Chapter 6: Pathways that Harvest and Store Chemical

Chapter 6: Pathways that Harvest and Store Chemical Energy
Chapter Review
1. The hydrolysis of ATP to support an anabolic process includes both endergonic and exergonic reactions,
depending on which perspective one takes: the hydrolysis of ATP versus the formation of anabolic products.
Discuss this statement.
ATP hydrolysis is a kind of “energy currency” in cells, as the accompanying energy transfers help drive many
different reactions and cell processes. Some of the energy in the chemical bonds of ATP is transferred as
hydrolysis proceeds, forming ADP and inorganic phosphate or phosphorylated proteins. Although it takes a
small amount of energy to “break off” a phosphate group from ATP (endergonic reaction), more energy is
transferred as hydrogen and oxygen atoms from water bind to the ADP and phosphate (exergonic reaction),
resulting in a net increase in free energy
Box 2A: Oxidation, because the molecule has lost a hydrogen atom.
Box 2B: Reduction, because the molecule gained a hydrogen atom.
Box 2C: Oxidation, because the molecule has lost a hydrogen atom.
Box 2D: Reduction, because the molecule gained a hydrogen atom.
3. Use the figure to the right to complete the following
questions based on the molecules:
propanoic acid
C 2 H 6 COOH
propanol
C 3 H 7 OH
propane
C3H8
Which compound is in the most reduced state? Propane
Which compound has the lowest free energy?
Propanoic acid
Which compound is in the most oxidized state? Propanoic acid
Which compound has the highest free energy? Propane
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
4. Explain how two different membrane-embedded proteins in mitochondria simultaneously influence the
gradient of hydrogen ions and ATP synthesis.
Proton Pump – The mitochondrial proton pump is driven by the electron transport system. An integral
membrane protein facilitates movement of protons across a cell membrane in an endergonic reaction,
creating a concentration gradient along the inner membrane due to the higher concentration of protons
outside the matrix than inside. The proton pump does not create energy; it sets up a bank of stored potential
+
energy in the resulting proton (H ) gradient.
ATP Synthase - ATP synthase is also an integral membrane protein that couples the movement of protons
-
back down their concentration gradient to binding ADP and HPO 4 , forming ATP. This enzyme provides
energy for the cell by facilitating the synthesis of ATP by utilizing the proton gradient described above. The
proton pump and ATP synthase form an interdependent energy-coupling complex. This process occurs in the
inner mitochondrial membrane where the energy needed for ATP formation by the enzyme ATP synthase is
transferred from the proton gradient.
5. The complete catabolism of glucose can yield 686 kcal/mol energy transfer. For each of the following
statements, indicate whether the statement is true or false and then explain your answer.
A. All 686 kcal/mol is directly transferred to ATP synthesis.
TRUE FALSE [choose one, then explain]
Slightly less than half (234 kcal/mol) is transferred to ATP synthesis; albeit imperfect, the efficiency of this
transfer is highly efficient compared to the best motors devised by humans.
B. Less than half of the 686 kcal/mol is directly transferred to ATP synthesis.
TRUE FALSE [choose one, then explain]
234 kcal/mol, or slightly less than half, is transferred to ATP synthesis.
C. Only 10% of the 686 kcal/mol is directly transferred to ATP synthesis, in accordance with the
principles of thermodynamics.
TRUE FALSE [choose one, then explain]
The first law of thermodynamics states that energy cannot be created nor destroyed, and therefore the total
amount of energy in a closed system remains constant. Even though less than half of the energy produced
during cellular respiration is transferred to ATP synthesis, the remainder is not destroyed but is given off as
heat. Therefore, energy changes form but is not “lost” by this metabolic process. The second law states that
the amount of available energy in a closed system is continually decreasing, or that entropy is increasing.
While the efficiency of ATP synthesis is greater than 10%, it is not 100% efficient, nor is any other physical
process or chemical reaction. Biological processes all tend to increase entropy, and the tendency gives
direction to these processes. Changes in entropy are mathematically related to changes in free energy, which
explains why some reactions proceed in one direction rather than another.
