Cell Metabolism
3/23/2016
Organisms live at the expense of free energy
The maximum amount of usable energy that can be harvested from
a particular reaction is the system’s free energy change from the
initial to the final state.
This change in free energy (ΔG) is given by the Gibbs-Helmholtz
equation at constant temperature and pressure:
ΔG = ΔH – TΔS
Where:
ΔG = change in free energy
ΔH = change in total energy (enthalpy)
ΔS = change in entropy
T = absolute temperature in K (C+273)
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Free energy and metabolism
Exergonic reaction
Chemical products have
less free energy than the
reactant molecules.
Endergonic reaction
Products store more free
energy than reactants
Reaction is energetically
downhill.
Reaction is energetically uphill.
Spontaneous reaction.
ΔG is negative.
Non-spontaneous reaction
(requires energy input)
ΔG is positive.
-ΔG is the maximum
amount of work the reaction
can perform
+ΔG is the minimum amount of
work required to drive the
reaction.
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An energy profile of a reaction
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Comparison of passive and active transport
In passive transport,
a substance diffuses
spontaneously down its
concentration gradient
with no need for the cell
to expend energy.
Hydrophobic molecules
and very small
uncharged polar
molecules diffuse directly
across the membrane.
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Comparison of passive and active transport
Hydrophilic substances
diffuse through transport
proteins in a process
called facilitated diffusion.
In active transport, a
transport protein moves
substances across the
membrane "uphill" against
their concentration
gradients.
Active transport requires
an expenditure of energy,
usually supplied by ATP.
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An electrogenic pump.
Proton pumps are examples of membrane proteins that store
energy by generating voltage (charge separation) across
membranes.
Using ATP for
power, a proton
pump
translocates
positive charge
in the form of
hydrogen ions.
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An electrogenic pump.
The voltage and H+ gradient represent a dual energy source
that can be tapped by the cell to drive other processes, such
as the uptake of sugar and other nutrients.
Proton pumps
are the main
electrogenic
pumps of
plants, fungi,
and bacteria.
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Cotransport.
An ATP-driven pump stores energy by concentrating a
substance (H+, in this case) on one side of the membrane.
As the substance
leaks back
across the membrane
through specific
transport proteins,
it escorts other
substances into the
cell.
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Cotransport.
The proton pump of the membrane is indirectly driving
sucrose accumulation by a plant cell,
with the help of a
protein that
cotransports the two
solutes.
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Energy flow and
chemical recycling in
ecosystems
The mitochondria of eukaryotes
(including plants)
use the organic products of
photosynthesis
as fuel for cellular respiration,
which also consumes the
oxygen produced by
photosynthesis.
Respiration harvests the energy
stored in organic molecules
to generate ATP, which powers
most cellular work.
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Energy flow and
chemical recycling in
ecosystems
The waste products of
respiration,
carbon dioxide and water,
are the very substances that
chloroplasts use as raw
materials for photosynthesis.
Thus, the chemical elements
essential to life are recycled.
But energy is not: It flows into
an ecosystem as sunlight and
leaves it as heat.
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How ATP drives cellular work
Phosphate-group transfer is the mechanism responsible for
most types of cellular work.
Enzymes shift a
phosphate group (P)
from ATP to some
other molecule,
and this
phosphorylated
molecule undergoes a
change that performs
work.
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How ATP drives
cellular work
For example, ATP drives
active transport by
phosphorylating
specialized proteins built
into membranes;
drives mechanical work by
phosphorylating motor
proteins, such as the ones
that move organelles along
cytoskeletal "tracks" in the
cell;
and drives chemical work by phosphorylating key reactants.
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How ATP drives
cellular work
The phosphorylated
molecules lose the
phosphate groups as
work is performed,
leaving ADP and
inorganic phosphate as
products.
Cellular respiration
replenishes the ATP
supply by powering the
phosphorylation of ADP.
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Cellular respiration and fermentation are catabolic
Fermentation – an ATP-producing catabolic pathway in which
both electron donors and acceptors are organic compounds.
Can be an anaerobic process
Results in partial degradation of sugars
Cellular respiration – an ATP-producing catabolic process in
which the ultimate electron acceptor is an inorganic molecule,
such as oxygen.
