Essential Cell Biology - Chapter 13, ESSENTIAL CELL BIOLOGY- CHAPTER 14: ENERGY GENERATION IN MITOCHONDRIA AND CHLOROPLASTS, Chapter 15 Cell Biology
In oxidative phosphorylation, ATP production is coupled to the events in the electron- transport chain. What is accomplished in the final electron-transfer event in the electron- transport chain? (a) OH- is oxidized to O2 (b) pyruvate is oxidized to CO2 (c) O2 is reduced to H2O (d) NAD+ is reduced to NADH
(c) O2 is reduced to H2O
Foods are broken down into simple molecular subunits for distribution and use throughout the body. Which type of simple subunits, listed below, is used preferentially as an energy source? (a) simple sugars (b) proteins (c) free fatty acids (d) glycerol
A
Several different classes of enzymes are needed for the catabolism of carbohydrates. Which of the following descriptions best matches the function of an isomerase? (a) An enzyme that catalyzes the rearrangement of bonds within a single molecule. (b) An enzyme that catalyzes a change in the position of a specific chemical group within a single molecule. (c) An enzyme that catalyzes the oxidation of a molecule by removing a hydride ion. (d) An enzyme that catalyzes the addition of phosphate groups to other molecules.
A
The oxygen-dependent reactions required for cellular respiration were originally thought to occur one in the in a linear pathway. By using a competitive inhibitor for enzyme pathway, investigators discovered that these reactions occur in a cycle. Which product in the reaction pathway builds up when the inhibitor is added? (a) citrate (b) succinate (c) fumarate (d) malate
B
Glyceraldehyde 3-phosphate dehydrogenase operates by stripping a hydride ion from its substrate. Which molecule is the recipient of the proton and two electrons during this transfer? (a) oxygen (b) acetyl CoA (c) NAD+ (d) FADH
C
The advantage to the cell of the gradual oxidation of glucose during cellular respiration compared with its combustion to CO2 and H2O in a single step is that ________________. (a) more free energy is released for a given amount of glucose oxidized. (b) no energy is lost as heat. (c) energy can be extracted in usable amounts. (d) more CO2 is produced for a given amount of glucose oxidized.
C
3 stages of metabolism
Stage 1:Digestion Stage 2:Acetyl CoA Production Stage 3:Energy Production
Your friend lost a lot of weight eating a low-calorie diet. Where did all the biomass go?
The mass was released as CO2 and H2O.
Metabolism = Catabolism + Anabolism
The sum total of the chemical reactions that take place in the cells of a living organism resulting in growth, division, energy production, excretion of waste, etc.
Modern eukaryotes depend on mitochondria to generate most of the cell's ATP. How many molecules of ATP can a single molecule of glucose generate? (a) 30 (b) 2 (c) 20 (d) 36
(a) 30
Which of the following statements describes the phosphorylation event that occurs during the process known as oxidative phosphorylation? (a) A phosphate group is added to ADP. (b) ATP is hydrolyzed in order to add phosphate groups to protein substrates. (c) A phosphate group is added to molecular oxygen. (d) Inorganic phosphate is transported into the mitochondrial matrix, increasing the local phosphate concentration.
(a) A phosphate group is added to ADP
In stage 1 of photosynthesis, a proton gradient is generated and ATP is synthesized. Where do protons become concentrated in the chloroplast? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane
(a) thylakoid space
In the absence of oxygen, yeast cells can switch to a completely anaerobic metabolism called fermentation. Which of the following is a final product of fermentation in yeast?
(b) Ethanol and CO2 are the products of fermentation in yeast when grown anaerobically. Option (a) is lactate, (c) is acetaldehyde, and (d) is pyruvate.
NADH contains a high-energy bond that, when cleaved, donates a pair of electrons to the electron-transport chain. What are the immediate products of this bond cleavage? (a) NAD+ + OH- (b) NAD+ + H- (c) NAD- + H+ (d) NAD + H
(b) NAD+ + H-
In the electron-transport chain in chloroplasts, ________-energy electrons are taken from __________. (a) high; H2O (b) low; H2O (c) high; NADPH. (d) low; NADPH.
(b) low; H2O
NADH and FADH2 carry high-energy electrons that are used to power the production of ATP in the mitochondria. These cofactors are generated during glycolysis, the citric acid cycle, and the fatty acid oxidation cycle. Which molecule below can produce the most ATP? Explain your answer. (a) NADH from glycolysis (b) FADH2 from the fatty acid cycle (c) NADH from the citric acid cycle (d) FADH2 from the citric acid cycle
(c) NADH from the citric acid cycle
Stage 1 of oxidative phosphorylation requires the movement of electrons along the electron-transport chain coupled to the pumping of protons into the intermembrane space. What is the final result of these electron transfers? (a) OH- is oxidized to O2 (b) pyruvate is oxidized to CO2 (c) O2 is reduced to H2O (d) H- is converted to H2
(c) O2 is reduced to H2O
Which of the following is not part of the process known as oxidative phosphorylation? (a) Molecular oxygen serves as a final electron acceptor. (b) FADH2 and NADH become oxidized as they transfer a pair of electrons to the electron-transport chain. (c) The electron carriers in the electron-transport chain toggle between reduced and oxidized states as electrons are passed along. (d) ATP molecules are produced in the cytosol as glucose is converted into pyruvate.
