Transcript Slide 1

Chapter 6
How Cells Harvest Chemical Energy
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
 In eukaryotes, cellular respiration
– harvests energy from food,
– yields large amounts of ATP, and
– Uses ATP to drive cellular work.
 A similar process takes place in many prokaryotic
organisms.
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Figure 6.0_1
Chapter 6: Big Ideas
Cellular Respiration:
Aerobic Harvesting
of Energy
Fermentation: Anaerobic
Harvesting of Energy
Stages of Cellular
Respiration
Connections Between
Metabolic Pathways
CELLULAR RESPIRATION:
AEROBIC HARVESTING
OF ENERGY
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6.1 Photosynthesis and cellular respiration
provide energy for life
 Life requires energy.
 In almost all ecosystems, energy ultimately comes
from the sun.
 In photosynthesis,
– some of the energy in sunlight is captured by
chloroplasts,
– atoms of carbon dioxide and water are rearranged, and
– glucose and oxygen are produced.
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6.1 Photosynthesis and cellular respiration
provide energy for life
 In cellular respiration
– glucose is broken down to carbon dioxide and water
and
– the cell captures some of the released energy to make
ATP.
 Cellular respiration takes place in the mitochondria
of eukaryotic cells.
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Figure 6.1
Sunlight energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2
Glucose
H2O
O2
Cellular respiration
in mitochondria
(for cellular
work)
ATP
Heat energy
6.3 Cellular respiration banks energy in ATP
molecules
 Cellular respiration is an exergonic process that
transfers energy from the bonds in glucose to form
ATP.
 Cellular respiration
– produces up to 32 ATP molecules from each glucose
molecule and
– captures only about 34% of the energy originally stored
in glucose.
 Other foods (organic molecules) can also be used
as a source of energy.
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Figure 6.3
C6H12O6
6
Glucose
Oxygen
O2
6 CO2
Carbon
dioxide
6
H2O
ATP
Water
 Heat
6.5 Cells tap energy from electrons “falling”
from organic fuels to oxygen
 The movement of electrons from one molecule to
another is an oxidation-reduction reaction, or
redox reaction. In a redox reaction,
– the loss of electrons from one substance is called
oxidation,
– the addition of electrons to another substance is called
reduction,
– a molecule is oxidized when it loses one or more
electrons, and
– reduced when it gains one or more electrons.
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6.5 Cells tap energy from electrons “falling”
from organic fuels to oxygen
 A cellular respiration equation is helpful to show
the changes in hydrogen atom distribution.
 Glucose
– loses its hydrogen atoms and
– becomes oxidized to CO2.
 Oxygen
– gains hydrogen atoms and
– becomes reduced to H2O.
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Figure 6.5A
Loss of hydrogen atoms
(becomes oxidized)
C6H12O6
6 O2
6 CO2
6 H2O
Glucose
Gain of hydrogen atoms
(becomes reduced)
ATP
 Heat
6.5 Cells tap energy from electrons “falling”
from organic fuels to oxygen
 Enzymes are necessary to oxidize glucose and
other foods.
 NAD+
– is an important enzyme in oxidizing glucose,
– accepts electrons, and
– becomes reduced to NADH.
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Figure 6.5B
Becomes oxidized
2H
Becomes reduced
NAD
2H
2 H
NADH
2
(carries
2 electrons)
H
6.5 Cells tap energy from electrons “falling”
from organic fuels to oxygen
 There are other electron “carrier” molecules that
function like NAD+.
– They form a staircase where the electrons pass from
one to the next down the staircase.
– These electron carriers collectively are called the
electron transport chain.
– As electrons are transported down the chain, ATP is
generated.
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Figure 6.5C
NADH
NAD
ATP
2
Controlled
release of
energy for
synthesis
of ATP
H
2
1 O
2 2
2 H
H 2O
STAGES OF CELLULAR
RESPIRATION
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6.6 Overview: Cellular respiration occurs in
three main stages
 Cellular respiration consists of a sequence of steps
that can be divided into three stages.
– Stage 1 – Glycolysis
– Stage 2 – Pyruvate oxidation and citric acid cycle
– Stage 3 – Oxidative phosphorylation
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6.6 Overview: Cellular respiration occurs in
three main stages
 Stage 1: Glycolysis
– occurs in the cytoplasm,
– begins cellular respiration, and
– breaks down glucose into two molecules of a threecarbon compound called pyruvate.
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6.6 Overview: Cellular respiration occurs in
three main stages
 Stage 2: The citric acid cycle
– takes place in mitochondria,
– oxidizes pyruvate to a two-carbon compound, and
– supplies the third stage with electrons.
