Cellular Respiration
Alyssa R Whitcher
Hodges University
Biology 1121
Mark Clifton
21 April 2015
Cellular Respiration
Introduction:
Cellular respiration is a series of metabolic processes by which living cells produce energy through the oxidation of organic substances. The harvesting of energy from glucose by cellular respiration is a cumulative function of three metabolic stages: Glycolysis, pyruvate oxidation, and oxidative phosphorylation. Glycolysis takes place in the cytosol of the cell. It begins the degradation process by breaking glucose into two molecules of a compound called pyruvate. In eukaryotic cells, pyruvate enters the mitochondrion and it is oxidized to a compound called acetyl CoA, which then enters the citric acid cycle. In the citric acid cycle, the breakdown of glucose to carbon dioxide is completed. Consequently, the carbon dioxide produced by respiration represents fragments of oxidized organic molecules. A few of the steps of glycolysis and the citric acid cycle are redox reactions in which dehydrogenase transfer electrons from substrates to NAD+ , forming NADH. In the third stage of cellular respiration, the electron transport chain accepts electrons from the breakdown products of the first two stages and passes these electrons from one molecule to another. At the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions, which in turn forms water. The energy released at each step of the chain is stored in a form the mitochondrion can use to make ATP from ADP. This mode of ATP synthesis is called oxidative phosphorylation. Oxidative Phosphorylation is powered by the redox reactions of the electron transport chain. In eukaryotes, the inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, these two processes constitute oxidative phosphorylation. Oxidative phosphorylation accounts for about 90% of the ATP generated by cellular respiration. A small amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle known by a mechanism called substrate-level phosphorylation. This type of ATP synthesis occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP like what takes place during oxidative phosphorylation. “Substrate molecule” refers to an organic molecule that is generated as an intermediate during the catabolism of glucose. Substrate-level phosphorylation occurs in both glycolysis and the citric acid cycle. Overall, roughly 32 molecules of ATP will be produced for each molecule of glucose that is degraded to carbon dioxide and water by cellular respiration. (Reece et al., 2014) Temperature can be described as the average kinetic energy of a given group of molecules. As temperature increases, molecules have more motion and activity. As temperature decreases, molecules have less motion and activity. The enzymes (which are proteins) that catalyze the reactions of cellular respiration are temperature sensitive. The enzymes in general become more and more active as the temperature increases. However, once the temperature reaches roughly 40º-45ºC their enzymes will begin to denature and their activity completely drops off. This is because high temperatures can damage the structure of a protein and if the shape of an enzyme’s active site is lost, then it cannot function properly. The temperature at which an enzyme works best is called the optimum temperature. The optimum temperature for enzymes is usually around the human body temperature at 37ºC. Germination is a process in which the seed develops into a seedling. The seeds of flowering plants are typically resistant structures in which embryonic plants are enclosed. The outer later of the seed is called the seed coat, which protects the embryo from adverse conditions. The structures of the seeds of flowering plants are similar in that each seed contains a seed coat, an