(d) ATP molecules are produced in the cytosol as glucose is converted into pyruvate.
Which of the following statements describes the mitochondrial matrix? (a) It is permeable to molecules with molecular mass as high as 5000 daltons. (b) It contains transporters for ATP molecules. (c) It contains proteins that are released during apoptosis. (d) It contains enzymes required for the oxidation of fatty acids
(d) It contains enzymes required for the oxidation of fatty acids
Stage 2 of photosynthesis, sometimes referred to as the dark reactions, involves the reduction of CO2 to produce organic compounds such as sucrose. What cofactor is the electron donor for carbon fixation? (a) H2O (b) NADH (c) FADH2 (d) NADPH
(d) NADPH
Photosynthesis is a process that takes place in chloroplasts and uses light energy to generate high-energy electrons, which are passed along an electron-transport chain. Where are the proteins of the electron-transport chain located in chloroplasts? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane
(d) thylakoid membrane
The ATP synthase found in chloroplasts is structurally similar to the ATP synthase in mitochondria. Given that ATP is being synthesized in the stroma, where will the F0 portion of the ATP synthase be located? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane
(d) thylakoid membrane
The enzyme ribulose bisphosphate carboxylase (Rubisco) normally adds carbon dioxide to ribulose 1,5-bisphosphate. However, it will also catalyze a competing reaction in which O2 is added to ribulose 1,5-bisphosphate to form 3-phosphoglycerate and phosphoglycolate. Assume that phosphoglycolate is a compound that cannot be used in any further reactions. If O2 and CO2 have the same affinity for Rubisco, what will be the lowest ratio of CO2 to O2 at which a net synthesis of sugar can occur?
3:1
Glycolysis is an anaerobic process used to catabolize glucose. What does it mean for this process to be anaerobic? (a) no oxygen is required (b) no oxidation occurs (c) it takes place in the lysosome (d) glucose is broken down by the addition of electrons
A
In step 4 of glycolysis, a six-carbon sugar (fructose 1,6-bisphosphate) is cleaved to produce two three-carbon molecules (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate). Which enzyme catalyzes this reaction? (a) aldolase (b) phosphoglucose isomerase (c) enolase (d) triose phosphate isomerase
A
In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. The energy of the thioester bond is harnessed in step 5. What is the energy used for? (a) to generate a molecule of GTP (b) to generate a molecule of ATP (c) to generate a proton gradient (d) to generate a molecule of NADH
A
In the final stage of the oxidation of food molecules, gradient of protons across the inner mitochondrial membrane, which is normally impermeable to protons. If cells were exposed to an agent that causes the membrane to become freely permeable to protons, which of the following effects would you expect to observe? (a) The ratio of ATP to ADP in the cytoplasm would fall. (b) NADH would build up. (c) Carbon dioxide production would cease. (d) The consumption of oxygen would fall.
A
Select the best option to fill in the blanks of the following statement: Fermentation is a/an _____________________ process that converts _____________ into carbon dioxide and _____________________. (a) anaerobic, pyruvate, ethanol (b) anaerobic, lactate, ethanol (c) eukaryotic, glyceraldehyde 3-phosphate, ethanol (d) prokaryotic, lactate, propanol
A
The citric acid cycle is a critical sequence of reactions energy production, place in the matrix of the mitochondria. The reaction cycle requires materials from the cytosol to be converted into acetyl CoA, which represents the starting point of a new cycle. Which of the following statements about acetyl CoA is true? (a) Amino acids can be converted into acetyl CoA. (b) Pyruvate is converted into acetyl CoA in the cytosol. (c) Triacylglycerol molecules are transported into the mitochondrial matrix and cleaved by lipases to produce acetyl CoA. (d) Oxaloacetate is converted directly into acetyl CoA to feed the citric acid cycle.
A
The citric acid cycle is a series of oxidation reactions that removes carbon atoms from substrates in the form of CO2. Where do the oxygen atoms in the carbon dioxide molecules come from? (a) water (b) phosphates (c) molecular oxygen (d) acetyl CoA
A
The conversion of glyceraldehyde 3-phosphate to 1,3 bisphosphoglycerate in step 6 of glycolysis generates a "high energy" phosphoanhydride bond. Which of the following best describes what happens to that bond in step 7? (a) It is hydrolyzed to drive the formation of ATP. (b) It is hydrolyzed to drive the formation of NADH. (c) It is hydrolyzed to generate pyruvate. (d) It is oxidized to CO2.
A
The oxygen-dependent reactions required for cellular respiration were originally thought to occur in a linear pathway. By using a competitive inhibitor for one enzyme in the pathway, investigators discovered that these reactions occur in a cycle. What compound served as the inhibitor? (a) malonate (b) malate (c) fumarate (d) succinate
A
What purpose does the phosphorylation of glucose to glucose 6-phosphate by the enzyme hexokinase serve as the first step in glycolysis? (a) It helps drive the uptake of glucose from outside the cell. (b) It generates a high-energy phosphate bond. (c) It converts ATP to a more useful form. (d) It by the enables the glucose 6-phosphate to be recognized phosphofructokinase, next enzyme in the glycolytic pathway.
