Download Cellular Respiration in More Depth Part 1: ATP—The

Cellular Respiration in More Depth
Part 1: ATP—The Free Energy Carrier
How does the ATP molecule capture, store, and release energy?
Why?
A sporting goods store might accept a $100 bill for the purchase of a bicycle, but the corner store will not take
a $100 bill when you buy a package of gum. That is why people often carry smaller denominations in their
wallets—it makes everyday transactions easier. The same concept is true for the energy transactions in cells.
Cells need energy (their “currency”) to take care of everyday functions, and they need it in many
denominations. As humans we eat food for energy, but food molecules provide too much energy for our cells
to use all at once. For quick cellular transactions, your cells store energy in the small molecule of ATP. This is
analogous to a $1 bill for your cells’ daily activities.
Model 1—Adenosine Triphosphate (ATP)
1. The diagram of ATP in Model 1 has three parts. Use your knowledge of biomolecules to label the molecule
with an “adenine” section, a “ribose sugar” section, and a “phosphate groups” section.
2. Refer to Model 1.
a. What is meant by the “tri-“ in the name adenosine triphosphate?
b. What might the structure of adenosine diphosphate (ADP) look like? Draw or describe your
conclusions.
Model 2—Hydrolysis of ATP
3. Model 2 illustrates a chemical reaction. Write the reaction as an equation, using the name or abbreviation
of each of the two reactants and each of the three products.
4. Consider the structural formulas of ATP, ADP, and phosphate in Model 2 carefully. What happens to the
atoms from the water molecule during the hydrolysis of ATP?
5. The word hydrolysis has two roots, hydro and lysis. Describe how this term relates to the chemical
reaction illustrated in Model 2.
6. Refer to Model 2.
a. Does the hydrolysis of ATP result in a net output or a net input of energy?
b. Which molecule, ATP or ADP, has a higher potential energy? Explain your reasoning.
Read This!
The conversion of ATP to ADP is not only exothermic, it is also exergonic. The hydrolysis of ATP provides free
energy for many processes needed to sustain life.
7. The reaction in Model 2 is reversible.
a. Write a reaction for the process that is the reverse of Model 2.
b. This reaction is called phosphorylation. Explain why this name is appropriate for the reaction
above.
Model 3—The ATP Cycle
8. Label the two arrows in Model 3 with “hydrolysis” and “phosphorylation.”
9. When ATP is hydrolyzed, free energy is available.
a. According to Model 3, what does that energy get used for?
b. Name at least two other cellular processes that could be fueled by the hydrolysis of ATP that are
not listed in Model 3.
10. After it is used, an ADP molecule is recycled back into ATP. What cellular, exergonic processes supply the
energy needed for the phosphorylation of ADP?
11. In the Why? Box at the start of this activity, an analogy was made between money and cellular energy.
a. What part(s) of the ATP cycle are analogous to earning money?
b. What part(s) of the ATP cycle are analogous to spending money?
c. What would be analogous to saving money in the context of ATP?
Part 2: Cellular Respiration—An Overview
What are the phases of cellular respiration?
Why?
All cells need energy all the time, and their primary source of energy is ATP. The methods cells use to make
ATP vary depending on the availability of oxygen and their biological make-up. In many cases the cells are in
an oxygen-rich environment. For example, as you sit and read this sentence, you are breathing in oxygen,
which is then carried throughout your body by red blood cells. But, some cells grow in environments without
oxygen (yeast in wine-making or the bacteria that cause botulism in canned food), and occasionally animal
cells must function without sufficient oxygen (as in running sprints). In this activity you will begin to look at
the aerobic processes that are used by all organisms to produce ATP.
Model 1—Cellular Respiration
1. According to Model 1, what are the reactants of cellular respiration?
2. According to Model 1, what are the products of cellular respiration?
3. Cellular respiration occurs in three phases: glycolysis, the Krebs cycle, and oxidative phosphorylation.
a. Which phase of cellular respiration occurs in the cytoplasm of the cell?
b. Which phases of cellular respiration occur in the mitochondria?
c. Which of the phases of cellular respiration require oxygen?
d. Which of the phases of cellular respiration produce carbon dioxide?
e. Which of the phases of cellular respiration produce water?
