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 Pi.

°             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.