Download Lecture 13: Fighting Entropy II: Respiration

BIO 2, Lecture 13
FIGHTING ENTROPY II:
RESPIRATION
• Respiration is the process whereby cells
break down complex organic molecules
(like starch) and convert them into ATP +
heat (not 100% efficient)
• The cell then uses the ATP to do work
$10,000 bill
(Starch; Unusable)
$1 bills
(ATP; Usable)
• There are two types of respiration:
• Anaerobic respiration, also called
fermentation, occurs without O2
• Example: Ethanol fermentation of glucose
C6H12O6 + ADP + Pi  2 C2H5OH + 2 CO2 + ATP + heat
• Aerobic respiration relies on O2
• Is more efficient than anaerobic respiration;
generates more ATP per organic molecule,
loses less energy as heat
C6H12O6 + 6O2 + ADP + Pi  6CO2 + 6H2O + ATP +
heat
• Cells do three types of work: mechanical,
transport, and chemical
• All must be coupled to the hydrolysis of
ATP
• Overall, the coupled reactions are
catabolic
• An example is the creation of the amino
acid glutamine from ammonia and glutamic
acid
NH2
Glu
Glutamic
acid
NH3
+
Glu
∆G = +3.4 kcal/mol
Glutamine
Ammonia
(a) Anabolic (endergonic) reaction
1 ATP phosphorylates
glutamic acid,
making the amino
acid less stable.
P
+
Glu
ATP
Glu
+ ADP
NH2
2 Ammonia displaces
the phosphate group,
forming glutamine.
P
Glu
+
NH3
Glu
+ Pi
(b) Coupled with ATP hydrolysis, a catabolic (exergonic) reaction
(c) Overall free-energy change
Adenine
Phosphate groups
Ribose
• The bonds between the phosphate groups
of ATP’s tail can be broken by hydrolysis
• Energy is released from ATP when the
terminal phosphate bond is broken
P
P
P
Adenosine triphosphate (ATP)
H20
Pi
+
P
P
+
Inorganic phosphate
Adenosine diphosphate (ADP)
Energy
• ATP is a renewable resource that is
regenerated by addition of a phosphate
group to adenosine diphosphate (ADP)
• The energy to re-phosphorylate ADP
comes from catabolic reactions in the cell
H2O
ATP
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP+ P i
Energy for cellular work
(endergonic, energyconsuming processes)
• Although carbohydrates, fats, and
proteins can all be broken down during
respiration to produce ATP, it is helpful
to trace cellular respiration with the
sugar glucose
• The step-wise transfer of electrons
(from high energy states in complex
organic molecules to lower energy states
in simple organic molecules) gently
releases the energy stored in glucose to
regenerate ATP from ADP + P
• Chemical reactions that transfer
electrons between reactants are called
oxidation-reduction 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)
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
• Some redox reactions do not transfer
electrons but change the electron
sharing in covalent bonds, thereby
changing the potential chemical energy
stored in the molecules
• Reduced organic molecules carry more
potential chemical energy than oxidized
forms of the same molecules
• Starch is a highly reduced form of water
and carbon dioxide
• Therefore, breaking starch down into water
and carbon dioxide releases energy
Reactants
Products
becomes oxidized
becomes reduced
Methane
Oxygen
Carbon dioxide
Water
• During cellular respiration, the fuel (such
as glucose) is oxidized, and another
molecule (such as O2) is reduced
• The chemical potential energy in the
reactants is greater than the chemical
potential energy in the products; thus
energy is released
becomes oxidized
becomes reduced
• Both anaerobic and aerobic respiration
begin with glycolysis
– Breaks down glucose into two molecules of
pyruvate) to produce 2 ATP per glucose
– Takes place in the cytoplasm
– In anaerobic respiration, the process stops
here and only 2 ATP are generated per
glucose
• Aerobic respiration has two additional
steps that break down the pyruvate to
carbon dioxide and water to produce an
additional 36 ATP
– The citric acid cycle (completes the
breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
• Both steps take place in the mitochondria
• Glycolysis (“splitting of sugar”) has two
major phases:
– Energy investment phase
– Energy payoff phase
• In the energy investment phase, 2 ATP
are consumed to “kick start” the process
• In the energy payoff phase, four ATP are
produced, yielding a net gain of 2 ATP
Energy investment phase
Glucose
2 ADP + 2
P
2 ATP used
Energy payoff phase
4 ADP + 4
P
2 NAD+ + 4 e– + 4 H+
4 ATP formed
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+
• In anaerobic respiration, the process
stops with the production of 2 ATP
• Pyruvate is converted to waste products
(like ethanol) but no more ATP is gained
• The energy stored in NADH is likewise
wasted
• It is important to note, however, that NADH
carries high energy electrons and can be
harvested to produce ATP if a cell has the
machinery to do so ...
Anaerobic Respiration
Electrons
carried
via NADH
Net = 2 NADH
Glycolysis
Pyruvate
Glucose
Cytosol
Net = 2
ATP
Substrate-level
phosphorylation
Potential source of
additional ATP;
WASTED in aerobic
respiration
Potential source of
additional ATP;
WASTED in aerobic
respiration
• In anaerobic respiration, the process
stops with the production of 2 ATP
• Pyruvate is converted to waste products
(like ethanol) but no more ATP is gained
• The energy stored in NADH is likewise
wasted
• It is important to note, however, that NADH
carries high energy electrons and can be
harvested to produce ATP if a cell has the
machinery to do so ...
• Aerobic respiration continues the process
of glycolysis to breakdown pyruvate and
utilize the high energy electrons stored in
NADH
• Takes place in cells that have mitochondria
– The citric acid cycle (breaks down pyruvate
to CO2 and H2O in the mitochondrial matrix to
produce additional ATP, NADH, and FADH2)
– Oxidative phosphorylation (harvests
electrons from NADH and FADH2 in the inner
mitochondrial membrane and accounts for
most of the ATP synthesis)
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
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
• The Citric Acid Cycle:
• In the presence of O2, pyruvate (generated
by glycolysis) enters the mitochondrion from
the cytoplasm
• Prior to the start of the cycle, pyruvate is
converted to acetyl CoA, generating one
molecule of NADH
• The cycle then oxidizes acetyl CoA,
generating 1 ATP, 3 NADH, and 1 FADH2 per
turn
Pyruvate
CO2
NAD+
NADH
+ H+
CoA
Acetyl CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD+
FADH2
3
FAD
NADH
+ 3 H+
ADP
+
ATP
P
i
• The cycle is “fed” by acetyl-coA
• In the first step, the acetyl-coA is
combined with oxaloacetate to form
citrate
• The citrate is then broken down in a series
of steps to produce energy (in the form of
ATP, NADH, and FADH2) + CO2 (gas)
• The end product of the cycle is
oxaloacetate, which can then combine with
another molecule of acetyl-coA to run the
cycle again ...
Acetyl CoA
CoA—SH
NADH
+H
NAD+
H2O
1
+
8
Oxaloacetate
2
Malate
H2O
Citrate
Isocitrate
NAD
+
Citric
acid
cycle
7
Fumarate
3
NADH
+ H+
CO2
CoA—SH
6
4
CoA—SH
5
FADH2
NAD
FAD
+
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
-Ketoglutarate
CO2
• Following glycolysis and the citric acid
cycle, NADH and FADH2 carry most of
the energy extracted from food
• These two molecules transport the high
energy electrons generated by the
breakdown of glucose to pyruvate (during
glycolysis) and pyruvate to oxaloacetate
and CO2 (during the citric acid cycle) and
donate them to the electron transport
chain, which powers ATP synthesis via
oxidative phosphorylation
• The electron transport chain is located in
the inner membrane 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 (become
reduced) and donate (become oxidized)
electrons down the chain
• Electrons drop in free energy as they go
down the chain and are finally passed to
O2 (gas), forming H2O
NADH
5
0
2 e–
NAD+
FADH2
2 e–
4
0
3
0
FM
N

