Cellular Respiration

Cellular Respiration – AKA Respiration The metabolic pathways by which carbohydrates are broken down into ATP The formula for respiration is: C6H12O6 + 6O2  6CO2 + 6 H2O + ATP Respiration allows the gradual build up of ATP, If a cell used all the potential energy in glucose at once, too much heat would be released, and not all the energy would be harvested.

Three Steps of Respiration 1. Glycolysis Yields 2 molecules of ATP 2. Citric Acid Cycle (Krebs Cycle) 3. Electron Transport Chain (Oxidative phosphorylation) Yields 32 molecules of ATP Though 36 molecules of ATP are produced, it is only 39% of the total energy in a molecule of glucose

The Process in General Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis 0 % Citric acid cycle Glycolysis Glucose Pyruvate ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

Glycolysis Glyco- of or relating to glucose Lysis- to split or burst Changes one molecule of glucose to 2 three Carbon molecules called pyruvate Occurs in the cytosol (cytoplasm) Does not require Oxygen

Glycolysis Glucose  2 pyruvate 2 NAD  2 NADH 2 ADP 2 ATP  Net gain of 2 ATP molecules (top=input, bottom= outputs) GlucoseFructosePGALPGAPEPpyruvate 4 ATP 2 ATP  2ADP P + 2NAD2NADH 2 ATP H2O 2 ATP

Glycolysis When Oxygen is present, glucose / pyruvate can be broken down further in a process called the Krebs cycle. However, when Oxygen is not present pyruvate can be broken down through the process of fermentation. Aerobic respiration- respiration with Oxygen Anaerobic respiration- respiration with out Oxygen

The Evolutionary Significance of Glycolysis Glycolysis occurs in nearly all organisms Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation Glycolysis + the reduction of pyruvate to lactic acid, alcohol, or CO2 Fermentation is inefficient. But when O2 is not present it is better than nothing. Glycolysis yields 2 ATP, fermentation yields 2 more Fermentation yields only 14.6 Kcal of a possible 686Kcal from 1 molecule of glucose that is about 2.1% efficient.

Fermentation In yeast produces alcohol and CO2  death In humans produces lactic Acid  soreness In athletes : because of training more mitochondria are produced in the muscle cells. b/c of higher # of mito., they utilize more O2 and can produce more ATP (less O2 wasted) Means less Oxygen debt or deficit Less fermentation Less soreness

Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two common types are alcohol fermentation and lactic acid fermentation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-18 Figure 9.18 Fermentation 2 ADP + 2 Pi 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD+ 2 NADH 2 CO2 + 2 H+ 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Pi 2 ATP Glucose Glycolysis Figure 9.18 Fermentation 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate 2 Lactate (b) Lactic acid fermentation

(a) Alcohol fermentation Fig. 9-18a 2 ADP + 2 P 2 ATP i Glucose Glycolysis 2 Pyruvate 2 NAD+ 2 NADH 2 CO2 + 2 H+ Figure 9.18a Fermentation 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation

In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(b) Lactic acid fermentation Fig. 9-18b 2 ADP + 2 P 2 ATP i Glucose Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate Figure 9.18b Fermentation 2 Lactate (b) Lactic acid fermentation

Fermentation and Aerobic Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Ethanol or lactate Citric acid cycle Fig. 9-19 Glucose Glycolysis CYTOSOL Pyruvate O2 present: Aerobic cellular respiration No O2 present: Fermentation MITOCHONDRION Ethanol or lactate Acetyl CoA Figure 9.19 Pyruvate as a key juncture in catabolism Citric acid cycle

Aerobic Respiration Occurs when Oxygen is present Produces 36 ATP molecules Begins after glycolysis The Transition reaction connects glycolysis to the Aerobic pathways (Krebs cycle & Electron transport chain).

