Download electron transport chain.

CELLULAR RESPIRATION
AND FERMENTATION
OVERVIEW: LIFE IS WORK
• Living cells require energy from outside
sources.
• Some animals, such as the chimpanzee,
obtain energy by eating plants, and some
animals feed on other organisms that eat
plants.
• Energy flows into an ecosystem as sunlight
and leaves as heat.
• Photosynthesis generates O2 and organic
molecules, which are used in cellular
respiration.
• Cells use chemical energy stored in
organic molecules to regenerate ATP,
which powers work.
Light
Light Energy
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2  H2O
Cellular respiration
in mitochondria
ATP
Heat
energy
Organic
 O2
molecules
ATP powers
most cellular work
Heat Energy
CATABOLIC PATHWAYS YIELD ENERGY
BY OXIDIZING ORGANIC FUELS
• Organic compounds store energy in the
arrangement of atoms.
• With the help of enzymes, the cell breaks down
large molecules that are rich in potential energy.
• Some of this energy can be used to perform
cellular activities, the rest is lost as heat.
• Several processes are central to cellular
respiration and related pathways.
CATABOLIC PATHWAYS AND
PRODUCTION OF ATP
• The breakdown of organic molecules is
exergonic.
• Fermentation is a partial degradation of sugars
that occurs without O2.
• Aerobic respiration consumes organic
molecules and O2 and yields ATP.
• Anaerobic respiration is similar to aerobic
respiration but consumes compounds other
than O2.
• Cellular respiration includes both aerobic and
anaerobic respiration but is often used to refer
to aerobic respiration.
• Although carbohydrates, fats, and proteins are
all consumed as fuel, it is helpful to trace
cellular respiration with the sugar glucose.
Organic Compounds + Oxygen  Carbon Dioxide + Water + Energy
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)
REDOX REACTIONS:
OXIDATION AND REDUCTION
Why do the catabolic pathways that decompose
glucose and other organic fuels yield energy?
• The transfer of electrons during chemical
reactions releases energy stored in organic
molecules.
• This released energy is ultimately used to
synthesize ATP.
THE PRINCIPLE OF REDOX
• Chemical reactions that transfer electrons
between reactants are called oxidationreduction reactions, or redox reactions.
• In oxidation, a substance loses electrons, or is
oxidized.
• In reduction, a substance gains electrons, or
is reduced (the amount of positive charge is
reduced).
Consider the reaction between sodium (Na) and chlorine (Cl)
that forms table salt.
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
becomes oxidized
becomes reduced
becomes oxidized
becomes reduced
• The electron donor is called the reducing
agent (X).
• The electron receptor is called the
oxidizing agent (Y).
• Some redox reactions do not transfer
electrons but change the electron sharing
in covalent bonds
• An example is the reaction between
methane and O2
Reactants
Products
becomes oxidized
Energy
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
Energy must be added to pull electrons away from atoms.
OXIDATION OF ORGANIC FUEL
MOLECULES DURING CELLULAR
RESPIRATION
• The energy yielding process that is most
important to us is cellular respiration.
• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced.
becomes oxidized
becomes reduced
*The status of electrons changes as H is transferred to O,
releasing energy.
STEPWISE ENERGY HARVEST VIA NAD+ AND
THE ELECTRON TRANSPORT CHAIN
• In cellular respiration, glucose and other
organic molecules are broken down in a series
of steps.
• Electrons from organic compounds (food) are
usually first transferred to NAD+, a coenzyme.
• As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration.
• NAD+ traps electrons from glucose using the
enzyme dehydrogenase.
• Each NADH (the reduced form of NAD+)
represents stored energy that is tapped to
synthesize ATP.
NAD
NADH
Dehydrogenase
Reduction of NAD
(from food)
Nicotinamide
NAD+
(oxidized form)
(from food)
Nicotinamide Adenine dinucleotide
Oxidation of NADH
Nicotinamide
NADH
(reduced
form)
• NADH passes the electrons to the electron
transport chain.
• Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series of
steps instead of one explosive reaction.
• O2 pulls electrons down the chain in an
energy-yielding tumble.