A
When glucose is being used up and not replaced from food intake, blood can be maintained by synthesizing glucose from smaller molecules such as pyruvate or lactate. This process is called gluconeogenesis. Which organ is principally responsible for supplying glucose to the rest of the body when glucose reserves are low? (a) liver (b) pancreas (c) spleen (d) gall bladder
A
Which of the following cells rely exclusively on glycolysis to supply them with ATP? (a) anaerobically growing yeast (b) aerobic bacteria (c) skeletal muscle cells (d) plant cells
A
Several different classes of enzymes are needed for the catabolism of carbohydrates. Which of the following descriptions best matches the function of a mutase? (a) An enzyme that catalyzes the rearrangement of bonds within a single molecule. (b) An enzyme that catalyzes a change in the position of a specific chemical group within a single molecule. (c) An enzyme that catalyzes the oxidation of a molecule by removing a hydride ion. (d) An enzyme that catalyzes the addition of phosphate groups to other molecules.
B
Step 6 of the citric acid cycle is catalyzed by succinate dehydrogenase. Keeping in mind that dehydrogenases catalyze redox reactions, which are the products of the reaction in which succinate is oxidized? (a) fumarate, NADH (b) fumarate, FADH2 (c) fumarate, FADH2 (d) succinyl CoA, NADH
B
Steps 7 and 10 of glycolysis result in substrate-level phosphorylation. Which of the following best describes this process? (a) ATP is being hydrolyzed to phosphorylate the substrate. (b) The energy derived from substrate oxidation is coupled to the conversion of ADP to ATP. (c) Two successive phosphates are transferred, first to AMP, then to ADP, finally forming ATP. (d) The substrate is hydrolyzed using ATP as an energy source.
B
The concentration of H+ ions inside the mitochondrial matrix is lower than it is in the cytosol or the mitochondrial intermembrane space. What would be the immediate effect of a membrane-permeable compound that carries and releases protons into the mitochondrial matrix? (a) inhibition of the electron-transport chain (b) inhibition of ATP synthesis (c) increased import of ADP into the matrix (d) inhibition of the citric acid cycle
B
The final metabolite produced by glycolysis is ___________. (a) acetyl CoA. (b) pyruvate. (c) 3-phosphoglycerate. (d) glyceraldehyde 3-phosphate
B
The first energy-generating steps in glycolysis begin when glyceraldehyde 3-phosphate undergoes an energetically favorable reaction in which it is simultaneously oxidized and to form 1,3-phosphorylated by the enzyme glyceraldehyde 3phosphate dehydrogenase bisphosphoglycerate, with the accompanying conversion of NAD+ to NADH. In a second energetically favorable reaction catalyzed by a second enzyme, the 1,3-bisphosphoglycerate is then converted to 3-phosphoglycerate, with the accompanying conversion of ADP to ATP. Which of the following statements is true about this reaction? (a) The reaction glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate should be inhibited when levels of NADH fall. (b) The ΔG° for the oxidation of the aldehyde group on glyceraldehyde 3-phosphate to form a carboxylic acid is more negative than the ΔG° for ATP hydrolysis. (c) The energy stored in the phosphate bond of glyceraldehyde 3-phosphate contributes to driving the reaction forward. (d) The cysteine side chain on the enzyme is oxidized by NAD+.
B
The simultaneous oxidation and phosphorylation of glyceraldehyde 3-phosphate forms a highly reactive covalent thioester bond between a cysteine side chain (reactive group -SH) on the enzyme (glyceraldehyde 3-phosphate dehydrogenase) and the oxidized intermediate (see arrow in Figure Q13-31A). If the enzyme had a serine (reactive group -OH) instead of a cysteine at this position, which could form only a much-lower-energy bond to the oxidized substrate (see arrow in Figure Q13-31B), how might this new enzyme act? Figure Q13-31 (a) It would oxidize the substrate and phosphorylate it without releasing it. (b) It would oxidize the substrate but not release it. (c) It would phosphorylate the substrate on the 2 position instead of the 1 position. (d) It would behave just like the normal enzyme.
B
Which of the following polymers of glucose is used as a vehicle to store energy reserves in animal cells? (a) glucagon (b) glycogen (c) starch (d) glycerol
B
Glycolysis generates more stored energy than it expends. What is the net activated carrier molecules produced in this process (number and type of molecules produced minus the number of those molecules used as input)? (a) 6 ATP, 2 NADH (b) 4 ATP, 4 NADH (c) 2 ATP, 2 NADH (d) 4 ATP, 2 NADH
C
In anaerobic conditions, skeletal muscle produces _____________. (a) lactate and CO2. (b) ethanol and CO2. (c) lactate only. (d) ethanol only.