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
6. Assume that the diagram on the right refers to the catabolism of one glucose molecule.
7. Starting with THREE molecules of glucose, insert the appropriate numbers in the blanks below, assuming
complete catabolism, with oxygen available.
6
30
6
18
18
18
molecules of ATP must be hydrolyzed to start the process.
molecules of NADH are produced.
molecules of FADH 2 are produced.
molecules of ATP are produced via substrate phosphorylation. (12 in glycolysis, 6 in Krebs)
molecules of water are produced in the electron transport chain.
molecules of carbon dioxide are released from the process.
Concept 6.3 examines how carbohydrate catabolism in the absence of oxygen releases a small amount of
energy. Organisms that live in conditions in which molecular oxygen is periodically unavailable or is never
available use the process called fermentation to re-oxidize NADH to NAD+. Without NAD+, glycolysis and
all subsequent steps of catabolism come to a halt and ATP production stops. Complex organisms like
ourselves, as well as some microbes, produce lactic acid or lactate as a byproduct of fermentation, whereas
yeasts and some plants produce an alcohol known as ethanol as a byproduct.
8. Lactic acid fermentation and alcoholic fermentation both result in alterations to three-carbon pyruvate
molecules. Which type of fermentation converts pyruvate to the smaller catabolites? Provide specific details.
Alcoholic fermentation converts three-carbon pyruvate into two smaller molecules, carbon dioxide (one
carbon) and ethanol (two carbons). This is a two-step process, where pyruvate is first converted to
acetaldehyde utilizing the enzyme pyruvate decarboxylase, and then it is reduced to ethanol using alcohol
dehydrogenase. Two molecules of carbon dioxide and two ATP are also produced in this anaerobic metabolic
pathway.
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
Concept 6.4 shows how catabolic and anabolic pathways are integrated. Changes in cellular activity and in
the availability of fuel molecules occur frequently in nature. During lean times or under high metabolic
activity, energy transfers occur by taking apart (catabolizing) a variety of fuel molecules, adding catabolism
of proteins and fats to whatever catabolism of carbohydrates is taking place. By contrast, in times of plenty,
cellular reserves are restored. For example, many types of smaller molecules can be converted to lipids. In
addition, the storage-polymer of glucose, called glycogen, is synthesized in muscle and liver.
9. Carbohydrates are just one source of fuel molecules. Identify two additional pools of fuel molecules whose
catabolism can yield energy transfers that result in ATP synthesis. For each of the two categories of fuel
molecules you’ve identified, briefly describe how the molecules are utilized and comment on their similarity
to glucose catabolism.
Fuel-molecule pool#1: Lipids
Lipids are broken down into their constituents: glycerol and fatty acids. Glycerol is converted into
dihydroxyacetone phosphate, an intermediate in glycolysis. Fatty acids are highly reduced molecules that are
converted to acetyl CoA in a process called β-oxidation. The acetyl CoA can then enter the citric acid cycle
and be catabolized to CO 2 .
Fuel-molecule pool#2: Amino Acids
Proteins are hydrolyzed to their amino acid building blocks. After deamination (removal of the amino group,
resulting in ammonia formation), the catabolites feed into glycolysis or the citric acid cycle at different points.
An example is the amino acid glutamate, which, after deamination, is converted into α-ketoglutarate, an
intermediate in the citric acid cycle.
Briefly define each term in each of the following pairs, and explain whether that process is more active during
times of plenty access to food or during times of limited access to food.
glycogenolysis vs. glycogenesis
Glycogenolysis is the process by which glycogen is broken down to form glucose-6-phosphate, which is
subsequently released into the bloodstream as glucose. This process is more active during times of limited
access to food. Glycogenesis is the formation of glycogen from glucose-6-phosphate, which is then stored in
the liver and in muscle cells. It occurs when food intake is plentiful.
lipolysis vs. lipogenesis
Lipolysis is the process whereby lipids are broken down, involving the hydrolysis of triglyerides into fatty acids
followed by further breakdown into acetyl-coA which can be used to fuel the body’s cells. This process is
more active during times of limited access to food. Lipogenesis, conversely, is the process of converting
acetyl-CoA to fats to be stored as energy, and it typically occurs when food intake is abundant.