• The energy created is used to regenerate ATP
H2  1/2 O2

2H
1/
Explosive
release of
heat and light
energy
Free energy, G
Free energy, G
(from food via NADH)
Controlled
release of
+

2H  2e
energy for
synthesis of
ATP
O2
ATP
ATP
ATP
2 e
2
1/
H+
H2O
H2O
(a) Uncontrolled reaction
2
(b) Cellular respiration
Food  NADH  ETC  Oxygen
2
O2
THE STAGES OF CELLULAR
RESPIRATION: A PREVIEW
• Harvesting of energy from glucose has three
stages:
1. Glycolysis (breaks down glucose into two molecules of
pyruvate).
2. The citric acid cycle (completes the breakdown of
glucose).
3. Oxidative phosphorylation (accounts for most of the ATP
synthesis).
The Stages of Cellular Respiration
1. Glycolysis (color-coded teal throughout the chapter)
2. Pyruvate oxidation and the citric acid cycle
(color-coded salmon)
3. Oxidative phosphorylation: electron transport and
chemiosmosis (color-coded violet)
The process that generates most of the ATP is oxidative phosphorylation
because it is powered by redox reactions.
• Oxidative phosphorylation accounts for
almost 90% of the ATP generated by cellular
respiration
• A smaller amount of ATP is formed in
glycolysis and the citric acid cycle by
substrate-level phosphorylation.
• For each molecule of glucose degraded to
CO2 and water by respiration, the cell
makes up to 38 molecules of ATP.
• Each has 7.3 kcal/mol of free energy.
Enzyme
Enzyme
ADP
P
Substrate
ATP
Product
During substrate-level phosphorylation,
some ATP is made by direct enzymatic transfer of a phosphate
group from an organic substrate to ADP.
CHECK YOURSELF!
In the following redox reaction, which compound is
oxidized and which is reduced?
C4H6O5 + NAD+  C4H4O5 + NADH + H+
GLYCOLYSIS HARVESTS CHEMICAL ENERGY BY
OXIDIZING GLUCOSE TO PYRUVATE
• Glycolysis (“splitting of sugar”) breaks down
glucose into two molecules of pyruvate.
• Glycolysis occurs in the cytoplasm and has ten
steps and two major phases:
1. Energy investment phase
2. Energy payoff phase
• Glycolysis occurs whether or not O2 is present.
Glycolysis: Energy Investment Phase
Glucose
ATP
Glucose 6-phosphate
Fructose 6-phosphate
ATP
ADP
ADP
Hexokinase
1
Fructose 1,6-bisphosphate
Phosphoglucoisomerase
Phosphofructokinase
2
3
Aldolase
Dihydroxyacetone
phosphate
4
Glyceraldehyde
3-phosphate
Isomerase
5
To
step 6
After the sugar splits, every reaction is doubled!
Glycolysis: Energy Payoff Phase
2 ATP
2 ATP
2 H2O
2 NADH
2 NAD
Triose
phosphate
dehydrogenase
6
+ 2 H
2 ADP
2
2
1,3-Bisphosphoglycerate
7
Enolase
Phosphoglyceromutase
Phosphoglycerokinase
2Pi
9
3-Phosphoglycerate
8
2 ADP
2
2-Phosphoglycerate
Pyruvate
kinase
Phosphoenolpyruvate (PEP)
10
Pyruvate
Overall, glycolysis uses 2 ATP in the first phase, but creates 4
ATP in return!
CHECK YOURSELF!
Glycolysis: Energy Investment Phase
Glucose
ATP
Glucose 6-phosphate
Fructose 6-phosphate
ATP
Fructose 1,6-bisphosphate
ADP
ADP
Hexokinase
1
Phosphoglucoisomerase
Phosphofructokinase
2
3
During the redox reaction in
glycolysis (step 6), which
molecule acts as the oxidizing
agent? The reducing agent?