4. The goal of cellular respiration is to provide the cell with energy in the form of ATP.
a. Which of the phases of cellular respiration result in the production of ATP?
b. How many ATPs (total) are produced for every glucose molecule that undergoes cellular
respiration?
c. What reactants of ATP must be available in the cell in order to produce ATP?
d. Brainstorm several cellular processes for which energy or ATP is necessary.
Read This!
Glucose can be burned in oxygen (oxidized) to produce carbon dioxide and water. Combustion reactions
release large amounts of energy. However, the energy release is uncontrolled. An organism would not be
able to handle all that energy at once to do the work of the cell. Cellular respiration is essentially the same
reaction as combustion, but the oxidation of glucose occurs in several controlled steps. The same amount of
energy is ultimately released, but it is gradually released in small, controlled amounts. High potential energy
molecules of ATP are produced while the carbon atoms are used to form various other molecules of lower
potential energy. Each of these steps is catalyzed by an enzyme specific to that step. Model 1 illustrates the
ideal circumstances for cellular respiration. In some situations, however, one glucose molecule may not result
in 38 ATP molecules being produced.
5. Consider Model 1. Besides ATP, what other molecules appear to be high potential energy molecules (free
energy carriers) during cellular respiration?
Model 2—Electron Acceptor Molecules
6. NAD+ and FAD are coenzymes used in cellular respiration to transport high potential energy electrons to
the electron transport chain (a step in oxidative phosphorylation) in the mitochondria. At the conclusion
of cellular respiration, oxygen is the final electron acceptor. The reactions in Model 2 show these electron
acceptors in the process of picking up an electron.
a. How many electrons are in a hydrogen ion (H+)?
b. Is a hydrogen ion with its positive charge likely to be attracted to NAD +, FAD or O2 without an input
of free energy? Explain your reasoning.
c. Consider the charges in the first reaction in Model 2. Can two positive particles combine to form a
neutral particle? Explain your reasoning.
7. The reactions in Model 2 are missing very important particles—electrons.
a. What is the charge of an electron?
b. For each reaction in Model 2, predict the number of electrons that must be involved in the reaction
to make the charges in the reaction balance.
c. Add electrons to each reaction in Model 2 on either the reactant or product side of the equation to
complete the reactions.
Read This!
Oxidation is a loss of electrons. Reduction is a gain of electrons. The two processes must go hand-in-hand. In
other words, electrons cannot be added to something from thin air, they must have been taken off of
something first.
8. Are the reactions in Model 2 oxidation or reduction reactions?
9. In terms of NAD+ and NADH, which is the reduced form? Explain your reasoning.
10. According to Model 1, glucose undergoes the following changes during cellular respiration. In each step
NADH or FADH2 is produced. Is glucose being oxidized or reduced during cellular respiration? Explain your
reasoning.
glucose  pyruvate  acetyl-CoA  carbon dioxide
11. Model 1 shows ten NADH molecules being produced during cellular respiration. What reactants must be
available in the cell for these molecules to be produced?
12. The hydrogen ions and electrons that were carried by NADH and FADH2 are used in oxidative
phosphorylation.
a. What molecules are produced as the hydrogen ions and electrons are removed from NADH and
FADH2?
b. Is the removal of hydrogen ions and electrons from NADH and FADH2 oxidation or reduction?
Explain your reasoning.
c. Predict what happens to those product molecules after they “drop off” their ions and electrons.
13. Cells can survive for short periods without oxygen. Only the glycolysis phase of cellular respiration occurs
in those circumstances.
a. Predict the number of ATP molecules that could be produced from one glucose molecule if oxygen
were not available.
b. Since oxidative phosphorylation no longer occurs when oxygen is not available, predict what would
eventually happen to the supply of NAD+ in the cell if only glycolysis were occurring.
Model 3—Energy in Anaerobic Environments
14. Examine the two anaerobic processes in Model 3. Is oxygen used in either process?
15. What is the definition of anaerobic?
16. Compare the energy output (in the form of ATP) for a single glucose molecule that undergoes glycolysis
and fermentation to that of a glucose molecule undergoing cellular respiration.