C
In step 1 of the citric acid cycle, citrate is generated by the enzyme citrate synthase. The enzyme combines the two-carbon acetyl group from acetyl CoA and the four-carbon oxaloacetate. What is the source of energy that drives this reaction forward? (a) a high-energy phosphodiester bond (b) a transfer of high-energy electrons (c) a high-energy thioester bond (d) the heat of molecular collision
C
In step 4 of the citric acid cycle, the reduction of NAD+ to NADH is coupled to the generation of CO2 and the formation of a high-energy thioester bond. Which molecule provides the sulfhydryl group necessary to form the thioester bond? (a) pyruvate (b) acetyl CoA (c) CoA (d) cysteine side chain in the catalytic pocket
C
Pyruvate is an important metabolic intermediate that can be converted into several other compounds, depending on which enzyme is catalyzing the reaction. Which of the following cannot be produced from pyruvate in a single enzyme-catalyzed reaction? (a) lactate (b) oxaloacetate (c) citrate (d) alanine
C
Several different classes of enzymes are needed for the catabolism of carbohydrates. Which of the following descriptions best matches the function of a dehydrogenase? (a) An enzyme that catalyzes the rearrangement of bonds within a single molecule. (b) An enzyme that catalyzes a change in the position of a specific chemical group within a single molecule. (c) An enzyme that catalyzes the oxidation of a molecule by removing a hydride ion. (d) An enzyme that catalyzes the addition of phosphate groups to other molecules.
C
The intermediates of the citric acid cycle are constantly being depleted because they are used to produce many of the amino acids needed to make proteins. The enzyme pyruvate carboxylase converts pyruvate to oxaloacetate to replenish these intermediates. Bacteria, but not animal cells, have additional enzymes that can carry out the reaction acetyl CoA + isocitrate = oxaloacetate + succinate. Which of the following compounds will not support the growth of animal cells when used as the major source of carbon in food, but will support the growth of nonphotosynthetic bacteria? (a) pyruvate (b) glucose (c) fatty acids (d) fructose
C
The reaction cycle that uses acetyl CoA to generate electron carrier molecules needed in the electron-transport chain is important for powering the cell. Which of the names below is not one of those commonly used to describe this reaction cycle? (a) tricarboxylic acid cycle (b) Krebs cycle (c) oxaloacetic acid cycle (d) citric acid cycle
C
Which of the following steps or processes in aerobic respiration include the production of carbon dioxide? (a) breakdown of glycogen (b) glycolysis (c) conversion of pyruvate to acetyl CoA (d) oxidative phosphorylation
C
Which reaction does the enzyme phosphoglucose isomerase catalyze? (a) glucose to glucose 6-phosphate (b) fructose 6-phosphate to fructose 1,6-bisphosphate (c) glucose 6-phosphate to fructose 6-phosphate (d) glucose to glucose 1-phosphate
C
The oxygen-dependent reactions required for cellular respiration were originally thought to occur in a linear pathway. By using a competitive inhibitor for one enzyme in the pathway, investigators discovered that these reactions occur in a cycle. Which enzyme was inhibited? (a) aconitase (b) isocitrate dehydrogenase (c) malate dehydrogenase (d) succinate dehydrogenase
D
Which of the following stages in the breakdown of the piece of toast you had for breakfast generates the most ATP? (a) the digestion of starch to glucose (b) glycolysis (c) the citric acid cycle (d) oxidative phosphorylation
D Is the correct answer. Oxidative phosphorylation produces about 28 ATP molecules. Choice (a) produces no ATP Choice (b) nets 2 ATP Choice (c) produces 1 GTP
Some bacteria can live both aerobically and anaerobically. How does the ATP synthase in the plasma membrane of the bacterium help such bacteria to keep functioning in the absence of oxygen?
In the absence of oxygen, the respiratory chain no longer pumps protons, and thus no proton electrochemical gradient is generated across the bacterial membrane. In these conditions, the ATP synthase uses some of the ATP generated by glycolysis in the cytosol to pump protons out of the bacterium, thus forming the proton gradient across the membrane that the bacterium requires for importing vital nutrients by coupled transport.
The Law of Conservation of Mass
Mass/matter cannot be created or destroyed by ordinary chemical means. In cellular metabolism, chemical reactions are accompanied by energy conversions.
Mitochondria can use both __________________ and __________________ directly as fuel.__________________ produced in the citric acid cycle donates electrons to the electron-transport chain. The citric acid cycle oxidizes __________________ and produces __________________ as a waste product. __________________ acts as the final electron acceptor in the electron-transport chain. The synthesis of ATP in mitochondria is also known as __________________.
Mitochondria can use both *pyruvate and fatty acids* directly as fuel. *NADH* produced in the citric acid cycle donates electrons to the electron-transport chain. The citric acid cycle oxidizes *acetyl groups* and produces *carbon dioxide* as a waste product. Oxygen acts as the final electron acceptor in the electron-transport chain. The synthesis of ATP in mitochondria is also known as *oxidative phosphorylation*
Do you expect the cell to produce more ATP from one glucose molecule or from one fatty acid molecule? Explain your answer.
More ATP is produced from fat catabolism than from glucose catabolism. Most ATP is generated in the mitochondria, and the amount depends on the production of the NADH and FADH2 cofactors in the Krebs cycle. The Krebs cycle relies on the input of acetyl CoA. Each glucose molecule can be converted into two acetyl CoA molecules. A molecule of fat will have three fatty acid chains, with an average length of 12-16 carbons. Even if we assume very short fatty acid chains of six carbons each (the length of a glucose molecule), this would mean the production of three acetyl CoA molecules per chain, and nine total for the triacylglycerol.
