Introduction to Cellular Respiration

1. Introduction to 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 refers to the 20 or so reactions that take place to free up the energy in glucose.

2. Oxidation of Glucose • C6H12O6 + 6O2 6H2O + 6CO2 • Oxygen oxidizes the C-H in glucose in two ways: • The 12 H are broken away to form 6 H2O, the 6 C to form 6 CO2. • A C-H bond is non-polar, the electrons are equally shared • In water and carbon dioxide the electrons are drawn closer to oxygen, therefore H is oxidized (LEO).

3. Oxidation of Glucose in the Lab • Oxygen and glucose are stable molecules. They do not readily react. • Lots of activation energy is needed (flame in lab conditions). • 2870 kJ/mol energy released as heat and light.

4. Oxidation of Glucose in the Cell • In the cell enzymes catalyze each reaction step, reducing the activation energy and making it easier for the cell to undergo aerobic cellular respiration. • 3012 kJ/mol released. 34 % is trapped in the form of ATP. The rest is lost as heat and light.

5. Aerobes and Anaerobes • Oxygen is not the only oxidizing agent 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 oxidizing agent. • Organisms that can tolerate the presence and absence of oxygen are called facultative aerobes (mostly bacteria).

6. 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 1 on page 94)

7. GLYCOLYSIS • First 10 reactions of cellular respiration • Name means sugar-splitting • It occurs in the cytoplasm and is anaerobic. • Glycolysis is thought to have been the earliest form of energy metabolism. • Each reaction is catalyzed by a specific enzyme. • (Figure 11, page 98, handouts).

8. Glycolysis Pathway

9. Steps in Glycolysis: Steps 1-5 • Two ATP are used (step 1 and step 3). This primes glucose for cleavage in steps 4 and 5.

10. Steps 4 and 5: • Fructose 1, 6-biphosphate is split into DHAP (dihydroxyacetone phosphate) and G3P (glyceraldehyde 3-phosphate) and immediately the enzyme isomerase changes DHAP into G3P.

11. Steps 6 through 10 • Steps 6 through 10 happen twice (one for each molecule of G3P). • In step 6, NAD+ is reduced to NADH + H+ • NAD+ is an electron carrier. Each NAD+ removes 2H+ and 2e- • it will transfer the electrons to another reaction later in cellular respiration (the ETC)

12. Steps 7-10 • Step 7: G3P is converted into PEP. Two ATP molecules are produced by substrate level phosphorylation. • Step 10: Phosphoenolpyruvate (PEP) is converted into pyruvate; Two ATP produced by substrate-level phosphorlyation. • Net reaction: Glucose + 2ADP + 2P + 2NAD --> 2 pyruvate + 2ATP + 2NADH + 2H+

13. Substrate Level Phosphorylation • Glycolysis produces 2.1% of the entire free energy of glucose in aerobic cellular respiration by Substrate-Level Phosphorylation • 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.