Transcript 8.2 What Happens During Glycolysis?

Chapter 8
Harvesting Energy:
Glycolysis and Cellular
Respiration
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 8 At a Glance
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8.1 How Do Cells Obtain Energy?
8.2 What Happens During Glycolysis?
8.3 What Happens During Cellular Respiration?
8.4 What Happens During Fermentation?
Biology: Life on Earth, 9e
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8.1 How Do Cells Obtain Energy?
 Most cellular energy is stored in the chemical
bonds of energy-carrier molecules like
adenosine triphosphate (ATP)
 Cells break down glucose in two stages:
glycolysis, which liberates a small quantity of
ATP, followed by cellular respiration, which
produces far more ATP
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8.1 How Do Cells Obtain Energy?
 Photosynthesis is the ultimate source of cellular
energy
– Photosynthetic organisms capture the energy of
sunlight and store it in the form of glucose
– The overall equation for photosynthesis is
6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2
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Author Animation: Overview: Photosynthesis and
Respiration
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Photosynthesis Provides the Energy
Released by Glycolysis and Cellular Respiration
energy from sunlight
photosynthesis
6 CO2
6 H2O
6 O2
cellular
respiration
C6H12O6
glycolysis
ATP
Fig. 8-1
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8.1 How Do Cells Obtain Energy?
 Glucose is a key energy-storage molecule
– All cells metabolize glucose for energy
– In humans, energy is stored as long chains of
glucose, called glycogen, or as fat
– These storage molecules are converted to
glucose to produce ATP for energy harvesting
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8.1 How Do Cells Obtain Energy?
 An overview of glucose breakdown
– The first stage of glucose breakdown is glycolysis
– Glycolysis begins by splitting glucose (a six-carbon
sugar) into two molecules of pyruvate (a three-carbon
sugar)
– Two ATP molecules are produced in glycolysis
– Glycolysis proceeds in the same way under aerobic
(with oxygen) or anaerobic (without oxygen)
conditions
– Glycolysis occurs in the cytoplasm
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8.1 How Do Cells Obtain Energy?
 An overview of glucose breakdown (continued)
– The second stage of glucose breakdown is cellular
respiration and occurs when oxygen is available
– In this stage, two pyruvate molecules produced by
glycolysis are broken down into six carbon dioxide
molecules and six water molecules
– For every two pyruvate molecules, an additional 34 or
36 ATP molecules are generated
– Cellular respiration occurs in mitochondria,
organelles specialized for the aerobic breakdown of
pyruvate
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8.1 How Do Cells Obtain Energy?
 An overview of glucose breakdown (continued)
– If oxygen is not available, the second stage of
glucose breakdown is fermentation
–Fermentation does not produce any ATP
–In fermentation, pyruvate remains in the
cytoplasm and is converted into lactate or
ethanol + CO2
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Author Animation: Fate of Pyruvate
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8.1 How Do Cells Obtain Energy?
 An overview of glucose breakdown (continued)
– The overall equation for the complete breakdown
of glucose is
C6H12O6 + 6 O2  6 CO2 + 6 H2O + ATP + heat
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A Summary of Glucose Breakdown
(cytoplasmic
fluid)
glucose
glycolysis
2
ATP
lactate
fermentation
2 pyruvate
If O2 is available
If no O2 is available
ethanol
+
CO2
6 O2
cellular
respiration
6 CO2
34
or ATP
36
6 H2O
mitochondrion
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Fig. 8-2
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8.2 What Happens During Glycolysis?
 Glycolysis has two parts, each with several
steps
– Glucose activation
– Energy harvesting
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8.2 What Happens During Glycolysis?
 Glucose activation
– A glucose molecule is activated when it receives
two phosphates from two ATPs, becoming
fructose bisphosphate
–Two ATPs are converted into two low-energy
adenosine diphosphate (ADP) molecules
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8.2 What Happens During Glycolysis?
 Glucose activation (continued)
– Although the formation of fructose bisphosphate
costs the cell two ATP molecules, this initial
investment of energy is necessary to produce
greater energy returns later
– The glucose molecule is relatively stable; the
added phosphates make fructose bisphosphate
highly reactive
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8.2 What Happens During Glycolysis?
