Transcript 8.3 What Happens During Cellular Respiration?

BIOLOGY Life on Earth
WITH PHYSIOLOGY Tenth Edition
Audesirk Audesirk Byers
8
Harvesting Energy:
Glycolysis and
Cellular Respiration
Lecture Presentations by
Carol R. Anderson
Westwood College, River Oaks Campus
© 2014 Pearson Education, Inc.
Chapter 8 At a Glance
 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?
© 2014 Pearson Education, Inc.
8.1 How Do Cells Obtain Energy?
 Most cellular energy is stored in the chemical bonds
of energy-carrier molecules such as 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
– Nearly all organisms use glycolysis and cellular
respiration to break down sugar molecules to capture
energy as ATP
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8.1 How Do Cells Obtain Energy?
 Photosynthesis is the ultimate source of cellular
energy (continued)
– Photosynthesis

6 CO2  6 H2O  light energy  C6H12O6  6 O2
– Complete glucose breakdown
– C6H12O6  6 O2  6 CO2  6 H2O  ATP energy  heat
energy
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Figure 8-1 Photosynthesis provides the energy released during glycolysis and cellular respiration
energy from sunlight
photosynthesis
6 CO2
6 H2O
6 O2
cellular
respiration
C6H12O6
glycolysis
ATP
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8.1 How Do Cells Obtain Energy?
 Glucose is a key energy-storage molecule
– All cells metabolize glucose for energy
– Plants convert glucose to sucrose or starch for
storage
– 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?
 Glucose is a key energy-storage molecule
(continued)
– The breakdown of glucose occurs in phases
– Glycolysis
– Fermentation
– Cellular respiration
– During glycolysis and cellular respiration, energy is
captured in ATP
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Figure 8-2 A summary of glucose breakdown
(cytosol)
1 glucose
glycolysis
ATP
2 lactate
fermentation
2 pyruvate
If O2 is available
If no O2 is available
2 ethanol

2 CO2
6 O2
cellular
respiration
6 CO2
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ATP
6 H2O
mitochondrion
8.2 What Happens During Glycolysis?
– Glycolysis has an etymological root from the Greek,
“glyco,” meaning “sweet,” and “lysis,” meaning to
“split apart”
– Glycolysis begins by splitting glucose (a six-carbon
sugar) into two molecules of pyruvate (a three-carbon
sugar)
– Glycolysis has energy investment and energy
harvesting stages
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8.2 What Happens During Glycolysis?
 Energy investment stage
– 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 6-carbon fructose bisphosphate is split into two
3-carbon molecules of glyceraldehyde-3-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 high-energy
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
– During the energy investment stage, phosphate groups
and energy from each of the two ATP are added to
glucose to produce fructose bisphosphate
– Fructose bisphosphate is broken down into two G3P
molecules
– During the energy harvesting stage, the two G3P
molecules are converted into two pyruvate molecules,
resulting in four ATP and two NADH molecules
– A net of two ATP molecules and two NADH (high-energy
electron carriers) are formed
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Animation: Glycolysis
The essentials of glycolysis
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Fig. 8-3
8.3 What Happens During Cellular Respiration?
 Cellular respiration breaks down the two pyruvate molecules into
six carbon dioxide molecules and six water molecules
– The chemical energy from the two pyruvate molecules aids in
the production of 32 ATP
– Cellular respiration occurs in mitochondria (powerhouses of
the cell), organelles specialized for the aerobic breakdown of
pyruvate
– Mitochondrion has two membranes
– 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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Figure 8-4 A mitochondrion
matrix
inner membrane
intermembrane space
outer membrane
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8.3 What Happens During Cellular Respiration?
 Cellular respiration occurs in three stages
1. Pyruvate breakdown
2. Transfer of electrons along the electron transport
chain
3. Generation of ATP by chemiosmosis
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8.3 What Happens During Cellular Respiration?
 During the first stage of cellular respiration, pyruvate
is broken down
– Pyruvate is synthesized in the cytoplasmic matrix
– Before cellular respiration can occur, the pyruvate is
actively transported from the cytoplasmic matrix to the
mitochondrial matrix
– The mitochondrial matrix is where cellular respiration
begins
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Figure 8-5 Reactions in the mitochondrial matrix
Formation of
acetyl CoA
coenzyme A
3 NADH
3 NAD
CO2
FADH2
coenzyme A
acetyl CoA
pyruvate
NAD
FAD
Krebs
cycle
NADH
ADP
ATP
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8.3 What Happens During Cellular Respiration?
 During the first stage of cellular respiration, pyruvate
is broken down (continued)
– 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?
 During the first stage of cellular respiration, pyruvate
is broken down (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?
 During the first stage of cellular respiration, pyruvate
is broken down (continued)
– The Krebs cycle
– Discovered by Hans Krebs
– Hans Krebs won the Nobel Prize in 1953 for his
discovery of the Krebs cycle
– The Krebs cycle is also known as the citric acid cycle
because citrate is produced first in the cycle
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8.3 What Happens During Cellular Respiration?
 During the first stage of cellular respiration, pyruvate
is broken down (continued)
– The Krebs cycle (continued)
– The Krebs cycle begins by combining acetyl CoA with a
four-carbon molecule to form six-carbon 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 four-carbon molecule
for use in future cycles
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8.3 What Happens During Cellular Respiration?
 During the first stage of cellular respiration, pyruvate
is broken down (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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8.3 What Happens During Cellular Respiration?
 During the first stage of cellular respiration, pyruvate
is broken down (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?
 During 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
– Depleted carriers are available for recharging by glycolysis and
the Krebs cycle
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8.3 What Happens During Cellular Respiration?
 During 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?
 During 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?
 During 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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Figure 8-6 The electron transport chain
(matrix)
ADP