proteolysis vs. proteins synthesis
Proteolysis is the breakdown of proteins into smaller polypeptides and amino acids by hydrolysis of their
peptide bonds. During times of limited access to food, proteolysis provides carbon skeletons for
gluconeogenesis in lieu of replenishing degraded proteins. Proteolysis of muscle protein, in particular,
provides some of the three-carbon precursors of glucose. Protein synthesis, on the other hand, occurs during
times of plentiful food intake, and it is the process by which proteins are created from food molecules. Excess
protein is not stored and must be eliminated, along with the nitrogenous wastes, ammonium ions and urea.
10. Cells of the brain and the heart are highly specialized to carry out specific functions in the body, and their
metabolic needs must always be met or death will soon follow. Part of this specialization includes reliance on
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
glucose as a key fuel molecule. Explain how glucose is made available to the tissues during times when no
carbohydrates are available as food.
The glucose provided by the intake of food can either be used by the cells immediately, or if there is already
sufficient glucose available, it can be converted to glycogen and stored in the liver or muscle cells for later
use. When cells in the body, including the brain and the heart, have an immediate need for glucose at some
point when there is no available carbohydrate food source, the stored glycogen can be broken down by the
process of glycogenolysis, which “retrieves” glucose and makes it available to these specialized cells. In
addition, the liver and the kidneys increase the amount of gluconeogenesis, or the synthesis of “new” (neo)
glucose molecules, using substrates such as deaminated amino acids
11. There are two photosystems, I and II, directly activated by different wavelengths of photon energy.
Describe each and discuss the interdependence between them.
Photosystem II
Photosystem II is a complex in the photosynthetic process with a chlorophyll that absorbs light at 680 nm,
passing electrons to the electron transport chain in the chloroplast. It produces ATP and oxidizes water
molecules. After chlorophyll has absorbed light in the reaction center of the cell, it gives up its energized
electron to reduce a chemical acceptor molecule, becoming highly unstable. It has a strong tendency to
“grab” an electron form another molecule to replace the one it lost, making it a strong oxidizing agent. The
replenishing electrons come from water, splitting the H-0-H bonds.
Photosystem I
Photosystem I is the photosynthetic complex with a chlorophyll that absorbs light at 700 nm and passes an
+
excited electron to NADP , reducing it to NADPH. Like in photosystem II, an excited electron from chlorophyll
in the reaction center reduces an acceptor. The oxidized chlorophyll now “grabs” an electron, but in this case
the electron comes from the last carrier in the electron transport system of photosystem II. This links the two
photosystems chemically. They are also linked spatially, with the two photosystems adjacent to one another
in the thylakoid membrane. The energetic electrons from photosystem I pass through several molecules
+
before finally reducing NADP to NADPH.
12. Briefly describe each of the three major segments of the Calvin cycle, noting the key ingredients needed
in each segment.
Carbon fixation – The initial reaction of the Calvin cycle adds the one-carbon CO 2 to an acceptor molecule,
the five-carbon ribose 1,5-biphosphate (RuBP), producing a six-carbon product that immediately breaks down
into two three-carbon molecules called 3-phophoglycerate (3PG). The enzyme, rubisco, is slow, and therefore
plants need a lot of rubisco to meet their growth and metabolic needs. Rubisco is said to be the most
abundant enzyme/protein in the world.
Reduction and sugar production – A series of reactions involves a phosphorylation (using the phosphate from
an ATP made in the light reactions) and a reduction (using an NADPH also made in the light reactions). The
product is glyceraldehyde 3-phosphate (G3P), which is a three-carbon sugar phosphate, also called triose
phosphate.
Regeneration of RuBP – Most of the G3P ends up as ribulose monophosphate (RuMP), and ATP is used to
convert this into RuBP. Thus, every “turn” of the Calvin cycle results in one CO 2 fixed and regeneration of the
CO 2 acceptor.
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
13. Explain the claim, “rubisco is the most abundant protein on the planet” by describing its key role in the
Calvin cycle.