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Cellular respiration
Most prevalent and efficient catabolic pathway
Is an exergonic process (ΔG = -2870 kJ/mol or -686 kcal/mol)
Can be summarized as:
Organic compounds + Oxygen  Carbon dioxide + Water +
Energy
Carbohydrates, proteins, and fats can all be metabolized as fuel,
but cellular respiration is most often described as the oxidation of
glucose:
C6H12O6 +6O2  6CO2 + 6 H2O + Energy (ATP + Heat)
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Cellular respiration
An introduction to redox reactions
Oxidation-reduction reactions – chemical reactions which
involve a partial or complete transfer of electrons from one
reactant to another; called redox reactions for short.
Oxidation – partial or complete loss of electrons
Reduction – partial or complete gain of electrons
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Cellular respiration and fermentation are catabolic
Generalized redox reaction:
Electron transfer requires both a donor and acceptor, so when
one reactant is oxidized the other is reduced.
Xe- + Y  X + Ye-
X = substance being oxidized, acts as
reducing agent because it reduces Y
Y = substance being reduced; acts as an
oxidizing agent because it oxidizes X.
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Methane combustion as an energy-yielding
redox reaction
During the reaction, covalently shared electrons move away from
carbon and hydrogen atoms and closer to oxygen, which is very
electronegative.
The reaction releases energy to the surroundings, because the
electrons lose potential energy as they move closer to
electronegative atoms.
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NAD+ as an electron shuttle.
Nicotinamide adenine dinucleotide molecule consists of two
nucleotides joined together.
The enzymatic transfer of two electrons and one proton from
some organic substrate to NAD+ reduces the NAD+ to NADH.
Most of the electrons removed from food are transferred initially to
NAD+.
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NAD+ as an electron shuttle.
During the oxidation of glucose, NAD+ functions as an oxidizing
agent by trapping energy-rich electrons from glucose or food.
These reactions are catalyzed by enzymes called
dehydrogenases, which:
Remove a pair of hydrogen atoms (two electrons and two
protons) from substrate
Deliver the two electrons and one proton to NAD+
Release the remaining proton into the surrounding solution
The high energy electrons transferred from substrate to NAD+ are
then passed down the electron transport chain to oxygen,
powering ATP synthesis (oxidative phosphorylation).
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Electron transport chains
Electron transport chains convert some of the chemical energy
extracted from food to a form that can be used to make ATP.
Are composed of electron-carrier molecules built into the inner
mitochondrial membrane.
Structure of this membrane correlates with its functional role.
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Electron transport chains
Accept energy-rich electrons from reduced coenzymes (NADH
and FADH2); and pass these electrons down the chain to
oxygen, the final electron acceptor.
The electronegative oxygen accepts these electrons, along
with hydrogen nuclei, to form water.
Release energy from energy-rich electrons in a controlled
stepwise fashion
Since electrons lose potential energy when they shift toward a
more electronegative atom, this series of redox reactions
releases energy.
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An introduction to electron transport chains.
(a) The exergonic reaction of hydrogen with oxygen to form
water releases a large amount of energy in the form of heat
and light: an explosion.
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Electron
transport
chains.
(b) In cellular
respiration, an
electron transport
chain breaks the "fall"
of electrons in this
reaction into a series
of smaller steps
It stores some of the released energy in a form that can be
used to make ATP (the rest of the energy is released as
heat).
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Cellular Respiration
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Respiration
is a cumulative function of three metabolic
stages:
1. Glycolysis
2. The Krebs cycle
3. The electron transport chain and
oxidative phosphorylation
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Respiration
is a cumulative function of three metabolic
stages:
Glycolysis is a catabolic pathway
that:
Occurs in the cytosol
Partially oxidizes glucose
(6C) into two pyruvate
(3C) molecules.
The Krebs cycle is a catabolic
pathway that:
Occurs in the
mitochondrial matrix
Completes glucose
oxidation by breaking
down a pyruvate
derivative (acetyl CoA)
into carbon dioxide
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Respiration
is a cumulative function of three metabolic
stages:
Glycolysis and the Krebs cycle produce:
A small amount of ATP by substrate-level
phosphorylation
NADH by transferring electrons from substrate to
NAD+ (Krebs cycle also produces FADH2 by
transferring electrons to FAD+)
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Respiration
is a cumulative function of three metabolic
stages:
The electron transport chain:
Is located at the inner membrane of the mitochondrion
Accepts energized electrons from reduced coenzymes
(NADH and FADH2) that are harvested during glycolysis
and Krebs cycle.
Couples this exergonic slide of electrons to ATP synthesis
or oxidative phosphorylation. This process produces most
(90%) of the ATP.
