Chapter 9

Cellular Respiration:
Harvesting Chemical Energy

Overview

· To perform their many tasks, living cells require energy from outside sources.

· Energy enters most ecosystems as __________ and leaves as heat.

· Photosynthesis generates _______ and organic molecules that the mitochondria of eukaryotes use as ______ for cellular respiration.

· Cells harvest the chemical energy stored in organic molecules and use it to regenerate ______, the molecule that drives most cellular work.

· Respiration has three key pathways: _________, the citric acid cycle, and oxidative ____________.

A. The Principles of Energy Harvest

1. Cellular respiration and fermentation are _________, energy-yielding pathways.

· Catabolic metabolic pathways release the energy stored in complex ________ molecules.

· One type of catabolic process, ___________, leads to the partial degradation of sugars in the absence of oxygen.

· A more efficient and widespread catabolic process, _________ respiration, consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules.

° In eukaryotic cells, ____________ are the site of most of the processes of cellular respiration.

° Food is the ________ for respiration. The exhaust is _______________ and water.

· The overall process is:

° organic compounds + O₂ à CO₂ + H₂O + energy (ATP + ______).

· Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider _________.

° C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + Energy (ATP + heat)

· The catabolism of glucose is _________ with a D G of -686 kcal per mole of glucose

2. Redox reactions release energy when electrons move closer to electronegative atoms.

· Redox reactions ________ both a donor and acceptor.

° Oxygen is very electronegative, and is one of the most potent of all ___________ agents.

· Energy must be _________ to pull an electron away from an atom.

· The more electronegative the atom, the more _________ is required to take an electron away from it.

· An electron loses potential energy when it shifts from a less electronegative atom toward a _______ electronegative one.

3. The “fall” of electrons during respiration is stepwise, via NAD⁺ and an electron transport chain.

· Cellular respiration does not oxidize ______ in a single step that transfers all the hydrogen in the fuel to oxygen at one time.

· Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific _______.

° At key steps, electrons are __________ from the glucose.

· The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called _____ (nicotinamide adenine dinucleotide).

· By receiving two electrons and only one proton, NAD⁺ has its charge neutralized when it is reduced to ______.

· Each NADH molecule formed during respiration represents stored ________. This energy is tapped to synthesize ATP as electrons “fall” from NADH to ______.

· How are electrons extracted from food and stored by NADH finally transferred to oxygen?

° cellular respiration uses an electron _________ chain to break the fall of electrons to O₂ into several steps.

· The electron transport chain consists of several molecules (primarily proteins) built into the inner __________ of a mitochondrion.

· At the “bottom” lower-energy end, ________ captures the electrons along with H⁺ to form water.

· Electron transfer from NADH to oxygen is an ________ reaction with a free energy change of -53 kcal/mol.

· Electrons are passed to increasingly electronegative molecules in the chain until they reduce oxygen, the most electronegative receptor.

· In summary, during cellular respiration, most electrons travel the following “downhill” route: food à _____ à electron transport chain à ________.

B. The Process of Cellular Respiration

1. These are the stages of cellular respiration: a preview.

· Glycolysis occurs in the __________.

° It begins catabolism by breaking glucose into two molecules of _________.

· The citric acid cycle occurs in the mitochondrial ______.

° It completes the breakdown of glucose by oxidizing a derivative of pyruvate to ________ _______.

· Several steps in glycolysis and the citric acid cycle are redox reactions in which dehydrogenase enzymes transfer electrons from substrates to NAD⁺, forming NADH.

· NADH _______ these electrons to the electron transport chain.

· In the electron transport chain, the electrons move from molecule to molecule until they combine with molecular ______ and hydrogen ions to form water.

· As they are passed along the chain, the energy carried by these electrons is transformed in the mitochondrion into a form that can be used to synthesize _____ via oxidative phosphorylation.

· The inner membrane of the mitochondrion is the site of electron transport and ___________, processes that together constitute oxidative phosphorylation.

° Oxidative phosphorylation produces almost __% of the ATP generated by respiration.

· Some ATP is also formed directly during glycolysis and the citric acid cycle by ________-_______ phosphorylation.

° Here an enzyme transfers a phosphate group from an organic substrate to _____, forming ATP.

