Mastering Concepts
6.1
1. Why do all organisms need ATP?
All organisms need ATP to provide the potential chemical energy that powers the
chemical reactions that occur in their cells.
2. What are the three general ways to generate ATP from food, and which organisms use
each pathway?
Anaerobic respiration: microorganisms; Aerobic respiration: plants, animals, and bacteria
in O2 rich environments; Fermentation: microorganisms
3. What is the overall equation for cellular respiration?
The overall equation that describes cellular respiration is:
6O2 + C6H12O6 >>>>>>>>>>>>> 6CO2 + 6H2O + 36ATP
4. How is cellular respiration related to breathing?
Breathing inhales air to bring in the needed O2 for aerobic respiration and exhales air to
remove the CO2 created in aerobic respiration.
5. How can plants release more O2 in photosynthesis than they consume in respiration?
Plants can release more O2 in photosynthesis than they consume in respiration because
they do not respire all of the glucose they produce. For example, plants may store
glucose as starch or cellulose.
6.2
1. Why do the reactions of respiration occur step-by-step instead of all at once?
If the reactions of respiration occurred all at once, the sudden release of heat energy
would harm or destroy cells.
2. What occurs in each of the three stages of cellular respiration?
Glycolysis splits glucose into two pyruvate molecules and produces ATP and some
NADH for later use. The Kreb’s cycle releases CO2 and produces additional ATP and
NADH, as well as FADH2. The electron transport chain uses energy stored in the
electron carriers NADH and FADH2 to create a gradient of hydrogen ions across the
inner membrane of the mitochondrion. ATP synthase uses this gradient to phosphorylate
ADP to form ATP.
6.3
1. What are the parts of a mitochondrion?
The parts of a mitochondrion are an outer membrane that envelops the mitochondrion; a
highly folded inner membrane; an intermembrane compartment between the two
membranes; and an inner fluid called the mitochondrial matrix.
2. Which respiratory reactions occur in each part of the mitochondrion?
The electron transport chain is located on the inner mitochondrial membrane. The Krebs
cycle occurs in the mitochondrial matrix. (In prokaryotes, the electron transport chain is
embedded in the cell’s outer membrane, and the Krebs cycle occurs in the cytoplasm).
6.4
1. What are the starting materials of glycolysis?
The starting materials of glycolysis are glucose and 2 molecules of ATP.
2. How is substrate-level phosphorylation different from chemiosmotic phosphorylation?
Substrate-level phosphorylation uses an enzyme to directly transfer a phosphate group
from a donor molecule to ADP, forming a molecule of ATP. Chemiosmosis uses an
electron transport chain to create a gradient of H+ ions; ATP synthase uses the potential
energy in the gradient to add a phosphate group to ADP.
3. What is the net gain of ATP and NADH for each glucose molecule undergoing
glycolysis?
There is a net gain of 2 ATPs and 2 NADHs for each glucose molecule that undergoes
glycolysis.
6.5
1. Pyruvate contains three carbon atoms; an acetyl group has only two. What happens to
the other carbon atom?
The third carbon atom from pyruvate is released as CO2.
2. How does the Krebs cycle generate CO2, ATP, NADH, and FADH2?
The Krebs cycle generates CO2, ATP, NADH, and FADH2 as it rearranges and oxidizes
citrate through several intermediate molecules. The energy and electrons derived from
these chemical reactions are stored in ATP, NADH, and FADH2. A molecule of CO2 is
released in two of these chemical reactions.
3. How do NADH and FADH2 power ATP formation?
These molecules deliver energy-rich electrons to the electron transport chain to power the
concentration of hydrogen ions (also delivered by NADH and FADH2). Those
concentrated hydrogen ions are used to power the addition of a phosphate group to ADP.
4. What is the role of O2 in the electron transport chain?
In the electron transport chain, O2 functions as the final electron acceptor. O2 binds to
electrons that have traveled to the end of the transport chain, which allows subsequent
electrons to move along the pathway.
6.6
1. Explain how to arrive at the estimate that each glucose molecule theoretically yields 36
ATPs.
Here is how you estimate that each glucose molecule yields 36 ATPs. Glycolysis yields
2 NADH and 2 ATP; the formation of acetyl CoA yields 2 NADH; the Krebs cycle yields
6 NADH, 2 ATP, 2 FADH2. When the NADH and FADH2 from glycolysis and the
Krebs cycle contribute their electrons to the electron transport chain; they yield a total of
34 ATP (derived from NADH and FADH2). Once 2 ATP are subtracted (the “cost” of
moving NADH from glycolysis to the intermembrane space), the net yield is 36 ATP per
molecule of glucose.
2. How does the actual ATP yield compare to the theoretical yield?
The theoretical yield is 36 ATP, but some protons leak across the membrane of the
mitochondria and energy is used to move pyruvate and ADP across the mitochondrial
membrane which would reduce the actual ATP yield to about 30 ATP per molecule of
glucose.
6.7
1. At which points do digested polysaccharides, proteins, and fats enter the energy
pathways?
Digested polysaccharides enter at glycolysis. Proteins enter at glycolysis, the transition
reaction and the Krebs cycle. Fats enter at both glycolysis and the Kreb’s cycle.
