Lecture Notes: Cellular Respiration PDF

CELLULAR RESPIRATION Cellular respiration is the metabolic pathway responsible for converting large increments of energy in the form of macromolecules into more efficiently used smaller increments of energy called ATP. C6H12O6 + 6O2  6CO2 + 6H20 + Energy (ATP) 4 - Stages of Cellular...

CELLULAR RESPIRATION Cellular respiration is the metabolic pathway responsible for converting large increments of energy in the form of macromolecules into more efficiently used smaller increments of energy called ATP. C6H12O6 + 6O2  6CO2 + 6H20 + Energy (ATP) 4 - Stages of Cellular Respiration: 1. Glycolysis 2. Oxidation of Pyruvate to Acetyl Coenzyme-A (Acetyl CoA) 3. Kreb’s Cycle 4. Oxidative Phosphorylation / Electron Transport Chain (OP/ETC) GLYCOLYSIS  Occurs in the Cytosol whether O2 is present (= aerobic) or absent (= anaerobic).  One 6-carbon molecule of Glucose is oxidized and split into two 3-carbon molecules of Pyruvate.  2-NAD+ are reduced to 2-NADH; each one is worth 2-ATP through OP/ETC.  2-ATP are consumed (= “Energy-Investment Phase”).  4-ATP are produced by SLP (= “Energy-Yielding Phase”).  No FADH2 or CO2 is produced during Glycolysis. FATE of PYRUVATE  During Glycolysis, one 6-carbon molecule of Pyruvate is oxidized and split into two 3-carbon molecules of Pyruvate.  What happens to the Pyruvate formed during Glycolysis depends on the presence/absence of Oxygen.  If O2 is present (aerobic), Pyruvate will enter the mitochondrial matrix where it will be oxidized further to Acetyl CoA.  If O2 is absent (anaerobic), Pyruvate will remain in the cytosol and become reduced to either Ethanol or Lactate through a metabolic pathway known as “Fermentation.” OXIDATION of PYRUVATE to ACETYL CoA  Occurs in the Mitochondrial Matrix only if O2 is present (aerobic).  Each 3-carbon molecule of Pyruvate is oxidized to a 2-carbon molecule of Acetyl CoA.  One molecule of NAD+ is reduced to NADH per pyruvate and is worth 3-ATP through OP/ETC.  One molecule of CO2 is produced per Pyruvate.  No ATP is consumed.  No ATP is produced by SLP.  No FADH2 is produced. KREB’s CYCLE  Occurs in the Mitochondrial Matrix only if O2 is present (aerobic).  The Kreb’s Cycle only accepts Acetyl CoA.  When Acetyl CoA enters the Kreb’s Cycle, it is completely oxidized to two molecules of CO2.  For every 2-carbon molecule of Acetyl CoA that enters the Kreb’s Cycle, the following products are produced: o 2-CO2 o 1-ATP by SLP o 1-FAD is reduced to 1-FADH2 and is worth 2-ATP by OP/ETC o 3-NAD+ are reduced to 3-NADH and each one is worth 3-ATP by OP/ETC for a total of 9-ATP. OXIDATIVE PHOSPHORYLATION (OP/ETC)  The ETC for cellular respiration is located embedded in the inner mitochondrial membrane (= “cristae”) and is only operational if O2 is present (aerobic).  Electrons are entered into the ETC, from the matrix side, by the reduced coenzymes NADH and FADH2 that were formed during Glycolysis, Oxidation of Pyruvate to Acetyl CoA and the Kreb’s Cycle.  Kinetic energy from the exergonic flow of electrons down the ETC is used to pump H+/protons from the matrix into the intermembrane space of the mitochondrion.  The potential energy of the H+/proton gradient in the intermembrane space is converted to kinetic energy as the H+/protons diffuse back into the matrix through membrane protein channels called ATP Synthase.   ATP Synthase uses the kinetic energy from the exergonic flow of H+/protons to phosphorylate ADP and form ATP.  The number of phosphorylation reactions possible by ATP Synthase is directly proportional to the number of H+/protons diffusing through ATP Synthase. o Since NADH enters its electrons into the very beginning of the ETC, its electrons provide enough kinetic energy to pump enough H+/protons into the intermembrane space to perform three phosphorylation reactions.  