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6.6 Overview: Cellular respiration occurs in
three main stages
 Stage 3: Oxidative phosphorylation
– involves electrons carried by NADH and FADH2,
– shuttles these electrons to the electron transport chain
embedded in the inner mitochondrial membrane,
– involves chemiosmosis, and
– generates ATP through oxidative phosphorylation
associated with chemiosmosis.
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Figure 6.6
CYTOPLASM
NADH
Electrons
carried by NADH
NADH
Glycolysis
Glucose
Pyruvate
Pyruvate
Oxidation
Citric Acid
Cycle
FADH2
Oxidative
Phosphorylation
(electron transport
and chemiosmosis)
Mitochondrion
ATP
Substrate-level
phosphorylation
ATP
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
6.7 Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
 In glycolysis,
– a single molecule of glucose is enzymatically cut in half
through a series of steps,
– two molecules of pyruvate are produced,
– two molecules of NAD+ are reduced to two molecules of
NADH, and
– a net of two molecules of ATP is produced.
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Figure 6.7A
Glucose
2 ADP
2 NAD
2 P
2 NADH
2
ATP
2 Pyruvate
2 H
6.7 Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
 ATP is formed in glycolysis by substrate-level
phosphorylation during which
– an enzyme transfers a phosphate group from a
substrate molecule to ADP and
– ATP is formed.
 The compounds that form between the initial
reactant, glucose, and the final product, pyruvate,
are called intermediates.
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Figure 6.7B
Enzyme
P
Enzyme
ADP
ATP
P
P
Substrate
Product
6.8 Pyruvate is oxidized prior to the citric acid
cycle
 The pyruvate formed in glycolysis is transported
from the cytoplasm into a mitochondrion where
– the citric acid cycle and
– oxidative phosphorylation will occur.
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6.8 Pyruvate is oxidized prior to the citric acid
cycle
 Two molecules of pyruvate are produced for each
molecule of glucose that enters glycolysis.
 Pyruvate does not enter the citric acid cycle, but
undergoes some chemical grooming in which
– a carboxyl group is removed and given off as CO2,
– the two-carbon compound remaining is oxidized while a
molecule of NAD+ is reduced to NADH,
– coenzyme A joins with the two-carbon group to form
acetyl coenzyme A, abbreviated as acetyl CoA, and
– acetyl CoA enters the citric acid cycle.
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Figure 6.8
NAD
NADH
H
2
CoA
Pyruvate
Acetyl coenzyme A
1
CO2
3
Coenzyme A
6.9 The citric acid cycle completes the oxidation of
organic molecules, generating many NADH
and FADH2 molecules
 The citric acid cycle
– is also called the Krebs cycle (after the German-British
researcher Hans Krebs, who worked out much of this
pathway in the 1930s),
– completes the oxidation of organic molecules, and
– generates many NADH and FADH2 molecules.
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Figure 6.9A
Acetyl CoA
CoA
CoA
2 CO2
Citric Acid Cycle
3 NAD
FADH2
3 NADH
FAD
3 H
ATP
ADP
P
6.9 The citric acid cycle completes the oxidation of
organic molecules, generating many NADH
and FADH2 molecules
 During the citric acid cycle
– the two-carbon group of acetyl CoA is added to a fourcarbon compound, forming citrate,
– citrate is degraded back to the four-carbon compound,
– two CO2 are released, and
– 1 ATP, 3 NADH, and 1 FADH2 are produced.
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6.9 The citric acid cycle completes the oxidation of
organic molecules, generating many NADH
and FADH2 molecules
 Remember that the citric acid cycle processes two
molecules of acetyl CoA for each initial glucose.
 Thus, after two turns of the citric acid cycle, the
overall yield per glucose molecule is
– 2 ATP,
– 6 NADH, and
– 2 FADH2.
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6.10 Most ATP production occurs by oxidative
phosphorylation
 Oxidative phosphorylation
– involves electron transport and chemiosmosis and
– requires an adequate supply of oxygen.
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6.10 Most ATP production occurs by oxidative
phosphorylation
 Electrons from NADH and FADH2 travel down the
electron transport chain to O2.
 Oxygen picks up H+ to form water.
 Energy released by these redox reactions is used
to pump H+ from the mitochondrial matrix into the
intermembrane space.
 In chemiosmosis, the H+ diffuses back across the
inner membrane through ATP synthase
complexes, driving the synthesis of ATP.