2. How does the body store extra calories as fat?
Acetyl CoA molecules can be diverted and combined to form fats.
6.8
1. What are some examples of alternative electron acceptors used in anaerobic
respiration?
Alternative electron acceptors in anaerobic respiration include nitrate, sulfate, and carbon
dioxide.
2. How many ATP molecules per glucose does fermentation produce?
A fermenting organism produces 2 ATP molecules per glucose molecule.
3. What are two examples of fermentation pathways?
Alcohol fermentation and lactic acid fermentation are two examples.
6.9
1. Which energy pathway is probably the most ancient? What is the evidence?
Glycolysis is probably the most ancient energy pathway. Virtually all cells have
glycolysis, a process that does not require O2 and could have arisen in an oxygen-free or
oxygen-poor environment.
2. Why must the first metabolic pathways have been anaerobic?
The first metabolic pathway must have been anaerobic because O2-producing
photosynthesis by cyanobacteria dates to 3.5 billion years ago. Life has been on Earth at
least a billion years longer than that.
3. What is the evidence that photosynthesis may have evolved from glycolysis?
Some of the reactions of the Calvin cycle are the reverse of some of those of glycolysis.
6.10
1. What hypothesis were the researchers testing, and what experiments did they design to
help them test the hypothesis?
The hypothesis was that heat helped attract pollinators. Their first experiment was to test
for temperature differences in regions of the flower. Next the researchers measured CO2
from active and resting beetles at different temperatures. Finally the researchers
calculated the energy saved by beetles as a result of the flower temperature.
2. Suppose you hold one group of active beetles at 20°C and another group at 30°C. After
several hours, you place each beetle in a device that measures how far the animal can fly
at 20°C. Which group of beetles do you predict will fly farther?
The beetles at 30oC used less energy initially and so have more energy to use for flight;
therefore, they will fly farther.
Write It Out
1. How are breathing and cellular respiration similar? How are they different?
Both involve gas exchange by taking in oxygen and releasing carbon dioxide. They differ
in where the process takes place. In breathing, the exchange takes place in the alveoli of
the lungs; in cellular respiration, it takes place in the cells themselves.
2. How do chemiosmotic phosphorylation and substrate-level phosphorylation each
generate ATP? In which pathways does each occur?
Substrate level phosphorylation generates ATP when a high-energy donor molecule
physically transfers a phosphate group to ADP. This process occurs in glycolysis and the
Kreb’s cycle. Chemiosmotic phosphorylation requires a proton gradient. As ATP
synthase channels protons across a membrane, ADP is phosphorylated to ATP. This
occurs in the electron transport chain.
3. How does aerobic respiration yield so much ATP from each glucose molecule,
compared with glycolysis alone?
The remaining H+ ions in glucose are removed and transported by NADH and FADH2 to
the electron transport chain. The accumulation of hydrogen ions by the electron transport
chain powers the production of ATP.
4. Cite a reaction or pathway that occurs in each of the following locations:
a. cytoplasm
b. mitochondrial matrix
c. inner mitochondrial membrane
d. intermembrane compartment
(a) Cytoplasm: glycolysis; (b) Mitochondrial matrix: Krebs cycle; (c) Inner mitochondrial
membrane: electron transport chain; (d) Intermembrane compartment: ATP synthase
dissipates the high concentration of protons in this compartment
5. Health-food stores sell a product called “pyruvate plus,” which supposedly boosts
energy. Why is this product unnecessary? What would be a much less expensive
substitute that would accomplish the same thing?
“Pyruvate plus” is unnecessary because cells make their own pyruvate. Glucose (or
another simple sugar) would be a cheaper alternative that would accomplish the same
thing, because glycolysis breaks glucose into two pyruvate molecules.
6. At what point does O2 enter the energy pathways of aerobic respiration? What is the
role of O2? Why does respiration stop if a person cannot breathe? What happens if
cellular respiration stops?
O2 enters the energy pathways at the electron transport chain; it is the final electron
acceptor. Cellular respiration requires oxygen to proceed. If an individual cannot
breathe, then his or her cells will not receive the oxygen necessary to complete cellular
respiration. Also, CO2 levels will rise in the blood if CO2 is not eliminated through
breathing. The cessation of cellular respiration deprives the cell of energy, eventually
killing the cell.
7. In a properly functioning mitochondrion, is the pH in the matrix lower than, higher
than, or the same as the pH in the intermembrane compartment? If you add one or more
poisons described in this chapter’s Apply It Now box, does your answer change?
The pH is higher in the matrix since the hydrogens are concentrated in the intermembrane
compartment. If, however, you add one of the electron transport inhibitors or DNP then
the electron transport chain would cease and the hydrogen ions in the intermembrane
space would equilibrate with the matrix creating equal pH. If you added oligomycin then
the hydrogen ions would stay concentrated, and the pH would still be higher in the
matrix.
8. A chemical works as a disinfectant by poking holes in bacterial cell membranes. Why
would this stop the cells from making ATP? Why would the inability to make ATP kill a
cell?