P
FADH2
NADH
NAD
ETC
(intermembrane space)
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FAD
ATP
synthase
(inner
membrane)
ATP
8.3 What Happens During Cellular Respiration?
 During the third stage of cellular respiration,
chemiosmosis generates ATP
– 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?
 During the third stage of cellular respiration,
chemiosmosis generates ATP (continued)
– The energy present in this nonuniform 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?
 During the third stage of cellular respiration,
chemiosmosis generates ATP (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
– Without this continuous recycling, life would cease
– People produce, use, and then regenerate the
equivalent of their body weight of ATP daily
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8.3 What Happens During Cellular Respiration?
 Summing up cellular respiration
– In the mitochondrial matrix, each pyruvate molecule is
converted into acetyl CoA, producing one NADH per
pyruvate molecule and releasing one CO2
– As each acetyl CoA passes through the Krebs cycle,
its energy is captured in one ATP, three NADH, and
one FADH2. The carbons of acetyl CoA are released
in two CO2 molecules
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8.3 What Happens During Cellular Respiration?
 Summing up cellular respiration (continued)
– During cellular respiration, the two pyruvate molecules
enter the mitochondrion and are completely broken
down, yielding two ATP and ten high-energy electron
carriers: eight NADH and two FADH2. The carbon
atoms from the pyruvates are released in six
molecules of CO2
– High-energy electrons release energy that is
harnessed to pump H into the intermembrane space
as they pass through the ETC
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8.3 What Happens During Cellular Respiration?
 Summing up cellular respiration (continued)
– The NADH and FADH2 molecules donate their
energetic electrons to the ETC embedded in the inner
mitochondrial membrane
– 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 ATP
– Energy-depleted electrons exiting the ETC are picked
up by H+ released from NADH and FADH2, and
combine with oxygen to form water
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
BioFlix Animation: Summary of Cellular Respiration
Figure 8-7 The energy sources and ATP harvest from glycolysis and cellular respiration
1 glucose
(cytosol)
2 NADH
glycolysis
ATP
2
2 pyruvate
mitochondrion
(matrix)
CoA
2 NADH
2 CO2
2 acetyl CoA
6 NADH
Krebs
cycle
2
ATP
2 FADH2
4 CO2
O2
H2O
electron transport chain
32
ATP
Total: 36 ATP
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8.3 What Happens During Cellular Respiration?
 Cellular respiration can extract energy from a variety
of molecules
– Glucose often enters the human body as starch or
table sugar, but energy can come from the
consumption of fats and proteins in the diet
– Intermediate molecules of cellular respiration can be
formed by other metabolic pathways
– Molecules enter at appropriate stages and then are
broken down to produce ATP
– Amino acids of protein serve as energy sources
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8.3 What Happens During Cellular Respiration?
 Cellular respiration can extract energy from a variety
of molecules (continued)
– Fats are excellent sources of energy
– Serve as major energy-storage molecule in animals
– Fatty acids combine with CoA then are broken down to
produce acetyl CoA molecules, which enter the first
stage of the Krebs cycle
– A limited intake of fats will allow this process to happen
– Overindulgence of fats will cause the body to store
excess fat
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8.4 What Happens During Fermentation?
– Glycolysis is used by virtually every organism on
Earth
– Earlier forms of life appeared under the anaerobic (no
oxygen) conditions existing before photosynthesis
– Some microbes lack enzymes for cellular respiration
and rely solely on fermentation
– Various microorganisms still thrive in places where
oxygen is limited or absent
– Stomach and intestines of animals
– Deep in soil
– Bogs and marshes
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8.4 What Happens During Fermentation?
 Fermentation allows NAD to be recycled when
oxygen is absent
– 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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8.4 What Happens During Fermentation?
 Fermentation allows NAD to be recycled when
oxygen is absent (continued)
– Under anaerobic conditions, with no oxygen to allow
the ETC to function, the cell must regenerate the
NAD for glycolysis using fermentation
– Under aerobic (with oxygen) conditions, NADH
donates its high-energy 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
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8.4 What Happens During Fermentation?
 Fermentation allows NAD to be recycled when oxygen is
absent (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
– Organisms use one of two types of fermentation to regenerate
NAD
– Lactic acid fermentation
– Alcohol fermentation
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8.4 What Happens During Fermentation?
 Some cells ferment pyruvate to form lactate
– Lactic acid fermentation produces lactic acid from
pyruvate
– 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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Figure 8-8 Glycolysis followed by lactic acid fermentation
2
NAD
2 NADH
2
NADH
(glycolysis)
2
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ADP
2
NAD
(fermentation)
2 pyruvate
1 glucose
2
ATP
2 lactate
Figure 8-9a Fermentation in action
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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
– Generates alcohol and CO2 from pyruvate
– 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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Animation: Fermentation
Figure 8-10 Glycolysis followed by alcoholic fermentation
2
NAD
2 NADH
2 NADH
(glycolysis)
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2 pyruvate
ADP
NAD
(fermentation)
1 glucose
2
2
2
ATP
2 ethanol
2 CO2
Figure 8-9b Fermentation in action
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