The process by which organisms such as some bacteria, green plants and some protistans convert sunlight's energy to produce cellular sugar is known as photosynthesis. Due the process, cellular respiration takes place converting sugar into ATP, which is the fuel used by living organisms for survival (Cramer, 2004). The process where unusable energy from sunlight is concerted to useful chemical energy is always associated with chlorophyll, which is a green-coloring matter. Water is used during the process to produce oxygen used by animals and food which is important for human survival. In this paper we will discuss how photosynthesis works and how certain organisms and plants have evolved to use the process to produce useful energy from sunlight.
How Photosynthesis Works
Different scientific journals and books have tried to explain how photosynthesis works and how plants evolved so that they can convert sunlight into useful chemical energy. The chemical process occurs in two major stages namely: Light dependent and light independent reactions. The first stage, as the name suggests, is dependent of sunlight and makes carrier molecules used during the second stage (Cramer, 2004). On the other hand, the dark or light independent reactions takes place after the products of the first stage have been used to form C-C covalent carbohydrates bonds. The dark reactions even take place in the absence of sunlight once the energy carriers are available from the light stage. Some of the recent studies have indicated that there is a major enzyme which is linked with the dark reaction, and which is indirectly stimulated by the presence of sunlight (Olson & Blankenship, 2004). Because of this new scientific finding, the process Dark Reaction is frequently of a misnomer. Another important thing about these stages is that light reaction occurs in the chloroplasts' grana and the dark reactions in the stroma.
In summary, the light reactions occur when sunlight strikes the green colored chlorophyll after which electrons are excited to achieve higher energy states. This converts energy into NADPH and ATP. Here water is also split during the process to release oxygen as an important by-product. The NADPH and ATP produced are used in making C-C bonds during the light independent reactions (Mulkidjanian, 2007). During the same light stage, CO2 derived from the atmosphere, or from water, is then captured after which Hydrogen molecules are added to form carbohydrates with the formula (CH2O) n. This process through which carbon dioxide is incorporated into organic compounds is commonly referred to as carbon fixation. The energy required for the process is derived from the very first phase of photosynthesis. It should be noted that living systems are not able to utilize light directly from sun's energy (Olson & Blankenship, 2004).
During the dark stage, carbon dioxide enters into single-celled organisms and plants after which it diffuses into the cells. For plants, leaves have stomata which have guard cells to prevent desiccation. This leads to the Calvin Cycle in the stromas of the green-colored chloroplasts. During this stage, carbon dioxide is usually captured by a chemical known as ribulose biphosphate (RuBP). Here six CO2 molecules enter the cycle to produce a molecule of glucose which is broken down to produce energy.
Evolution towards Photosynthesis
As we know today, evolution is a process which is believed to have led to our current universe. Photosynthesis, for instance, is an important process which must have evolved together to sustain life (Nelson & Ben-Shem, 2005). According to scientists and biologists, photosynthesis is a process that evolved from bacteria organisms, making them cyanobacteria, which are known as algae. However, this process was not at all complex and sophisticated as it is today. The production of oxygen also led to natural selection and gene variation which resulted in organisms requiring the by-products of photosynthesis (Cramer, 2004). This was something very important because it allowed around four millions years of gradual change. As evolution continued to take place, lower plants such as organisms and algae developed gradually to other plants as indicated by the phylogenetic trees of evolution. It is therefore agreeable that life must have evolved in such a way that it would exploit the new gas hence allowing breathing by other organisms (Swindell, 2009).
Based on available fossil evidences, it is indicated that there are several indications that prove that biochemically microbial and advanced life existed on our earth's surface over 3.5 billion years ago (Raymond et al, 2002). It is also notable that these precursors of prokaryotic organisms existed even much earlier, and therefore have been referred to as photocells or progenotes. Basically, the evolution of the progenotes into the prokaryotes has been indicated to have occurred about three to five billion years ago (Swindell, 2009). The initial metabolic activities presented include anaerobic fermentation, followed by primitive bacterial activities and photosynthesis, followed by complex photosynthesis type in plants as it is the case in Cyanobacteria. According to phylogentic trees, the earliest prokaryotes must have been fermenting bacteria and lived in environments where organic compounds were available for free. Also, the atmosphere must have also been reducing (Olson & Blankenship, 2004).
As well, the available bacteria were also anaerobic scavengers and capable of breaking down several compounds with very high energy content. These organic compounds were thereby used by the organisms as the sources of energy. The produced molecules having lower content of free energy were released as by-products (Raymond et al, 2002). Therefore the process of fermentation has always remained as an important source of energy and is used by anaerobic bacteria such as Clostridium.
The next step included the development of photosynthetic process whereby solar energy could be trapped by plants organisms to synthesize free energy contents. This also led to the production of chemical energy which can very easily be converted into ATP (Mulkidjanian, 2007). According to biologists, the energy produced is used in other activities in the organisms. For instance, sulphate respiration must have evolved in the response to 'sulphate' releasing photosynthetic activity which occurred earlier on the primitive world (Nelson & Ben-Shem, 2005).
Looking at the above evolution process, plants evolved slowly from lower organisms such as cyanobacteria and algae and later to higher plants. As well, different higher plants evolved differently with different metabolic pathways which help the plants to adapt to their environments (Swindell, 2009). Since evolution is something complex, the evolution of other organisms was also determined by that of plants and especially because of the evolution process. However, from most of the articles which have been used to write the paper, it should be noted that there is still a lot of contradicting information regarding the evolutionary process of photosynthesis as we understand it today. Today photosynthesis is a very important biochemical process which produces energy capable of supporting life on earth.