What Is the Purpose of Photosynthesis?
Photosynthesis is a naturally occurring event by which some bacteria, protistans and plants use sunlight energy to make sugar or chemical energy. The overall purpose of photosynthesis is to change inorganic carbon dioxide and water molecules into organic compounds or carbohydrates. Photosynthesis's general equation is 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O. Does this Spark an idea?
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Stage 1 - Light Dependent Process
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Photosynthesis is a two-stage process. The first process--the light dependent process--requires the direct energy of light to make energy carrier molecules that are used in the second process. In the light dependent process, green plant chlorophyll is hit by sunlight in such a way as to stimulate electrons to a higher state of energy. In a series of reactions, the energy is changed into adenosine triphosphate (ATP) and nicotine adenine dinucleotide p (NADPH) and water molecules are divided. When water molecules split, oxygen necessary to human life is released. Also, during the light dependent process, carbon dioxide is captured from the atmosphere and used to form an organic compound when hydrogen from the water molecules is added to form carbohydrates.
Stage 2 - Dark Reactions/Calvin Cycle
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The second process of photosynthesis is referred to as carbon-fixing reactions, dark reactions or light independent reactions. For this process to occur, carbon dioxide must enter and diffuse into single-celled and aquatic autotrophs. When carbon dioxide enters the leaves of land plants, it prevents their drying out. Science refers to this carbon dioxide plant invasion as the Calvin cycle. In plant leaves, the 5-C chemical ribulose biphosphate (RuBP) is extracted from the carbon dioxide. The enzyme catalyzing the carbon dioxide fixation is called RuBisCO. Six molecules of carbon dioxide during the Calvin cycle eventually will make one molecule of glucose. As with humans, glucose is used for energy.
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Photorespiration
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RuBisCO, the enzyme that begins carbon dioxide fixation, binds to oxygen instead of carbon dioxide in high oxygen, low carbon dioxide conditions. In these instances, photosynthetic energy does not make organic carbohydrates. During photorespiration, oxygen is used and carbon dioxide is produced due to the unusual RuBisCO fixation, thereby mimicking natural plant respiration.
Preliminary C-4 Cycle
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For some plants, before the Calvin cycle can begin, a preliminary step known as C-4 has evolved. These kinds of plants include sugar cane, crabgrass and corn as they tend to grow close together, thereby decreasing access to carbon dioxide. While most carbon attachment begins with RuBP, C-4 begins with a new molecule, phosphoenolpyruvate (PEP), as a result of convergent evolution. Carbon dioxide from the plant's mesophyll cell forms an attraction to PEP. When carbon dioxide is combined with PEP, a chemical called oxaloacetic acid (OAA) is made, forming sugars for energy that lie in plant leaf veins, ready to be transported through the plant.
Carbon Dioxide Equilibrium
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In photosynthesis, plants remove carbon dioxide from the atmosphere and bodies of water by changing it into organic chemicals. Carbon dioxide released from plant respiration also is used by photosynthesis. In turn, animals use carbohydrates produced by plants by photosynthesis and produce carbon dioxide, contributing to the carbon cycle. The balance between plants removing carbon dioxide and animals producing carbon dioxide actually removes excess carbon dioxide from the air and water (both of which are in equilibrium with regard to carbon dioxide).
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References
- Photo Credit Biene Blume image by Mediz from Fotolia.com
