Types of Photosynthesis
Ultimately, life on Earth depends on the sun's energy. From a biological point of view, most of this energy is harnessed through a process known as photosynthesis. Through this process, organisms use sunlight to build carbon compounds that require energy input and store that energy as chemical bonds. Photosynthesis also releases oxygen into the atmosphere. Additionally, many of our current sources of energy, such as fossil fuels, ultimately derive from ancient photosynthetic activity.
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Brief Description
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Photosynthetic organisms use the sun's energy to combine carbon dioxide (CO2) and water to form sugars. In addition to sequestering CO2 they release oxygen (O2) into the atmosphere. Sunlight is captured by pigments, the most important of which are the different types of chlorophyll. Both CO2 and O2 are exchanged with the environment thorough pores known as stomata, which can be opened or closed
Photorespiration
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The catalyst responsible for introducing CO2 molecules into the cycle, an enzyme known as rubisco, can also mediate the incorporation of O2. This process is known as photorespiration, and it releases CO2 and consumes oxygen. Thus, photosynthesis and photorespiration are opposing processes, and plants have evolved mechanisms to minimize the amount and consequences of photorespiration. The following sections discuss several means that plants use to minimize CO2 loss.
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C3 Photosynthesis
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The name derives from the fact that CO2 is incorporated into a 3-carbon compound. It is the most common. Under cool and moist conditions it is very efficient. Although some photorespiration may occur at higher temperatures, the system scavenges the CO2 released and recovers up to 75% of it.
C4 Photosynthesis
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In this type of photosynthesis CO2 is incorporated into a 4-carbon compound. This mechanism is found in plants that live in hot environments. It works by concentrating CO2 in the site of activity of Rubisco. This has the consequence of drastically reducing photorespiration, because the enzyme will preferentially use the more abundant CO2 over oxygen. It requires special anatomic structures.
CAM Photosynthesis
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This process is found in desert plants such as cacti. It enables these plants to use water more efficiently, an obvious advantage in a desert environment. In order to exchange CO2 with the environment, plants have to open their pores, but this exposes them to water loss through evaporation. The CAM mechanism works by allowing the plants to incorporate CO2 from the environment at night, minimizing water loss. The CO2 is stored and used during the day, when light is again available to power photosynthesis.
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