Chapter 9: Cellular Respiration and Fermentation

*Steps of Pyruvate Oxidation*

(1) Pyruvate's carboxyl group is removed and given off as a molecule of CO2. (2) The remaining 2-carbon fragment is oxidized, forming acetate. The extracted electrons are transferred to NAD+, storing energy in the form of NADH. (3) CoA is attached via its sulfur atom to the acetate, forming acetyl CoA, which has a high potential energy.

*Glycolysis can be divided into two phases...*

(1) energy investment: cell actually spends ATP (2) energy payoff: investment is repaid with interest; ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electrons released from the oxidation of glucose

*The net energy yield from glycolysis, per glucose molecule, is...*

2 ATP plus 2 NADH

The end products of glycolysis are...

2 pyruvate, 2 NADH, 2 ATP

*About ______ of the energy in a glucose molecule is _________, making about _______ ATP.*

34%; transferred to ATP during cellular respiration; 32

How do electrons that are extracted from glucose and stored as potential energy in NADH finally reach oxygen?

FIGURE 9.5 • simple version is reaction b/w H and O to form water: mix H2 and O2, provide spark for activation energy and gases combine explosively; explosion represents a release of energy as the electrons of H "fall" closer to the electronegative O atoms • cellular respiration also brings H and O together to form water but there are two important differences: (1) H that reacts with O is derived from organic molecules rather than H2 (2) instead of occurring in one explosive reaction, respiration uses an electron transport chain to break the fall of electrons to O into several energy-releasing steps

Figure 9.3: Methane combustion as an energy-yielding redox reaction

The covalent electrons in methane are shared nearly equally between the bonded atoms because C and H are about equally electronegative. But when methane reacts with oxygen, forming CO2, electrons end up shared less equally between the C atom and its new covalent partners, the O atoms, which are very electronegative. In effect, the C atom has partially "lost" its shared electrons; thus, methane has been oxidized.

*NAD+ as an Electron Shuttle*

The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH. The second proton (H+) is released. Most of the electrons removed from food are transferred initially to NAD+.

Why does oxygen perform the role of final electron acceptor of an ETC so well?

because it is extremely electronegative

*Fermentation*

catabolic process that is a partial degradation of sugars occurring without the use of oxygen

After pyruvate is oxidized, the citric acid cycle... (START 9.3)

completes the energy-yielding oxidation of organic molecules

During oxidative phosphorylation, chemiosmosis...

couples electron transport to ATP synthesis

*Oxidizing Agent*

electron acceptor in a redox reaction

*Reducing Agent*

electron donor in a redox reaction

*Distinction between fermentation and anaerobic respiration*

electron transport chain is used in anaerobic respiration but not in fermentation

*Fermentation and anaerobic respiration...* (START 9.5)

enable cells to produce ATP without the use of oxygen

When electrons move closer to a more electronegative atom, what happens?

energy is released and the more electronegative atom is reduced

*In the presence of oxygen, pyruvate...*

enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed

Breakdown of organic molecules...

exergonic

During cellular respiration, most electrons travel the following "downhill" route:

glucose > NADH > electron transport chain > oxygen

*During respiration, most energy flows in this sequence:*

glucose > NADH > electron transport chain > proton-motive force > ATP

*Metabolic Stages of Cellular Respiration*

harvesting of energy from glucose has three stages: (1) glycolysis (2) pyruvate oxidation and citric acid cycle (3) oxidative phosphorylation: electron transport and chemiosmosis

When skeletal muscle cells are oxygen-deprived, the heart still pumps. What must the heart muscle cells be able to do?

heart muscle must continue to carry out aerobic metabolism even though muscle cells cannot

The reaction of acetyl CoA to yield lower-energy products is...

highly exergonic

*Which stage of cellular respiration accounts for most of the ATP synthesis and why?*

oxidative phosphorylation because it is powered by redox reactions

*Glycolysis harvests chemical energy by....* (START 9.2)

oxidizing glucose to pyruvate

Catabolic pathways yield energy by... (START 9.1)

oxidizing organic fuels

*Fermentation uses _____________ instead of an ETC to generate ATP.*

substrate-level phosphorylation

*NAD+*

• a coenzyme that is well-suited as an electron carrier because it can cycle easily between oxidized (NAD+) and reduced (NADH) states • each electron in an oxidation reaction travels with a proton (H atom); the H atoms are not transferred directly to oxygen but instead are passed first to NAD+ • as an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration • consists of two nucleotides joined together at their phosphate groups • most versatile electron acceptor in cellular respiration