NADH donates electrons to the __________________ of the three respiratory enzyme complexes in the mitochondrial electron-transport chain. __________________ is a small protein that acts as a mobile electron carrier in the respiratory chain. __________________ transfers electrons to oxygen. Electron transfer in the chain occurs in a series of __________________ reactions. The first mobile electron carrier in the respiratory chain is __________________.
NADH donates electrons to the *first* of the three respiratory enzyme complexes in the mitochondrial electron-transport chain. Cytochrome is a small protein that acts as a mobile electron carrier in the respiratory chain. Cytochrome oxidase transfers electrons to oxygen. Electron transfer in the chain occurs in a series of *oxidation- reduction* reactions. *The first mobile electron carrier in the respiratory chain is ubiquinone*
Figure Q13-19 represents a cell lining the gut. Draw numbered, labeled lines to indicate exactly where inside a cell the following processes take place. Figure Q13-19 1. glycolysis 2. citric acid cycle 3. conversion of pyruvate to activated acetyl groups 4. oxidation of fatty acids to acetyl CoA 5. glycogen breakdown 6. release of fatty acids from triacylglycerols 7. oxidative phosphorylation
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Explain how the F0 complex of ATP synthase harnesses the proton-motive force to help synthesize ATP. What would happen if the proton gradient were reversed?
Protons flow through a channel that exists between the subunits of the transmembrane H+ carrier, which forms a ring (the rotor). The flow of protons through this carrier makes the rotor and its attached stalk rotate. As the stalk rotates, it rubs against proteins in the stationary F1 portion of the ATP synthase. The resulting mechanical deformation produces a conformational change in the subunits of the F produce ATP. When the proton gradient is reversed, the Fcatalyzes the hydrolysis of ATP to ADP and Pi, rather than the revers reaction of ATP synthesis; this causes protons to be pumped out of the matrix against their electrochemical gradient, as the rotor and its stalk rotate in the direction opposite to that involved in ATP synthesis.
Anabolism:
Reactions that assemble small molecules (in many cases intermediates/ products of catabolism) into *macromolecules and biomass*. *REQUIRES ENERGY*
Catabolism
Reactions that break down larger, high-energy molecules into *smaller, low(er) energy molecules. RELEASES ENERGY.*
It can be useful to analyze the steps of glycolysis with respect to the four basic types of enzymes required by this central catabolic pathway and to consider whether each enzyme produces or harvests the energy of an activated carrier. For each step of glycolysis (see Figure 13-5 or Panel 13-1), indicate which type of enzyme (of the four listed below and in Table 13-1 is required—or if none apply). Also, indicate whether an activated energy carrier is involved, and, if so, how. Enzyme types: kinase, isomerase, mutase, dehydrogenase Step 1 ___________ Step 2 ___________ Step 3 ___________ Step 4 ___________ Step 5 ___________ Step 6 ___________ Step 7 ___________ Step 8 ___________ Step 9 ___________ Step 10 ___________
Step 1 kinase, energy in the form of ATP consumed Step 2 isomerase Step 3 kinase, energy in the form of ATP consumed Step 4 none of the above Step 5 isomerase Step 6 dehydrogenase, energy in the form of NADH produced Step 7 kinase; energy in the form of ATP produced Step 8 mutase Step 9 none of the above
The citric acid cycle generates NADH and FADH2, which are then used in the process of oxidative phosphorylation to make ATP. If the citric acid cycle (which does not use oxygen) and oxidative phosphorylation are separate processes, as they are, then why is it that the citric acid cycle stops almost immediately when O2 is removed?
The citric acid cycle stops almost immediately when oxygen is removed because several steps in the cycle require the oxidized forms of NAD+ and FAD. In the absence of oxygen, these electron carriers can be reduced by the reactions of the citric acid cycle but cannot be reoxidized by the electron-transport chain that participates in oxidative phosphorylation.
True or False -The intermembrane space of the mitochondria is chemically equivalent to the cytosol with respect to pH and the small molecules present.
True
True or False -The number and location of mitochondria within a cell can change, depending on both the cell type and the amount of energy required.
True
In step 7 a molecule to a of the citric acid cycle, fumarase catalyzes the addition of water carbon-carbon double bond (see Panel 13-2). Can this be considered an oxidation reaction? Explain your answer.
Yes, citric acid cycle, the overall process oxidizes carbon molecules to producecarbon dioxide. Although, fumarase does not directly remove electrons from its substrate (as enzymes that catalyze steps 3, 4, 6, and 8), the addition of water across the double bond in fumarate leaves one of its carbon atoms in a more oxidized state. This is because one of the carbons becomes bonded to an oxygen atom. In this arrangement, the carbon atom shares its electrons unequally across the new bond (see Figure 3-11).