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GLYCOLYSIS
During glycolysis, each glucose molecule is broken down into
two molecules of the compound pyruvate.
The pyruvate crosses the double membrane of the
mitochondrion to enter the matrix, where the Krebs cycle
decomposes it to carbon dioxide.
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Respiration
is a cumulative function of three metabolic
stages:
The electron transport chain converts the chemical energy to a
form that can be used to drive oxidative phosphorylation, which
accounts for most of the ATP generated by cellular respiration.
A smaller amount of ATP is formed directly during glycolysis
and the Krebs cycle by substrate-level phosphorylation.
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Substrate-level
phosphorylation
Enzyme
OC =O
C
=
O
CH2
OC =O
C
P
P
P
P
Phosphoenolpyruvate
(PEP) is formed from
breakdown of sugar
during glycolysis
P
Substrate
(PEP)
+
ADP
Some ATP is made by
direct enzymatic
transfer of P group
from a substrate to
ADP.
P
Adenosine
Adenosine
=
O
CH3
Product
(pyruvate)
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ATP
Glycolysis
Harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis – catabolic pathway during which six-carbon
glucose is split into two three-carbon sugars, which are
then oxidized and rearranged by a step-wise process that
produces two pyruvate molecules.
Each reaction is catalyzed by specific enzymes
dissolved in the cytosol.
No CO2 is released as glucose is oxidized to
pyruvate; all carbon in glucose can be accounted
for in the two molecules of pyruvate.
Occurs whether or not oxygen is present.
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The energy input and
output of glycolysis
Electrons carried
via NADH
And FADH2
GLYCOLYSIS
Krebs
cycle
ATP
ATP
Electron transport
chain and oxidative
phosphorylation
Glucose Energy-investment phase
ATP
2 ADP
2ATP
Energy-payoff phase
4ADP
2NAD+
4ATP
2NADH
2Pyruvate
NET
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Glucose
2Pyruvate + 2H2O
2ADP +2 P i
2ATP
2NAD+
2NADH +2H+
Glycolisis:
steps 1-5
The orientation diagram
at the right relates
glycolysis to the whole
process of respiration.
Steps 1-5 are the
energy-investment phase
of glycolysis.
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Steps 6-10 are the energy-payoff phase of
glycolysis.
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Glycolisis
Ten reactions, each catalyzed by a specific enzyme, makeup the
process we call glycolysis.
ALL organisms have glycolysis occurring in their cytoplasm.
At steps 1 and 3 ATP is converted into ADP, inputting energy into
the reaction as well as attaching a phosphate to the glucose.
At steps 7 and 10 ADP is converted into the higher energy ATP. At
step 6 NAD+ is converted into NADH + H+.
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Glycolisis
The process works on glucose, a 6-C, until step 4 splits the 6-C
into two 3-C compounds.
The end of the glycolysis process yields two pyruvic acid (3-C)
molecules, and a net gain of 2 ATP and two NADH per glucose.
The process is exergonic (ΔG = -140 kcal/mol or -586 kJ/mol);
most of the energy harnessed is conserved in the high-energy
electrons of NADH and in the phosphate bonds of ATP.
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Glycolisis
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Krebs Cycle (Citric Acid Cycle)
Most of the chemical energy originally stored in glucose still
resides in the two pyruvate molecules produced by glycolysis.
The fate of pyruvate depends upon the presence or absence of
oxygen.
If oxygen is present, pyruvate enters the mitochondrion where it
is completely oxidized by a series of enzyme-controlled
reactions.
The junction between glycolysis and the Krebs cycle is the
oxidation of pyruvate to acetyl CoA.
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Acetyl Co-A: The Transition Reaction from
glycolysis and the Krebs cycle
Pyruvic acid is first altered in the transition reaction by removal of
a carbon and two oxygens (which form CO2).
When the carbon dioxide is removed, energy is given off, and
NAD+ is converted into the higher energy form NADH.
Cytosol
Transport protein
OC =O
C
Mitochondrion
NAD+
NADH
+ H+
2
=
=
O
CH3
1
CO2
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S CoA
C= O
CH3
3
Coenzyme A
Acetyl CoA
Acetyl Co-A: The Transition Reaction from
glycolysis and the Krebs cycle
Coenzyme A attaches to the remaining 2-C (acetyl) unit, forming
acetyl Co-A. This process is a prelude to the Krebs Cycle.
