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Life on earth depends on the sun's energy. Not only does the sun provide warmth and light, it is also the source of energy for the process of photosynthesis that is the most crucial chemical reaction on Earth (Deatsman, 2006). All living organisms obtain the chemical energy through the process of respiration that take place in the living cells. Therefore both photosynthesis and respiration are very crucial in sustaining the life of living organisms on planet Earth. According to Kalman (2005), photosynthesis is defined as a chemical process by which green plants make their own food in the presence of sunlight. This phenomenon takes place in the plant cells that contain chlorophyll. The leaves of most plants are green in color due to chlorophyll that is a green pigment. Cellular respiration is defined as the process by which simplest food molecules are chemically broken down in the living cells into water and carbon dioxide (Karp, 2009). Respiration can take place with or without oxygen. The type of respiration that takes place without the presence of oxygen is referred to as anaerobic respiration while aerobic respiration takes place in the presence of oxygen. Energy is released during respiration and is used during various cellular activities.  


Some organisms such as green plants are capable of making their own food in the presence of sunlight. The process of photosynthesis requires water and carbon dioxide as the raw materials, and releases oxygen, glucose, and water as the byproducts (Kalman, 2005). Photosynthesis makes it possible for the solar energy to be converted into chemical energy that is stored as glucose. The following is the chemical equation for photosynthesis:


6CO2 + 12H2O                    C6H12O6 + 6O2 + 6H2O


According to Kalman (2005), photosynthesis takes place in two stages, namely, the light reactions and the dark reactions. Light reactions occur in the presence of sunlight but in the dark stage direct sunlight is not a requirement. However in most plants, dark reactions take place during the day. Light stage takes place in the grana where sunlight is absorbed to trigger a chain of steps that lead into the production of NADP (nicotinamide adenine dinucleotide diphosphate), ATP (adenosine triphosphate), and oxygen in the process of photolysis. Oxygen as a byproduct is released into the atmosphere through the stomata while the NADP and ATP move into the stroma of the chloroplast to be used in the dark stage for the production of sugar. The dark reactions take place in the stroma where ATP and NADP convert carbon dioxide into sugar through a process called carbon fixation. The process of aerobic respiration is makes use of the manufactured sugar and the released oxygen (Kalman, 2005).

Aerobic respiration

Aerobic respiration is the type of cellular respiration by which simple sugar molecules are broken down in the presence of oxygen (Karp, 2009). During the process of aerobic respiration, electrons are released as energy is created. According to Metzler et al. (2001), the process of aerobic respiration results into the formation of a substance called adenosine triphosphate (ATP) that is responsible for carrying energy to other cell of the body. ATP releases the energy of respiration in the process of hydrolysis. The first step of aerobic respiration in organic cells involves the breakdown of carbohydrates and complex sugars into glucose. The second step takes place in the mitochondrion where glucose reacts with oxygen to produce carbon dioxide, water, and energy (Karp, 2009). Aerobic respiration is represented by the following equation:

   C6H12O6 + 6O2                    6CO2 + 6H2O + energy (ATP)


According to Karp (2009), the ATP is hydrolyzed in living cells to release energy as shown by the equation below:

ATP + H2O                  ADP + Pi   %u2206 = 7.2kcal/mol

Therefore the processes of photosynthesis and aerobic respiration are linked to each other. Due to photosynthesis glucose and oxygen are formed and these two products are used in the process of respiration to produce energy in both animal and plant cells. The process of aerobic respiration in both plants and animals produces carbon dioxide, water, and energy. The carbon dioxide and water are absorbed by green plants as the raw materials for the process of photosynthesis. In photosynthesis ATP is used in the formation of glucose while in respiration is formed for purposes of energy storage and transportation to other cells in the body.

Glycolysis and fermentation

Glycolysis is defined as the process whereby one molecule of glucose is broken down into two pyruvate molecules (Metzler et al., 2001). The process of glycolysis involves numerous steps that occur in the cytoplasm of plant cells, animal cells, and microorganisms' cells. The process of glycolysis involves more than six different enzymes. Fermentation is defined as the process by which energy is derived from the oxidation of food substrate by use of an endogenous electron acceptor (Starr et al., 2008). Fermentation takes place in anaerobic conditions in which oxidative phosphorylation is not used to sustain the production of ATP by the process of glycolysis (Metzler et al., 2001). In the process of fermentation, pyruvate undergoes metabolism to yield a variety of compounds. Typical products of fermentation include lactic acid, hydrogen, and ethanol. Fermentation occurs as each glucose molecule is split into two pyruvate molecules. This process is referred to as glycolysis. Glycolysis is an important process because it leads to extraction of chemical energy from glucose. According to Metzler et al. (2001), two molecules of ATP and NADH are generated during glycolysis and can be represented by the equation below:

 C6H12O6 +2ADP + 2Pi + 2NAD+              2ATP + 2NADH + 2H2O + 2H+ + 2CH3COCOO-

Therefore the process of fermentation results into the availability of the chemical energy originally stored in food substrate. This is apparent that sun's energy finally gets utilized by the cells of anaerobic organisms because glucose that is a food substrate is the product of photosynthesis.

Enzymes as biological catalysts  

Enzymes are very important to living organisms because they increase the rate of chemical reactions within the living cells. Enzymes are protein in nature and consist of active sites over which interactions with specific substrates take place (Taggart & Starr, 2006). Substrates are initially bound to the active sites of enzymes by non-covalent interactions that include ionic bonds, hydrogen bonds, and the hydrophobic interactions. While on the active site on the corresponding enzyme, a substrate is converted to appropriate products, and the process is accelerated due multiple mechanisms. After the reactions have taken place successfully at the active site, the products of the reactions are released and the enzymes become free to accept new substrate. Enzymatic action therefore involves three interactions. The first interaction is to accept a substrate, the second interaction is to accelerate the reactions over the substrate to produce the required products, and the third interaction is to release the end products after the reactions are over (Starr et al., 2008).    

According to Karp (2009)The in the living cells enzymatic activities can be regulated to meet various physiological needs that arise in the course of a cell's life. Feedback inhibition is a form of enzyme regulation in which the activity of an enzyme is inhibited by the product of the metabolic reaction. For instance, the amino acid isoleucine is created through a series of reactions with amino acid threonine as the initial substance. This conversion is catalysed by an enzyme called threonine deaminase whose reaction is inhibited by isoleucine that is the end product (Karp, 2009). When the concentration of the end product decreases, feedback inhibition becomes inactive and the enzymatic action continues. Activities of enzymes can be regulated as they interact with other proteins as well as due to covalent modifications (Taggart & Starr, 2006).

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