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.

Oxidation of Glucose C 6 H 12 O 6 + 6O 2  6H 2 O + 6CO 2 Oxygen oxidizes the C-H in glucose in two ways:  The 12 H are broken away to form 6 H 2 O, the 6 C to form 6 CO 2.  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).

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) kJ/mol energy released as heat and light.

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 kJ/mol released. 34 % is trapped in the form of ATP. The rest is lost as heat and light.

Aerobes and Anaerobes Oxygen is not the only oxidizing agent at the end of the respiration process, other molecules such as NO 2, SO 4, CO 2, and Fe 3 + 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).

Aerobic Respiration In aerobic respiration there are three main goals:  break the bonds of glucose freeing the carbon to make CO 2  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)

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).

Glycolysis Pathway

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.

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.

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)

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 +

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.