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47 Cards in this Set
- Front
- Back
chemical recycling processes
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• Energy stored in organic molecules of food ultimately comes from the sun
• Photosynthesis generates oxygen and organic molecules used by the mitochondria of eukaryotes (including plants and algae) as fuel for cellular respiration • Respiration breaks this fuel down, generating ATP • The waste products of this type of respiration, carbon dioxide and water, are the raw materials for photosynthesis |
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fermentation
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• A catabolic process that makes a limited amount of ATP from glucose (or other organic molecules) without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid
• Simply put, occurs without the use of oxygen |
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aerobic respiration
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• A catabolic pathway for organic molecules, using oxygen (O2) as the final electron acceptor in an electron transport chain and ultimately producing ATP
• This is the most efficient catabolic pathway and is carried out in most eukaryotic cells and many prokaryotic organisms |
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cellular respiration
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The catabolic pathways of aerobic and anaerobic respiration, which break down organic molecules and use an electron transport chain for the production of ATP
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degradation of glucose
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• C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + heat)
• Breakdown of glucose is exergonic, having a large negative free energy change |
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redox reaction
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• A chemical reaction involving the complete or partial transfer of one or more electrons from one reactant to another; short for reduction-oxidation reaction
• Not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds |
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oxidation
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The complete or partial loss of electrons from a substance involved in a redox reaction
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reduction
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• The complete or partial addition of electrons to a substance involved in a redox reaction
• Adding electrons is called reduction because negatively charged electrons added to an atom reduce the amount of positive charge of the atom |
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reducing agent
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The electron donor in a redox reaction
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oxidizing agent
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The electron acceptor in a redox reaction
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oxygen & electronegativity
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• Oxygen is so electronegative that it is one of the most potent oxidizing agents
• Energy must be added to pull an electron away from an atom • The more electronegative the atom (the stronger its pull on electrons), the more energy is required to take an electron away from it • An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one |
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oxidation of organic fuel molecules during cellular respiration
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• Organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of "hilltop" electrons
• Oxidation of glucose transfers electrons to a lower energy state, liberating energy • Carbohydrates and fats, the main energy-yielding foods, are reservoirs of electrons associated with hydrogen • Oxidation of glucose within the body is initiated by enzymes that lower the barrier of activation energy |
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NAD+ (nicotinamine adenine dinucleotide)
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• A coenzyme that cycles easily between oxidized (NAD) and reduced (NADH) states, thus acting as an electron carrier
• The most versatile electron acceptor in cellular respiration |
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how NAD+ traps electrons from organic molecules
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• Dehydrogenase removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate
• Enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+ • The other proton is released as a hydrogen ion (H+) into the surrounding solution • By receiving 2 negatively charged electrons but only 1 positively charged proton, NAD+ has its charge neutralized when it is reduced to NADH • Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their "fall" down an energy gradient from NADH to oxygen |
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electron transport chain
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• A sequence of electron carrier molecules (membrane proteins) that shuttle electrons down a series of redox reactions that release energy used to make ATP
• Consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes • Electrons removed by glucose are shuttled by NADH to the "top," higher-energy end of the chain • At the "bottom," lower-energy end, O2 captures these electrons along with hydrogen nuclei (H+), forming water • Electron transfer from NADH to oxygen is an exergonic reaction • Each "downhill" carrier is more electronegative than, and thus capable of oxidizing, its "uphill" neighbor, with oxygen at the bottom of the chain • Oxygen pulls electrons down the chain in an energy yielding tumble • During cellular respiration, most electrons travel the following "downhill" route: glucose → NADH → electron transport chain → oxygen |
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the stages of cellular respiration
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• Includes three stages: (1) glycolysis, (2) pyruvate oxidation and the citric acid cycle, and (3) oxidative phosphorylation: electron transport and chemiosmosis