In prokaryotes, the production of ATP occurs at the cell membrane. A membrane with
holes could not maintain the H+ gradient necessary to produce ATP. The inability to
produce ATP would kill a cell because ATP is the energy source that drives its processes.
9. Describe the energy pathways that are available for cells living in the absence of O2.
Without O2, cells can use anaerobic respiration, in which an inorganic molecule (not O2)
is used as the final electron acceptor. In fermentation an organic molecule is used as the
acceptor (acetaldehyde or pyruvate itself).
10. Ben decides to bake bread. The recipe says to dissolve yeast in a mixture of sugar and
hot water. Shortly after he does so, the mixture begins to bubble. What is happening?
How would the outcome change if Ben forgets to add the sugar?
The yeast is using the glucose in the sugar as a fuel for fermentation. One byproduct of
alcohol fermentation is CO2 gas, which makes the mixture bubble. Without the sugar
there would be no fermentation and no bubbling.
11. How are photosynthesis, glycolysis, and cellular respiration interrelated?
Respiration and photosynthesis each generates products required as the starting materials
for the other. Photosynthesis uses CO2 and H2O to generate glucose and O2; glycolysis
splits the glucose and passes the products to the reactions of cellular respiration, which
uses the glucose and O2 to generate CO2 and H2O.
12. A student runs 5 kilometers each afternoon at a slow, leisurely pace. One day, she
runs 2 km as fast as she can. Afterward she is winded and feels pain in her chest and leg
muscles. She thought she was in great shape! What, in terms of energy metabolism, has
she experienced?
When she increased her pace, her muscles depleted the available O2 and used lactic acid
fermentation to generate ATP instead. The resulting buildup of lactic acid caused the
soreness.
13. Explain the fact that species as diverse as humans and yeasts use the same
biochemical pathways to extract energy from nutrient molecules.
All species share a common ancestor and therefore share many of the same biochemical
pathways.
14. A seed is a plant embryo packaged with a food supply. Soaking a seed in water
prompts the embryo to begin respiring, metabolizing its food supply to fuel its growth.
Suppose that Anna has 50 soaked seeds. She boils half of them, killing their embryos, and
lets them return to room temperature. She then places the dead seeds in one container and
live seeds in another. If she later measures the temperature in the two containers, will
they be different? Explain your answer.
The temperatures would be higher in the container that has living seeds. Heat is a
byproduct of metabolic activity.
15. Birds and mammals are endotherms: they maintain a constant internal body
temperature no matter whether the environment is cold or hot (within limits, of course).
An endotherm that gets too cold will increase its metabolic rate to generate heat. An
ectothermic animal such as a reptile, on the other hand, allows its body temperature to
fluctuate with the environment. If you own a pet rat and a pet snake of equal weight,
which will require more food and why?
The endotherm (mouse) would require more food because it has to maintain a constant
body temperature. Since it must increase its metabolic rate to maintain that temperature
in a cold environment, it will need more food to provide energy.
Pull It Together
1. Where in the cell does each of stage of respiration occur? (note: preceding question
contains error; found in confirming pages)
In a eukaryotic cell, respiration takes place in many different places. Glycolysis takes
place in the cytoplasm where glucose is converted into pyruvate. Also in the cytoplasm,
pyruvate is converted to Acetyl CoA, which enters the matrix of the mitochondria.
Within the mitochondrial matrix, the Kreb’s cycle reactions take place; the results of the
Kreb’s cycle are carbon dioxide, NADH, and FADH2 molecules. The electron transport
chain uses electrons from NADH and FADH2 to pump hydrogen ions across the inner
membrane of the mitochondria to the intermembrane compartment. From that
compartment, hydrogen returns to the mitochondrial matrix though the ATP pump,
producing ATP from ADP. ATP is then shuttled throughout the cell.
2. Where do the O2 and glucose used in respiration come from?
Oxygen is provided from the environment and enters the cell through the plasma
membrane. Glucose is provided from digesting other organisms or from photosynthesis.
3. How many ATP, NADH, CO2, FADH2, and H2O molecules are produced at each
stage?
Converting glucose to 2 pyruvates produces 2 ATP and 2 H2O.
Shuttling 2 pyruvates into the mitochondrion produces 2 NADH.
The 2 Acetyl CoA enter the Krebs Cycle and produce 2 ATP, 6 NADH and 2 FADH2.
From all these products, the electron transport chain can produce up to 36 ATP.
4. What happens to the CO2 and H2O waste products?
Excess carbon dioxide and water leave the cell through the plasma membrane; water is
used throughout the organism, and CO2 is returned to the external environment.
5. What do cells do with the ATP they generate in respiration?
ATP is the source of energy to power most of the chemical reactions that take place in the
cell.
6. Where would photosynthesis, fermentation, anaerobic respiration, and ATP synthase
fit into this concept map?
“Photosynthesis” could lead to “Glucose” with “produces”. “Photosynthesis could also
lead to “CO2” with “requires”. “ATP synthase” could lead to “ATP” with “catalyzes the
formation of”. “Anaerobic respiration” could lead to O2 with “occurs in the absence of”.
“Fermentation” could lead to “Glycolysis” with “uses only”.