Chapter 9

9.1 Catabolic pathways yield energy by oxidizing organic fuelsCatabolic Pathways and Production of ATPCells degrade complex organic molecules that are rich in potential energy to simpler waste products that have less energy with enzymes. Some energy is used to do work, the rest is dissipated as heat. Fermentation – catabolic process that is a partial degradation of sugars without the use of oxygen.

Cellular Respiration - most efficient catabolic pathway where oxygen is a reactant along with organic fuel. Breakdown of glucose is exergonic, having a free-energy change of -

686 kcal (-2,870 kJ) per mole of glucose decomposed. Therefore, respiration’s products store less energy than the reactants and it can happen spontaneously.Redox Reactions: Oxidation and Reduction

Redox reactions are oxidation-reduction reactions: a transfer of one or more electrons from one reactant to another. The loss of electrons is called oxidation while the addition of

electrons is called reduction. In these generalized reactions, substance X, the electron donor, is called the reducing agent. The reducing agent reduces Y, which accepts the donated electron.

Substance Y is the electron acceptor, the oxidizing agent. Since an electron transfer requires both a donor and an acceptor, oxidation and reduction always go together.

In some redox reactions, the degree of electron sharing in covalent bonds is changed. This reaction between methane and oxygen, for example, oxidizes methane because the carbon atom has partially lost its shared electrons. Energy must be added to pull an electron away from an atom. The more electronegative the atom, the more energy is required to take the electron. Oxidation of methane by oxygen is the main combustion reaction that occurs at the burner of a gas stove, similarly to an automobile engine.

Organic molecules with an abundance of hydrogen are good fuels because their bonds are a source of “hilltop” electrons

whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen. In cellular respiration, glucose is not oxidized in one step. They are broken down in a series of steps, each step catalyzed by an enzyme. Usually first passed to a coenzyme called NAD+ (nicotinamide adenine dinucleotide, derivative of niacin):

it’s an electron acceptor!Respiration uses an electron transport chain to break the fall of electrons into oxygen into several energy-releasing steps. Chain is proteins on the inner membrane of a mitochondria. At the top, higher-energy end, NADH is. At the bottom, lower energy level, oxygen captures electrons with H+ to form water. Free energy change is -53 kcal/mol (-222 kJ/mol)Oxygen basically pulls electrons down the chain that yields energy. Food > NADH > electron transport chain > oxygen

Chapter 9The Stages of Cellular Respiration

Glycolysis occurs in the cytosol = breaking down glucose into two molecules of pyruvate. Citric acid cycle takes place in the mitochondrial matrix, oxidizes a derivative of pyruvate to carbon dioxide. The carbon dioxide produced by respiration represents fragments of oxidized organic molecules.

Some steps include redox reactions where dehydrogenase enzymes transfer electrons from substrates to NAD+ into NADH. The electron transport chain accepts electrons from the first two stages and passes electrons from other oxygens. The energy released at each step of the chain is stored in a form the mictochondrion can use to make ATP: called oxidative phosphorylation. Smaller amount of ATP is formed directly by glycolysis and citric acid cycle by a method called substrate-level phosphorylation: enzyme transfers phosphate group from substrate molecule to ADP instead of adding an inorganic phosphate to ATP. Substrate molecule = organic molecule generated during catabolism of glucose. Upto about 38 molecules of ATP, each with 7.3 kcal/mol of free energy.

9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvateGlycolysis means “splitting of sugar”.

Glucose 6-carbon is split into 2 3-carbon sugars. Each of those is then oxidized and rearranged to form two molecules of pyruvate

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9.3 The citric acid cycle completes the energy-yielding oxidation of organic moleculesGlycolysis releases about a quarter of the chemical energy stored in glucose: most is still in pyruvate. If molecular oxygen is present, pyruvate enters the mitochondrion via active transport.Pyruvate is firect converted to a compound called acetyl coenzyme A, or acetyl CoA.

1. Pyruvate’s carboxyl group is removed and released as a molecule of CO2.

2. Remaining two-carbon fragment is oxidized, forming acetate (ionized form of acetic acid)

3. Enzyme transfers extracted electrons to NAD+, creating NADHs.

4. Coenzyme A, a sulfur-containing compound derived from a B vitamin, is attached to acetate by an unstable bond, making acetyl group very reactive. This acetyl group is not going into the citric acid cycle for more oxidation.

Also known as the tricarboxylic acid cycle, or Krebs.

