Biology

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The Chemistry of Photosynthesis Introduction The sun is primary source of energy in the universe for existence of life on earth. The enormous amount of light energy present in the sun is being utilized in various forms thereby promoting life and survival of certain living organisms. The conversion of light energy emitting from sun into chemical energy and storing it in the forms of sugar is a unique process which is known as photosynthesis. Photosynthesis is carried out by many different organisms, ranging from plants to bacteria. It may be defined as the process in which plants, some bacteria and some protistans consume the light energy of sun along with CO2 and H2O to produce sugar. The process of photosynthesis takes place in the Chloroplasts with the help of green pigment present in plants, which is known as chlorophyll. A pigment is any substance that absorbs light. The process of photosynthesis mainly occurs in plant leaves with negligible amount of process in stems. Therefore we can call leaves as the food factories of plant. Oxygen is released as by-product due to this phenomenon. There is misconception in the minds of many people that plant takes nutrition from the soil but that is not the main food of the plant. Plants do absorb minerals from the soil but the main source of plant nutrition is dependent upon the photosynthesis. More than 10% CO2 of atmosphere is utilized by photosynthesis process therefore it has profound impact on the earth’s atmosphere and climate. Mechanism of Photosynthesis In this process plants absorb water from the soil through the roots and channel it through the vascular bundles in the stems to the leaves. The leaves absorb carbon dioxide through the stomata. The sunlight is absorbed by the leaves with the help of chlorophyll (green pigment). In the presence of all these ingredients plant produces glucose which is the main food for its growth and nutrition. Plants unlike humans and animals produce food for themselves and eventually the source of nutrition for humans as well as animals. The overall chemical reaction involved in photosynthesis can be written as under.- 6 CO2 + 6 H2O gt. C6H12O6+ 6 O2 Carbon dioxide + Water + Light energy gt. Glucose + Oxygen The oxygen released as by-product is utilized by human beings for breathing therefore significance of plants for our existence cannot be undermined. Steps of Photosynthesis It takes place in two stages. The first stage involves light reaction and the second stage is Calvin Benson cycle. In light reactions the light energy is captured and utilized to make high-energy molecules, which are subsequently used by the Calvin-Benson Cycle to detain carbon dioxide and make the precursor of carbohydrates. 1. Light Reactions It is the initial phase of photosynthesis in which solar energy is converted into potential energy. Ample amount of sunlight is required in this phase and therefore it is known as light dependent phase of photosynthesis process. The energy produced due to the light reaction is stored by forming a chemical substance called ATP (adenosine triphosphate). This compound is used by cells for storage of energy. This chemical is made of the nucleotide adenine bonded to a ribose sugar, and that is bonded to three phosphate groups. This molecule is very similar to the building blocks for our DNA. 2. Calvin Benison Cycle This phase of photosynthesis reactions occurs in the stroma. Stroma is the dense fluid within the chloroplast, which is the site of conversion of carbon dioxide to sugar. All plants and algae remove CO2 from the environment and reduce it to carbohydrate by the Calvin cycle. The process is a sequence of biochemical reactions that reduce carbon and rearrange bonds to produce carbohydrate from CO2 molecules. These reactions do not require the presence of solar light and hence known as light-independent reactions or dark reactions. In this cycle the end products of the initial phase of light dependent reaction like ATP and another chemical called NADPH are used. It consists of three phases and immediate product of the Calvin-Benison cycle is water and glyceraldehyde-3-phosphate. The glyceraldehydes 3 phosphate molecules that have exited the cycle are then used to make larger carbohydrates. This process is known as carbon fixation or the Calvin cycle. Classification of Photosynthetic Organisms There are basically two categories of photosynthetic organisms, which include oxygenic Photosynthetic organisms and anoxygenic photosynthetic organisms. 1. Oxygenic Photosynthetic Organisms The photosynthetic process in all plants and algae as well as in certain types of photosynthetic bacteria involving the reduction of CO2 to carbohydrate and removal of electrons from H20, which results in the release of O2 in the atmosphere are known as oxygenic photosynthetic organisms. 2. Anoxygenic Photosynthetic Organisms Some photosynthetic bacteria can use light energy to extract electrons from molecules other than water. These organisms are of ancient origin, presumed to have evolved before oxygenic photosynthetic organisms. Anoxygenic photosynthetic organisms occur in the domain Bacteria and have representatives in four phyla – Purple Bacteria, Green Sulfur Bacteria, Green Gliding Bacteria, and Gram Positive Bacteria. Global Photosynthesis and the Atmosphere The estimated amount of CO2 removed from the atmosphere each year by the process of oxygenic photosynthetic organisms is enormous. However due to deforestation this quantity is reducing . Dependence on predicted fossil fuel use and land management, it is estimated that the amount of CO2 in the atmosphere will reach 700 ppm within the next century resulting a rapid change in our atmosphere. Because CO2 acts as a greenhouse gas, some climate models predict that the temperature of the earth’s atmosphere may increase by 2-8 degrees. Such a large temperature increase would lead to significant changes in rainfall patterns and atmospheric changes. Works cited 1. Wim Vermaas. An Introduction to Photosynthesis and Its Applications 2. Alvin Silverstein, Virginia B. Silverstein, Virginia Silverstein – Photosynthesis 2007 3. David Oakley Hall, K. K. Rao, Institute of Biology – Photosynthesis 1999 Web http://www.buzzle.com/articles/steps-of-photosynthesis.html http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookPS.html

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