Chapter 9 continued Electron Transport

Cellular Respiration Fig. 9.6 p 166 Electrons carried via NADH via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which is less electronegative. It then returns to its oxidized form as it passes electrons to its more electronegative “downhill” neighbor. For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files. For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Pathway of Electron Transport Fig. 9.13 p173 The electron transport chain is in the highly folded cristae of the mitochondrion Most of the chain’s components are proteins, which exist in multiprotein complexes numbered I-IV. The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The steps of Electron Transport Fig. 9.13 p173 Electrons removed from glucose by NAD+ during glycolysis and the citric acid cycle are transferred from NADH to flavoprotein(FMN), the first molecule of the transport chain in Complex I. (flavoprotein is reduced). In the next redox reaction flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (Fe-S in Complex I). The Fe-S protein then passes the electrons ubiquinone (Q), a small hydophobic molecule that is mobile within the membrane. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electron Transport Fig. 9.13 p173 Electrons are then passed through cytochrome proteins (Cyt), each with a heme group that contains an iron atom, to O2. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution to form water. FADH2 is another source of electrons for the chain. FADH2 adds electrons in Complex II at a lower energy than NADH does. Even though both NADH and FADH2 contribute 2 electrons to the chain to reduce O2, FADH2 provides 1/3 of the energy that NADH does. 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.

Electron Transport results in H+ release into the intermembrane space Fig. 9.13 p173 Fig. 9.16 p175 Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space The electron transport chain is responsible for establishing the H+ gradient across the inner mitochondrial membrane. 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

Electron Transport Chain + Chemiosmosis = Oxidative Phosphorylation Fig. 9.16 p175 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 makes ATP from ADP + Pi ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy stored in the form of an H+ gradient across a membrane to drive cellular work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chemiosmosis and ATP Synthase Fig. 9.14 p174 Main steps of ATP Synthase: 1. H+ flowing down their gradient enter the stator H+ bind one at a time to the rotor, causing it to spin in a way the catalyzes ADP + Pi → ATP Each H+ makes one turn then leaves the rotor, re-entering the stator and traveling through it to enter the mitochondrial matrix. Spinning of the rotor also causes spinning of an internal rod. The turning of the internal rod activates catalytic sites in the knob that catalyze ADP + Pi → ATP Chemiosmosis is an energy-coupling mechanism that uses stored energy in the form of an H+ gradient across a membrane to drive cellular work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

ATP production by Cellular Respiration Fig. 9.17 p176 Substrate-level phosphorylation Substrate-level phosphorylation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

BioFlix: Cellular Respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate. H2S (hydrogen sulfide) is produced as a by-product rather than H2O (water). Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation Fermentation is an expansion of glycolysis that allows for the continuous generation of ATP by substrate-level phosphorylation -as long as there is a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. Instead of an electron chain to recycle NAD+ fermentation employs the transfer of electrons from NADH to pyruvate, the end product of glycolysis. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation pathways recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. In alcohol fermentation, pyruvate is converted to ethanol in two steps. 1. Pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO2. 2. Acetaldehyde is reduced by NADH to ethanol. This process regenerates the supply of NAD+ needed for the continuation of glycolysis. Step 1 Step 2 Fig. 9.18 p178 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation pathways recycle NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce. Excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells. Fig. 9.18 p178 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation and Cellular Respiration Fermentation and cellular respiration are anaerobic and aerobic alternatives, respectively, for producing ATP from sugars. Both use glycolysis to oxidize sugars to pyruvate with a net production of 2 ATP by substrate-level phosphorylation. Both use NAD+ as an oxidizing agent to accept electrons from food during glycolysis. Fermentation and cellular respiration differ in their mechanism for oxidizing NADH to NAD+, which is required to sustain glycolysis. In fermentation, the electrons of NADH are passed to an organic molecule such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation), in order to regenerate NAD+. In cellular respiration, the electrons of NADH are ultimately passed to O2, generating ATP by oxidative phosphorylation. Under aerobic respiration, a molecule of glucose yields 38 ATP, but the same molecule of glucose yields only 2 ATP under anaerobic respiration. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation Fig. 9.18 p178 AnimationChapter_09\C_Animation_and_Video_Files\09_Animations\09_18FermentationOverview_A.swf: Fermentation Overview Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Organisms vary in the pathways available to them to break down sugars. Obligate anaerobes carry out only fermentation or anaerobic respiration and cannot survive in the presence of oxygen. A few cell types, such as the cells of the vertebrate brain, can carry out only aerobic oxidation of pyruvate, not fermentation. Yeast and many bacteria are facultative anaerobes that can survive using either fermentation or respiration. At a cellular level, human muscle cells can behave as facultative anaerobes. For facultative anaerobes, pyruvate is a fork in the metabolic road that leads to two alternative routes.