· For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to ____ ATP, each with 7.3 kcal/mol of free energy.

2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate.

· During glycolysis, glucose, a six carbon-sugar, is split into two _______-______ sugars.

· Each of the ten steps in glycolysis is catalyzed by a specific enzyme.

· These steps can be divided into two phases: an energy investment phase and an energy _______ phase.

· In the energy investment phase, the cell invests ATP to provide activation energy by _____________ glucose.

° This requires __ ATP per glucose.

· In the energy payoff phase, ATP is produced by substrate-level __________ and NAD⁺ is ________ to NADH by electrons released by the oxidation of glucose.

· The net yield from glycolysis is __ ATP and __ NADH per glucose.

° No CO₂ is produced during _________.

· Glycolysis can occur whether O₂ is present or _____.

3. The citric acid cycle ((Krebs cycle) completes the energy-yielding oxidation of organic molecules.

· More than three-quarters of the original _______ in glucose is still present in the two molecules of pyruvate.

· If _______ is present, pyruvate enters the mitochondrion where enzymes of the citric acid cycle complete the ________ of the organic fuel to carbon dioxide.

· After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A or _______________.

· This step is accomplished by a multienzyme complex that catalyzes three reactions:

1. A carboxyl group is removed as _____.

2. The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers the pair of electrons to NAD+ to form ______.

3. Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA.

° The citric acid cycle has eight steps, each catalyzed by a specific enzyme.

° The acetyl group of acetyl CoA joins the cycle by combining with the compound ____________, forming citrate.

° The next seven steps decompose the citrate back to ____________. It is the regeneration of oxaloacetate that makes this process a cycle. Much like RUBP in the Calvin cycle

° Three ____ molecules are released, including the one released during the conversion of pyruvate to acetyl CoA.

· The cycle generates one ____ per turn by substrate-level phosphorylation.

· Most of the chemical energy is transferred to NAD⁺ and ____ during the redox reactions.

· The reduced coenzymes NADH and FADH₂ then transfer high-energy __________ to the electron transport chain.

· Each cycle produces one ATP by substrate-level phosphorylation, ________ NADH, and one FADH₂ per acetyl CoA.

4. The inner mitochondrial membrane couples electron transport to ATP synthesis.

· Only 4 of 38 ATP ultimately produced by respiration of glucose are produced by substrate-level phosphorylation.

° Two are produced during ________, and 2 are produced during the citric acid cycle.

· NADH and FADH₂ account for the vast majority of the energy extracted from the food.

· The electron transport chain is a collection of molecules embedded in the cristae, the folded inner membrane of the mitochondrion.

· Electrons drop in ______ energy as they pass down the electron transport chain.

· Electrons carried by NADH are transferred to the ______ molecule in the electron transport chain, a flavoprotein.

· The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier.

· The last cytochrome of the chain, cyt a₃, passes its electrons to ________, which is very electronegative.

° Each oxygen atom also picks up a pair of ________ ions from the aqueous solution to form water.

° For every two electron carriers (four electrons), one O₂ molecule is reduced to two molecules of _______.

· The electrons carried by FADH₂ have ________ free energy and are added at a lower energy level than those carried by NADH.

· A protein complex, ATP ________, in the cristae actually makes ATP from ADP and P_i.

° The proton gradient develops between the intermembrane space and the ________.

· The chain is an energy converter that uses the exergonic flow of electrons to pump H⁺ from the _______ into the intermembrane space.

· The protons pass back to the matrix through a channel in ATP synthase, using the exergonic flow of H⁺ to drive the ______________ of ADP.

· The ATP synthase molecules are the only place that H⁺ can diffuse back to the matrix.

° This coupling of the redox reactions of the electron transport chain to ATP synthesis is called ___________.

° The H⁺ gradient that results is the ________-motive force.

· Chemiosmosis in chloroplasts also generates ATP, but _______ drives the electron flow down an electron transport chain and H⁺ gradient formation.

5. Here is an accounting of ATP production by cellular respiration.

_______ ATP molecules are produced by substrate-level phosphorylation during glycolysis and the citric acid cycle.

· Many more ATP molecules are generated by ____________ phosphorylation.

· Each NADH from the citric acid cycle and the conversion of pyruvate contributes enough energy to the proton-motive force to generate a maximum of __ ATP.