Fifty percent of the protein in leaves of plants is composed of rubisco, the enzyme responsible for the first
major step in carbon fixation during the initial phase of the Calvin cycle. Without rubisco, carbon dioxide
cannot be converted to energy-rich molecules in plants. The enzyme rubisco catalyzes the reaction of CO 2
and RuBP to form 3PG, which is necessary for the cycle to continue to its next phase, reduction and sugar
production. Since all life on earth depends directly or indirectly on primary producers, which form the base of
the food chain, the claim that “rubisco is the most abundant protein on the planet” seems convincing.
14. Help Nathan and Elijah settle an argument. Elijah says that since plants can carry out photosynthesis, they
do not need cellular respiration. Nathan says that photosynthesis without respiration is a wasted effort. Who is
correct? Explain your answer.
Nathan is correct. Both photosynthesis and respiration are reduction-oxidation reactions. Respiration is the
process by which organisms oxidize organic molecules (sugars) and obtain energy (APT) from the breaking of
chemical bonds, with CO 2 and water as the end products. Photosynthesis is the opposite chemical process,
where carbon dioxide and water combine with the input of light energy to form sugars, with oxygen being
released as an end product of this reaction. The two chemical processes rely upon one another, with the
molecules necessary for one reaction coming from the end products of the opposite reaction. They are
integral parts of one large cycle, and plant life could not exist without both photosynthesis and respiration.
Science Practices & Inquiry
15. In the absence of electron transport, an artificial H+ gradient is sufficient for ATP synthesis in cellular
organelles. In an experiment, chloroplasts were isolated from plant cells and incubated at pH 7. The
chloroplasts were then subjected to six different conditions. They were incubated with ADP, phosphate (P i ),
and magnesium ions (Mg2+) at pH 7 and at pH 3.8. The chloroplasts were then incubated at pH 3.8 with one
of the four elements (ADP, P i , Mg2+, chloroplasts) missing.
ATP formation was measured using luciferase, which catalyzes the formation of a luminescent (lightemitting) molecule if ATP is present. Here are the data from the original paper:
a) Identify which reaction mixture is the control by circling it in the table above.
b) Use the control data to correct the raw data for the other, experimental reaction mixtures and fill in
the table.
c) Summarize the results of the experiment.
d) Why did ATP production go down in the absence of P i ?
e) Explain why ATP production could be negative in this experiment.
f) Discuss where the free energy comes from to drive the production of ATP.
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key
a) The control reaction mixture is “complete, pH 3.8, minus chloroplasts”
b)
Reaction mixture
Complete, pH 3.8
Complete, pH 7.0
Complete, pH 3.8 - P i
“
“
- ADP
2+
“
“
- Mg
“
“
- chloroplasts
Luciferase activity
(light emission)
Raw data
141
12
12
4
60
7
Corrected data
134
5
5
-3
53
0
c) This experiment shows that the greatest production of ATP, as measured by luciferase activity, occurs when
2+
all reaction components (P i , ADP, Mg , and chloroplasts) are present and under conditions of high acidity
(pH 3.8). A more neutral pH (7.0) and the lack of phosphate ions both reduce ATP production. Removing
2+
Mg under conditions of high acidity improves ATP production, but the elimination of ADP causes negative
ATP production.
d) The formation of ATP from ADP requires a phosphate ion, as suggested by the names of these two
molecules: “adenosine tri-phosphate,” and “adenosine di-phosphate.”
e) The Calvin cycle’s carbon fixation phase of photosynthesis requires ATP in order to reduce 3PG to G3P, and
this ATP is normally derived from the light reactions of photosynthesis. In the absence of ADP in the reaction
mixture, ATP cannot be produced. ATP is, therefore, consumed rather than produced, hence the negative ATP
production in the absence of ADP.
f) The free energy driving the production of ATP ultimately comes from the sun. The energy from the sun, in
the form of a photon, is absorbed by the molecule which causes its energy level to jump to an excited state.
This is the basis for the hydrogen concentration gradient that provides the energy for ATP production in cells.
Chapter 6: Pathways that Harvest and Store Chemical Energy
Answer Key

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