Download Chapter 3: Energy, Catalysis, and Biosynthesis

CHAPTER 13
HOW CELLS OBTAIN ENERGY FROM FOOD
 2009 Garland Science Publishing
The Breakdown and Utilization of Sugars and Fats
13-1
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.
13-2
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
13-3
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
13-4
The final metabolite produced by glycolysis is ___________.
(a)
acetyl CoA
(b)
pyruvate
(c)
3-phosphoglycerate
(d)
glyceraldehyde 3-phosphate
13-5
Glycolysis generates more stored energy than it expends. What is the net number of
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
13-6
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
13-7
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 3phosphate). Which enzyme catalyzes this reaction?
(a)
aldolase
(b)
phosphoglucose isomerase
(c)
enolase
(d)
triose phosphate isomerase
13-8
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
13-9
Figure Q13-9 represents a cell lining the gut. Draw numbered labeled lines to indicate
exactly where inside a cell the following processes take place.
Figure Q13-9
1.
2.
3.
4.
5.
6.
7.
glycolysis
citric acid cycle
conversion of pyruvate to activated acetyl groups
oxidation of fatty acids to acetyl CoA
glycogen breakdown
release of fatty acids from triacylglycerols
oxidative phosphorylation
13-10 Fill in the spaces in the table below. For steps 1, 4, 5, and 8, name the correct substrates,
enzyme, or products. For all the other steps, name the enzyme and draw the missing
structure.
13-11 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
13-12 Which reaction does the enzyme phosphoglucose isomerase catalyze?
(a)
glucose → glucose 6-phosphate
(b)
fructose 6-phosphate → fructose 1,6-bisphosphate
(c)
glucose 6-phosphate → fructose 6-phosphate
(d)
glucose → glucose 1-phosphate
13-13 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 enables the glucose 6-phosphate to be recognized by phosphofructokinase, the
next enzyme in the glycolytic pathway.
13-14 A.
B.
C.
How does the generation of ATP by oxidative phosphorylation differ from ATP
generation by substrate-level phosphorylation?
What catabolic process uses substrate-level phosphorylation, and how many ATP
molecules are generated in this way in the reaction pathway?
Where does oxidative phosphorylation take place, and what other processes are
required for this to occur?
13-15 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
13-16 In anaerobic conditions, skeletal muscle produces _____________.
(a)
lactate and CO2
(b)
ethanol and CO2
(c)
lactate only
(d)
ethanol only
13-17 Anaerobically growing yeast further metabolizes the pyruvate produced by glycolysis to
CO2 and ethanol as part of a series of fermentation reactions.
A.
What other important reaction occurs during this fermentation step?
B.
Why is this reaction (i.e., the answer to part A) essential for the anaerobically
growing cell?
13-18 The first energy-generating steps in glycolysis begin when glyceraldehyde 3-phosphate
undergoes an energetically favorable reaction in which it is simultaneously oxidized and
phosphorylated by the enzyme glyceraldehyde 3-phosphate dehydrogenase to form 1,3bisphosphoglycerate, with the accompanying conversion of NAD+ to NADH. In a second
energetically favorable reaction catalyzed by a second enzyme, the 1,3bisphosphoglycerate is then converted to 3-phosphoglycerate, with the accompanying
conversion of ADP to ATP. Which of the following statements is true?
(a)
The reaction glyceraldehyde 3-phosphate → 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 high-energy bond to the phosphate group in glyceraldehyde 3-phosphate
contributes to driving the reaction forward.
(d)
The cysteine side chain on the enzyme is oxidized by NAD+.
13-19 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 Q13-19A). 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 Q13-19B), how might this new
enzyme act?
Figure Q13-19
(a)
(b)
(c)
(d)
It would oxidize the substrate and phosphorylate it without releasing it.
It would oxidize the substrate but not release it.
It would phosphorylate the substrate on the 2 position instead of the 1 position.
It would behave just like the normal enzyme.
13-20 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?
13-21 Pyruvate must move from the cytosol into the mitochondria, where it 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
13-22 In the absence of oxygen, mammalian cells will endure for a short time using a process
called fermentation. Which of the following is a final product of fermentation in
mammalian cells?
13-23 In the reaction cycle involved in the oxidation of pyruvate, what are the advantages of
having three enzyme activities contained in a single large complex instead of having
three smaller and physically independent enzymes?