17. The efficiency of a cell is dependent on the cyclic nature of NAD + and NADH. Building NAD+ molecules
from raw materials each time one was needed would require a huge amount of free energy and resources.
Even though the fermentation steps shown in Model 3 do not provide any ATP, they are critical to the
energy production of the cell. Predict what would happen to the energy supply in a cell if fermentation did
not happen under anaerobic conditions.
18. The muscle “burn” that you feel when doing strenuous activity (sprints for example) is caused by a build up
of lactic acid in the muscle tissue of your body. Explain this phenomenon in the context of cellular
respiration and fermentation.
19. The evolution of photosynthesizing organisms on Earth and the development of an oxygen-rich
environment led to a rapid diversification of life. Explain why there is an evolutionary advantage to an
organism that requires oxygen to live compared to one that does not require oxygen.
20. Under laboratory conditions muscle cells were broken up and separated into factions of mitochondria and
cytoplasm in an attempt to learn more about cellular respiration. Each fraction was incubated with
glucose or pyruvate. Tests were carried out during incubation for the presence of either carbon dioxide or
lactic acid. The results are shown below:
Cell Fraction
CO2
Lactic Acid
Mitochondria incubated with glucose Absent Absent
Mitochondria incubated with pyruvate Present Absent
Cytoplasm incubated with glucose
Absent Present
Cytoplasm incubated with pyruvate
Absent Present
a. What does the presence of lactic acid in a sample indicate about what process is occurring in each
cell fraction?
b. Explain why lactic acid was produced by the cytoplasm fraction incubated with glucose, but not the
mitochondrial fraction.
c. Why was no carbon dioxide produced by either fraction incubated with glucose?
d. Why did the cytoplasm fraction produce lactic acid in the presence of both glucose and pyruvate?
e. Why did the mitochondria produce carbon dioxide in the presence of pyruvate but not in the
presence of glucose?
Part 3: Glycolysis and the Krebs Cycle
What reactions occur in the cell to turn glucose into carbon dioxide?
Why?
Glucose is a high potential energy molecule. Carbon dioxide on the other hand is a very stable, low potential
energy molecule. When a glucose molecule is converted to carbon dioxide and water during cellular
respiration, energy is released and stored in high potential energy ATP molecules. The three phases of cellular
respiration that oxidize the glucose molecule to carbon dioxide are glycolysis, the Link reaction, and the Krebs
cycle.
Model 1—Glycolysis
1. Refer to Model 1.
a. What molecule from food is the primary reactant for glycolysis?
b. How many carbon atoms are in that reactant molecule?
2. The carbon atoms from glucose end up in pyruvate molecules as a product of glycolysis.
a. How many carbon atoms are in a pyruvate molecule?
b. How many pyruvate molecules are made from each glucose molecule?
3. Does the process of glycolysis require an input of energy? Provide specific evidence from Model 1 to
support your answer.
4. Does pyruvate have more or less potential energy than glucose? Provide specific evidence from Model 1
to support your answer.
5. What is the net production of ATP by glycolysis?
6. What molecule acts as an electron acceptor in glycolysis?
7. In the last steps of glycolysis 4 ATP molecules are produced. Analyze Model 1 to find the source of the four
inorganic phosphates (Pi) that are added to the ADP molecules to make the four ATP molecules. Describe
the origins of the four inorganic phosphates.
Model 2—The Link Reaction
8. According to Model 2, where in a cell does the link reaction take place?
9. During the link reaction, the pyruvic acid molecule is decarboxylated. What molecule is removed during
this process?
10. Coenzyme A carries the remainder of the pyruvate molecule to the site of the Krebs cycle.
a. What is the name of decarboxylated pyruvic acid?
b. How many carbons of the pyruvate molecule remain when it is attached to Coenzyme A?
11. Has any ATP been used or produced during the link reaction?
12. Have any other high potential energy molecules been produced during the link reaction?
13. How many acetyl-CoA, carbon dioxide, and NADH molecules are produced in the link reaction for each
glucose molecule that undergoes cellular respiration?
Model 3—The Krebs Cycle
14. Where in the cell does the Krebs cycle take place?
15. What molecule is introduced to the Krebs cycle from the link reaction?
16. Is oxygen needed as a reactant in the Krebs cycle?
17. How many “turns” of the Krebs cycle occur for every glucose molecule that undergoes cellular respiration?
18. How many of each of the of the molecules below are produced in the Krebs cycle of every glucose
molecule that undergoes cellular respiration?