 Energy harvesting
– The six-carbon fructose bisphosphate is split into
two, three-carbon molecules of glyceraldehyde3-phosphate (G3P)
– In a series of reactions, each of the two G3P
molecules 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
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8.2 What Happens During Glycolysis?
 Energy harvesting (continued)
– As each G3P is converted to pyruvate, two highenergy electrons and a hydrogen ion are added
to an “empty” electron-carrier nicotinamide
adenine dinucleotide (NAD+) to make the highenergy electron-carrier molecule NADH
– Because two G3P molecules are produced per
glucose molecule, two NADH carrier molecules
are formed as well
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8.2 What Happens During 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
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Author Animation: Glycolysis
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The Essentials of Glycolysis
2 ATP
C C C C C C
glucose
4 ADP
2 ADP
C C C C C C
P
fructose P
bisphosphate
1 Glucose activation
4
ATP
2 C C C
G3P
P
2 C C C
pyruvate
2 NAD+
2 NADH
2 Energy harvest
Fig. 8-3
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8.3 What Happens During Cellular Respiration?
 Cellular respiration in eukaryotic cells occurs in
mitochondria in three stages
– First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
– Keep in mind that each glucose molecule
produces two pyruvate molecules
– Second, high-energy electrons travel through the
electron transport chain
– Third, ATP is generated by chemiosmosis
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
(continued)
– 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 forms the outer surface
of the organelle, and an intermembrane
space lies between the two membranes
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A Mitochondrion
matrix
inner membrane
intermembrane space
outer membrane
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Fig. 8-4
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
(continued)
– Glucose is first broken down into pyruvate,
through glycolysis, in the cell cytoplasm
– Pyruvate is next transported into the
mitochondrion matrix (in eukaryotes), where
further breakdown occurs in two stages:
–The formation of acetyl coenzyme A (acetyl
CoA)
–The Krebs cycle
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
(continued)
– The formation of acetyl CoA
–To generate acetyl CoA, pyruvate is split,
forming an acetyl group and releasing CO2
–The acetyl group reacts with CoA, forming
acetyl CoA
–During this reaction, two high-energy electrons
and a hydrogen ion are transferred to NAD+,
forming NADH
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
(continued)
– The Krebs cycle
–The Krebs cycle begins by combining acetyl
CoA with a four-carbon molecule to form sixcarbon citrate, and coenzyme A is released
–As the Krebs cycle proceeds, enzymes in the
matrix break down the acetyl group, releasing
two CO2 molecules and regenerating the fourcarbon molecule for use in future cycles
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the mitochondrial
matrix, releasing energy and CO2 (continued)
– The Krebs cycle (continued)
– Chemical energy released by breaking down each
acetyl group is captured in energy-carrier molecules
– Each acetyl group produces one ATP, three NADH,
and one FADH2
– Flavin adenine dinucleotide (FAD), a high-energy
electron carrier similar to NAD, picks up two electrons
and two H+, forming FADH2
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Author Animation: Acetyl CoA and the Krebs Cycle
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Reactions in the Mitochondrial Matrix
1 Formation of
acetyl CoA
coenzyme A
3 NADH
3 NAD+
FADH2
C CO2 coenzyme A
C C – CoA
acetyl CoA
C C C
pyruvate
NAD+
FAD
2 Krebs
cycle
NADH
ADP
ATP
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2 C CO2
Fig. 8-5
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8.3 What Happens During Cellular Respiration?
 First, pyruvate is broken down in the
mitochondrial matrix, releasing energy and CO2
(continued)
– During the mitochondrial reactions, CO2 is
generated as a waste product
– CO2 diffuses out of cells and into the blood,
which carries it to the lungs, where it is exhaled
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8.3 What Happens During Cellular Respiration?
 In the second stage of cellular respiration, high-energy
electrons travel through the electron transport chain
– During glycolysis and the mitochondrial matrix reactions,
the cell captures many high-energy electrons in carrier
molecules: 10 NADH and 2 FADH2 for every glucose
molecule that was broken down
– These carriers each release two electrons into an
electron transport chain (ETC), many copies of which are
embedded in the inner mitochondrial membrane
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8.3 What Happens During Cellular Respiration?