° The NADH from glycolysis may also yield __ ATP.

· Each FADH₂ from the citric acid cycle can be used to generate about __ ATP.

1. The ATP yield varies slightly depending on the type of shuttle used to transport electrons from the ________ into the mitochondrion.

° The mitochondrial inner membrane is impermeable to ______, so the two electrons of the NADH produced in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems.

° If all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose molecule could generate a maximum of 34 ATP by oxidative phosphorylation plus 4 ATP (net) from substrate-level phosphorylation to give a total yield of 36–38 _____ (depending on the efficiency of the shuttle).

° Efficiency of respiration is 7.3 kcal/mol times 38 ATP/glucose divided by 686 kcal/mol glucose, which equals 0.4 or __%.

° Approximately 60% of the energy from glucose is lost as _______.

§ Some of that heat is used to maintain our high body temperature (37°C).

C. Related Metabolic Processes

1. Fermentation enables some cells to produce ATP without the help of oxygen.

· Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation ________.

· However, _________ provides a mechanism by which some cells can oxidize organic fuel and generate ATP without the use of oxygen.

° In glycolysis, glucose is oxidized to two __________ molecules with NAD⁺ as the oxidizing agent.

° Glycolysis is ___________ and produces 2 ATP (net).

° If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron __________ chain.

· Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (__________).

· Anaerobic catabolism of sugars can occur by ___________.

· Fermentation can generate ATP from glucose by substrate-level phosphorylation as long as there is a supply of ____ to accept electrons.

° If the NAD⁺ pool is exhausted, ___________ shuts down.

° Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, __________ NAD⁺.

· In alcohol fermentation, _________ is converted to ethanol in two steps.

° First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of ____.

° Second, acetaldehyde is reduced by _______ to ethanol.

· During ________ acid fermentation, pyruvate is reduced directly by NADH to form lactate (the ionized form of lactic acid) without release of CO₂.

° Human muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when ___ is scarce.

§ The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to ________ in the liver.

° In fermentation, the electrons of NADH are passed to an organic molecule to __________ NAD⁺.

° In respiration, the electrons of NADH are ultimately passed to ____, 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, from glycolysis.

· Yeast and many bacteria are ____________ anaerobes that can survive using either fermentation or respiration.

· The fact that glycolysis is a __________ metabolic pathway and occurs in the cytosol without membrane-enclosed organelles suggests that glycolysis evolved early in the history of life.

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

· Glycolysis can accept a wide range of carbohydrates for ____________.

° Polysaccharides like starch or _________ can be hydrolyzed to glucose monomers that enter glycolysis.

· The other two major fuels, proteins and fats, can also enter the respiratory pathways used by carbohydrates.

· Proteins must first be digested to individual ______ acids.

· Catabolism can also harvest energy stored in fats.

· Fats must be digested to _________ and fatty acids.

° Glycerol can be converted to ____________ phosphate, an intermediate of glycolysis.

· A gram of fat oxides by respiration generates ________ as much ATP as a gram of carbohydrate.

° A human cell can synthesize about half the ____ different amino acids by modifying compounds from the citric acid cycle.

3. Feedback mechanisms control cellular respiration.

· Basic principles of supply and demand regulate the metabolic economy.

° If a cell has an excess of a certain amino acid, it typically uses feedback inhibition to prevent the diversion of intermediary molecules from the citric acid cycle to the synthesis pathway of that amino acid.

· The rate of catabolism is also regulated, typically by the level of _____ in the cell.

° If ATP levels ____, catabolism speeds up to produce more ATP.

· One strategic point occurs in the third step of glycolysis, catalyzed by _________.

· Allosteric regulation of phosphofructokinase sets the pace of respiration.

° This enzyme catalyzes the earliest step that ____________ commits the substrate to glycolysis.

° Phosphofructokinase is an ___________ enzyme with receptor sites for specific inhibitors and activators.

° It is inhibited by ____ and stimulated by ____ (derived from ADP).

§ When ATP levels are ______, inhibition of this enzyme slows glycolysis.

§ As ATP levels drop and ADP and AMP levels _____, the enzyme becomes active again and glycolysis speeds up.

· Citrate, the first product of the citric acid cycle, is also an __________ of phosphofructokinase.