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WEB TUTORIAL 7.1
Harvesting Energy
Text Sections
Section 7.6 Third Stage of Respiration: The Electron Transport Chain, p. 117
Introduction
The energy derived from food is transferred to ATP in a process that has three
major stages: glycolysis, the Krebs cycle, and the electron transport chain. Up to
36 molecules of ATP can be obtained from a single molecule of glucose in these
three stages. Glycolysis and the Krebs cycle also yield electron carriers (NADH
and FADH2) that transport electrons to the electron transport chain. Oxygen is consumed in electron transport chain metabolism, while water and carbon dioxide are
products. Glycolysis takes place in the cytosol of the cell, but the Krebs cycle and
electron transport chain take place in the mitochondria.
Learning Objectives
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Know the meaning of the term “metabolism.”
Understand how cellular energy is produced from a glucose molecule during cellular respiration.
Discuss how the metabolic state of a cell determines the pathway by which
ATP is produced.
Narration
Overview of Glucose Metabolism
Metabolism refers to all the chemical reactions that occur in living cells. The cells
of your body use molecules of adenosine triphosphate (ATP) as their primary
source of energy.
How does the cell produce a continual supply of ATP? The answer lies in the
process called cellular respiration, which occurs partly in the cytoplasm and partly
in mitochondria.
During cellular respiration, a glucose molecule is completely oxidized. In the
process, it releases enough energy to produce up to 36 molecules of ATP. The first
phase of respiration, glycolysis, occurs in the cytoplasm. During glycolysis, glucose is broken down into two molecules of pyruvic acid. Glycolysis also generates
two ATP molecules and two molecules of an electron carrier called NADH.
Pyruvic acid can be metabolized in two possible pathways, depending on whether
or not oxygen is present. In the absence of oxygen, pyruvic acid undergoes fermentation. During fermentation, pyruvic acid might be converted into lactate (as in
highly active human muscles) or ethanol (as in yeast), but there is no ATP production beyond the two ATP molecules generated by glycolysis.
In the presence of oxygen, pyruvic acid is metabolized by cellular respiration,
which produces a great deal of energy. Pyruvic acid is transported into the mitochondrial matrix, where it is stripped of a carbon atom and combined with the molecule coenzyme A, or CoA, to make acetyl CoA. This reaction produces two
NADH electron carriers. Acetyl CoA then enters the next phase of cellular respiration, called the Krebs cycle.
In the Krebs cycle, the remaining carbon atoms from glucose are incorporated into
molecules of carbon dioxide. This process produces two molecules of ATP and a
series of electron carriers. The electron carriers essentially transport energy from
glucose to the next phase of cellular respiration.
The electron carriers donate their electrons to a series of complexes within the
inner mitochondrial membrane. These complexes, together called the electron
transport chain, use the donated energy from the electron carriers to pump protons
into the intermembrane space, forming a concentration gradient of protons across
the membrane.
The protons flow down their concentration gradient through the enzyme ATP synthase, which is a membrane protein. ATP synthase uses the energy released by the
proton flow to produce ATP. From this process, ATP synthase produces an additional 34 ATP molecules, but 2 are used in moving the NADH produced in glycolysis into the mitochondria, thus bringing the net ATP yield from one glucose molecule to 36.
Glycolysis
The initial reactions in the conversion of glucose to energy are called glycolysis.
We will look at the overall process in terms of energy yield and final products.
It may seem strange, but to get energy in the form of ATP from glucose, we first
have to expend energy in the form of ATP. During this energy-investment phase,
two ATP molecules are consumed.
Later in glycolysis, a series of energy-yielding reactions produce four ATP molecules and two NADH molecules.
The energy-investment reactions cost the cell two ATP molecules but are necessary
to prepare glucose for the reactions that follow. The end result of these reactions is
a 6-carbon sugar. This molecule is the starting point for the energy-yielding reactions.