*Redox Reactions*

• a.k.a oxidation-reduction reactions • transfer of electrons from one reactant to another • OXIDATION: loss of electrons from one substance • REDUCTION: addition of electrons to another substance • releases energy stored in organic molecules and the released energy is then used for ATP synthesis • some do not transfer electrons but change the electron sharing in covalent bonds (ex. reaction b/w methane and oxygen)

Cytochromes

• an iron-containing protein that is a component of electron transport chains • ETC has several types, each a different protein with a slightly different electron-carrying heme group • last cytochrome of the chain passes its electrons to oxygen; each O atom also picks up a pair of H ions from the aqueous solution, forming water • electron carriers between ubiquinone and oxygen (figure 9.13)

Evolutionary Significance of Glycolysis

• ancient prokaryotes are though to have used glycolysis to make ATP long before oxygen was present in the Earth's atmosphere • very little oxygen was available until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP

Obligate Anaerobes

• carry out only fermentation or anaerobic respiration • cannot survive in presence of oxygen

*Electron Transport Chain*

• consists of a number of molecules, mostly proteins, built into inner membrane of mitochondria of eukaryotic cells and plasma membrane of aerobically respiring prokaryotes • a sequence of electron carrier molecules that shuttle electrons down a series of redox reactions that release energy used to make ATP

*Oxidative Phosphorylation* (START 9.4)

• electron transport and chemiosmosis together make up oxidative phosphorylation, so it occurs in inner membrane of mitochondria in eukaryotes (and in the plasma membrane for prokaryotes) • production of ATP using energy derived from the redox reactions of electron transport chain • accounts for almost 90% of ATP generated

*Chemiosmosis*

• energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP • in mitochondria, the energy for gradient formation comes from exergonic redox reactions, and ATP synthesis is the work performed • refers to the flow of H+ across a membrane • under aerobic conditions, most ATP synthesis in cells occurs by chemiosmosis

*How does NAD+ trap electrons from glucose and other organic molecules?*

• enzymes called dehydrogenases remove a pair of H atoms (2 electrons 2 protons) from the substrate, thereby oxidizing it • the enzyme delivers the 2 electrons with 1 proton to its coenzyme, NAD+ (the other proton is released as a H ion into surrounding solution) • by receiving 2 negatively charged electrons but only 1 proton, NAD+ has its charge neutralized when it is reduced to NADH

*Regulation of Cellular Respiration via Feedback Mechanisms*

• feedback inhibition: end product of anabolic pathway inhibits enzyme that catalyzes an early step of the pathway, which prevents the needless diversion of key metabolic intermediates from uses that are more urgent • 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

*Oxidation of Pyruvate to Acetyl CoA*

• formula of pyruvate: CH3COCOOH or C3H4O3 • upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called ACETYL CoA (acetyl coenzyme A)

Versatility of Catabolism (START 9.6)

• glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways • glycolysis can accept carbohydrates, fats, and proteins as forms of fuel for cellular respiration

*Cellular Respiration*

• includes both aerobic and anaerobic processes • often used to refer to aerobic respiration • it is helpful to learn the steps of cellular respiration by tracing the degradation of the sugar glucose • negative G indicates that products store less energy than reactants; exergonic/spontaneous

*Glycolysis*

• means "sugar splitting" • occurs in the cytosol • begins the degradation process by breaking glucose into two molecules of a compound called pyruvate • in eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which then enters the citric acid cycle

*Substrate-Level Phosphorylation*

• mechanism that allows smaller amount of ATP to be formed directly in a few reactions of glycolysis and citric acid cycle • occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP (as in oxidative)

*Citric Acid Cycle/Krebs Cycle*

• occurs in the eukaryote's mitochondrial matrix and the prokaryote's cytosol • breakdown of glucose to CO2 is completed; thus the CO2 produced by respiration represents fragments of oxidized organic molecules • functions as a metabolic furnace that oxidizes organic fuel derived from pyruvate, generating 1 ATP (substrate-level phos), 3 NADH and 1 FADH2 per turn • most of the chemical energy is transferred to NAD+ and the coenzyme FAD during the redox reactions • for each turn of the cycle, 2 carbons enter in the relatively reduced form of an acetyl group and 2 different carbons leave in the completely oxidized form of CO2 molecules

