Download CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL

CHAPTER 9
CELLULAR RESPIRATION:
HARVESTING CHEMICAL ENERGY
1. Catabolic pathways yield energy by Oxidizing Organic
Fuels.
2. Glycolysis harvests chemical energy by oxidizing
glucose to pyruvate.
3. The citric acid cycle completes the energy-yielding
oxidation of organic molecules.
4. During
g Oxidative Phosphorylation,
y
chemiosmosis
couples electron transport to ATP synthesis.
Life is work
•
cells require transfusions of
energy from outside sources to
perform
f
their
h i tasks.
k
– In most ecosystems, energy
enters as sunlight
sunlight.
– Light energy trapped in
organic
g
molecules is
available to both
photosynthetic organisms and
others that eat them.
them
Fig. 9.2
Catabolic pathways yield energy by Oxidizing Organic
Fuels
Catabolic Pathways and Production of ATP
• Organic
O
i molecules
l l store energy in
i their
h i arrangement off atoms.
• Enzymes catalyze the degradation of organic molecules that are
rich in energy to simpler waste products with less energy.
• Some of the released energy is used to do work and the rest is
di i
dissipated
d as heat.
h
• Cellular respiration is a catabolic process in which oxygen is
usedd as a reactant
t t to
t complete
l t the
th breakdown
b kd
off a variety
i t off
organic molecules.
– Most of the processes in cellular respiration occur in
mitochondria.
• One type of catabolic process, fermentation, leads to the
partial degradation of sugars in the absence of oxygen
oxygen.
• Carbohydrates, fats, and proteins can all be used as the fuel, but
it is traditional to start learning with glucose.
C6H12O6 + 6O2
6CO2 + 6H2O + Energy (ATP + heat)
• The catabolism of glucose is exergonic with :
a ∆G of - 686 kcal / mole of glucose.
– Some of this energy is used to produce ATP that will perform
cellular work
• The price of most cellular work is the conversion of ATP to ADP
and inorganic phosphate (Pi).
• An animal cell regenerates ATP from ADP and Pi by the
catabolism of organic molecules.
• The transfer of the terminal phosphate group from ATP to another
molecule is phosphorylation.
– This changes the shape of the receiving molecule, performing
work (transport, mechanical, or chemical).
– When the phosphate group leaves the molecule,
the molecule returns to its alternate shape. Fig 9.2.
Fig.9.2
Adenosine denosine TTri
ri‐‐Phosphate (
hosphate (ATP
ATP))
Adenosine
P
P
+ H2O
P
Triphosphate
Energy
Pi
+
P
P
Adenosine Di‐Phosphate(ADP)
٧
Fig. 6.8
Redox Reactions: Oxidation and Reduction
• Catabolic pathways relocate the electrons stored in food
molecules releasing energy that is used to synthesize ATP.
molecules,
ATP
• Reactions that result in the transfer of one or more electrons from
one reactant to another are oxidation-reduction reactions, or
redox reactions.
– The loss of electrons is called oxidation.
– The addition of electrons is called reduction.
• O
Oxygen
yg is one of the most ppotent oxidizing
g agents.
g
• An electron looses energy as it shifts from a less electronegative
atom to a more electronegative one.
• A redox reaction that relocates electrons closer to oxygen releases
chemical energy that can do work.
• The formation of table salt from sodium and chloride is a redox
reaction.
– Na + Cl
Na+ + Cl-
– Here sodium is oxidized and chlorine is reduced (its charge
drops from 0 to -1).
• More generally: Xe- + Y
X + Ye-
– X, the electron donor, is the reducing agent, reduces Y and
it self is oxidized.
– Y, the electron recipient, is the oxidizing agent, oxidizes X
and itself is reduced.
• Redox reactions require both a donor and acceptor.
Oxidation of Organic Fuel Molecules During Cellular
Respiration
•
In cellular respiration, glucose and other fuel molecules are oxidized,
releasing energy.
•
Cellular respiration:
C6H12O6 + 6O2
6CO2 + 6H2O
•
Glucose is oxidized, oxygen is reduced, and electrons loose potential
energy.
•
Molecules that have an abundance of hydrogen(carbohydrates and fats)
are excellent fuels because their bonds are a source of “hilltop” electrons
th t “fall”
that
“f ll” closer
l
to
t oxygen.