Mitochondrial electron carriers with the ___ ___ ___ generally contain copper ions and/or heme groups.
highest redox potential
In the carbon-fixation process in chloroplasts, carbon dioxide is initially added to the sugar _____ ___ ____ . The final product of carbon fixation in chloroplasts is the three-carbon compound ___ ___ __. This is converted into ___ (which can be used directly by the mitochondria), into ___ (which is exported to other cells), and into ___ (which is stored in the stroma). The carbon- fixation cycle requires energy in the form of ___ and reducing power in the form of ___.
ribulose ,5-bisphosphate. glyceraldehyde 3-phosphate (G3P) pyruvate; sucrose; starch ATP; NADPH
In humans, glycogen is a more useful food-storage molecule than fat because _____________________. (a) a gram of glycogen produces more energy than a gram of fat. (b) it can be utilized to produce ATP under anaerobic conditions, whereas fat cannot. (c) it binds water and is therefore useful in keeping the body hydrated. (d) for the same amount of energy storage, glycogen occupies less space in a cell than does fat.
B
In the final step of the citric acid cycle, oxaloacetate is regenerated through the oxidation of malate and this is coupled with the production of which other molecule? (a) FADH (b) NADH (c) GTP (d) CO2
B
Fatty acids can easily be used to generate energy for the cell. Which of the following fatty acids will yield more energy? Explain your answer. (a) CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-COOH (b) CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH (c) CH3-CH=CH-CH2-CH2-CH2-CH2-CH=CH-COOH (d) CH3-CH2-CH2-CH2-CH2-CH2-CH2-COOH
B
True or False -The inner mitochondrial membrane is actually a series of discrete, flattened, membrane-enclosed compartments called cristae, similar to what is seen in the Golgi apparatus.
False. -Although the cristae do look like individual compartments on the basis of the images of the inner structure of the mitochondria, the inner membrane is a
True or False The inner mitochondrial membrane contains porins, which allow pyruvate to enter for use in the citric acid cycle.
False. -The outer mitochondrial membrane contains porins, allowing the passage of all molecules with a mass of less than 5000 daltons. Although pyruvate must
Based upon what you know about metabolism, explain how electrons are stripped from food molecules and used to drive the electron-transport chain.
Food molecules are ultimately converted into acetyl CoA. Electrons removed during the generation of acetyl CoA are added to the cofactors NAD+ and FAD to generate the reduced cofactors NADH and FADH2, respectively. The two carbon atoms in the acetyl group of acetyl CoA are then fed into the citric acid cycle, where they are oxidized to two molecules of CO2. The electrons removed during this oxidation are also captured by the activated carriers NADH and FADH2. The high-energy electrons in all of these activated carriers, which derived from carbons that were formerly part of food molecules are now transferred to the proteins in the electron-transport chain.
Which of the following processes do not take place in the mitochondria? (a) citric acid cycle (b) conversion of pyruvate to activated acetyl groups (c) oxidation of fatty acids to acetyl CoA (d) glycogen breakdown
D
The citric acid cycle is outlined in Figure Q13-44. Some of these reactions produce small molecules that are used in the electron-transport chain or as energy for other reactions. Select from the list below to fill in the empty boxes. Keep in mind that some choices may be used more than once and others not used at all. Figure Q13-44 A. ATP B. ADP C. GTP D. GDP E. NAD+ F. NADH G. FADH H. FADH2
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Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The proteins of the electron-transport chain remove a pair of high-energy electrons from the cofactors NADH and FADH2, after which the electrons move across the inner mitochondrial membrane to maintain the voltage gradient. B. Gluconeogenesis is a linear reaction pathway that the cell employs to generate glucose from pyruvate and is exactly the reverse of the reactions in the glycolytic pathway. C. With respect to the amount of energy stored in molecules of the body, 6g of glycogen is the equivalent of 1g of fat. D. Glycogen phosphorylase cleaves glucose monomers from the glycogen polymer, phosphorylating them at the same time so that they can be fed unchanged into the glycolytic pathway.
A. False. Although the proteins of the electron-transport chain collect electrons from the NADH and FADH2 cofactors, these high-energy electrons go through a series of transfers along the electron-transport chain. The energy released with each transfer moves protons across the inner mitochondrial membrane. It is this proton B. gradient that provides the energy to synthesize ATP. B. False. Gluconeogenesis can begin with pyruvate as a building block to make glucose, but there are three reactions in glycolysis that are irreversible because of a large free-energy barrier. Alternative enzymes and reaction pathways are used to bypass this problem, and they require the input of energy in the form of ATP and GTP. C. True. D. False. When glycogen phosphorylase cleaves a glucose monomer from glycogen, the product is glucose 1-phosphate. Before it can be used in glycolysis, it needs to be isomerized to glucose 6-phosphate.
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. During glycolysis, glucose molecules are broken down to yield CO2 and H2O. B.The cleavage of fructose 1,6-bisphosphate yields two molecules of glyceraldehyde 3-phosphate. C. Anaerobic respiration is not the same as fermentation, as only the former requires an electron-transport chain. D. When subjected to anaerobic conditions, glycolysis in mammalian cells continues and causes a buildup of pyruvate in the cytosol. E. The pyruvate dehydrogenase complex catalyzes three different, but linked, enzymatic reactions. F. Amino acids can be transported into the mitochondria and converted into acetyl CoA.