*Electron Transport Chain in Cellular Respiration Process*

• occurs in the inner membrane of mitochondria • most components are proteins, which are tightly bound to prosthetic groups (nonprotein components essential for the catalytic functions of certain enzymes) • accepts electrons from the breakdown products of the first two stages and passes those electrons from one molecule to another • during electron transport along the chain, electron carriers alternate b/w reduced and oxidized states as they accept and donate electrons. 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 "downhill", more electronegative neighbor • at the end of the chain, the electrons are combined with molecular oxygen and hydrogen ions forming water

*Anaerobic Respiration and Production of ATP*

• organisms that live in environments without oxygen have electron transport chains but do not use oxygen as the final electron acceptor at the end of the chain • other, less electronegative substances can also serve as final electron acceptors

*Aerobic Respiration*

• oxygen is consumed as a reactant along with the organic fuel (ex. glucose), yields ATP • the most prevalent and efficient catabolic pathway

*ATP Synthase*

• protein complex that populates the inner membrane of mitochondria (eu) or plasma membrane (pro) • the enzyme that actually makes ATP from ADP and inorganic phosphate • works like an ion pump in reverse: in respiration it uses the energy of an existing ion gradient to power ATP synthesis • power source: difference in concentration of H+ on opposite sides of the inner mitochondrial membrane

Alcohol Fermentation

• pyruvate is converted to ethanol in two steps: (1) CO2 is released from pyruvate, which is converted to the two-carbon compound acetaldehyde (2) acetaldehyde is reduced by NADH to ethanol, which regenerates the supply of NAD+ needed for the continuation of glycolysis • many bacteria carry it out under anaerobic conditions; also yeast does

Lactic Acid Fermentation

• pyruvate is reduced directly by NADH to form lactate as an end product with no release of CO2 • performed by certain fungi and bacteria in the dairy industry to make cheese and yogurt • human muscle cells make ATP this way when oxygen is scarce

*Anaerobic Respiration*

• similar to aerobic respiration but consumes compounds other than oxygen • some prokaryotes do this

Facultative Anaerobes

• some organisms, including yeasts and many bacteria, can make enough ATP to survive using either fermentation OR respiration • pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

Biosynthesis/Anabolic Pathways

• the body uses small molecules to build other substances • these small molecules may come directly from food, from glycolysis, or from the citric acid cycle

*Oxidation of Organic Fuel Molecules During Cellular Respiration*

• the fuel (glucose) is oxidized to CO2 and oxygen is reduced to H2O • electrons lose potential energy along the way and energy is released (exergonic) • the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis

*NADH*

• the name NADH shows the hydrogen that has been received in the reaction • each NADH formed during respiration represents stored energy that is tapped to synthesize ATP when the electrons complete their "fall" down an energy gradient from NADH to oxygen

*Proton-Motive Force*

• the potential energy stored in the form of a proton electrochemical gradient, generated by the pumping of hydrogen ions across the membrane during chemiosmosis • drives H+ back across membrane through the H+ channels provided by ATP synthases

*Comparing Fermentation with Anaerobic and Aerobic Respiration*

• three alternative cellular pathways for producing ATP by harvesting chemical energy of food • similarities: all three use glycolysis to oxidize glucose and other organic fuels to pyruvate with a net production of 2 ATP by substrate-level phosphorylation, and in all three, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis • differences: have different final electron acceptors—an organic molecule (such as pyruvate or acetaldehyde) in fermentation, oxygen in aerobic respiration, and another electronegative molecule in anaerobic respiration; respiration harvests much more energy from each sugar molecule than fermentation can...respiration produces 32 ATP per glucose, fermentation produces 2 ATP per glucose

*Fermentation and Production of ATP*

• way of harvesting chemical energy without use of oxygen OR electron transport chain...in other words, without cellular respiration • glycolysis occurs with neither oxygen nor an ETC; it generates 2 ATP whether oxygen is present or not (whether conditions are aerobic or anaerobic) • fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis

Beta Oxidation

• when catabolism harvests energy stored in fats (as opposed to carbohydrates) this process is used • breaks fatty acids down to two-carbon fragments, which enter the citric acid cycle as acetyl CoA • NADH and FADH2 are also generated during beta oxidation; these are electron carriers and will transfer electrons to the ETC leading to ATP synthesis