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CELLULAR RESPIRATION

CELLULAR RESPIRATION. The majority of organisms on earth use glucose as their main energy source. Through a series of redox reactions glucose is broken down and free energy is released Aerobic Cellular Respiration is the most often used method of converting glucose to free energy.

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CELLULAR RESPIRATION

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  1. CELLULAR RESPIRATION • The majority of organisms on earth use glucose as their main energy source. Through a series of redox reactions glucose is broken down and free energy is released • Aerobic Cellular Respiration is the most often used method of converting glucose to free energy. • Aerobic means that oxygen is used in the process. Respiration does not refer to the act of breathing or gas exchange in the lungs, but the 20 or so reactions that take place to free up the energy in glucose.

  2. OXIDATION OF GLUCOSE • Overall the oxidation of glucose moves electrons from a higher free energy state to a lower free energy state, thereby decreasing potential energy. • Oxidation of glucose produces 2870 kJ/mol of glucose @ 25 degrees Celsius and 101.3 kPa (Lab conditions; 3012 kJ/mol in a real cell).

  3. About 34% of this energy is trapped by the cell and used to fuel endergonic processes. The rest dissipates as heat or light. • Oxygen and glucose are stable molecules. They do not readily react with one another. Lots of activation energy is needed. • Enzymes catalyze each reaction step thereby reducing the activation energy and making it easier for the cell to undergo aerobic cellular respiration.

  4. AEROBES AND ANAEROBES • Oxygen is not the only primary electron acceptor at the end of the respiration process, other molecules such as NO2, SO4, CO2, and Fe3+ are used in some forms of bacteria to help undergo respiration (obligate anaerobes) • Animals are obligate aerobes since they use oxygen as their final electron acceptor. • Organisms that can tolerate the presence and absence of oxygen are called facultative aerobes (mostly bacteria).

  5. AEROBIC RESPIRATION • In aerobic respiration there are three main goals: • break the bonds of glucose freeing the carbon to make CO2 • break the bonds of glucose freeing H to form water • to trap as much free energy as possible in the form of ATP • The entire process occurs in 4 main stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle and the Electron Transport Chain (Figure 2 on page 169) and involves 2 types of phosphorylation: substrate-level phosphorylation and oxidative phosphorylation.

  6. Two Types of Phosphorylation • Substrate-Level Phosphorylation is the formation of ATP directly in an enzyme-catalyzed reaction. A phosphate containing compound transfers its phosphate group to ADP (forming ATP) directly on an enzyme. • 4 molecules of ATP are formed this way in glycolysis and 2 in the Krebs cycle for every one glucose. • Oxidative Phosphorylation is the indirect formation of ATP. It involves a series of redox reactions in which oxygen is the final electron acceptor. It is more complex than Substrate-Level Phosphorylation and creates more ATP. • The reduction of the electron carrying molecules NAD+ and FAD to NADH and FADH2 are energy harvesting reactions that will transfer the majority of their free energy to the creation of ATP.

  7. GLYCOLYSIS • Name means sugar-splitting • First 10 reactions of cellular respiration • It occurs in the cytoplasm and is anaerobic. • Each reaction is catalyzed by a specific enzyme. • (The reactions are shown in figure 2 on page 173, as well as your handouts). • Glycolysis produces 2.1% of the entire free energy of glucose in aerobic cellular respiration. • Glycolysis is thought to have been the earliest form of energy metabolism.

  8. Endothermic Glycolysis I • Steps 1-5: Two ATP are used (step 1 and step 3). This primes glucose for cleavage in steps 4 and 5. (6 carbons) • Step 4/5: Fructose 1, 6-biphosphate is split into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P/PGAL) and immediately the enzyme isomerase changes DHAP into G3P/PGAL. (3 carbons)

  9. Exothermic Glycolysis II • Steps 6 through 10 happen twice (one for each molecule of G3P/PGAL). • In step 6, NAD+ is reduced to NADH + H+ • Step 7: Two ATP molecules are produced by substrate level phosphorylation. One for each 1,3-bisphophoglycerate (PGAP) processed. • Step 10: Phosphoenolpyruvate (PEP) is converted into pyruvate; this produces ATP by substrate-level phosphorlyation. (3 carbon) • Net-reaction: glucose + 2ADP + 2P + 2NAD --> 2 pyruvate + 2ATP + 2NADH + 2H+

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