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Figure 6.10
H
Intermembrane
space
H
H
H
H Mobile
electron
carriers
Protein
complex
of electron
carriers
H ATP
synthase
IV
I
II
FADH2
Electron
flow
NADH
Mitochondrial
matrix
H
H
III
Inner mitochondrial
membrane
H
NAD
FAD
2 H
1
2 O2
H2O
H
ADP
P
ATP
H
Electron Transport Chain
Oxidative Phosphorylation
Chemiosmosis
6.12 Review: Each molecule of glucose yields
many molecules of ATP
 Recall that the energy payoff of cellular respiration
involves
1. glycolysis,
2. alteration of pyruvate,
3. the citric acid cycle, and
4. oxidative phosphorylation.
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6.12 Review: Each molecule of glucose yields
many molecules of ATP
 The total yield is about 32 ATP molecules per
glucose molecule.
 This is about 34% of the potential energy of a
glucose molecule.
 In addition, water and CO2 are produced.
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Figure 6.12
CYTOPLASM
Electron shuttles
across membrane
2 NADH
Mitochondrion
2 NADH
or
2 FADH2
6 NADH
2 NADH
Glycolysis
2
Pyruvate
Glucose
Pyruvate
Oxidation
2 Acetyl
CoA
Citric Acid
Cycle
2 FADH2
Oxidative
Phosphorylation
(electron transport
and chemiosmosis)
Maximum
per glucose:
2
ATP
by substrate-level
phosphorylation
2
ATP
by substrate-level
phosphorylation
 about
28 ATP
by oxidative
phosphorylation
About
32 ATP
FERMENTATION: ANAEROBIC
HARVESTING OF ENERGY
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6.13 Fermentation enables cells to produce ATP
without oxygen
 Fermentation is a way of harvesting chemical energy
that does not require oxygen. Fermentation
– takes advantage of glycolysis,
– produces two ATP molecules per glucose, and
– reduces NAD+ to NADH.
 The trick of fermentation is to provide an anaerobic
path for recycling NADH back to NAD+.
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6.13 Fermentation enables cells to produce ATP
without oxygen
 Your muscle cells and certain bacteria can oxidize
NADH through lactic acid fermentation, in which
– NADH is oxidized to NAD+ and
– pyruvate is reduced to lactate.
Animation: Fermentation Overview
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Figure 6.13A
2 ADP
2 P
2 ATP
Glycolysis
Glucose
2 NAD
2 NADH
2 Pyruvate
2 NADH
2 NAD
2 Lactate
6.13 Fermentation enables cells to produce ATP
without oxygen
 The baking and winemaking industries have used
alcohol fermentation for thousands of years.
 In this process yeasts (single-celled fungi)
– oxidize NADH back to NAD+ and
– convert pyruvate to CO2 and ethanol.
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Figure 6.13B
Glucose
2 NAD
Glycolysis
2 ADP
2 P
2 ATP
2 NADH
2 Pyruvate
2 NADH
2 CO2
2 NAD
2 Ethanol
CONNECTIONS BETWEEN
METABOLIC PATHWAYS
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6.15 Cells use many kinds of organic molecules
as fuel for cellular respiration
 Although glucose is considered to be the primary
source of sugar for respiration and fermentation,
ATP is generated using
– carbohydrates,
– fats, and
– proteins.
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6.15 Cells use many kinds of organic molecules
as fuel for cellular respiration
 Fats make excellent cellular fuel because they
– contain many hydrogen atoms and thus many energyrich electrons and
– yield more than twice as much ATP per gram than a
gram of carbohydrate or protein.
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Figure 6.15
Food, such as
peanuts
Carbohydrates
Sugars
Fats
Proteins
Glycerol Fatty acids
Amino acids
Amino
groups
Glucose
G3P
Pyruvate
Glycolysis
Pyruvate
Oxidation
Acetyl CoA
ATP
Citric
Acid
Cycle
Oxidative
Phosphorylation
Figure 6.16
ATP needed
to drive
biosynthesis
Citric
Acid
Cycle
ATP
Pyruvate
Oxidation
Acetyl CoA
Glucose Synthesis
Pyruvate
G3P
Glucose
Amino
groups
Amino acids
Proteins
Fatty acids Glycerol
Fats
Cells, tissues, organisms
Cells use intermediates from cellular
respiration for the biosynthesis of
other organic molecules.
Sugars
Carbohydrates
Figure 6.UN02
Cellular
respiration
has three stages
generates
oxidizes
uses
produce
some
produces
many
energy for
glucose and
organic fuels
(a)
(b)
(d)
to pull
electrons down
(c)
cellular work
(f)
by a process called
chemiosmosis
uses
(g)
H diffuse
through
ATP synthase (e)
uses
pumps H to create
H gradient
to