•
However, these fuels do not spontaneously combine with O2 because they
lack the activation energy
energy.
•
Enzymes lower the barrier of activation energy, allowing these fuels to be
oxidized slowly.
Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
• Cellular respiration does not oxidize glucose in a single
step that transfers all the hydrogen in the fuel to oxygen
at one time.
• Rather
Rather, glucose and other fuels are broken down
gradually in a series of steps, each catalyzed by a
specific
p
enzyme.
y
• At key steps, hydrogen atoms are stripped from glucose
and passed first to a coenzyme,
coenzyme like NAD+ (nicotinamide
adenine dinucleotide).
• During the oxidation of glucose:
– Dehydrogenase enzymes strip two hydrogen
atoms from the glucose (2 electrones and 2
protons).
– pass two
t electrons
l t
andd one proton
t to
t its
it
coenzyme NAD+.
– Release the remaining proton (H+) into the
surrounding.
Oxidation
– H-C-OH +
NAD+
dehydrogenae
C=O + NADH + H+
Reduction
• This changes the oxidized form, NAD+, to the reduced
form NADH.
• NAD + functions as the oxidizing agent in many of the
redox steps during the catabolism of glucose.
• The electrons carried by NADH loose very little of
their potential energy in this process.
• This energy is stored in NADH to synthesize ATP as
electrons “fall”
fall from NADH to oxygen.
oxygen
•Unlike the explosive release of heat energy that would
occur when H2 and O2 combine, cellular respiration
p
uses
an electron transport chain to break the fall of electrons
to O2 into several steps.
p
Fig. 9.5
• The electron transport chain, consisting of several
molecules (primarily proteins),
proteins) is built into the inner
membrane of a mitochondrion.
• NADH shuttles
h l electrons
l
ffrom ffood
d to the
h ““top”” off the
h
chain.
• At the “bottom”, oxygen captures the electrons and H+
to form water.
• The free energy change from “top” to “bottom” is -53
kcal/mole of NADH.
• Electrons are passed by increasingly electronegative
molecules in the chain until they are caught by oxygen,
oxygen
the most electronegative.
During cellular respiration, most electrons travel this downhill pathway:
Food
NADH
electron transport chain
oxygen
Stages of Cellular respiration
The three Stages of Cellular Respiration are :
1. Glycolysis is the process that harvest chemical energy by
oxidizing glucose to pyruvate .
2.The Citric acid cycle is the process that completes the
energy-yielding oxidation of organic molecules.
3. During oxidative phosphorylation, chemiosmosis couples
electron transport to ATP synthesis .
The Stages of Cellular Respiration:
a Preview
• Respiration occurs
in three metabolic
stages:
1. Glycolysis
2. The Citric Acid Cycle
3. Oxidative
phosphorylation:
electron transport
p
and
chemiosmosis
Fi 9.6
Fig.
96
1.Glycolysis:
– occurs in the cytosol
cytosol.
– It doesn’t require O2.
– It begins
b i catabolism
t b li
b
by b
breaking
ki glucose
l
iinto
t ttwo
molecules of pyruvate.
2 Citric Acid cycle:
2.
– occurs in the mitochondrial matrix.
– It is a catabolic pathway that degrades pyruvate to
carbon dioxide.
• Several steps in glycolysis and the citric acid cycle
transfer electrons from substrates to NAD+, forming
NADH.
• NADH passes these electrons to the electron transport
chain.
chain
3. Electron transport chain:
− the electrons move from molecule to molecule until
theyy combine with oxygen
yg and hydrogen
y g ions to form
water.
− the energy released at each step of the chain is stored in
the mitochondrion in a form that can be used to make
ATP.
ATP
• The mode of ATP synthesis is called oxidative
phosphorylation:
•
– Oxidative phosphorylation produces almost 90% of the ATP generated
by respiration.
Some ATP is also generated in glycolysis and the citric acid cycle by
substrate-level phosphorylation.
– Here an enzyme
y
transfers a phosphate
group from an
organic
g
molecule
(the substrate)
to ADP, forming
ATP.
• Ultimately 38 ATP are produced
per a molecule of glucose that
is degraded to carbon dioxide and
water by respiration.