• For each molecule of glucose degraded to carbon dioxide, the cell makes up to about 32 molecules of ATP |
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glycolysis
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• A series of reactions that ultimately splits glucose into pyruvate
• Occurs in almost all living cells in the cytosol, serving as the starting point for fermentation or cellular respiration |
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citric acid cycle
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A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules begun in glycolysis by oxidizing acetyl CoA (derived from pyruvate) to carbon dioxide; occurs within the mitochondrion in eukaryotic cells and in the cytosol of prokaryotes; together with pyruvate oxidation, the second major stage in cellular respiration
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oxidative phosphorylation
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• The production of ATP using energy derived from the redox reactions of an electron transport chain; the third major stage of cellular respiration
• Accounts for almost 90% of the ATP generated by respiration when the NADH and FADH2 produced by the citric acid cycle relay the electrons extracted from food to the electron transport chain |
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substrate-level phosphorylation
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The enzyme-catalyzed formation of ATP by direct transfer of a phosphate group to ADP from an intermediate substrate in catabolism
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difference between oxidative phosphorlyation and substrate-level phosphorylation
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Substrate-level phosphorylation is a mode of ATP synthesis that occurs when an enzyme transfers a phosphate group to a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation
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the process of glycolysis
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• During the energy investment phase, the cell actually spends ATP
• This investment is repaid with interest during the energy payoff phase, when 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 • No carbon is released as CO2 during glycolysis • Glycolysis occurs whether or not O2 is present; however, if O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation |
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acetyl CoA (acetyl coenzyme A)
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The entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme
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oxidation of acetyl coA to pyruvate
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• Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A
• This step, linking glycolysis and the citric acid cycle, is carried out by a multienzyme complex that catalyzes three reactions: (1) Pyruvate's carboxyl group (--COO-), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2 -- this is the first step in which CO2 is released during respiration (2) The remaining two-carbon fragment is oxidized, forming acetate (CH3COO-, the ionized form of acetic acid) and the extracted electrons are transferred to NAD+ storing energy in the form of NADH (3) Finally, coenzyme A, a sulfur-compound derived from a B vitamin, is attached via its sulfur atom to the acetate, forming acetyl CoA, which has a high potential energy (the reaction of acetyl CoA to yield lower-energy products is highly exergonic) |
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a closer look at the citric acid cycle
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• The citric acid cycle functions as a metabolic furnace that oxidizes organic fuel derived from pyruvate
• Pyruvate is broken down to three CO2 molecules, including the molecule of CO2, released during the conversion of pyruvate to acetyl CoA • The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred to NAD+ and a related electron carrier, the coenzyme FAD (flavin adenine dinucleotide, derived from riboflavin, B vitamin) • Reduced coenzymes NADH and FADH2 shuttle their cargo of high-energy electrons into the electron transport chain • For each turn of the citric acid cycle, two carbons enter in the relatively reduced form of an acetyl group, and two different carbons leave in the completely oxidized form of CO2 molecules • The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate • For each acetyl group entering the cycle, 3 NAD+ are reduced to NADH (steps 3, 4, and 8) • In step 6, electrons are transferred not to NAD+, but to FAD, which accepts 2 electrons and 2 protons to become FADH2 • In many animal tissue cells, step 5 produces a guanosine triphosphate (GTP) molecule by substrate-level phosphorylation |
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the pathway of electron transport
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• Electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells
• The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion • Most components of the chain are proteins and tightly bound to these proteins are prosthetic groups, nonprotein components essential for the catalytic functions of certain enzymes • Along the chain, electron carriers alternate between 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 has a lower affinity for electrons (is less electronegative) • It then returns to its oxidized form as it passes electrons to its "downhill," more electronegative neighbor |
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a closer look at the electron transport chain
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• Electrons removed from glucose by NAD+ during glycolysis and the citric acid cycle, after transferred from NADH to the first molecule of the electron transport chain in complex 1
