Biology: Life on Earth

Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 8
Harvesting Energy:
Glycolysis and Cellular Respiration
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 8 Outline
•  8.1 How Do Cells Obtain Energy? p. 134
•  8.2 How Is the Energy In Glucose Captured
During Glycolysis? p. 135
•  8.3 How Does Cellular Respiration Capture
Additional Glucose Energy From Glucose? p. 138
•  8.4 Putting It All Together, p. 142
Section 8.1 Outline
•  8.1 How Do Cells Obtain Energy?
–  Photosynthesis Is the Ultimate Source of Energy
–  Glucose Is a Key Energy-Storing Molecule
–  An Overview of Glucose Breakdown
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 à 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
Glucose
•  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 à 6CO2 + 6H2O + ATP
Overview of Glucose Breakdown
•  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
Section 8.2 Outline
•  8.2 How Is the Energy in Glucose
Captured During Glycolysis?
–  Glycolysis Breaks Down Glucose to Pyruvate,
Releasing Chemical Energy
–  In The Absence of Oxygen, Fermentation
Follows Glycolysis
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 enzymecatalyzed reactions, using 2 ATPs
Glycolysis
2.  Energy harvesting phase
–  Fructose bisphosphate is split into two threecarbon molecules of glyceraldehyde 3phosphate (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
high-energy 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)
Fermentation
• 
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
Section 8.3 Outline
• 
8.3 How Does Cellular Respiration
Capture Additional Energy from
Glucose?
–  Cellular Respiration in Eukaryotic Cells Occurs
in Mitochondria
–  Pyruvate Is Broken Down in the Mitochondrial
Matrix, Releasing More Energy
–  High-Energy Electrons Travel Through the
Electron Transport Chain
–  Chemiosmosis Captures Energy Stored in a
Hydrogen Ion Gradient and Produces ATP
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
Section 8.4 Outline
• 
8.4 Putting It All Together
–  A Summary of Glucose Breakdown in
Eukaryotic Cells
–  Glycolysis and Cellular Respiration Influence
the Way Organisms Function
Summary of Glucose Breakdown
•  Figure 8-9, p. 142, 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