Therefore, each NADH is said to be worth 3-ATP by OP/ETC. o Since FADH2 enters its electrons further down the ETC, its electrons only provide enough kinetic energy to pump enough H+/protons into the intermembrane space to perform two phosphorylation reactions.  Therefore, each FADH2 is said to be worth 2-ATP by OP/ETC. EXCEPTION:  The NADH produced during Glycolysis are worth only 2-ATP each.  This is because they are formed in the cytosol outside the mitochondrion and the inner mitochondrial membrane is impermeable to NADH.  Since the NADH from Glycolysis cannot enter the mitochondrial matrix and enter its own electrons into the ETC, it sends its electrons across the inner mitochondrial membrane and into the mitochondrial matrix where they are picked-up by a molecule of FAD which reduces it to FADH2 which then enters the electrons into the ETC.  Since it is ultimately FADH2 and not NADH that enters the electrons into the ETC, the NADH formed during Glycolysis are said to be worth only worth 2-ATP each. SUMMARY for CELLULAR RESPIRATION (per Glucose) PROCESS CO2 ATP REDUCED ATP TOTAL EVOLVED by COENZYME by ATP SLP ETC PRODUCED Glycolysis None 4-total 2-NADH 4 8 (2-net gain) Oxidation 2 2-NADH 6 6 of Pyruvate None to Acetyl CoA Kreb’s 4 2 6-NADH 18 24 Cycle 2-FADH2 4 GLYCOLYSIS:  4-ATP total by Substrate Level Phosphorylation (SLP)  2-ATP total net gain after energy investment phase  One 6-carbon Glucose converted to two 3-carbon Pyruvate molecules  2-NADH worth 2-ATP each by OP/ETC  No CO2 or FADH2 produced OXIDATION of PYRUVATE to ACETYL CoA:  No ATP produced directly by SLP  1-NADH per Pyruvate oxidized; therefore 2-NADH per Glucose  Each NADH is worth 3-ATP by OP/ETC  1-CO2 evolved per Pyruvate; therefore 2-CO2 per Glucose KREB’S CYCLE:  1-ATP by SLP per Acetyl CoA; therefore 2-ATP by SLP per Glucose  3-NADH per Acetyl CoA; therefore 6-NADH per Glucose o Each NADH is worth 3-ATP by OP/ETC (therefore 9-ATP per Acetyl CoA or 18-ATP per Glucose)  1-FADH2 per Acetyl CoA; therefore 2-FADH2 per Glucose o Each FADH2 is worth 2-ATP by OP/ETC (therefore 2-ATP per Acetyl CoA or 4-ATP per Glucose)  2-CO2 evolved per Acetyl CoA; therefore 4-CO2 per Glucose FERMENTATION  Occurs in the Cytosol if O2 is absent (anaerobic).  When O2 is absent, Glycolysis will still occur and split Glucose into two molecules of Pyruvate.  Instead of entering the Mitochondrial Matrix, without O2 the Pyruvate will remain in the Cytosol and become reduced through Fermentation to either a 2- carbon molecule of Ethanol (alcohol fermentation) or a 3-carbon molecule of Lactate (lactic acid fermentation). o If the terminal product is Ethanol, one molecule of CO2 will also be produced per Pyruvate.  Fermentation serves as a way of regenerating NAD+ (the oxidizing agent for Glycolysis) from NADH under anaerobic conditions. o Under aerobic conditions, NAD+ is regenerated from NADH when the electrons are sent to the ETC. BETA OXIDATION of LIPIDS  Occurs in the mitochondrial matrix if O2 is present.  Lipids are not processed like carbohydrates, instead, they undergo a process called “Beta Oxidation” that effectively hydrolyzes the covalent bond between every other carbon atom in the hydrocarbon chains making up the fatty acids of the lipid.  This process creates 2-carbon molecules of Acetyl CoA that directly enter the Kreb’s Cycle.  For each 2-carbon molecule of Acetyl CoA that enters the Kreb’s Cycle, the following are produced: o 2-CO2 o 1-ATP (by SLP) o 1-FADH2 (= 2-ATP by OP/ETC) o 3-NADH (= 9-ATP by OP/ETC)