Fig. 9.7
1. Glycolysis harvests chemical energy by oxidizing
glucose
l
to
t pyruvate
t
•
During glycolysis, glucose, a six-carbon sugar, is split into
two, three-carbon sugars.
•
These smaller
Th
ll sugars are oxidized
idi d and
d rearranged
d tto fform
two molecules of pyruvate.
•
Each
E
h off the
th ten
t steps
t
i glycolysis
in
l
l i iis catalyzed
t l
db
by a
specific enzyme.
•
These steps
Th
t
can be
b divided
di id d iinto
t two
t
phases:
h
1. energy investment phase
2. energy payoff phase.
• In the energy investment phase, ATP provides activation
energy by phosphorylating glucose.
– This requires 2 ATP per glucose.
• In the energy payoff
phase, ATP is
produced by
substrate-level
phosphorylation
and NAD+ is
reduced to NADH.
• 2 ATP ((net)) and
2 NADH are produced
per glucose.
Fig. 9.8
Fig. 9.9a
Energy Investment Phase (steps 1-5)
Fig. 9.9b
Energy-Payoff Phase (Steps 6-10)
• The net yield from glycolysis is 2 ATP and 2 NADH
per glucose.
l
• No CO2 is pproduced during
g gglycolysis.
y y
• Glycolysis occurs whether O2 is present or not.
– If O2 is present, pyruvate moves to the citric acid
cycle
y and the energy
gy stored in NADH can be
converted to ATP by the electron transport system
and oxidative phosphorylation.
p p y
2. The Citric acid cycle (Krebs cycle)
•
•
If oxygen is present
present, pyruvate enters the mitochondrion where enzymes of
the citric acid cycle complete the oxidation of the organic fuel to carbon
dioxide.
As pyruvate enters the mitochondrion, a multienzyme complex modifies
pyruvate to acetyl CoA which enters the Krebs cycle in the matrix:
– A carboxyl group is removed as CO2.
– A pair of electrons is transferred from the remaining two-carbon
fragment to NAD+ to form NADH.
– The oxidized
fragment, acetate,
combines with
coenzyme A to
form acetyl CoA.
Fig. 9.10
– This cycle begins when acetate from acetyl CoA
combines
bi
with
ith oxaloacetate
l
t t to
t form
f
citrate.
it t
– The oxaloacetate is recycled
y
and the acetate is
broken down to CO2.
–E
Eachh cycle
l produces
d
one ATP by
b substrate-level
bt t l l
phosphorylation, three NADH, and one FADH2
( th electron
(another
l t
carrier)
i ) per acetyl
t l CoA.
C A
• The conversion of
pyruvate
t andd th
the Citirc
Citi
Acid Cycle produces
l
large
quantities
titi off
electron carriers:
6 NADH
2 FADH2
per glucose molecule
Fig. 9.12
3.Oxidative phosphorylation:
( chemiosmosis couples electron transport to ATP synthesis)
•
The majority of the ATP comes from the energy in the electrons carried by
NADH and
d (FADH2).
)
•
This energy is used in the electron transport system to power ATP
synthesis.
synthesis
•
Thousands of copies of the electron transport chain (ETC) are found in the
extensive surface of the cristae,, the inner membrane of the mitochondrion.
– Most components of the chain are proteins that can alternate between
reduced and oxidized states as they accept and donate electrons.
•
Electrons drop in free energy as they pass down the electron transport
chain.
• Electrons carried by NADH are
transferred to the first molecule
in the electron transport chain,
flavoprotein.
– The
h electrons
l
continue
i
along the chain which
includes several
cytochrome proteins and
one lipid carrier.
• The electrons carried by
FADH2 have lower free energy
and are added to a later point in
the chain.
Fig. 9.13
• Cytochrome = Type of protein molecule that contains
a heme
h
which
hi h acts
t as an electron
l t
carrier
i in
i the
th
electron transport chains of mitochondria .
• Electrons from NADH or FADH2 ultimately pass to
oxygen.
– For every two electron carriers (four electrons), one
O2 molecule is reduced to two molecules of water.
• The electron transport chain generates no ATP directly.
• The movement of electrons along the electron
transport chain does contribute to ATP synthesis.