Transition Reactions Connects glycolysis to the Krebs cycle The pyruvate from glycolysis is converted to a 2 Carbon compound called an acetyl group. The acetyl group is attached to Coenzyme A The resulting complex is Acetyl Coenzyme A (Acetyl CoA) Coenzyme- protein that acts as a carrier molecule in biochemical processes. This process releases CO2

Transition Reaction Occurs twice for each glucose molecule Occurs in the matrix of the mitochondria 2 pyruvate + 2 CoA  2 Acetyl CoA + 2 CO2 Mitochondrial structure Cristae - location of the electron transport chain Cytosol – location of glycolysis Matrix- location of the transition reactions and the Krebs cycle. 2 NAD  2 NADH + H-

CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Pyruvate Fig. 9-10 CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle Pyruvate Coenzyme A CO2 Transport protein

Krebs Cycle The products of the Transition RXN are the reactants of the Krebs Cycle. The Krebs Cycle is also known as the Citric Acid Cycle. Citrate is the first metabolite produced in the Krebs Cycle. During the Krebs Cycle: the acetyl group on Acetyl CoA is oxidized to CO2. Some of the electrons (H ions) are accepted by NAD, but 1 is picked up by another electron carrier, FAD. Some ATP is produced by phosphorylation like in glycolysis. The Krebs Cycle turns twice for each original molecule of glucose.

Krebs Cycle Inputs Outputs 2 acetyl groups 2 ADP + 2 P 6 NAD 2 FAD 4 CO2 2 ATP 6 NADH 2FADH2 See Figure 9.7

Citric acid cycle Succinyl CoA Acetyl CoA CoA—SH NADH H2O NAD+ Oxaloacetate Isocitrate Malate Citrate NAD+ Citric acid cycle NADH CO2 H2O Fumarate CoA—SH -Keto- glutarate CoA—SH FADH2 CO2 NAD+ FAD Succinate NADH P Succinyl CoA ADP ATP

Substrate-level phosphorylation: Fig. 9-7 Substrate-level phosphorylation: Use of an enzyme to produce ATP and another product Enzyme Enzyme + ADP Figure 9.7 Substrate-level phosphorylation P Substrate ATP Product

The Electron Transport Chain Located in the cristae Series of electron carriers that pass an electron from one to another. The electrons in the electron transport chain come from NADH and FADH2 in the Krebs Cycle Drives the process that generates most of the ATP . The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions

The electron transport chain generates no ATP Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 The electron transport chain generates no ATP The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 9.13 Free-energy change during electron transport NADH 50 2 e– NAD+ FADH2 2 e– FAD Multiprotein complexes  40 FMN FAD Fe•S  Fe•S Q  Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 Figure 9.13 Free-energy change during electron transport e– 10 2 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O

Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space H+ then moves back across the membrane, passing through channels in ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

i INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Cata- lytic knob Fig. 9-14 INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Figure 9.14 ATP synthase, a molecular mill Cata- lytic knob ADP + P ATP i MITOCHONDRIAL MATRIX

Oxidative Phosphorylation The process by which ATP production is tied to an electron transport system that uses Oxygen as a final electron receptor. After Oxygen accepts the electron it binds with Hydrogen from the matrix and form H2O Figure 9.8

Complexes of the Electron Transport Chain NADH dehydrogenase - causes the oxidation of NADH. Cytochrome b-c complex - the complex which receives electrons and pumps H ions into the inter-membrane space. Cytochrome oxidase complex - receives electrons and passes them to oxygen.

Electron Transport Chain

The Process in General Electrons carried via NADH Electrons carried via NADH and FADH2 0 % Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis Glucose Pyruvate ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

The process with details Electron shuttles span membrane 2 NADH MITOCHONDRION or CYTOSOL 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Acetyl CoA 2 Pyruvate Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP Maximum per glucose: About 36 or 38 ATP

Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fatty acids are broken down by beta oxidation and yield acetyl CoA Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Fatty acids are broken down by beta oxidation and yield acetyl CoA An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Citric acid cycle Oxidative phosphorylation Fig. 9-20 Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Figure 9.20 The catabolism of various molecules from food Citric acid cycle Oxidative phosphorylation

Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings