Chapter 8
Harvesting Energy:
Glycolysis and Cellular Respiration
Chapter 8 Outline
• 8.1 How Do Cells Obtain Energy?
• 8.2 How Is the Energy In Glucose Captured
During Glycolysis?
• 8.3 How Does Cellular Respiration Capture
Additional Glucose Energy From Glucose?
• 8.4 Putting It All Together
8.1 Photosynthesis
• Photosynthetic organisms capture the energy of
sunlight and store it in the form of glucose
• The overall equation for photosynthesis is:
6 CO2 + 6H2O  C6H12O6 + 6O2
Glucose
• Glucose is a key energy-storing molecule:
– Nearly all cells metabolize glucose for energy
– Glucose metabolism is fairly simple
– Other organic molecules are converted to
glucose for energy harvesting
– During glucose breakdown, all cells release the
solar energy that was originally captured by
plants through photosynthesis, and use it to
make ATP
Overview of Glucose Breakdown
• The overall equation for the complete
breakdown of glucose is:
C6H12O6 + 6O2  6CO2 + 6H2O + ATP
The main stages of glucose metabolism are:
– Glycolysis
– Cellular respiration
Overview of Glucose
Breakdown
• Glycolysis
–
–
–
–
Occurs in the cytosol
Does not require oxygen
Breaks glucose into pyruvate
Yields two molecules of ATP per molecule of
glucose
Overview of
Glucose Breakdown
• If oxygen is absent
fermentation occurs
– pyruvate is converted
into either lactate, or
into ethanol and CO2
• If oxygen is present
cellular respiration
occurs
Overview of Glucose Breakdown
• Cellular respiration
– Occurs in mitochondria
(in eukaryotes)
– Requires oxygen
– Breaks down pyruvate into
carbon dioxide and water
– Produces an additional 32
or 34 ATP molecules,
depending on the cell type
8.2 Glycolysis
Overview of the two major phases of
glycolysis
1. Glucose activation phase
2. Energy harvesting phase
Glycolysis
1. Glucose activation phase
– Glucose molecule converted to highly reactive
fructose bisphosphate by two enzyme-catalyzed
reactions, using 2 ATPs
Glycolysis
2. Energy harvesting phase
–
–
–
Fructose bisphosphate is split into two three-carbon
molecules of glyceraldehyde 3-phosphate (G3P)
In a series of reactions, each G3P molecule is converted
into a pyruvate, generating two ATPs per conversion, for a
total of four ATPs
Because two ATPs were used to activate the glucose
molecule there is a net gain of two ATPs per glucose
molecule
Glycolysis
2. Energy harvesting phase (continued)
– As each G3P is converted to pyruvate, two
high-energy electrons and a hydrogen ion are
added to an “empty” electron-carrier NAD+ to
make the high-energy electron-carrier
molecule NADH
– Because two G3P molecules are produced per
glucose molecule, two NADH carrier
molecules are formed
Glycolysis
•
Summary of glycolysis:
– Each molecule of glucose is broken down to
two molecules of pyruvate
– A net of two ATP molecules and two NADH
(high-energy electron carriers) are formed
Fermentation
•
•
Pyruvate is processed differently under
aerobic and anaerobic conditions
Under aerobic conditions, the high energy
electrons in NADH produced in glycolysis
are ferried to ATP-generating reactions in
the mitochondria, making NAD+ available
to recycle in glycolysis
Fermentation
•
•
Under anaerobic conditions, pyruvate is
converted into lactate or ethanol, a
process called fermentation
Fermentation does not produce more ATP,
but is necessary to regenerate the highenergy electron carrier molecule NAD+,
which must be available for glycolysis to
continue
Fermentation
• Some cells ferment
pyruvate to form acids
• Human muscle cells can
perform fermentation
– Anaerobic conditions
produced when muscles
use up O2 faster than it
can be delivered (e.g.