Facultative Anaerobes: Two Catabolic Routes Under aerobic conditions, pyruvate is converted to acetyl CoA and oxidation continues in the citric acid cycle. Under anaerobic conditions, pyruvate serves as an electron acceptor to recycle NAD+. To make the same amount of ATP, a facultative anaerobe must consume sugar at a much faster rate when fermenting than when respiring. Fig. 9.19 p179 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways Glycolysis can accept a wide range of carbohydrates for catabolism. Polysaccharides like starch or glycogen can be hydrolyzed to glucose monomers that enter glycolysis and the citric acid cycle. The digestion of disaccharides, including sucrose, provides glucose and other monosaccharides as fuel for respiration. The other two major fuels, proteins and fats, can also enter the respiratory pathways used by carbohydrates. Fig. 9.20 p180 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Proteins and fats can also enter respiratory pathways Proteins must first be digested to individual amino acids.. Amino acids that will be catabolized must have their amino groups removed. The nitrogenous waste is excreted as ammonia, urea, or another waste product. The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the citric acid cycle, depending on their structure. Catabolism can also harvest energy stored in fats obtained from food or from storage cells in the body. Fats must be digested to glycerol and fatty acids. Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis. The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via beta oxidation. These molecules enter the citric acid cycle as acetyl CoA. NADH and FADH2 are also generated during beta oxidation; they can enter the electron transport chain, leading to further ATP production. A gram of fat oxidized by respiration generates twice as much ATP as a gram of carbohydrate. Fig. 9.20 p180 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Respiration also plays a role in anabolic pathways of the cell Food provides the carbon skeletons that cells require to make their own molecules. Some organic monomers obtained from digestion can be used directly. Intermediaries in glycolysis and the citric acid cycle can be utilized in anabolic pathways as precursors for cells to synthesize molecules. A human cell can synthesize about half the 20 different amino acids by modifying compounds from the citric acid cycle. The rest are “essential amino acids” that must be obtained in the diet. Glucose can be synthesized from pyruvate; fatty acids can be synthesized from acetyl CoA. Anabolic, or biosynthetic, pathways do not generate ATP but instead consume it. Excess carbohydrates and proteins can be converted to fats through intermediaries of glycolysis and the citric acid cycle. If we eat more food than we need, we store fat even if our diet is fat-free.

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

Feedback Mechanisms Phosphofructokinase- catalyzes the earliest step that irreversibly commits the substrate to glycolysis. Phosphofructokinase has receptor sites for specific inhibitors and activators. Phosphofructokinase is inhibited by ATP and stimulated by AMP (derived from ADP). When ATP levels are high, inhibition of this enzyme slows glycolysis. As ATP levels drop and ADP and AMP levels rise, the enzyme becomes active again and glycolysis speeds up. Citrate- the first product of the citric acid cycle, is also an inhibitor of phosphofructokinase. This synchronizes the rate of glycolysis and the citric acid cycle. Fig. 9.21 p181 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings