Name_________________________________
Class_________
Date____________
Station 1: Using Energy and ATP
You know that your body needs energy to function. Does food, which contains sugar and other carbonbased molecules, give you energy? The answer to this question is yes and no. All of the carbon-based
molecules in food store chemical energy in their bonds. Carbohydrates and lipids are the most important
energy sources in foods you eat. However, this energy is only usable after these molecules are broken down
by a series of chemical reactions. Your energy does come from food, but not directly.
All cells use chemical energy carried by ATP – adenosine
triphosphate. ATP is a molecule that transfers energy from the
breakdown of food molecules to cell processes. You can think of
ATP as a wallet filled with money. Cells use ATP for energy in the
same way you use cash to purchase things. ATP is used by cells for
functions such as building and breaking down molecules, and
moving materials by active transport.
The energy carried by ATP is released when a phosphate group is removed from the molecule. ATP has
three phosphate group bonded together, but the bond between the second and third phosphate is unstable
and is easily broken. This is because each phosphate group in ATP is negatively charged. Since these groups
are crowded together, and at the same time repel each other due to their like charges, the bonds between the
phosphate groups are unstable. The removal of the third phosphate group is a chemical reaction that releases
energy.
When the phosphate group is removed, energy is released and ATP becomes ADP- adenosine
diphosphate. ADP has only two phosphate groups and is a lower energy molecule. ADP can be converted back
into ATP by the addition of a phosphate group during the process of respiration. You can think that if ATP is a
wallet filled with money, ADP is a wallet that is nearly empty. The breakdown of ATP into ADP to power cell
function and the conversion of ADP into ATP by using the energy contained in food represent a continuous
cycle going on in cells.
Another way to think about ATP is as a
compressed spring that stores potential energy.
When the third phosphate group is removed,
the “spring” relaxes and energy is released. ADP
is analogous to the relaxed spring that has
released its potential energy, but can be
compressed again to store energy.
Station 1 Questions (Please answer all questions below in complete sentences):
1. Name the molecule that carries energy for the cell to use and the process that creates it.
2. Explain how the energy in ATP is released.
3. Why are the bonds that connect the phosphate groups in ATP unstable?
4. Describe the relationship between energy stored in food and ATP.
5. Describe the relationship between the molecules ATP and ADP.
Name_________________________________
Class_________
Date____________
Station 2: Cellular Respiration Overview
In all organisms, the energy stored in organic molecules may be released during the life process of
cellular respiration. Cellular respiration is a biochemical process that includes the chemical reactions used by
cells to release energy from organic molecules such as glucose. This energy is temporarily stored in ATP
molecules. The chemical bonds between atoms of food molecules provide the energy used by all living
organisms to sustain life. Like other reactions that occur in the cell, the reactions involved in respiration are
controlled by enzymes. The formula for respiration is shown below, and you may notice that it is the opposite
of photosynthesis.
C6H12O6 + O2  CO2 + H2O + ATP
There are two main forms of cellular respiration carried out by living things. They are known as aerobic
(with oxygen) and anaerobic (without oxygen). However, both types begin with the same process. Cellular
respiration begins with glycolysis, a process in which glucose is broken in half to form pyruvate. Glycolysis is
an anaerobic process, meaning it does not need oxygen. Since both prokaryotes and eukaryotes perform
glycolysis, this process does not occur in a specialized organelle but in the cell’s cytoplasm.
To simplify the chemical reactions of glycolysis, we first need to remember that glucose contains 6
carbon atoms. Glycolysis begins by the cell using two molecules of ATP to power the reaction. The energy
supplied by ATP is used to split the 6-carbon glucose into two 3-carbon molecules called pyruvate. By
rearranging the atoms in these molecules the cell is able to produce 4 ATP’s and 2 NADH (another energy
carrier). Since the cell used 2 ATP to begin the process, and
produced 4 ATP, we can say that cell has a net gain of 2 ATP
molecules.
At this point, if the organism is an anaerobe then it will
perform fermentation. Fermentation recycles the NADH into
NAD+ and does not produce any more ATP. If the organism is
an aerobe, then the pyruvates that are produced will be
transported to the cells mitochondria, where it will undergo
further breakdown to release even more ATP.
Station 2 Questions (Please answer all questions below in complete sentences):
1. What is the life process that creates ATP by releasing the energy stored in foods?
2. Explain the major difference between aerobic and anaerobic respiration.
3. Where in the cell does glycolysis occur?
4. Why is glycolysis considered an anaerobic process?
5. In your own words, describe the process of glycolysis. Be sure to identify any molecules that are used
and produced during the process.
6. Explain what happens to the products of glycolysis if the organism is an aerobe and there is oxygen
available.
Name_________________________________
Class_________
Date____________
Station 3: Anaerobic Respiration
Anaerobic respiration takes place when oxygen is low or not present. Some prokaryotes that do not
need oxygen use anaerobic respiration all the time. Other cells use anaerobic respiration when oxygen levels
are too low to perform aerobic respiration. This process operates by means of chemical reactions catalyzed by
enzymes located in the cytoplasm of the cell. Organisms that typically use this form of respiration include
certain bacteria and fungi (yeasts). Human beings have learned to take advantage of this process by using
these organisms in the manufacturing of foods such as cheese, buttermilk, and yogurt. These organisms also
play an important role in the baking, wine-making, and brewing industries.
