FINAL: Chapter 9
Terms in this set (47)
catabolic process that is a partial degradation of sugars occurring without the use of oxygen
• oxygen is consumed as a reactant along with the organic fuel (ex. glucose), yields ATP
• the most prevalent and efficient catabolic pathway
• similar to aerobic respiration but consumes compounds other than oxygen
• some prokaryotes do this
• includes both aerobic and anaerobic processes
• 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
• a.k.a oxidation-reduction reactions
• transfer of electrons from one reactant to another
n*: loss of electrons from one substance
n*: 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)
electron donor in a redox reaction
electron acceptor in a redox reaction
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
Redox reaction that moves electrons closer to oxygen...
releases chemical energy that can be put to work
• 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
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+.
• 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
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
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
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
Which stage of cellular respiration accounts for most of the ATP synthesis and why?
oxidative phosphorylation because it is powered by redox reactions
• 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
• 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)
Glycolysis harvests chemical energy by....
oxidizing glucose to pyruvate
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
• also 2 pyruvate + 2 H2O
In the presence of oxygen, pyruvate...
enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed
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)
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.
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
Products of Citric Acid Cycle
8 NADH, 2 FADH2, 2 ATP, 6 CO2
How are the products of the Krebs cycle involved in ATP production?
• step 5: CoA is displaced by a phosphate group which is transferred to a GDP molecule forming GTP by substrate-level phosphorylation...GTP is similar to ATP and may be used to make an ATP molecule
• in some cases, step 5 forms an ATP molecule directly by substrate-level phosphorylation
• 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
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
• 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
• 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
• 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
How does the proton gradient connect the electron transport chain to ATP synthesis?
(1) NADH and FADH2 shuttle high-energy electrons into an electron transport chain; electrons finally pass to oxygen at "downhill" end of chain, forming water; as complexes I, II, and IV accept and then donate electrons, they pump protons from mitochondrial matrix into intermembrane space; chemical energy originally harvested from food is transformed into proton-motive force, a gradient of H+ across the membrane
(2) during chemiosmosis, protons flow back down their gradient via ATP synthase; ATP synthase harnesses proton-motive force to phosphorylate ADP, forming ATP
During respiration, most energy flows in this sequence:
glucose > NADH > electron transport chain > proton-motive force > ATP
About ______ of the energy in a glucose molecule is _________, making about _______ ATP.
34%; transferred to ATP during cellular respiration; 32
ATP yield per molecule of glucose at each stage of cellular respiration
• see study guide for more
• one glucose could generate maximum of 28 ATP by oxidative phosphorylation plus 4 ATP from substrate-level phosphorylation to give total yield of about 32 ATP
Fermentation and anaerobic fermentation...
enable cells to produce ATP without the use of oxygen
Distinction between fermentation and anaerobic respiration
electron transport chain is used in anaerobic respiration but not in fermentation
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
Fermentation uses _____________ instead of an ETC to generate ATP.
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
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
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
Control of cellular respiration
• Allosteric enzymes at certain points in respiratory pathway respond to inhibitors and activators that help set pace of glycolysis and citric acid cycle
• Phosphofructokinase, which catalyzes an early step in glycolysis, is one such enzyme...it is stimulated by AMP but is inhibited by ATP and citrate
• This feedback regulation adjusts rate of respiration as the cell's catabolic and anabolic demands change
Flow of Carbon in Citric Acid Cycle
two carbon atoms enter cycle in reduced for of acetyl CoA and two different carbons leave in completely oxidized form of CO2 molecules
Proteins in Energy Metabolism
• proteins must be digested to their constituent amino acids before they can be used for fuel
• amino acids present in excess are converted by enzymes to intermediates of glycolysis and citric acid cycle
• before amino acids can feed into glycolysis or citric acid cycle their amino groups must be removed, a process called
Fats in Energy Metabolism
• catabolism can also harvest energy stored in fats obtained from food or storage cells in body
• after fats are digested to fatty acids and glycerol, the glycerol is converted to glyceraldehyde 3-phosphate, an intermediate of glycolysis
• metabolic sequence called
n* breaks fatty acids down to two-carbon fragments which enter citric acid cycle as acetyl CoA; NADH and FADH2 are also generated and can enter electron transport chain, leading to further ATP production
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