Fe•
S
Q
FAD
FAD
Fe• 
S
Cyt
b
Multiprotein
complexes

Fe•
S
Cyt c1
I
V
Cyt c
Cyt
a
2
0
1
0
0
Cyt
a3
2 e–
(from
NADH
or FADH2)
2 H+ + 1/2
O2
H2O
• As electrons are transferred down the
electron transport chain, the energy
released at each step is used by the
protein complexes to pump H+ from the
mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane, with
its diffusion gradient, passing through channels
in a protein complex called ATP synthase
• ATP synthase uses the exergonic flow of
H+ to drive phosphorylation of ATP from
ADP and Pi
• This is an example of chemiosmosis, the
use of energy in a H+ gradient to drive
cellular work
• The H+ gradient is referred to as a
proton-motive force, emphasizing its
capacity to do work
H+
H+
H+
Protein complex
of electron
carriers
Cyt c
V
Q



FADH2
NADH
(carrying
electrons
from food)
H+
ATP
synthase
2 H+ + 1/2O2
FAD
NAD+
H 2O
Why we
breathe O2!!
1 Electron transport
chain
Oxidative phosphorylation
ADP + P i
ATP
H+
2 Chemiosmosis
• During aerobic respiration, most energy
flows in this sequence:
glucose  NADH  electron transport
chain  proton-motive force  ATP
• About 40% of the energy in a glucose
molecule is transferred to ATP during
aerobic respiration, generating about 38
ATP
• The rest is lost as heat
Anaerobic phase
Electron shuttles
span membrane
CYTOSOL
Aerobic phase
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
Glycolysis
Glucose
2
Pyruvate
2 NADH
2
Acetyl
CoA
+ 2 ATP
6 NADH
Citric
acid
cycle
+ 2 ATP
Maximum per glucose:
About
38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 34 ATP
• Comparing aerobic and anaerobic
respiration:
• Both processes use glycolysis to oxidize
glucose and other organic fuels to pyruvate
• However, the processes have different final
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
Proteins
Amino
acids
Respiration can
use many
different fuels
(not just
glucose!)
Carbohydrates
Sugars
Glycolysis
Glucose
Glyceraldehyde-3-P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Glycerol
Fatty
acids
H2 + 1/2 O2
1/
2 H
(from food)
2 H+ + 2 e–
2
O2
Controlled
release of
energy for
synthesis of
ATP
Explosive
release of
heat and
light
energy
1/
2
O2
(a) Uncontrolled reaction
(b) Cellular respiration
Light
energy
ECOSYSTEM
CO2 + H2O
Photosynthesis
in chloroplasts
Cellular respiration
in mitochondria
ATP
ATP powers most cellular work
Heat
energy
Organic
molecules
+ O2