Aldolase
Dihydroxyacetone
phosphate
4
Glyceraldehyde
3-phosphate
Isomerase
To
step 6
5
Glycolysis: Energy Payoff Phase
2 ATP
2 ATP
2 H2O
2 NADH
2 NAD
Triose
phosphate
dehydrogenase
6
+ 2 H
2 ADP
2
2
1,3-Bisphosphoglycerate
7
Enolase
Phosphoglyceromutase
Phosphoglycerokinase
2Pi
2 ADP
2
Pyruvate
kinase
9
3-Phosphoglycerate
8
2-Phosphoglycerate
Phosphoenolpyruvate (PEP)
10
Pyruvate
ALL OF THAT ONLY
OCCURRED HERE!
THE CITRIC ACID CYCLE COMPLETES THE
ENERGY-YIELDING OXIDATION OF ORGANIC
MOLECULES
• In the presence of O2, pyruvate enters the
mitochondrion (in eukaryotic cells) where the
oxidation of glucose is completed.
OXIDATION OF PYRUVATE TO ACETYL COA
• Before the citric acid cycle can begin, pyruvate
must be converted to acetyl Coenzyme A
(acetyl CoA), which links glycolysis to the citric
acid cycle.
• This step is carried out by a multi-enzyme
complex that catalyzes three reactions.
MITOCHONDRION
CYTOSOL
CO2
Coenzyme A
3
1
2
Pyruvate
NAD
NADH + H
Acetyl CoA
Transport protein
1. Pyruvate’s carboxyl group is removed and turned into CO2.
2. The remaining two-carbon fragment is oxidized forming acetate
and the electrons are transferred to NAD+  NADH.
3. Coenzyme A is attached, forming an acetyl group, which is highly
reactive.
THE CITRIC ACID CYCLE
• The citric acid cycle,
also called the Krebs
cycle, completes the
break down of pyruvate
to CO2
• The cycle oxidizes
organic fuel derived
from pyruvate,
generating 1 ATP, 3
NADH, and 1 FADH2 per
turn.
THE CITRIC ACID CYCLE
• The citric acid cycle has eight steps, each
catalyzed by a specific enzyme.
• The acetyl group of acetyl CoA joins the cycle
by combining with oxaloacetate, forming
citrate.
• The next seven steps decompose the citrate
back to oxaloacetate, making the process a
cycle.
• The NADH and FADH2 produced by the cycle
relay electrons extracted from food to the
electron transport chain.
THE
CITRIC
ACID
CYCLE
Acetyl CoA
CoA-SH
NADH
+ H
H2O
1
NAD
8
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD
Citric
acid
cycle
7
H2O
Fumarate
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
6
CoA-SH
5
FADH2
NAD
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H
CO2
SECTION QUESTIONS
1. In which molecules is most of the energy form the
citric acid cycle's redox reactions conserved? How
will these molecules convert their energy to a form
that can be used to make ATP?
2. What cellular processes produce the carbon dioxide
that you exhale?
DURING OXIDATIVE PHOSPHORYLATION,
CHEMIOSMOSIS COUPLES ELECTRON
TRANSPORT TO ATP SYNTHESIS
• Following glycolysis and the citric acid cycle,
NADH and FADH2 account for most of the energy
extracted from food.
• These two electron carriers donate electrons to the
electron transport chain, which powers ATP
synthesis via oxidative phosphorylation.
THE PATHWAY OF ELECTRON TRANSPORT
• The electron transport chain is in the inner
membrane (cristae) of the mitochondrion.
• Most of the chain’s components are proteins,
which exist in multi-protein complexes.
• The carriers alternate reduced and oxidized
states as they accept and donate electrons.
• Electrons drop in free energy as they go down
the chain and are finally passed to O2, forming
H2O.
NADH
50
2 e
NAD
FADH2
Free energy (G) relative to O2 (kcal/mol)
2 e
40
FMN
FeS
FeS
II
Q
III
Cyt b
30
Multiprotein
complexes
FAD
I
FeS
Cyt c1
IV
Cyt c
Cyt a
20
10
0
Cyt a3
2 e
(originally from
NADH or FADH2)
2 H + 1/2 O2
H2O
THE
PATHWAY
OF
ELECTRON
TRANSPORT
THE PATHWAY OF ELECTRON TRANSPORT
• Electrons are transferred from NADH or FADH2 to
the electron transport chain.