 In the second stage of cellular respiration, high-energy
electrons travel through the electron transport chain
(continued)
– These high-energy electrons jump from molecule to
molecule in the ETC, losing small amounts of energy at
each step
– This resembles the process that occurs in the
thylakoid membrane of chloroplasts during
photosynthesis
– Much of this energy is harnessed to pump H+ from the
matrix across the inner membrane and into the
intermembrane space, producing a concentration
gradient of H+
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8.3 What Happens During Cellular Respiration?
 In the second stage of cellular respiration, high-energy
electrons travel through the electron transport chain
(continued)
– The buildup of H+ in the intermembrane space is used to
generate ATP during chemiosmosis
– At the end of the ETC, the energy-depleted electrons are
transferred to oxygen, which acts as an electron acceptor
– Energy-depleted electrons, oxygen, and hydrogen ions
combine to form water
– One water molecule is produced for every two
electrons that traverse the ETC
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8.3 What Happens During Cellular Respiration?
 In the second stage of cellular respiration, highenergy electrons travel through the electron
transport chain (continued)
– Without oxygen, electrons would be unable to
move through the ETC, and H+ would not be
pumped across the inner membrane
– The H+ gradient would dissipate, and ATP
synthesis by chemiosmosis would stop
– ATP generation continues only when there is a
steady supply of oxygen
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The Electron Transport Chain
(matrix)
ADP

P H+
3
1
NADH 2 e–
2
e–
FADH2
NAD+
1
2
O2  2
H+
 2
FAD
e–
ATP
4
H2O
ATP
synthase
(inner
membrane)
ETC
H+
2
H+
H+
H+
(intermembrane space)
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H+
H+
Fig. 8-6
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8.3 What Happens During Cellular Respiration?
 The third stage of cellular respiration generates
ATP by chemiosmosis
– Chemiosmosis is the process by which energy
is first used to generate a gradient of H+, and
then captured in the bonds of ATP as H+ flows
down its gradient
– As the ETC pumps H+ across the inner
membrane, it produces a high concentration of
H+ in the intermembrane space and a low
concentration in the matrix
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8.3 What Happens During Cellular Respiration?
 The third stage of cellular respiration generates
ATP by chemiosmosis (continued)
– The energy present in this non-uniform H+
distribution across the inner membrane is
released when hydrogen ions flow down their
concentration gradient
– The H+ ions flow across the membrane through
the ATP synthase channels, and their movement
generates ATP from ADP and phosphate
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8.3 What Happens During Cellular Respiration?
 The third stage of cellular respiration generates
ATP by chemiosmosis (continued)
– The flow of H+ through the synthase channel
provides the energy to synthesize 32 or 34
molecules of ATP for each molecule of glucose
– The newly formed ATP leaves the mitochondrion
and enters the cytoplasm, where it provides the
energy needed by the cell
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Author Animation: The Electron Transport Chain
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8.3 What Happens During Cellular Respiration?
 A summary of glucose breakdown in eukaryotic
cells
– Glycolysis occurs in the cytoplasm, with one
glucose molecule producing two three-carbon
pyruvate molecules and releasing a small
fraction of the energy stored in glucose
– Some of the energy is used to generate two ATP
molecules, and some is captured in two NADH
molecules, which will feed their electrons into the
ETC during cellular respiration, generating more
ATP
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8.3 What Happens During Cellular Respiration?
 A summary of glucose breakdown in eukaryotic
cells (continued)
– During cellular respiration, the two pyruvate
molecules enter the mitochondrion
– First, each reacts with coenzyme A (CoA), a
process that captures high-energy electrons in
two NADH, produces two molecules of acetyl
CoA, and liberates two molecules of CO2
– The acetyl CoA enters the Krebs cycle
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8.3 What Happens During Cellular Respiration?
 A summary of glucose breakdown in eukaryotic
cells (continued)
– The Krebs cycle releases four molecules of CO2,
produces two ATP, and captures high-energy
electrons in six NADH and two FADH2
– These electrons are passed to the ETC, where
their energy is used during chemiosmosis to
generate a gradient of H+, yielding a net of 32 or
34 ATP
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8.3 What Happens During Cellular Respiration?