• This is flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN) • Flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (FeS) in complex 1 • The iron-sulfur protein then passes the electrons to a compound called ubiquinone, a small hydrophobic molecule, which is the only member of the electron transport chain that is not a protein • Ubiquinone, an electron carrier, is invidivually mobile within the membrane rather than residing in a particular complex • Most of the remaining electron carriers between upbiquinone and oxygen are proteins called cytochromes • The last cytochrome of the chain, cyt a3, passes its electrons to oxygen • FADH2 adds its electrons to the electron transport chain from within complex 2, at a lower energy level than NADH does • The electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH |
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cytochrome
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An iron-containing protein that is a component of electron transport chains in the mitochondria and chloroplasts of eukaryotic cells and the plasma membranes of prokaryotic cells
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ATP synthase
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• A complex of several membrane proteins that functions in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion (proton) concentration gradient to make ATP
• Found in the inner mitochondrial membranes of eukaryotic cells and in the plasma membranes of prokaryotes • Works like an ion pump running in reverse • The power source is a difference in the concentration of H+ on opposite sides of the inner mitochondrial membrane (a different in pH, essentially) • The smallest molecular rotary motor known in nature |
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chemiosmosis
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• An 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
• Under aerobic conditions, most ATP synthesis in cells occurs by chemiosmosis |
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how the flow of H+ through ATP synthase powers ATP production
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• ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides
• Protons move one by one into binding sites on one of the parts (the rotor), causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate |
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establishing the H+ gradient
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• The electron transport chain is a major energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane, from the mitochondrial matrix into the intermembrane space
• The H+ has a tendency to move back across the membrane, diffusing down its gradient • ATP synthases are the only sites that provide a route through the membrane for H+ • The passage of H+ through ATP synthase uses the exergonic flow of H+ to drive the phosphorylation of ADP • Thus, energy stored in an H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis, an example of chemiosmosis |
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how the electron transport chain pumps H+
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• Certain members of the ETC accept and release protons along with electrons
• At certain steps along the chain, electron transfers cause H+ to be taken up and released into the surrounding solution • In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited in the intermembrane space • The resulting H+ gradient is known as a proton-motive force • The force drives H+ back across the membrane through the H+ channels provided by ATP synthesis • Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work |
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proton-motive force
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The potential energy stored in the form of a proton electrochemical gradient, generated by the pumping of hydrogen ions (H+) across a biological membrane during chemiosmosis
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an accounting of ATP production by cellular respiration
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• During respiration, most energy flows in this sequence: glucose → NADH → electron transport chain → proton-motive force → ATP
• 4 ATP produced directly by substrate-level phosphorlyation during glycolysis and the citric acid cycle to the many more molecules of ATP generated by oxidative phosphorylation • Phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole number • 1 NADH results in 10 H+ being transported out across the inner mitochondrial membrane and about 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP • So, a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP • The citric acid cycle also supplies electrons to the electron transport chain via FADH2, but each molecule is responsible for transport of only enough H+ for the synthesis of 1.5 ATP • The proton-motive force generated by the redox reactions of respiration reduce the yield of ATP by driving other kinds of work (ex. powering the mitochondrion's uptake of pyruvate from the cytosol) • If all the proton-motive force generated by the ETC were used to drive ATP synthesis, one glucose molecule could generate a maximum of 28 ATP produced by oxidative phosphorylation plus 4 ATP (net) from substrate-level phosphorylation to give a total yield of about 32 ATP (or about 30 ATP if the less efficient shuttle were functioning) • About 34% of the potential chemical energy in glucose is transferred to ATP |
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fermentation
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• An extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis
• For this to occur, there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis • Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete the cell's pool of NAD+ by reducing it all to NADH and would shut itself down for lack of an oxidizing agent • Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the ETC • An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis |
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alcohol fermentation
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• Glycolysis followed by the reduction of pyruvate to ethyl alcohol, regenerating NAD and releasing carbon dioxide
• In the first step, carbon dioxide is released from pyruvate, which is converted to the two-carbon compound acetaldehyde • The second step uses NADH to reduce acetaldehyde to ethanol |