while sprinting)
– Lactate (lactic acid)
produced from pyruvate
Fermentation
• Some microbes
ferment pyruvate to
other acids (as seen in
making of cheese,
yogurt, sour cream)
• Some microbes
•
perform fermentation
exclusively (instead of
aerobic respiration)
Yeast cells perform
alcoholic
fermentation
Fermentation
•
•
•
Glucose is fermented to ethanol and CO2
Sparkling wine is made by adding yeast with
the sugar in grapes; CO2 produces the fizz
Bread is made by adding yeast, sugar, and
flour; CO2 bubbles cause the dough to rise
8.3 Cellular Respiration
•
In eukaryotic cells, cellular
respiration occurs within
mitochondria, organelles
with two membranes that
produce two compartments
– The inner membrane
encloses a central
compartment containing
the fluid matrix
– The outer membrane
surrounds the organelle,
producing an
intermembrane space
Cellular Respiration
Overview of Aerobic Cellular
Respiration:
1. Glucose is first broken down
into pyruvate, through
glycolysis, in the cell
cytoplasm
2. Pyruvate is transported into
the mitochondrion
(eukaryotes) and split into
CO2 and a 2 carbon acetyl
group
Cellular Respiration
3. The acetyl group is further
broken down into CO2 in the
Krebs Cycle (matrix space)
as electron carriers are
loaded
4. Electron carriers loaded up
in glycolysis and the Krebs
Cycle give up electrons to
the electron transport
chain (ETC) along the inner
mitochondrial membrane
Cellular Respiration
5. A hydrogen ion gradient
produced by the ETC is used
to make ATP
(chemiosmosis)
6. ATP is transported out of the
mitochondrion to provide
energy for cellular activities
Pyruvate Breakdown in Mitochondria
1. After glycolysis, pyruvate
diffuses into the
mitochondrion into the matrix
space
2. Pyruvate is split into CO2 and
a 2-carbon acetyl group,
generating 1 NADH per
pyruvate
Pyruvate Breakdown in Mitochondria
3. Acetyl group is carried by a
helper molecule called
Coenzyme A, now called
Acetyl CoA
4. Acetyl CoA enters the Krebs
Cycle and is broken down into
CO2
Pyruvate Breakdown in Mitochondria
5. Electron carriers NAD+
and FAD are loaded with
electrons to produce 3
NADH & 1 FADH2 per
Acetyl CoA
6. One ATP also made per
Acetyl CoA in the Krebs
Cycle
Electron Transport Chain
• Most of the energy in
glucose is stored in
electron carriers NADH
and FADH2
– Only 4 total ATP produced
per glucose after complete
breakdown in the Krebs
Cycle
Electron Transport Chain
• NADH and FADH2 deposit
electrons into electron
transport chains in the inner
mitochondrial membrane
• Electrons join with oxygen
gas and hydrogen ions to
made H2O at the end of the
ETCs
Chemiosmosis
1. Energy is released from
electrons as they are
passed down the
electron transport chain
2. Released energy used
to pump hydrogen ions
across the inner
membrane
–
Hydrogen ions
accumulate in
intermembrane space
Chemiosmosis
3. Hydrogen ions form a
concentration gradient
across the membrane, a
form of stored energy
4. Hydrogen ions flow back
into the matrix through an
ATP synthesizing enzyme
– Process is called
chemiosmosis
Chemiosmosis
5. Flow of hydrogen ions
provides energy to link 32-34
molecules of ADP with
phosphate, forming 32-34
ATP
6. ATP then diffuses out of
mitochondrion and used for
energy-requiring activities in
the cell
8.4 Summary of Glucose Breakdown
• Figure 8-9
summarizes the
process of glucose
metabolism in a
eukaryotic cell with
oxygen present…
Summary of Glucose Breakdown
• Figure 8-10, p. 143,
shows the energy
produced by each
stage of glucose
breakdown…
Influence on How Organisms Function
• Metabolic processes in cells are heavily
dependent on ATP generation (cyanide kills
by preventing this)
• Muscle cells switch between fermentation
and aerobic cell respiration depending on O2
availability