Cytosol
Transport protein
OC =O
C
Mitochondrion
NAD+
NADH
+ H+
2
=
=
O
CH3
1
CO2
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S CoA
C= O
CH3
3
Coenzyme A
Acetyl CoA
Krebs Cycle (Citric Acid Cycle)
The Krebs cycle reactions oxidize the remaining acetyl
fragments of acetyl CoA to CO2.
Energy released from this exergonic process is used to reduce
coenzyme (NAD+ and FAD) and to convert ADP to ATP
(substrate-level phosphorylation).
A German-British scientist, Hans Krebs, elucidated this
catabolic pathway in the 1930s.
The Krebs cycle, which is also known as the citric acid cycle,
has eight enzyme-controlled steps that occur in the
mitochondrial matrix.
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Summary of the
Krebs cycle.
The cycle functions as a
metabolic "furnace" that
oxidizes organic fuel
derived from pyruvate, the
product of glycolysis.
The cycle generates 1 ATP
per turn by substrate
phosphorylation, but most
of the chemical energy is
transferred during the
redox reactions to NAD+
and FAD.
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Summary of the
Krebs cycle.
The reduced coenzymes,
NADH and FADH2,
shuttle high-energy
electrons to the electron
transport chain,
which uses the energy to
synthesize ATP by
oxidative phosphorylation.
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The inner mitochondrial membrane couples
electron transport to ATP synthesis
Only few molecules of ATP are produced by substrate-level
phosphorylation:
2ATPs per glucose from glycolysis
2ATPs per glucose from the Krebs cycle
Most molecules of ATP are produced by oxidative
phosphorylation.
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The inner mitochondrial membrane couples
electron transport to ATP synthesis
At the end of the Krebs cycle, most of the energy extracted
from glucose is in molecules of NADH and FADH2.
These reduced coenzymes link glycolysis and the Krebs
cycle to oxidative phosphorylation by passing their electrons
transport chain to oxygen.
The exergonic transfer of electrons down the ETC to oxygen
is coupled to ATP synthesis.
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Cell metabolism, Part II
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Mitochondria
outer membrane
inner membrane
Cristae
Matrix
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Cellular respiration and fermentation are catabolic
Oxidation-reduction reactions – chemical reactions which
involve a partial or complete transfer of electrons from one
reactant to another; called redox reactions for short.
Oxidation – partial or complete loss of electrons
Reduction – partial or complete gain of electrons
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Electron Transport Phosphorylation
The electron transport chain is made of electron carrier
molecules embedded in the inner mitochondrial membrane.
Each successive carrier in the chain has a higher
electronegativity than the carrier before it, so the electrons
are pulled downhill towards oxygen, the final electron
acceptor and the molecule with the highest electronegativity.
Except for ubiquinone (Q), most of the carrier molecules are
proteins and are tightly bound to prosthetic groups (nonprotein cofactors).
Prosthetic groups alternate between reduced and oxidized
states as they accept and donate electrons.
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Electron
transport
chains.
In cellular respiration,
an electron transport
chain breaks the "fall"
of electrons in this
reaction into a series
of smaller steps
It stores some of the released energy in a form that can be used
to make ATP (the rest of the energy is released as heat).
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Electron Transport Phosphorylation
Protein Electron Carriers
Flavoproteins
(FMN)
Iron-sulfur proteins
Cytochromes
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Prosthetic Group
flavin mononucleotide
iron and sulfur
heme group
Electron Transport Phosphorylation
Heme group – prosthetic group composed of four organic rings
surrounding a single iron atom.
Cytochrome – type of protein molecule that contains a heme
prosthetic group and functions as an electron carrier in the
electron transport chains of mitochondria and chloroplasts
There are several cytochromes, each a slightly different
protein with heme group.
It is the iron of cytochromes that transfers electrons.
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Electron Transport Chain
Each member of the chain oscillates
between a reduced state and an
oxidized state.
A component of the chain becomes
reduced when it accepts electrons
from its "uphill" neighbor (which has a
lower affinity for the electrons).
Each member of the chain returns to
its oxidized form as it passes
electrons to its "downhill" neighbor
(which has a greater affinity for the
electrons).
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Electron Transport Chain
At the bottom of the chain is
oxygen, which is very
electronegative.
The overall energy drop for
electrons traveling from NADH to
oxygen is 53 kcal/mol,
but this fall is broken up into a
series of smaller steps by the
electron transport chain.