ATP Production by Cellular Respiration
• During respiration, most energy flows from:
glucose
NADH
electron transport chain
proton-motive
t
ti force
f
ATP
ATP.
• Considering the fate of carbon, one six-carbon glucose molecule is
oxidized to six CO2 molecules.
•
Each NADH from the Citric Acid cycle and the conversion of
pyruvate generate a maximum of 3 ATP .
• The NADH from gglycolysis
y y mayy also yield
y
3 ATP.
• Each FADH2 from the Citric Acid cycle generate about 2ATP.
Process ATP Produced
Directly
y by
y
Substrate-Level
Phosphorylation
Glycolysis
Net 2 ATP
Oxidation
of
Pyruvate
Citric
Acid Cycle
2 ATP
Reduced
Coenzyme
y
ATP Produced by
Oxidative
Phosphorylation
Total
2 NADH
6 ATP
8
2 NADH
6 ATP
6
6 NADH
2 FADH2
18 ATP
4 ATP
24
38
Fig. 9.16
ATP-synthase,an
ATPsynthase,an enzyme in the cristae of the mitochondria that makes ATP from ADP and
Pi.
ATP--synthase uses the energy of an existing proton gradient to power ATP synthesis.
ATP
This proton gradient develops between the intermembrane space and the matrix.
proton--motive force
force..
This difference in the concentration of H+ is the proton
The p
power source for the ATP synthase
y
is a difference in the concentration of H+
on the opposite sides of the inner mitochondrial membrane.
The process ,in
in which energy stored in the form of
a hydrogen ion gradient (difference in
concentration on the two sides) across a membrane
i used
is
d to
t drive
d i cellular
ll l work
k such
h as ATP ,is
i called
ll d
CHEMIOSMOSIS
Fig. 9.14
Energy of NADH and FADH2 give a maximum yield of ATP p
production by oxidative production by y oxidative phosphorylation
phosphorylation.
p
p y
.
Fig. 9.15
٣٧
Fermentation:
Some cells produce ATP without the use of oxygen
• Oxidation refers to the loss of electrons to any electron acceptor,
acceptor
not just to oxygen.
– In gglycolysis,
y y , gglucose is oxidized to two py
pyruvate molecules
with NAD+ as the oxidizing agent, not O2.
– Some energy from this oxidation produces 2 ATP (net).
– If oxygen is present, additional ATP can be generated when
NADH delivers its electrons to the electron transport chain.
• Glycolysis
Gl l i generates
t 2 ATP whether
h th oxygen iis presentt (aerobic)
(
bi )
or not (anaerobic).
• Anaerobic catabolism of sugars can occur by
fermentation.
• Fermentation can generate ATP from glucose by
substrate-level
substrate
level phosphorylation as long as there is a
supply of NAD+ to accept electrons.
– If the NAD+ pool is exhausted, glycolysis shuts
down.
– Under aerobic conditions, NADH transfers its
electrons to the electron transfer chain, recycling
NAD+.
• Under anaerobic conditions, various fermentation
pathways generate ATP by glycolysis and recycle
NAD+ by transferring electrons from NADH to
pyruvate or derivatives of pyruvate.
• In alcohol fermentation, pyruvate is converted to ethanol in
two steps.
steps
– First, pyruvate is converted to a two-carbon compound,
acetaldehyde by the removal of CO2.
– Second, acetaldehyde is reduced by NADH to ethanol.
– Alcohol fermentation
by yeast is used in
brewing and
winemaking.
Fig. 9.17a
• During lactic acid fermentation, pyruvate is reduced directly
by NADH to form lactate (ionized form of lactic acid).
– Lactic acid fermentation by some fungi and bacteria is used
to make cheese and yogurt.
– Muscle cells switch from aerobic respiration to lactic acid
fermentation to generate ATP when O2 is scarce.
• The waste product,
product
lactate, may cause
muscle fatigue, but
it is converted back to
pyruvate in the liver.
Fig. 9.17b
Some organisms (
Some organisms (facultative anaerobes facultative anaerobes , , including yeast and many bacteria, can survive using either fermentation or respiration.
At a cellular level, human muscle cells can b h
behave as facultative anaerobes, but f l i
b b
nerve cells cannot.
Fig. 9.18