• Electrons are passed through a number of
proteins including cytochromes (each with an
iron atom) to O2.
• The electron transport chain generates no ATP
directly.
• It breaks the large free-energy drop from food
to O2 into smaller steps that release energy in
manageable amounts.
CHEMIOSMOSIS:
THE ENERGY-COUPLING MECHANISM
• Electron transfer in the electron transport chain
causes proteins to pump H+ from the
mitochondrial matrix to the intermembrane
space.
• H+ then moves back across the membrane,
passing through the proton, ATP synthase.
• ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP.
• Chemiosmosis is the use of energy in a H+
gradient to drive cellular work.
INTERMEMBRANE SPACE
H
Stator
Rotor
Internal
rod
Catalytic
knob
ADP
+
Pi
ATP
MITOCHONDRIAL MATRIX
CHEMIOSMOSIS:
THE ENERGYCOUPLING
MECHANISM
H
H

H
Protein
complex
of electron
carriers
Cyt c
Q
I
IV
III
II
FADH2 FAD
NADH
H
2 H + 1/2O2
ATP
synthase
H2O
NAD
ADP  P i
(carrying electrons
from food)
ATP
H
1 Electron transport chain
2 Chemiosmosis
Oxidative phosphorylation
OXIDATIVE PHOSPHORYLATION
OXIDATIVE PHOSPHORYLATION
• The energy stored in a H+ gradient across a
membrane couples the redox reactions of
the electron transport chain to ATP synthesis.
• The H+ gradient is referred to as a protonmotive force, emphasizing its capacity to do
work
AN ACCOUNTING OF ATP PRODUCTION BY
CELLULAR RESPIRATION
• During cellular respiration, most energy flows in
this sequence:
glucose  NADH  electron transport chain 
proton-motive force  ATP
• About 34% of the energy in a glucose molecule
is transferred to ATP during cellular respiration,
making about 32 ATP.
• There are several reasons why the number of
ATP is not known exactly.
1. The numbers are not whole numbers.
2. ATP yield varies depending on electron transport.
3. Amount of proton-motive force varies.
Electron shuttles
span membrane
2 NADH
Glycolysis
2 Pyruvate
Glucose
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
Pyruvate oxidation
2 Acetyl CoA
 2 ATP
Maximum per glucose:
6 NADH
2 FADH2
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
 2 ATP
 about 26 or 28 ATP
About
30 or 32 ATP
CYTOSOL
ATP Yield per Molecule of Glucose
SECTION QUESTIONS
1. What would occur if O2 was not present in this
picture?
2. In the absence of O2, what do you think would
happen if you decreased the pH of the
intermembrane space of the mitochondria? Explain.
FERMENTATION AND ANAEROBIC
RESPIRATION ENABLE CELLS TO PRODUCE
ATP WITHOUT THE USE OF OXYGEN
• Most cellular respiration requires O2 to produce
ATP.
• Without O2, the electron transport chain will cease
to operate.
• In that case, glycolysis couples with fermentation
or anaerobic respiration to produce ATP.
CREATING ATP WITHOUT THE USE OF OXYGEN
• Anaerobic respiration uses an electron transport
chain with a final electron acceptor other than
O2, for example sulfate.
• Fermentation uses substrate-level phosphorylation
instead of an electron transport chain to
generate ATP
TYPES OF FERMENTATION
• Fermentation consists of glycolysis plus
reactions that regenerate NAD+, which can be
reused by glycolysis.
• Two common types are:
1. Alcohol Fermentation
2. Lactic Acid Fermentation
TYPES OF FERMENTATION
• In alcohol fermentation, pyruvate is
converted to ethanol in two steps, with the
first releasing CO2
• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
TYPES OF FERMENTATION
2 ADP  2 P
Glucose
i
2 ADP  2 P
2 ATP
Glycolysis
Glucose
i
2 ATP
Glycolysis
2 Pyruvate
2 NAD 
2 Ethanol
(a) Alcohol fermentation
2 NADH
 2 H
2 NAD 
2 CO2
2 Acetaldehyde
2 NADH
 2 H
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
TYPES OF FERMENTATION
• In lactic acid fermentation, pyruvate is
reduced to NADH, forming lactate as an end
product, with no release of CO2.
