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3 types of photosynthesis

 

Photosynthesis is a fascinating process that has puzzled scientists for centuries. It is the mechanism through which plants and other photosynthetic organisms transform the energy from sunlight into chemical energy, which is stored in organic molecules, primarily carbohydrates. This process is possible thanks to the magical interplay between the Greek words “ph?s” (light) and “sýnthesis” (synthesis), which gave birth to the term photosynthesis.

Most plant species engage in photosynthesis, with some exceptions, such as parasitic plants. Similarly, most algae and cyanobacteria also conduct photosynthesis, making them photoautotrophic organisms. The type of photosynthesis they carry out is known as oxygenic photosynthesis, named for its byproduct of molecular oxygen (O2).

This remarkable reaction combines carbon dioxide (CO2) and water (H2O) with the help of light energy, ultimately generating glucose and oxygen as the end products. It’s the opposite of what happens in the human body’s mitochondria, where glucose is “burned” to produce carbon dioxide and water.

Despite this knowledge, there are still unanswered questions in the realm of photosynthesis. Scientists have discovered that other types of photosynthesis exist that don’t use water as a substrate, and generate waste products other than oxygen. These kinds of photosynthesis fall under the category of anoxygenic photosynthesis, and they present a mystery that scientists are eager to unravel. For example, sulfur bacteria use hydrogen sulfide and produce sulfur instead of oxygen, raising more questions than answers.

Intriguingly, photosynthesis is not only an essential process for the survival of plants and other photosynthetic organisms, but it also plays a crucial role in our lives. The carbohydrates and other organic molecules produced by photosynthesis serve as a source of food for humans and animals. Without photosynthesis, life as we know it would not be possible.

The 3 types of photosynthesis in plants

Plant photosynthesis is a complex process that can be broken down into different types depending on the method and conditions under which it occurs. While all types of photosynthesis share common basic features, they each have unique characteristics that allow plants to adapt and thrive in different environments. In this article, we explore the three main types of photosynthesis used by plants: C3 photosynthesis, C4 photosynthesis, and CAM photosynthesis.

C3 photosynthesis

The most common type of photosynthesis is C3 photosynthesis, which is used not only by plants but also by algae and bacteria. In this process, plants use a photosynthetic route that incorporates CO2 from the atmosphere in a reaction whose first organic molecule consists of three carbon atoms, phosphoglyceric acid (PGA). The enzyme responsible for catalyzing this first photosynthetic reaction is rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), which is estimated to be the most abundant protein enzyme in the biosphere.

Rubisco catalyzes the same process in the opposite direction, carboxylating and oxygenating during photosynthesis and photorespiration, respectively. Photorespiration is activated in conditions of high light and high temperature. C3 photosynthesis is the most common type observed in nature under normal light and temperature conditions. Photosynthesis takes place throughout the leaf, and the stomata remain open during the day, which can result in water loss through transpiration.

C4 photosynthesis

The C4 photosynthesis pathway, also known as the Hatch-Slack pathway, is followed by many tropical and warm-weather plants. Carbon dioxide is fixed in a first molecule with four carbon atoms, which can be malic acid or aspartic acid depending on the exact type of plant. In tropical climates, there is more solar radiation and higher temperatures, and the activation of photorespiration could pose a great problem for the survival of the plant.

In C4 plants, photosynthesis does not take place throughout the leaf, but rather in inner cells arranged in a specialized anatomical structure called the Kranz anatomy. CO2 is absorbed more quickly than in C3 plants and is fixed by the enzyme PEP carboxylase. Subsequently, CO2 is transferred to the rubisco in the internal structures of Kranz’s anatomy where the activation of photorespiration is very limited.

Compared to C3 plants, C4 plants have a higher photosynthetic rate under high light intensity and high-temperature conditions. Because PEP carboxylase absorbs CO2 faster, plants do not need to keep their stomata open as long as C3 plants, thus reducing transpiration and improving the efficiency of water use. C4 plants include thousands of plants belonging to at least 19 families.

CAM photosynthesis

The third type of photosynthesis is CAM (Crassulacean Acid Metabolism) photosynthesis, which refers to a special photosynthetic pathway first observed in the Crassulaceae family that differs considerably from C3 and C4 photosynthesis. In CAM plants, the absorption and fixation of CO2 occur at different times.

CAM plants absorb CO2 overnight and store it in vacuoles in the form of malic acid. The next day, when it is light, CO2 is released from malic acid and supplied to the Calvin cycle to synthesize carbohydrates. One of the main adaptive advantages of these plants is that they can keep their stomata closed during the hottest hours of the day, thus minimizing water loss through transpiration.

In addition, these plants can survive in conditions of low concentration of CO2, since they take it in and store it during the night, highlighting submerged freshwater plants as an example. CAM plants include many succulents .

 

References

  1. Christine and Edwards. The C4 plant lineages of planet Earth . Journal of Experimental Botany .

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