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lactic acid fermentation
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• Glycolysis followed by the reduction of pyruvate to lactate, regenerating NAD with no release of carbon dioxide
• Human muscle cells make ATP by lactic acid fermentation when oxygen is scare (such as during strenuous exercise, when sugar catabolism for ATP production outspaces the muscle's supply of oxygen from the blood) • That lactate accumulates was thought to cause muscle fatigue and pain, but recent research suggests instead that increased levels of potassium ions (K+) may be to blame, while lactate appears to enhance muscle performance • Excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells |
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comparing fermentation with anaerobic and aerobic respiration
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• All three use glycolysis to oxidize glucose and other organic fuels to pyruvate, with a net product of 2 ATP by substrate-level phosphorylation
• In all three pathways, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis |
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contrasting fermentation with aerobic and anaerobic respiration
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• A key difference among the three pathways is the contrasting mechanisms for oxidizing NADH back to NAD+, which is required to sustain glycolysis
• In fermentation, the final electron acceptor is an organic molecule such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation) • In cellular respiration, by contrast, electrons carried by NADH are transfered to an electron transport chain, where they move stepwise down a series of redox reactions to a final electron acceptor • In aerobic respiration, the final electron acceptor is oxygen; in anaerobic respiration, the final electron acceptor is another electronegative molecule • Aerobic respiration yields up to 16 times as much ATP per glucose molecule as does fermentation -- up to 32 molecules of ATP for respiration, compared with 2 molecules of ATP produced by substrate-level phosphorylation in fermentation |
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obligate anaerobe
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• An organism that only carries out fermentation or anaerobic respiration
• Such organisms cannot use oxygen and in fact may be poisoned by it |
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facultative anaerobe
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An organism that makes ATP by aerobic respiration if oxygen is present but that switches to anaerobic respiration or fermentation if oxygen is not present
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pyruvate in catabolism
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• Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle via aerobic respiration
• Under anaerobic conditions, lactic acid fermentation occurs: Pyruvate is diverted from the citric acid cycle, serving instead as an electron acceptor to recycle NAD+ |
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the evolutionary significance of glycolysis
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• Ancient prokaryotes are thought to have used glycolysis to make ATP long before oxygen was present in the Earth's atmosphere
• The oldest known fossils of bacteria date back 3.5 billion years, but appreciable quantities of oxygen probably did not begin to accumulate in the atmosphere until about 2.7 billion years ago • Cyanobacteria produced this O2 as a by-product of photosynthesis • Early prokaryotes may have generated ATP exclusively from glycolysis • The fact that glycolysis is the most widespread metabolic pathway among Earth's organisms suggests that it evolved very early in the history of life |
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the versatility of catabolism
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• Before amino acids can feed into glycolysis or the citric acid cycle, their amino groups must be removed, called deamination
• The nitrogenous refuse is excreted from the animal in the form of ammonia (NH3) urea, or other waste products • After fats are digested to glycerol and fatty acids, the glycerol is converted to glyceraldehyde 3-phosphate, an intermediate of glycolysis; most of the energy of a fat is stored in the fatty acids • Beta oxidation then breaks down the fatty acids to two-carbon fragments, which enter the citric acid cycle as acetyl CoA • NADH and FADH2 are also generated during beta oxidation; they can enter the electron transport chain, leading to further ATP production |
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beta oxidation
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A metabolic sequence that breaks fatty acids down to two-carbon fragments that enter the citric acid cycle as acetyl CoA
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regulation of cellular respiration via feedback mechanisms
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• The cell does not waste energy making more of a particular substance than it needs -- it controls this process via feedback inhibition: the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway
• Phosphofructokinase, which catalyzes step 3 of glycolysis, the first step that commits the substrate irreversibly to the glycolytic pathway, can be considered the pacemaker of respiration • Phosphofructokinase is inhibited by ATP and stimulated by AMP (adenosine monophosphate), which the cell derives from ADP • As ATP accumulates, inhibition of the enzyme slows down glycolysis • The enzyme becomes active again as cellular work converts ATP to ADP (and AMP) faster than ATP is being regenerated • Phosphofructokinase is sensitive to citrate -- if citrate accumulates in the mitochondria, some of it passes into the cytosol and inhibits phosphofrucktokinase • If citrate consumption increases, either because of a demand for more ATP or because anabolic pathways are draining off intermediates of the citric acid cycle, glycolysis accelerates and meets the demand |