There are two principal types of anaerobic respiration. The first is called alcoholic or ethanol
fermentation because one of the major by-products produced in this process is ethyl alcohol.
Glucose  2 ethyl alcohol + carbon dioxide
The second type is called Lactic Acid fermentation because it produces lactic acid as one of its by-products.
This takes place in human muscle cells during strenuous exercise and causes a condition known as muscle
fatigue. Muscle fatigue can be described as the burning feeling in the muscle – it then becomes difficult to
keep using the muscle.
Glucose  2 lactic acid
Both types of anaerobic respiration are considered inefficient. The end products of both (ethyl alcohol and
lactic acid) contain considerable amounts of energy that are not effectively released for use by the cell. We
take advantage of this chemical energy when we burn ethanol as a biofuel.
All of the energy (ATP) from anaerobic respiration comes from glycolysis. The second step,
fermentation, does not create any additional ATP for the cell. So what’s the point? During glycolysis, NAD+
accepts electrons from the glucose during the transition from glucose to pyruvate and becomes NADH. During
fermentation, NADH releases the electron and turns back into NAD+. Without this exchange of electrons,
glycolysis would get “backed up” like a traffic jam and the process would stop. This is important because if
glycolysis stopped, then the cell would not be able to produce energy to run its life process, homeostasis
would be disrupted, and the organism may die.
Station 3 Questions (Please answer all questions below in complete sentences):
1. In what conditions would cells perform anaerobic respiration?
2. How do humans benefit from the fermentation process?
3. Describe the difference between the two major types of aerobic fermentation.
4. Why is fermentation considered inefficient?
5. Explain the effect of a low concentration of oxygen in human muscle cells?
6. Describe the reason cells will perform fermentation even though it does not produce any additional
ATP for the cell to use.
Name_________________________________
Class_________
Date____________
Station 4: Aerobic Respiration
Aerobic respiration is a form of cellular respiration carried on by certain cells in the presence of
molecular oxygen. This process operates by means of chemical reactions catalyzed by enzymes located
primarily in the mitochondria of the cell. As in anaerobic respiration, the principal purpose of aerobic
respiration is to release energy from the chemical bonds of organic molecules. This energy is used by to carry
out life processes such as synthesis, digestion, active transport, and growth. Unlike anaerobic respiration
however, aerobic respiration uses oxygen to release substantially more energy per glucose molecule that is
broken down. For this reason, aerobic respiration is a more efficient form of respiration than anaerobic
respiration. In general, aerobic respiration may be illustrated by the following equation:
C6H12O6 + O2  CO2 + H2O + 36 ATP
Remember, all forms of cell respiration begin with the process of glycolysis, an anaerobic process. In
glycolysis, glucose is broken in half using 2 ATP to form the products: 2 pyruvate, 2 ATP, and 2 NADH. In the
presence of oxygen, the final two stages of cellular respiration can occur. However, before these aerobic
processes begin, pyruvate undergoes a series of reactions after it is transported into the mitochondria. In this
reaction, pyruvate combines with a molecule called Coenzyme A. The result is that one of the carbon atoms in
pyruvate (a 3-carbon molecule) is broken off and excreted as CO2 and the molecule Acetyl-CoA forms. The 2carbon molecule acetyl CoA enters the next stage of cell respiration called the Krebs cycle which is also known
as the Citric Acid Cycle.
The Krebs cycle is a series of chemical reactions that breaks down a molecule of acetyl-CoA to form an
ATP molecule and carbon dioxide. Since glycolysis produces two molecules of pyruvate from one glucose
molecule, the cycle needs to “turn” twice for each glucose molecule. This means the overall products of the
Krebs cycle are 2 ATP and 4 CO2 molecules. The Krebs cycle also uses two electron carriers (NADH and FADH2)
to pass energized electrons on to the electron transport chain (ETC) in the inner membranes of mitochondria.
The ETC is the last stage in aerobic cellular respiration. It is similar to the ETC used in photosynthesis.
Basically, energized electrons are passed between proteins embedded in the mitochondrial membrane. As
they pass from protein to protein, energy is slowly released and used to form ATP molecules. Oxygen is
important in this process because it is the final electron acceptor at the end of the ETC chain. The oxygen
reacts with hydrogen ions (H+) and electrons to form molecules of water (H2O). The full breakdown of one
molecule of glucose in the ETC results in 32 molecules of ATP produced.
To put it all together,
glycolysis produces 2 ATP, the
Krebs cycle produces 2 ATP, and
the ETC produces 32 ATP for a
grand total of 36 ATP molecules
per glucose. Carbon dioxide and
water are waste products of this
process.
Station 4 Questions (Please answer all questions below in complete sentences):
1. What conditions are necessary for aerobic respiration and where in the cell does the process occur?
2. Describe what happens to the products of glycolysis before they enter the Krebs cycle.
3. Identify the products of glycolysis, the Krebs cycle, and the ETC.
4. How is the Krebs cycle related to the ETC?
5. Why is oxygen needed for aerobic respiration?