 A summary of glucose breakdown in eukaryotic
cells (continued)
– Energy-depleted electrons exiting the ETC are
picked up by H+ released from NADH and
FADH2, and combine with oxygen to form water
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8.3 What Happens During Cellular Respiration?
 A summary of glucose breakdown in eukaryotic
cells (continued)
– The total energy captured from the breakdown of a single
glucose molecule from glycolysis and cellular respiration is
36 or 38 ATP
– The reason two different numbers exist for ATP synthesis
is that some cell types have to expend two ATPs to
transport the two NADH molecules created during
glycolysis into the mitochondrion; others are more
efficient and don’t require two
– The ATP enters the cytoplasm for cellular metabolic
activities
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Author Animation: Overview of Glucose
Metabolism
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Author Animation: Summary of Cellular
Respiration
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The Energy Sources and ATP Harvest from
Glycolysis and Cellular Respiration
glucose
(cytoplasmic
fluid)
glycolysis
2 NADH
2
ATP
2 pyruvate
mitochondrion
CoA
2 NADH
2 CO2
2 acetyl CoA
Krebs
cycle
6 NADH
2
ATP
2 FADH2
4 CO2
O2
H2O
electron transport chain
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32
or ATP
34
total: 36 or 38 ATP
Fig. 8-7
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8.4 What Happens During Fermentation?
 Why is fermentation necessary?
– For glycolysis to continue, the NAD+ used to generate
NADH must constantly be regenerated
– Under aerobic conditions, NADH donates its highenergy electrons and hydrogen produced in glycolysis
to ATP-generating reactions in the mitochondria,
ultimately being donated to oxygen during the creation
of water and making NAD+ available to recycle during
glycolysis
– Under anaerobic conditions, with no oxygen to allow
the ETC to function, the cell must regenerate the
NAD+ for glycolysis using fermentation
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8.4 What Happens During Fermentation?
 Why is fermentation necessary? (continued)
– Fermentation does not produce more ATP, but is
necessary to regenerate NAD+, which must be
available for glycolysis to continue
– If the supply of NAD+ were to be exhausted,
glycolysis would stop, energy production would
cease, and the organism would rapidly die
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8.4 What Happens During Fermentation?
 Why is fermentation necessary? (continued)
– Organisms use one of two types of fermentation to
regenerate NAD+
– Lactic acid fermentation produces lactic acid from
pyruvate
– Alcohol fermentation generates alcohol and CO2
from pyruvate
– Because fermentation does not break down glucose
completely and does not use the energy of NADH to
produce more ATP, organisms that rely on
fermentation must consume far more sugar to
generate the same amount of ATP than do those
relying on cellular respiration
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8.4 What Happens During Fermentation?
 Some cells ferment pyruvate to form lactate
– Muscles that are working hard enough to use up
all the available oxygen ferment pyruvate to
lactate
– To regenerate NAD+, muscle cells ferment
pyruvate to lactate, using electrons from NADH
and hydrogen ions
– A variety of microorganisms use lactic acid
fermentation, including the bacteria that convert
milk into yogurt, sour cream, and cheese
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Glycolysis Followed by Lactic Acid Fermentation
2
NAD+
2 NADH
2
NADH
2
NAD+
2 C C C
2 C C C
C C C C C C
(glycolysis)
(fermentation)
glucose
pyruvate
lactate
2
ADP
2
ATP
Fig. 8-8
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8.4 What Happens During Fermentation?
 Some cells ferment pyruvate to form alcohol
and carbon dioxide
– Many microorganisms, such as yeast, engage in
alcohol fermentation under anaerobic conditions
– As in lactic acid fermentation, the NAD+ must be
regenerated to allow glycolysis to continue
– During alcohol fermentation, H+ and electrons
from NADH are used to convert pyruvate into
ethanol and CO2; this releases NAD+, which can
accept more high-energy electrons during
glycolysis
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Author Animation: Fermentation
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Glycolysis Followed by Alcohol Fermentation
2
NAD+
2 NADH 2 NADH 2
NAD+
2 C C C
C C C C C C
2 C C +2 C
(glycolysis)
(fermentation)
glucose
pyruvate
ethanol CO2
2
ADP
2
ATP
Fig. 8-9
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Fermentation in Action
Fig. 8-10
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