Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
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 is the overall equation for cellular respiration?
The overall equation that describes cellular respiration is
6O2 + C6H12O6 >>>>>>>>>>>>> 6CO2 + 6H2O + 36ATP
3. 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.
4. 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 Krebs 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 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 and end products of glycolysis?
The starting materials of glycolysis are glucose and 2 molecules of ATP. At the end of
glycolysis there are 4 ATPs, 2 NADHs, and 2 pyruvate molecules for each molecule of
glucose.
2. 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 undergoing
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. 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 Krebs cycle.
2. How does the body store extra calories as fat?
Acetyl CoA molecules can be diverted and combined to form fats, which are stored in fat
tissue.
6.8
1. How many ATP molecules per glucose does fermentation produce?
A fermenting organism produces 2 ATP molecules per glucose molecule.
2. What are two examples of fermentation pathways?
Alcohol fermentation and lactic acid fermentation are two examples.
6.9
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 measure how far each animal can fly at 20°C. Which group should fly
farther?
The beetles at 30oC used less energy initially and so have more energy to use for flight;
therefore, they will fly farther.
Answers to Write It Out Questions
1. How are breathing and cellular respiration similar and 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 are photosynthesis and cellular respiration interrelated?
Each process produces the materials needed for the other process. Photosynthesis
produces glucose and oxygen that are required for cellular respiration to function.
Cellular respiration produces the carbon dioxide needed for photosynthesis.
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. 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.
5. 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? Why would a cell die if
it could not make ATP?
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.
6. 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 Why We Care 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.
7. Edgar is making bread. He begins by adding yeast to sugar and hot water; the mixture
begins to bubble. What is happening? How would the outcome change if he forgot 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.
8. 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.
9. Explain the fact that species as diverse as humans and yeasts use the same
biochemical pathways to extract energy from nutrient molecules.
The energy extraction pathways are fundamental to life. They must have arisen early in
life’s history, in a common ancestor shared by humans, yeasts, and most other species.
10. Birds and mammals are endotherms: they maintain a constant internal body
temperature. An endotherm that gets too cold will increase its metabolic rate to generate
heat. An ectotherm such as a lizard or snake, 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 (rat) 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.
Answers to Pull It Together Questions
1. Where in the cell does each stage of respiration occur?
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 matrix of the mitochondria, the Krebs cycle reactions take place; the results of
the Krebs 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 cellular respiration come from?
Oxygen and glucose are both produced during photosynthesis. Organisms feed on plants
or other organisms to obtain the glucose and the oxygen is obtained by the organism
through breathing.
3. How many ATP and CO2 molecules are produced at each stage?
Glycolysis yields: 2 ATP
Conversion of pyruvate to Acetyl CoA yields: 2 CO2
Krebs Cycle yields: 2 ATP and 4 CO2
Electron Transport Chain yields: 34 ATP
4. What happens to the CO2 and H20 waste products?
Excess carbon dioxide and water leave the cell through the plasma membrane; H20 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. Put photosynthesis, fermentation, and ATP synthase on 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”. “Fermentation” could also
lead to “O2” with the phrase “does not use”.