbon dioxide, allowing it to scatter in mesophyll cells.

Take a look at the following two-part cycle that will help you in comprehending the CAM pathway better.

• During night time

As said earlier, at night CAM plants have their stomata open which allows the entry of CO₂ that gets fixed like organic acids through phosphoenolpyruvate (PEP) reaction. Vacuoles store the final organic acids, which are to be used later because the Calvin cycle cannot function in the absence of NADPH and ATP.

• During day time

To store water, stomata remain shut throughout the daytime, and the organic acids which hold CO₂ get released. Furthermore, an enzyme present in the stroma produces carbon dioxide, which moves into the Calvin cycle to enable photosynthesis.

Plants that show CAM are mostly found in places where there are alternatively shortage and availability of water. Cacti, agave, and clusia pratensis are a few CAM plants examples. This type of carbon fixation is seen in aquatic plants as well. Isoetes, Littorella, Crassula, and Sagittaria are four genera of marine species which follow CAM photosynthesis.

Now that you have been enlightened with C₃ C₄ and CAM carbon fixation separately let us move on with the difference between C₃, C₄, and CAM pathways.

Difference Between C₃, C₄, and CAM Pathway

The main distinction between C₃ C₄ and CAM photosynthesis is that C₃ photosynthesis produces a three-carbon compound via the Calvin cycle, while C₄ photosynthesis produces an intermediate four-carbon compound that splits into a three-carbon compound for the Calvin cycle, whereas CAM photosynthesis collects sunlight during the day and fixes carbon dioxide at night. Furthermore, C₃ photosynthesis occurs in the majority of plants, whereas C₄ photosynthesis occurs in just around 3% of vascular plants, such as crabgrass, sugarcane, and corn. Meanwhile, plants that have adapted to arid settings, such as cactus and pineapples, use CAM photosynthesis.

Photosynthesis pathways C₃, C₄, and CAM are three main types of photosynthesis pathways with differing Calvin cycle modes. To counteract photorespiration, they use a variety of techniques. C₃ plants have no specific traits to combat photorespiration, but C₄ plants do so by executing carbon dioxide fixation and the Calvin cycle in distinct cells. By executing carbon dioxide fixation and the Calvin cycle at different times, CAM plants reduce photorespiration.

C₃, C₄, and CAM Photosynthesis Pathway Similarities

Plants use three different types of photosynthetic processes: C₃, C₄, and CAM photosynthesis.

• In general, photosynthesis is the plant’s biological process for storing energy from the sun to synthesize tiny organic compounds like glucose from carbon dioxide and water.

• In each kind of photosynthesis, the light reaction and the Calvin cycle proceed in the same way. They differ, however, in terms of carbon fixation mechanisms.

• Increases in daily, monthly, and annual mean temperatures, as well as the severity, frequency, and duration of exceptionally low and high temperatures, are all contributing to global climate change. Temperature and other environmental changes have a direct impact on plant growth and are important determinants of plant dispersal. Knowing how effectively plants can survive or acclimatize to the new environmental order is important since humans rely on plants for sustenance both directly and indirectly.

Effects Of The Environment On Photosynthesis

Through the process of photosynthesis, all plants take atmospheric carbon dioxide and convert it to sugars and starches, but they do so in different ways. The photosynthetic mechanism (or pathway) of each class of plant, is a variation of a set of chemical reactions known as the Calvin Cycle. These processes have an impact on the number and kind of carbon molecules a plant produces, the storage locations for those molecules, and, most critically for climate change research, a plant’s ability to endure low carbon atmospheres, higher temperatures, and lower water and nitrogen.

When C₃ and C₄ plants respond differently to variations in atmospheric carbon dioxide concentration, temperature, and water availability, these photosynthetic processes—designated by botanists as C₃, C₄, and CAM—are directly relevant to global climate change studies.

Humans rely on plant species that do not survive in hotter, dryer, and more unpredictable climates. Researchers have begun investigating how plants can adapt to the changing climate as the world continues to warm. One approach to do so might be to alter photosynthetic mechanisms.

Possibility Engineering And Evolution

Global food poverty is already a major issue, so continuing to rely on inefficient food and energy sources is a risky course to take, especially since we don’t know how plant cycles will be altered as our atmosphere becomes more carbon-rich. The evolution of C₄ and CAM is assumed to have been aided by a decrease in atmospheric CO₂ and a drying of the Earth’s climate, raising the worrying potential that increased CO₂ will reverse the conditions that favoured these alternatives to C3 photosynthesis.

Photosynthesis In The Future

The promise to improve food and energy security has prompted a surge in photosynthesis research. Photosynthesis feeds us with food and fiber, as well as the majority of our energy sources. Even the bank of hydrocarbons found in the Earth’s crust was formed by photosynthesis.

The globe will face the issue of replacing fossil fuels with renewable resources as fossil fuels become depleted—or if humans limit their usage of fossil fuels to avoid global warming. It is unrealistic to expect human evolution to keep pace with climatic change over the next 50 years. Plants, scientists hope, will be on a different level thanks to improved genomes.

Conclusion

This is all about the difference between C₃, C₄, and CAM Pathway. Understand the differences between these processes along with the similarities to get hold of the concept of these enzymatic processes.