A. False. At the end of a series of the 10 different reactions involved in glycolysis, the final products are two molecules of pyruvate. Pyruvate will later be broken down into CO2 and H2O in the citric acid cycle. B. False. When fructose 1,6-bisphosphate is cleaved, the products are dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Only after the subsequent isomerization of dihydroxyacetone phosphate is the second molecule of glyceraldehyde 3-phosphate produced. C. True. D. False. Under anaerobic conditions, mammalian cells convert pyruvate to lactate in a fermentation process. The lactate is subsequently excreted from the cell. E. True. F. True.
In which of the four compartments of a mitochondrion are each of the following located? A. porin B. the mitochondrial genome C. citric acid cycle enzymes D. proteins of the electron-transport chain E. ATP synthase F. membrane transport protein for pyruvate
A. Porin is in the outer membrane. B. The mitochondrial genome is in the matrix. C. The citric acid cycle enzymes are in the matrix. D. The proteins of the electron-transport chain are in the inner membrane. E. ATP synthase is in the inner membrane. F. The transport protein for pyruvate is in the inner membrane.
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. CO2 and H2O are generated during the oxidation of food molecules. B. Activated carrier molecules store heat energy for the cell to use later. C. Catabolism is a general term that refers to the processes by which large molecules are synthesized from smaller molecules. D. The oxidation of sugar is an energetically favorable process.
A. True. B. False. Activated carriers have high-energy bonds that can drive other reactions when broken. Heat may be released during these reactions and may increase the reaction rates, but is not a form of energy that is stored in biological systems. C. False. Catabolism comprises the metabolic reactions that are involved in breaking large molecules into smaller molecules. Anabolism encompasses the reverse types of reactions: synthesizing larger molecules from smaller molecules. D. True.
Although the outer mitochondrial membrane is permeable to all small molecules, the inner mitochondrial membrane is essentially impermeable in the absence of specific transport proteins. Consider this information and what you have learned about the citric acid cycle to address the following questions. A. The ATP generated by oxidative respiration is used throughout the cell. The majority of ATP production occurs in the mitochondrial matrix. How do you think ATP is made accessible to enzymes in the cytosol and other organelles?
ATP must be transported across the inner mitochondrial membrane, after which it freely diffuses into the cytosol through the permeable outer membrane. Embedded in the inner membrane are dedicated ADP/ATP antiporters that serve dual purpose of exporting ATP and bringing in new ADP, which can then be converted into ATP during oxidative phosphorylation.
Although the outer mitochondrial membrane is permeable to all small molecules, the inner mitochondrial membrane is essentially impermeable in the absence of specific transport proteins. Consider this information and what you have learned about the citric acid cycle to address the following questions. C. Present two types of benefits derived from separating the reactions of glycolysis in the cytosol from those that occur during the citric acid cycle in the mitochondrion.
C. Compartmentalization provides a basic mechanism for the regulation of independent sets of reactions, including the citric acid cycle, fatty acid oxidation, glycolysis, and gluconeogenesis. In some cases, metabolic reactions are physically compartmentalized to separate anabolic from catabolic reactions. For example, in the mitochondrial matrix, oxaloacetate is used in the citric acid cycle to help oxidize the acetyl carbons of acetyl CoA. However, oxaloacetate in the cytosol tends to be consumed by biosynthetic enzymes that use the molecule as a precursor for the production of amino acids such as aspartate. By keeping these or in an anabolic reactions separate, the cell can control whether a molecule is used catabolic reaction. A second advantage of compartmentalization is the colocalization and concentration of enzymes with their substrates, which can enhance reaction rates.
Glucose is broken down to CO2 and H2O (by glycolysis + TCA cycle + respiration) by the following net reaction: C6H12O6 + 6O2 --> 6H2O + 6CO2
C6H12O6 + 6O2 --> 6H2O + 6CO2 + energy (enough to make 38 ATP!)
In step 2 of the citric acid cycle, the enzyme aconitase generates isocitrate from citrate. Which of the following statements about this reaction is true? (a) There is a substantial free-energy difference between the reactants and products of this reaction. (b) this reaction. The unbonded electrons from hydroxide ions provide energy for (c) The aconitase enzyme functions as a mutase in this reaction. (d) The reaction sequence first generates one molecule of water and then consumes one molecule of water.
D
In step 3 of the citric acid cycle, the oxidation of isocitrate and the production of CO2 are coupled to the reduction of NAD+, generating NADH and an α-ketoglutarate molecule. In the isocitrate molecule shown in Figure Q13-47, which carbon is lost as CO2 and which is converted to a carbonyl carbon? (a) 4 and 6 (b) 6 and 5 (c) 5 and 4 (d) 6 and 4
D
On a diet consisting of nothing but protein, which of the following is the most likely outcome? (a) loss of weight because amino acids cannot be used for the synthesis of fat (b) muscle gain because the amino acids will go directly into building muscle (c) tiredness because amino acids cannot be used to generate energy (d) excretion of more nitrogenous (ammonia-derived) wastes than with a more balanced diet
D
Pyruvate can be converted into many other molecules by various biosynthetic and metabolic pathways, which makes it a central hub in the regulation of cellular metabolism. Which of the following molecules is not made from pyruvate? (a) oxaloacetate (b) ethanol (c) lactate (d) NADH
D
Pyruvate must move from the cytosol into the mitochondria, where it is oxidized to form CO2 and acetyl CoA by the pyruvate dehydrogenase complex. How many different enzymes and what total number of polypeptides, respectively, are required to perform this oxidation process in the mitochondrion? (a) 1; 60 (b) 3; 3 (c) 3; 26 (d) 3; 60
D
Several different classes of enzymes are needed for the catabolism of carbohydrates. Which of the following descriptions best matches the function of a kinase? (a) An enzyme that catalyzes the rearrangement of bonds within a single molecule. (b) An enzyme that catalyzes a change in the position of a specific chemical group within a single molecule. (c) An enzyme that catalyzes the oxidation of a molecule by removing a hydride ion. (d) An enzyme that catalyzes the addition of phosphate groups to other molecules.
