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Photosynthesis Photosynthesis (photo=light; synthesis=build) is the crucial link between the sun and the chemical energy stored in all living organisms. It involves the removal of carbon dioxide from the atmosphere by plants and the combination of it with water and other nutrients from the soil to form carbohydrates. Carbohydrate molecules contain carbon (carbo-), hydrogen (hydr-), and oxygen (ate). In the process, oxygen is released into the atmosphere. For photosynthesis to occur, sunlight is needed. The overall reaction can be written as: or more generally: carbon dioxide + water + sunlight carbohydrate + oxygen In this reaction, two important tasks are accomplished. Firstly, carbon is “fixed”, that is, converted from its inorganic form (carbon dioxide) to its organic form (carbohydrates in the form of glucose or sugar). Secondly, the sun’s dispersed light-energy is transformed into concentrated chemical energy. Once sugar is formed, it can be converted to starch for storage or combined with other nutrients such as nitrogen, phosphorous and sulfur to create more complex molecules such as proteins. Proteins are the building blocks of life, essential to the growth of the body as well as the brain. The best sources of proteins are meat, fish, eggs and milk. But does all of the energy in the sunlight participate in photosynthesis? The light required for photosynthesis must be of the right frequency, around the red end of the visible light spectrum. To capture light for 6 CO2 + 6 H2O C6H12O6 + 6 O2 (6-1)12 CO2 + 11 H2O C12H22O11 + 12 O2 (6-2)SunlightSunlight 121 Chapter 6 - Biomass Energy photosynthesis, plants have special pigments called chlorophyll; these absorb only the red and blue portions of sunlight, reflecting the green. This is why many leaves appear green. Lower frequency light does not have sufficient energy so plants do not absorb it. Higher frequency light is too strong and the excess energy is wasted. The result is that photosynthetic efficiency is at a maximum for red light,3 drops abruptly to zero for lower energies (infrared light), and falls slowly for photons of higher energies (blue and green lights). The net effect is that only 32% of solar energy participates in photosynthesis.4 In addition to chlorophyll, plants have two other pigments to trap light in regions that chlorophyll misses. Carotenes absorb blue and blue-green light and anthocyanins absorb green and yellow light and are responsible for the leaves’ yellow, orange, brown or red colors common during the fall season.5


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