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Photosynthesis can be defined as the metabolic process whereby plants convert inorganic material into organic material using sunlight, atmospheric carbon dioxide and water. Oxygen and glucose are produced as a byproduct of photosynthesis. Respiration is the metabolic process in which the cells of organisms use organic material to release energy. Respiration requires oxygen and glucose (by-products of photosynthesis) and produces carbon dioxide and water, the raw reactants used by plants for photosynthesis. Thus, photosynthesis and respiration are linked and are dependent on each other to make energy available for both plants and animals.
Chlorophyll molecules, in the leaves of plants, capture photons of light energy from sunlight then convert it to chemical energy, in form of ATP (adenosine triphosphate) and NADPH. Next, a number of enzymes catalyze reactions that utilize the energy stored in ATP and NADPH molecules in the production of glucose (Audesirk, Audesirk, E, & Byers, 2007). Body cells contain mitochondria which burn sugar for fuel during cellular respiration and harness the energy from the sugar so that the cell can utilize that energy for other functions.
Fermentation is an anaerobic process and therefore it does not require oxygen to take place. Through fermentation, organisms and cells are able to generate energy from sugar molecules in the absence of oxygen. Fermentation in cells involves conversion of NADH into NAD+ which is an electron carrier. NAD+ enables glycolysis to continue taking place and thereby supply ATP (Audesirk, Audesirk, E, & Byers, 2007). There are two main types of fermentation; alcoholic and lactic acid fermentation. Alcoholic fermentation takes place in microorganisms such as yeast. Alcoholic fermentation after the process of glycolysis is represented by the equation:
Pyruvic + NADH %u279E alcohol + CO2 + NAD+ acid
In lactic acid fermentation, a 3-carbon compound called lactic acid is produced from pyruvate:
pyruvate + NADHlactic acid + NAD+
Carbohydrates are oxidized in part and chemical energy is given out during anaerobic respiration.
Enzymes reduce the activation energy required to start a reaction. Without enzymes, it would be impossible for most reactions to take place at a useful rate. All aspects of cell metabolism (digestion of food, conservation and conversion of chemical energy; and the production of cellular materials and components) are catalyzed by enzymes. The three main steps of the cycle of enzyme-substrate interactions are:
i. Enzyme + substrate
ii. Enzyme-substrate complex
iii. Enzyme + product
Enzyme activity is regulated by end-product inhibition (Audesirk, Audesirk, E, & Byers, 2007). The end-products of a metabolic process inhibit the enzyme catalyzing one of the steps in the pathway. Subsequently, if the quantity of product increases, the pathway is decelerated and less is produced. If the quantity decreases, the inhibition is decreased and more is synthesized. Enzyme activity can also be regulated by gene regulation. The gene that synthesizes the enzyme is switched on or off by messenger molecules.