CO2
ATP
NADH
FADH2
19. Was oxygen used as a reactant in any of these processes explored above—glycolysis, the link reaction or
the Krebs cycle?
Read This!
Glycolysis will occur in a cell with or without oxygen present. If oxygen is present, the link reaction, Krebs
cycle and oxidative phosphorylation will complete the process of oxidizing glucose and maximizing the energy
output. However, without oxygen, glycolysis is coupled with fermentation processes to provide a continual
supply of energy to the cell. It is important to note that even though the link reaction and Krebs cycle do not
use oxygen as a reactant, they will not occur in the cell without oxygen present.
Part 4: Oxidative Phosphorylation
How are the electrons in NADH and FADH2 used to make ATP during cellular respiration?
Why?
The final phase of cellular respiration is oxidative phosphorylation. Both the electron transport chain and
chemiosmosis make up oxidative phosphorylation. During this phase of cellular respiration, all of the NADH
and FADH2 that were produced in other phases of cellular respiration (glycolysis, the link reaction, and Krebs
cycle) are used to make ATP. The process occurs in the protein complexes embedded in the inner
mitochondrial membrane.
Model 1—Electron Transport Chain
1. Consider the membranes illustrated in Model 1. Circle a section of the mitochondria below where this site
might be located.
2. Refer to Model 1.
a. Describe the region in Model 1 where the highest concentration of hydrogen ions (H+) is located.
b. According to Model 1, how do the hydrogen ions reach this area?
3. Explain why energy is required to move the hydrogen ions across the membrane in the direction indicated
in Model 1.
4. High potential energy electrons provide the energy necessary to pump hydrogen ions across the inner
mitochondrial membrane.
a. What molecules carry these high potential energy electrons?
b. Where did these electron acceptor molecules come from?
c. When the electrons are released from the electron acceptor molecules, what else is produced?
d. Is the release of an electron from these electron acceptor molecules oxidation or reduction?
5. Refer to Model 1
a. What molecule is the final electron acceptor after the electron has moved through the electron
transport chain?
b. What compound is formed as a final product of the electron transport chain?
6. Is any ATP produced in the electron transport chain?
7. Is any ATP used in the electron transport chain?
Model 2—Chemiosmosis
8. Describe the movement of hydrogen ions through the membrane illustrated in Model 2.
9. Would free energy be required for the hydrogen ions to move in the direction shown in Model 2? Explain
your reasoning.
10. What is the name of the embedded protein that provides a channel for the hydrogen ions to pass through
the membrane?
11. The flow of hydrogen ions through the protein channel provides free energy to do work. What process in
Model 2 requires energy?
Read This!
The embedded protein complex, ATP synthase, is more of a machine than a chemical enzyme. Research has
shown that a protein “rotor” down the middle of the ATP synthase complex turns as hydrogen ions flow
through. This rotates other proteins, which then “squeeze” the ADP and inorganic phosphate groups together
to form ATP.
12. During oxidative phosphorylation, what molecule is being phosphorylated?
13. Under ideal conditions, each NADH molecule will result in three ATP molecules, and each FADH2 molecule
will result in two ATP molecules during oxidative phosphorylation. Calculate the total number of ATP
molecules that might be produced in this phase of cellular respiration from one glucose molecule.
14. Considering all the stages of cellular respiration (glycolysis, link, Krebs cycle, and oxidative
phosphorylation) how many ATP molecules are produced from one glucose molecule, assuming ideal
circumstances?
15. Because of its role in aerobic respiration, oxygen is essential for most living things on Earth. In complete
sentences, describe the role of molecular oxygen (O2) in aerobic respiration.
16. Consider the overall chemical reaction for cellular respiration.
C6H12O6 + 6O2  6CO2 + 6H2O
Complete the table below to identify the phase of cellular respiration where each of the reactants are
used, the produced are produced and the location in the cell where that phase occurs.
Reactants
C6H12O6
Phase(s) at
which it is used
or produced
Location
6O2
6CO2
Products
6H2O
38 ATP