13-24 Glycolysis and the citric acid cycle comprise two different sets of oxidation reactions.
The reaction sequence for glycolysis is linear, whereas the reaction sequence for the citirc
acid cycle forms a circle. How does this difference in the arrangement of reactions
influence the rare of these processes when an excess amount of a single intermediate is
added?
13-25 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.
D.
E.
F.
Anaerobic respiration is not the same as fermentation, as only the former requires
an electron-transport chain.
When subjected to anaerobic conditions, glycolysis in mammalian cells continues
and causes a buildup of pyruvate in the cytosol.
The pyruvate dehydrogenase complex catalyzes three different, but linked,
enzymatic reactions.
Amino acids can be transported into the mitochondria and converted into acetyl
CoA.
13-26 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
13-27 Two molecules of CO2 are produced after the completion of a single citric acid reaction
cycle. Where does the required oxygen come from?
(a)
water
(b)
phosphates
(c)
molecular oxygen
(d)
acetyl CoA
13-28 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
13-29 For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
may be used more than once.
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
ATP
cytosol
electrons
FADH2
favorable
glucose
GTP
H+
higher
inner membrane
lower
matrix
moderately
NAD+
NADH
Pi
protons
severely
slightly
unfavorable
13-30 The citric acid cycle is outlined in Figure Q13-30. Some of these reactions produce small
molecules. 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 Q13-30
1. ATP
2. ADP
3. GTP
4. GDP
5. NAD+
6. NADH
7. FADH
8. FADH2
13-31 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 Q13-31, which carbon is lost as CO2 and which is
converted to a carbonyl carbon?
a)
b)
c)
d)
4 and 6
6 and 5
5 and 4
6 and 4
Figure Q13-31
13-32 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. What 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
13-33 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
13-34 In the final step of the citric acid cycle, oxaloacetate is regenerated through the oxidation
of malate and coupled with the production of what other molecule?
(a)
FADH
(b)
NADH
(c)
GTP
(d)
CO2
How We Know: Unraveling the Citric Acid Cycle
13-35 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)
(c)
(d)
malate
fumarate
succinate
13-36 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
13-37 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 product in
the reaction pathway builds up when the inhibitor is added?
(a)
citrate
(b)
succinate
(c)
fumarate
(d)
malate
13-38 The oxidative reactions of cellular respiration were the focus of intense study in the
1930s. These reactions are represented in a linear pathway, as they were thought to occur.
Each product is designated as a lettered compound (A through H) in Figure Q13-38.
Figure Q13-38
A.
B.
C.
What was the first observation that Krebs made when he added malonic acid to
the minced muscle samples, and what was his conclusion about how and where it
was acting in the reactions he was studying?
What happens when the malonate block is introduced and subsequently
compound A is added in excess? What is the result if compound G is added after
the block, instead of A? How did Krebs attempt to reconcile these two results?
What additional observation led Krebs to hypothesize that what was previously
thought to be a linear sequence of reactions is actually a cyclic sequence of
reactions? How did this idea further explain the earliest observations that the
addition of any single compound in the pathway greatly increases oxygen uptake
by the muscle tissue?
13-39 Do you expect the cell to produce more ATP from one glucose molecule or from one
fatty acid molecule? Explain your answer.
13-40 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, 6 g of
glycogen is the equivalent of 1 g of fat.
D.
Glycogen phosphorylase cleaves glucose monomers from the glycogen polymer,
phosphorylating it at the same time so that it can be fed unchanged into the
glycolytic pathway.
13-41 In the final stage of the oxidation of food molecules, a gradient of protons is formed
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.
13-42 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
13-43 For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or phrase
should be used only once.
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
carbon fixation
citric acid cycle
fatty acid
fermentation
galactose
glucose
glycogen
insulin
lactate
nucleotides
oxidative phosphorylation
pyruvate
starch
triacylglycerol
13-44 When glucose is being used up and not replaced from food intake, the blood sugar level
can be maintained by synthesizing it 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
13-45 Step 3 in glycolysis requires the activity of phosphofructokinase to convert fructose 6phosphate 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
13-46 The conversion of fructose 1,6-bisphosphate is catalyzed by a fructose 1,6bisphosphatase 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
13-47 Which of the following polymer of glucose is used as a vehicle to store energy reserves in
animal cells?
(a)
glucagon
(b)
glycogen
(c)
starch
(d)
glycerol
13-48 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
13-49 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
13-50 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