Chapter 9

Cycle based on the decomposition then regeneration of oxaloacetate. For each acetyl group that enters, 3 NADH, 1 FADH, and 1 ATP. Total for one glucose: 10.

Chapter 9

9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

Oxidative phosphorylation uses energy released by electron transport chain to power ATP synthesis.

The Power of Electron TransportElectron transport chain is a collection of molecules on inner memberane of mitochondrion. Membrane folds to form cristae, mostly proteins in multi-protein complexes. Prosthetic groups are nonprotein complements essential for catalytic functions of certain enzymes tightly bound to these proteins.Flavoprotein > Iron-sulfer protein – ubiquinone (electron carrier, small hydrophobic molecule, mobile in membrane)Most other electron carriers are cytochromes. They have a prosthetic group called heme which has an iron atom which accepts and donates electrons. Electron transport chain makes no ATM directly – mitochondrion makes it by chemiosmosis.

Chemiosmosis: The Energy-Coupling MechanismATP Synthase is everywhere. Ion pump in reverse. Since ion gradient formed by electron transport chain Power source for ATP synthase is a difference in the concentration of H+ on opposite sides of the mitochondrial membrane (aka difference in pH)Energy stored in the form of a hydrogen ion gradient macros a membrane is called chemiosmosis. H+ gradient result of electron transport chain is called proton-motive force. Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work. Energy for gradient formation comes from exergonic redox reactions.

Chloroplasts use chemiosmosis to generate ATP during photosynthesis. Light energy causes H+ gradient. Prokaryotes make H+ gradients across plasma membranes, to make ATP as well as pump nutrients and waste products across membrane.

An Accounting of ATP Production by Cellular RespirationGlucose > NADH > electron transport chain > proton-motive force > ATPGlycolysis and Citric Acid = 4 ATP through substrate-level phosphorylation. NADH contributes about 3 ATP. 2.5-3.3 ATP per NADH. FADH2 is 1.5-2 ATP. Total yield: 38 ATP or 26 ATP for less efficient shuttle.

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9.5 Fermentation enables some cells to produce ATP without the use of oxygenAerobic Respiration = normal, 38 ATP.Anaerobic Respiration = no oxygen, 2 net ATP with substrate-level phosphorylation.

Types of FermentationAlcohol fermentation is for pyruvate converted to ethanol (ethyl alcohol) in two steps. Yeast carries out alcohol fermentation. Lactic acid fermentation is for pyruvate directly reduced to NADH to form lactate as an end product, with no CO2.Muscle cells make ATP with lactic acid when they’re tired. Lactate later moved to liver then liver converts back to pyruvate. In fermentation, NAD+ is the oxidizing agent. In fermentation, final electron acceptor is organic molecule such as pyruvate (lactic) or acetaldehyde (alcohol). In respiration, final acceptor for electrons from NADH is oxygen. This regenerates NAD+ for glycolysis and helps another ATP. Species able to make enough ATP to survive with either fermentation or respiration are called facultative anaerobes. Muscle cells count as facultative anaerobe.

9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways

Versatility of CatabolismCalories can also be obtained from fast, proteins, sucrose (other disaccharaides), starch (polysaccharide). In digestive tract, starch is hydrolyzed to glucose. Glycogen can be hydrolyzed to glucose between meals. Proteins must be digested to amino acids for fuel. Many amino acids are used to build more proteins, in excess are converted by enzymes to intermediates of glycolysis and the citric acid cycle. Deamination is then an amino acid’s amino group is removed. Nitrogenous refuse is excreted in ammonia, urea, or other waste. After fats digested to glycerol and fatty acids, glycerol is converted to glyceraldehyde-3-phosphate.

Chapter 9Beta oxidation is a metabolic sequence that breaks the fatty acids down to two-carbon fragments: acetyl CoA.

BiosynthesisCells need substance. Food provides carbon skeletons, for example compounds in intermediates of glycolysis can be diverted into anabolic pathways as precursors from which the cell can synthesize the molecules it requires.

Regulation of Cellular Respiration via Feedback MechanismsSupply and demand = metabolic “economy”. Feedback inhibition – end product of anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway. If the cell is working hard and its ATP concentration begins to drop, respiration speeds up. When there is plenty of ATP to meet demand, respiration slows down. Phosphofructokinase is an allosteric enzyme with receptor sites for specific inhibitors and activators. It’s inhibited by ATP and stimulated by AMP which is derived from ADP.