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Electron Transport Chain
As molecular oxygen is reduced it also picks up two protons
from the medium to form water. For every two NADHs, one
O2 is reduced to two H2O molecules.
FADH2 also donates electrons to the electron transport
chain, but those electrons are added at a lower energy level
than NADH.
The electron transport chain does not make ATP directly.
It generates a proton gradient across the inner mitochondrial
membrane, which stores potential energy that can be used
to phosphorylate ADP.
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Chemiosmosis: the energy-coupling
mechanism
Cytochromes are molecules that pass the "hot potatoes"
(electrons) along the ETC.
Energy released by the "downhill" passage of electrons.
The ADP is reduced by the gain of electrons.
ATP formed in this way is made by the process of oxidative
phosphorylation.
The mechanism for the oxidative phosphorylation process is the
gradient of H+ ions discovered across the inner mitochondrial
membrane.
This mechanism is known as chemiosmotic coupling.
This involves both chemical and transport processes.
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Chemiosmosis: the energy-coupling
This protein complex,
which uses the energy of
an H+ gradient to drive ATP
synthesis,
resides in mitochondrial
and chloroplast membranes
and in the plasma
membranes of prokaryotes.
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Chemiosmosis: the energy-coupling
ATP synthase has three
main parts: a cylindrical
component within the
membrane,
a protruding knob (which, in
mitochondria, is in the
matrix), and
a rod (or "stalk") connecting
the other two parts.
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Chemiosmosis: the energy-coupling
The cylinder is a rotor that spins
clockwise when H+ flows
through it down a gradient.
The attached rod also spins,
activating catalytic sites in the
knob,
the component that joins
inorganic phosphate to ADP to
make ATP.
The chemiosmosis hypothesis
was proposed by Peter Mitchell
in 1961, later he would win the
Nobel Prize for his work.
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Electron transport chain
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ATP synthase
Chemiosmosis: the energy-coupling mechanism
The mechanism for coupling exergonic electron flow from the
oxidation of food to the endergonic process of oxidative
phosphorylation is chemiosmosis.
Chemiosmosis – the coupling of exergonic electron flow down
an electron transport chain to endergonic ATP production by the
creation of a protein gradient across membrane.
The proton gradient drives ATP synthesis as protons diffuse
back across the membrane.
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Chemiosmosis: the energy-coupling mechanism
The term chemiosmosis emphasizes a coupling between
(1) chemical reactions (phosphorylation) and
(2) transport processes (proton transport).
Process involved in oxidative phosphorylation and
photophosphorylation.
Potential energy is captured by ADP and stored in the pyrophosphate
bond.
NADH enters the ETS chain at the beginning, yielding 3 ATP per NADH.
FADH2 enters at Co-Q, producing only 2 ATP per FADH2.
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Chemiosmosis: the energy-coupling mechanism
How does the electron transport chain pump hydrogen ions from
the matrix to the intermembrane space?
The process is based on spatial organization of the ETC in the
membrane:
Some electron carriers accept and release protons along
with electrons. These carriers are spatially arranged so that
protons are picked up from the matrix and are released into
the intermembrane space.
As complexes transport electrons, they also harness energy
from this exergonic process to pump protons across the
inner mitochondrial membrane.
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Chemiosmosis: the energy-coupling mechanism
When the transport chain is operating: the pH in the
intermembrane space is one or two pH units lower than
in the matrix.
The H+ gradient that results is called a proton-motive force
– to emphasize that the gradient represents potential
energy.
Proton-motive force – potential energy stored in the proton
gradient created across biological membranes that are
involved in chemiosmosis.
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Chemiosmosis: the energy-coupling mechanism
This force is an electrochemical gradient with two
components:
1. Concentration gradient of protons (chemical gradient)
2. Voltage across the membrane because of a higher
concentration of positively charged protons on one side
(electrical gradient)
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Chemiosmosis: the energy-coupling
Cristae, or infoldings of the inner mitochondrial membrane,
increase the surface area available for chemiosmosis to occur.
Membrane structure correlates with the prominent functional role
membranes play in chemiosmosis:
Using energy from exergonic electron flow, the electron
transport chain creates the proton gradient by pumping H+s
from the mitochondrial matrix, across the inner membrane to
the intermembrane space.
This proton gradient is maintained, because the membrane’s
phospholipid bilayer is impermeable to H+s and prevents
them from leaking back across the membrane by diffusion.