• Lactic acid fermentation by some fungi and
bacteria is used to make cheese and yogurt.
• Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce.
• The lactate that accumulates may cause muscle
fatigue and pain.
• Lactate is converted back to pyruvate by liver cells.
Fermentation
1. Produces 2 ATP per
Glucose Molecule
2.
Pyruvate/Acetaldehyde
(final electron acceptor)
Cellular
Respiration
Glycolysis
NAD+
(oxidizing agent)
1. Produces 32 ATP per
Glucose Molecule
2. O2
(final electron acceptor)
COMPARING FERMENTATION & CELLULAR RESPIRATION
• Obligate anaerobes carry out fermentation
or anaerobic respiration and cannot survive
in the presence of O2.
• Yeast and many bacteria are facultative
anaerobes, meaning that they can survive
using either fermentation or cellular
respiration.
• In a facultative anaerobe, pyruvate is a fork
in the metabolic road that leads to two
alternative catabolic routes.
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol,
lactate, or
other products
Acetyl CoA
Citric
acid
cycle
THE EVOLUTIONARY SIGNIFICANCE OF
GLYCOLYSIS
• Ancient prokaryotes are thought to have
used glycolysis long before there was
oxygen in the atmosphere.
• Very little O2 was available in the
atmosphere until about 2.7 billion years
ago, so early prokaryotes likely used only
glycolysis to generate ATP.
• Glycolysis is a very ancient process.
SECTION QUESTIONS
1. What is the final acceptor for NADH’s electrons during
fermentation? What is the final acceptor for its
electrons during respiration?
2. If a glucose fed yeast cell is moved from an aerobic
environment to an anaerobic one, how would its rate
of glucose consumption need to change?
GLYCOLYSIS AND THE CITRIC ACID CYCLE
CONNECT TO MANY OTHER METABOLIC
PATHWAYS
• Glycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
THE VERSATILITY OF CATABOLISM
• Catabolic pathways funnel electrons from
many kinds of organic molecules into cellular
respiration.
• Glycolysis accepts a wide range of
carbohydrates.
• Proteins must be digested to amino acids;
amino groups can feed glycolysis or the citric
acid cycle.
THE VERSATILITY OF CATABOLISM
• Fats are digested to glycerol (used in glycolysis)
and fatty acids (used in generating acetyl CoA).
• Fatty acids are broken down by beta oxidation
and yield acetyl CoA.
• An oxidized gram of fat produces more than
twice as much ATP as an oxidized gram of
carbohydrate.
Proteins
Carbohydrates
Amino
acids
Sugars
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
THE
VERSATILITY
OF
CATABOLISM
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Glycerol Fatty
acids
BIOSYNTHESIS (ANABOLIC PATHWAYS)
• The body uses small molecules to build other
substances.
• These small molecules may come directly
from food, from glycolysis, or from the citric
acid cycle.
• They can be converted by cells to other
types of molecules.
• If we eat more food than we need, we store
excess fat.
• Metabolism is adaptable.
REGULATION OF CELLULAR RESPIRATION VIA
FEEDBACK MECHANISMS
• Feedback inhibition is the most common
mechanism for control.
• If ATP concentration begins to drop,
respiration speeds up; when there is plenty of
ATP, respiration slows down.
• Control of catabolism is based mainly on
regulating the activity of enzymes at
strategic points in the catabolic pathway.
Glucose
AMP
Glycolysis
Fructose 6-phosphate

Stimulates

Phosphofructokinase

Fructose 1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
THE
CONTROL
OF
CELLULAR
RESPIRATION
SECTION QUESTIONS
1. Compare the structure of a fat with that of a
carbohydrate. What features make fat a much
better fuel?
2. Under what circumstances might your body
synthesize fat molecules?
3. What will happen to a muscle cell that has used up its
supply of oxygen and ATP?