The 6-carbon sugar is split in half, resulting in two molecules with three carbons
each. These two molecules are then transformed by a series of reactions into two
molecules of pyruvic acid. The process yields four molecules of ATP and converts
two molecules of NAD+ to NADH. Because two ATP molecules were consumed in
the energy-investment phase, the net yield at the end of glycolysis is two ATP molecules.
The Fate of Pyruvic Acid
The fate of pyruvic acid after glycolysis depends on the metabolic state of the cell.
The presence or absence of oxygen determines the metabolic state.
Consider the metabolism in your muscle cells. At rest or during light exercise,
when oxygen is plentiful, pyruvic acid enters the Krebs cycle and continues to be
metabolized through cellular respiration.
During heavy exercise, your lungs and circulatory system can't transport oxygen to
your muscles rapidly enough to keep electron transport chains active. The lack of
oxygen forces a shift in energy metabolism toward the anaerobic pathway, fermentation. In muscle cells, fermentation results in lactate production. In some organisms, fermentation produces ethanol and carbon dioxide.
In the presence of oxygen, the pyruvic acid molecules from glycolysis are transported into the mitochondrion. Each pyruvic acid molecule then enters a two-phase
process: the formation of acetyl CoA, and the Krebs cycle.
A pyruvic acid molecule is first joined to a CoA molecule. In this reaction, CO2 is
released and NADH is formed. The third product is acetyl CoA, which contains the
remaining two carbon atoms of pyruvic acid bound to CoA. This becomes a substrate for the later reactions of the Krebs cycle.
The acetyl group, now a two-carbon remnant of the original glucose molecule,
enters the Krebs cycle.
The Krebs Cycle
The Krebs cycle is a series of reactions that remove electrons, donating them to the
electron carriers NAD+ and FAD to form NADH and FADH2. These carriers will
contribute their electrons to an electron transport chain that will then generate significantly more ATP than is produced by the Krebs cycle.
The two carbon atoms contributed by acetyl CoA are each converted to CO2. Thus,
every carbon atom from the original glucose is ultimately incorporated into a CO2
molecule.
Finally, one molecule of ATP is released for each pyruvic acid molecule that enters
the Krebs cycle. Because each glucose molecule contributes two pyruvic acid molecules, the Krebs cycle generates a total of two ATP molecules for each glucose
molecule metabolized.
The Electron Transport Chain and ATP Synthase
The electron transport chain and the enzyme ATP synthase participate in the final
phase of cellular respiration, electron transport. The earlier phases provide energetic electrons in the form of the electron carriers NADH and FADH2.
Here you can follow the fate of electrons donated by NADH. The electrons are
passed to the first carrier in the chain and then move down the chain, losing energy as they go. The energy is used to pump protons across the membrane into the
intermembrane space. This creates a proton gradient that is a source of potential
energy.
The last electron acceptor is oxygen, which reacts with the electrons and protons
to form H2O, or water. The bonding of oxygen with electrons and protons is the
reason cellular respiration depends on oxygen and works only under aerobic conditions.
Finally, ATP synthase allows protons to diffuse across the membrane down their
concentration gradient. It uses the energy released by the flow to synthesize ATP
from ADP and inorganic phosphate.
Let's examine the total energy generated by cellular respiration—as it is stored in
high energy molecules. The total includes the ATP from glycolysis, the NADH
from glycolysis, the NADH from acetyl CoA formation, the ATP from the Krebs
cycle, and the NADH and FADH2 from the Krebs cycle. From the energy of all the
NADH and FADH2 molecules that enter the electron transport chain, approximately 32 ATP molecules are produced. So, cellular respiration yields a total of about
36 ATP molecules—much larger than the yield—two ATP molecules—from the
partial oxidation of glucose by fermentation.
You should now be able to…
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Compare and contrast the processes of cellular respiration and fermentation.
Name the three stages of cellular respiration and describe the major events
in each one.
Explain the role of the electron carriers NADH and FADH2 in cellular respiration.
Describe how a proton gradient facilitates the production of ATP in the
electron transport chain.
Discuss the origin of the carbon dioxide and water produced during cellular respiration.