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One molecule of glucose – 38 ATP
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Anaerobic versus aerobic
O2
Pyruvic acid
- lactic acid fermentation;
- alcohol fermentation;
- cellular (anaerobic)
respiration.
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O2
- Krebs cycle;
- electron transport.
Fermentation enables some cells to
produce ATP without the help of oxygen
Food can be oxidized under anaerobic conditions
Aerobic – existing in the presence of oxygen
Anaerobic – existing in the absence of free oxygen
Fermentation – anaerobic catabolism of organic nutrients
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Fermentation
Pyruvate, the end-product of
glycolysis,
serves as an electron acceptor
for oxidizing NADH back to
NAD+.
The NAD+ can then be reused
to oxidize sugar during
glycolysis,
which yields two net molecules
of ATP by substrate-level
phosphorylation.
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Fermentation
Two of the common waste
products formed from
fermentation are
(a) Ethanol
(b) lactate, the ionized form of
lactic acid.
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Pyruvate as key
juncture in
catabolism
Glycolysis is common to
fermentation and
respiration.
The end-product of
glycolysis, pyruvate,
represents a fork in the
catabolic pathways of
glucose oxidation.
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Pyruvate as key
juncture in
catabolism
In a cell capable of both
respiration and
fermentation,
pyruvate is committed to
one of those two
pathways,
depending on oxygen
presence.
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Anaerobic pathways
Humans cannot ferment alcohol in their own bodies, we lack
the genetic information to do so.
Many organisms will also ferment pyruvic acid into other
chemicals, such as lactic acid.
Humans ferment lactic acid in muscles where oxygen
becomes depleted, resulting in localized anaerobic
conditions.
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Anaerobic pathways
This lactic acid causes the muscle stiffness couchpotatoes feel after beginning exercise programs.
The stiffness goes away after a few days since the
cessation of the physical activity allows aerobic conditions
to return to the muscle,
and the lactic acid can be converted into ATP via the
normal aerobic respiration pathways.
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Fermentation/respiration
Similarity:
Use glycolysis to oxidize glucose and other substrates
to pyruvate, producing a net of two ATPs by substrate
level of P
Use NAD+ as the oxidizing agent that accepts electrons
from food during glycolysis
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Fermentation/respiration
Differences:
How NADH is oxidized back to NAD+ (necessary for
glycolysis to continue)
During fermentation, NADH passes electrons to
pyruvate. As pyruvate is reduced, NADH is oxidized to
NAD+.
Electrons transferred from NADH to pyruvate or other
substrates are not used to power ATP production.
ETC not only drives oxidative P, but regenerates NAD+
Final electron acceptor: pyruvate, acetaldehyde; in cellular
respiration: oxygen
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Fermentation/respiration
Differences:
Amount of energy harvested
Cellular respiration yields 18 times more ATP per
glucose
Requirement for oxygen
Fermentation does not require
Cellular respiration does
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The evolutionary significance of glycolysis
The first prokaryotes probably produced ATP by
glycolysis:
The oldest known bacterial fossils date back to 3.5
billion years ago when oxygen was not present
Glycolysis is the most widespread metabolic pathway,
so it probably evolved early
Glycolysis occur in cytosol and does not require
membrane-bound organelles.
Eukaryotic cell with organelles probably evolved about
two billion years after prokaryotes.
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The catabolism of
various food molecules.
Carbohydrates, fats, and proteins
can all be used as fuel for cellular
respiration.
Monomers of these food molecules
enter glycolysis or the Krebs cycle at
various points.
Glycolysis and the Krebs cycle are
catabolic funnels through which
electrons from all kinds of food
molecules flow on their exergonic fall
to oxygen.
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The catabolism of
various food molecules.
Catabolism can harvest energy
stored in fats obtained either
from food or from storage cell in
the body.
Most of the energy of a fat is
stored in the fatty acids.
A metabolic sequence called
beta oxidation breaks the fatty
acids down to two-carbon
fragments.
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The control of cellular
respiration
Allosteric enzymes at certain
points in the respiratory
pathway respond to inhibitors
and activators that help set the
pace of glycolysis and the
Krebs cycle.
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The control of cellular
respiration
Phosphofructokinase, the
enzyme that catalyzes step 3 of
glycolysis, is one such enzyme.
It is stimulated by AMP that is
derived from ADP, but it is
inhibited by ATP and by citrate.
This feedback regulation
adjusts the rate of respiration as
the cell's catabolic and anabolic
demands change.
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Reading
Ch. 9 pp. 162-184
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