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Overview: Life Is Work
• Living systems require energy from outside
sources
• Different organisms have different strategies
Light
An ecosystem
energy
is an open
ECOSYSTEM
system
CO2 + H2O
Photosynthesis in
chloroplasts and
cyanobacteria
Organic
+O
molecules 2
Cellular respiration
in mitochondria
Energy enters
as light and
leaves as heat
or entropy
Energy enters as light
ATP
ATP powers most cellular work
Heat
energy
• Energy harvesting pathways provide great
examples of metabolic pathways in general
• Also they are central to life
• And they are VERY well-studied (about 125
years)
• They serve as paradigm for all metabolism
• We usually use energy harvesting pathways
that are catabolic pathways as our examples
• Chemotrophic pathways
Concept 9.1: Energy-harvesting catabolic pathways
• The breakdown of organic molecules (sugars) is
exergonic and involves several multistep pathways
• Fermentation is a partial degradation of organic molecules
that occurs without O2
• Aerobic respiration complete degradation of organic
molecules and requires O2
• Anaerobic respiration is similar to aerobic respiration but
requires compounds other than O2 -used by many types of
microbes but has lower energy yield
• Cellular respiration includes both aerobic and anaerobic
respiration but is usually used to refer to aerobic respiration
Aerobic respiration is most relevant
to humans
Discussion Outline
• Aerobic Respiration
• Anaerobic Respiration
• Fermentation
We trace aerobic respiration by following the path
of the atoms of glucose (although many other
substances are also consumed as fuel) as it is
used as a source of chemical energy.
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
(ATP + heat)
• The movement of electrons during chemical
reactions helps to release and move energy
stored in organic molecules
• The electrons carry energy (key point) as they
are transferred from one substance to another
• This released energy is ultimately used to
synthesize ATP
• In electron transfer reactions, one substance
loses electrons and another gains them.
• In an oxidation, a substance loses electrons, or
is oxidized
• In a reduction, a substance gains electrons, or is
reduced
• They must occur together
• Chemical reactions that transfer electrons
between reactants are called oxidation-reduction
reactions, or redox reactions
Example
becomes oxidized
becomes reduced
In an aqueous environment, protons from water follow the
electrons.
Therefore the reduced form of Y might be written as YH
And the reduced form of X as XH
• The electron donor is called the reducing
agent
• The electron receptor is called the oxidizing
agent
• During aerobic respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced
• The reactions occur in a series or chain
• Electrons carry the energy in these transfer
reactions but something is needed to carry the
electrons from reaction to reaction
• Electrons from organic compounds are usually
first transferred to NAD+, a coenzyme (other
coenzymes can be used as well)
• As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
• Each NADH (the reduced form of NAD+)
represents stored energy that can be tapped to
synthesize ATP or do something else
Oxidation/Reduction
of NAD
2 e– + 2 H+
2 e– + H+
NADH
H+
Dehydrogenase
Reduction of NAD+
NAD+
+
+ H+
2[H]
Oxidation of NADH
Nicotinamide
(reduced form)
Nicotinamide
(oxidized form)
Hydrogen (H) follows
electrons in an aqueous
environment
An enzyme that catalyzes an
oxidation is a
“dehydrogenase”
• NADH passes high energy electrons to the
electron transport chain
• This chain hands off electrons in a series of
exergonic steps
• Finally they reach O2 which becomes reduced
• This is the last stop for the electrons so O2 is
called the terminal electron acceptor
• The energy given off is used to regenerate ATP
• There are three important stages:
– Glycolysis (breaks down glucose into two
molecules of pyruvate) and yields a little ATP
directly by non-oxidative reaction-substrate
level phosphorylation
– The Citric acid cycle (completes the
breakdown of glucose) and also yields a little
ATP directly reaction-substrate level
phosphorylation
– Oxidative phosphorylation (accounts for
most of the ATP synthesis) includes electron
transport and reduction of oxygen
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
• Oxidative phosphorylation involves
transfer of electrons from reduced
coenzymes to oxygen, the terminal
electron acceptor.
• A smaller amount of ATP is formed in
glycolysis and the citric acid cycle by
substrate-level phosphorylation
Substrate Level Phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
+
Product
Direct transfer of high energy phosphate
from substrate to substrate
ATP
Oxidative phosphorylation involves a membrane
and the movement of electrons (and protons)
Concept 9.2: Glycolysis harvests chemical energy
by converting glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down
glucose into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two
major phases:
– Energy investment phase
– Energy payoff phase
Overview of Glycolysis
Glucose
Energy investment phase
2 ADP + 2 P
2 ATP
used
4 ADP + 4 P
4 ATP
formed
Energy payoff phase
2 NAD+ + 4 e– + 4 H+
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
Concept 9.3: The citric acid cycle completes the
energy-yielding oxidation of organic molecules
(except for coenzymes)
• In the presence of O2 , pyruvate is transported
into the mitochondrion
• First, pyruvate must be converted to acetyl
CoA, which links the citric acid cycle to
glycolysis
Aerobic Metabolism of Pyruvate
CYTOSOL
MITOCHONDRION
NAD+
NADH + H+
2
1
Pyruvate
3
CO2
Coenzyme A
Acetyl CoA
Transport protein
CoA is derived from a vitamin- Vitamin B5 or pantothenic acid
Acetyl CoA links carbohydrate and fatty acid catabolism (beta oxidation)
• The citric acid cycle, also called the Krebs
cycle, takes place within the mitochondrial
matrix
• The cycle oxidizes organic fuel via Acetyl-CoA,
generating 1 ATP, 3 NADH, and 1 FADH2 per
turn
Pyruvate
CO2
NAD+
CoA
NADH
+ H+
Acetyl CoA
CoA
CoA
Citric
acid
cycle
FADH2
2 CO2
3 NAD+
3 NADH
FAD
+ 3 H+
ADP + P i
ATP
Acetyl CoA
CoA—SH
NADH
+H+
H2O
1
NAD+
8
Oxaloacetate
2
Malate
Citrate
Isocitrate
Carbon
Skeletons
NAD+
Citric
acid
cycle
7
H2O
NADH
+ H+
3
CO2
Fumarate
CoA—SH
6
-Ketoglutarate
4
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
Pi
Succinyl
CoA
NADH
+ H+
ADP
ATP
You do not need to memorize this
CO2
Can be
made into
many
substances
Concept 9.4: During oxidative phosphorylation,
chemiosmosis couples electron transport to ATP
synthesis
• Following glycolysis and the citric acid cycle,
NADH and FADH2 carry most of the energy
extracted from food in the form of high energy
electrons
• These two electron carriers hand off the
electrons to the electron transport chain, which
powers ATP synthesis via oxidative
phosphorylation
• The electron transport chain is in the cristae of
the mitochondrion
• Most of the chain’s components are proteins,
which exist in the form of huge multiprotein
“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
H 2O
NADH
50
2 e–
NAD+
FADH2
2 e–
40

