Harvesting Energy:
Glycolysis and Cellular
Respiration
Chapter 8
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
Glycolysis
•
Glucose activation phase
– Glucose molecule converted to highly reactive
fructose bisphosphate by two enzymecatalyzed reactions, using 2 ATPs
Glycolysis
•
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
1. 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)
 Yeast cells perform alcoholic fermentation
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
Pyruvate Breakdown in
Mitochondria
•
•
After glycolysis, pyruvate diffuses into the
mitochondrion into the matrix space
Pyruvate is split into CO2 and a 2-carbon
acetyl group, generating 1 NADH per
pyruvate
Pyruvate Breakdown in
Mitochondria
•
•
Acetyl group is carried by a helper
molecule called Coenzyme A, now called
Acetyl CoA
Acetyl CoA enters the Krebs Cycle and is
broken down into CO2
Pyruvate Breakdown in
Mitochondria
•
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
 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
1. Hydrogen ions form a concentration
gradient across the membrane, a form of
stored energy
• Hydrogen ions flow back into the matrix
through an ATP synthesizing enzyme
– Process is called chemiosmosis
Chemiosmosis
•
•
Flow of hydrogen ions provides energy to
link 32-34 molecules of ADP with
phosphate, forming 32-34 ATP
ATP then diffuses out of mitochondrion
and used for energy-requiring activities in
the cell
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