D
Step 3 in glycolysis requires the activity of phosphofructokinase to convert fructose 6-phosphate into fructose 1,6-bisphosphate. Which of the following molecules is an allosteric inhibitor of this enzyme? (a) Pi (b) AMP (c) ADP (d) ATP
D
The citric acid cycle is a series of oxidation reactions that removes carbon atoms from substrates in the form of CO2. Once a molecule of acetyl CoA enters the citric acid cycle, how many complete cycles are required for both of the carbon atoms in its acetyl groupto be oxidized to CO2? (a) 1 (b) 2 (c) 3 (d) 4
D
The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate is catalyzed by a fructose 1,6-bisphosphatase and is one of the final steps in gluconeogenesis. Which of the following molecules is an allosteric activator of this enzyme? (a) Pi (b) AMP (c) ADP (d) ATP
D
Although the outer mitochondrial membrane is permeable to all small molecules, the inner mitochondrial membrane is essentially impermeable in the absence of specific transport proteins. Consider this information and what you have learned about the citric acid cycle to address the following questions. B. If the inner mitochondrial membrane were rendered as permeable as the outer membrane, how would that affect oxidative phosphorylation? Which specific processes would stop and which remain?
During oxidative phosphorylation, the NADH and FADH2 generated by the citric cycle donate their electrons to an electron-transport chain in the inner mitochondrial membrane. As the electrons move along this chain, the energy released is used to drive protons across the inner mitochondrial membrane. This movement of protons produces a proton gradient across the membrane, which then serves as a source of energy for the generation of ATP. If the inner mitochondrial membrane were made "leaky," the proton gradient would dissipate. Thus although acetyl CoA would continue to be oxidized by the citric acid cycle, and electrons donated to the electron-transport chain, these processes could no longer promote the production of ATP.
Oxidative phosphorylation is a process that occurs in the __________________ of mitochondria. It requires an electron-transport chain that operates on the high-energy electrons taken from the activated carrier molecules __________________ and __________________ that are produced by glycolysis and the citric acid cycle. These electrons are transferred through a series of molecules, and the energy released during these transfers is used to generate a gradient of __________________, or __________________. Because their concentration is much __________________ outside than inside the mitochondria, the flow of __________________, or __________________, down the concentration gradient is energetically very __________________ and can thus be coupled to the production of ATP from ADP. Thus, oxidative phosphorylation refers to the oxidation of __________________ and __________________ molecules and the phosphorylation of __________________. Without this process, the yield of ATP from each glucose molecule would be __________________ decreased. ADP GTP NAD+ ATP H+ NADH cytosol higher Pi electrons inner membrane protons FADH2 lower severely favorable matrix slightly glucose moderately unfavorable
inner membrane NADH and FADH2 protons or H+ higher protons H+ favorable NADH and FADH2 ADP severely
An electron bound to a molecule with ____ for electrons is a _____. Transfer of an electron from a molecule with low affinity to one with higher affinity has a ____ ____ and is thus favorable under standard conditions. If the reduced form of a redox pair is a strong electron donor with a low affinity for electrons, it is ____ ____; the oxidized member of such a redox pair is a ___ ___ ___
low affinity; high-energy electron negative ΔG° easily oxidized; weak electron acceptor
For molecules that have a strong tendency to pass along their electrons, the standard redox potential is ____
negative
A carbon atom in a CO2 molecule in the atmosphere eventually becomes a part of one of the enzymes that catalyzes glycolysis in one of your cells. The CO2 first enters a cell in a corn leaf, where photosynthesis fixes the carbon to make it part of a sugar molecule; this travels from the leaf to an ear of corn, where it is stored as part of a polysaccharide __________________ molecule in the corn seed. You then eat a corn chip made from the corn seed. You digest the corn seed, and the free __________________ travels in your bloodstream, eventually being taken up by a liver cell and stored as __________________. When required, this storage molecule breaks down into glucose 1-phosphate, which enters the glycolytic pathway. Glycolysis produces __________________, which is converted into acetyl CoA, which enters the __________________. Several intermediates in this process can provide the carbon skeleton for the production of __________________, which are then incorporated into the enzymes that catalyze steps in glycolysis. amino acids insulin carbon fixation lactate citric acid cycle nucleotides fatty acid oxidative phosphorylation fermentation pyruvate galactose starch glucose triacylglycerol glycogen
starch glucose glycogen pyruvate citric acid cycle amino acids
