Harvesting Chemical Energy
· 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.