FMN
FAD
Multiprotein
complexes
FAD
Fe•S 
Fe•S
Q

Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
2 e–
(from NADH
or FADH2)
0
You do not need to memorize this
2 H+ + 1/2 O2
H2O
• 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) and iron-sulfur proteins
• The electron transport chain generates no ATP
directly
• The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps
that release energy in manageable amounts
• 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 channels in ATP synthase
• ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP
• This is an example of chemiosmosis, the use of
energy in a H+ gradient to drive cellular work
(Note-this is an important concept)
• The energy stored in the electrochemical H+
gradient across a membrane connects or
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
H+
H+
H+
H+
Protein complex
of electron
carriers
Cyt c
V
Q


ATP
synthase

FADH2
NADH
2 H+ + 1/2O2
H2O
FAD
NAD+
ADP + P i
(carrying electrons
from food)
ATP
H+
1 Electron transport chain
Oxidative phosphorylation
2 Chemiosmosis
How much ATP is produced?
• During cellular respiration, most energy flows in
this sequence:
glucose  NADH  electron transport chain
 proton-motive force  ATP
• Roughly 40% of the energy in a glucose
molecule is transferred to ATP during cellular
respiration, making about 38 ATP
Electron shuttles
span membrane
CYTOSOL
2 NADH
Glycolysis
Glucose
2
Pyruvate
MITOCHONDRION
2 NADH
or
2 FADH2
6 NADH
2 NADH
2
Acetyl
CoA
+ 2 ATP
Citric
acid
cycle
+ 2 ATP
Maximum per glucose:
About
36 or 38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
Concept 9.5: Without oxygen-cells use fermentation or
anaerobic respiration to produce ATP
• Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
• In the presence of O2 glycolysis couples with
aerobic respiration and uses oxygen as
terminal electron to produce ATP
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
• Anaerobic respiration uses an electron
transport chain with an electron acceptor other
than O2, for example sulfate, ending with H2S
not H2O
• Fermentation uses phosphorylation instead of
an electron transport chain to generate ATP
• Note: Fermentation is defined to consist of
reactions that regenerate NAD+-these vary
from organism to organism
Types of Fermentation
• alcohol fermentation
• lactic acid fermentation
• butyric acid fermentation
• biohydrogen fermentation
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 Ethanol
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
Alcohol fermentation
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
Lactic acid fermentation
• Fermentation vs. aerobic respiration
• Both processes use glycolysis to oxidize
glucose and other organic fuels to pyruvate
• The processes have different terminal electron
acceptors: an organic molecule (such as
pyruvate or acetaldehyde) in fermentation and
O2 in cellular respiration
• Cellular respiration produces 38 ATP per
glucose molecule; fermentation produces 2
ATP per glucose molecule
• 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
Proteins
Carbohydrates
Amino
acids
Sugars
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Glycerol
Fatty
acids
Glucose
AMP
Glycolysis
Fructose-6-phosphate
–
Stimulates
+
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation