Transcript Cellular Respiration: Harvesting Chemical Energy

Cellular Respiration:
Harvesting Chemical Energy
AP Biology
Ms. Haut
Energy Flow
• Energy flows into an
ecosystem as sunlight and
leaves as heat
• Photosynthesis generates
oxygen and organic
molecules, which are used
in cellular respiration
• Cells use chemical energy
stored in organic molecules
to regenerate ATP, which
powers work
Catabolic Pathways and
Production of ATP
• The breakdown of organic molecules is exergonic
• Fermentation is a partial degradation of sugars that
occurs without oxygen
• Cellular respiration consumes oxygen and organic
molecules and yields ATP
• Although carbohydrates, fats, and proteins are all
consumed as fuel, it is helpful to trace cellular
respiration with the sugar glucose:
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP + heat)
Cellular Respiration
• ATP-producing catabolic process in which
the ultimate electron acceptor is an
inorganic compound, Oxygen
• Most efficient catabolic pathway
• Is an exergonic process (ΔG = -686kcal/mol)
Electrons “fall” from Organic Molecules to
Oxygen during Cellular Respiration
Oxidation
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP + Heat)
Reduction
The Stages of Cellular
Respiration: A Preview
• Cellular respiration has three stages:
– Glycolysis (breaks down glucose into two molecules
of pyruvate)
– The Krebs Cycle (citric acid cycle) (completes the
breakdown of glucose)
– Oxidative phosphorylation (accounts for most of the
ATP synthesis)
• The process that generates most of the ATP is
called oxidative phosphorylation because it is
powered by redox reactions
Glycolysis
• Catabolic pathway
• Occurs in the cytosol
• Partially oxidizes
glucose (6C) into two
pyruvate (3C)
molecules
Glycolysis
Pyruvate
Glucose
Cytosol
Mitochondrion
ATP
Substrate-level
phosphorylation
Krebs Cycle
• Catabolic pathway
• Occurs in
mitochondrial matrix
• Completes glucose
oxidation by breaking
down a acetyl-CoA
into CO2
Glycolysis
Pyruvate
Glucose
Cytosol
Citric
acid
cycle
Mitochondrion
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Glycolysis and Krebs Cycle
Together produce:
• 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+
Electron Transport Chain
• Located near inner
membrane of mitochondrion
• Accepts energized electrons
from reduced coenzymes
(NADH and FADH2) that are
harvested during glycolysis
and Krebs Cycle
– Oxygen pulls electrons
down ETC to a lower
energy state
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Glycolysis
Pyruvate
Glucose
Cytosol
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
Oxidative Phosphorylation
• Accounts for almost 90% of the ATP generated by
cellular respiration
• Energy released at each step of the ETC is
stored in a form the mitochondrion can use to
make ATP
– Powered by redox reactions that transfer electrons
from food to oxygen
• Small amount of ATP is produced directly by the
enzymatic transfer of phosphate from an
intermediate substrate in catabolism to ADP
(substrate-level phosphorylation)
Glycolysis
• Glycolysis (“splitting of
sugar”) breaks down
glucose into two
molecules of pyruvate
• Two major phases:
– Energy investment
phase
– Energy payoff phase
Glycolysis
Glycolysis
Glycolysis
Glycolysis
Conversion of Pyruvate to Acetyl CoA:
Pyruvate Oxidation
LE 9-12_1
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
Citric
acid
cycle
LE 9-12_2
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
CO2
Citric
acid
cycle
NAD+
NADH
+ H+
a-Ketoglutarate
NAD+
Succinyl
CoA
NADH
+ H+
CO2
LE 9-12_3
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
H2O
Oxaloacetate
Citrate
Isocitrate
CO2
Citric
acid
cycle
NAD+
NADH
+ H+
Fumarate
a-Ketoglutarate
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
LE 9-12_4
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidation
phosphorylation
ATP
Acetyl CoA
NADH
+ H+
H2O
NAD+
Oxaloacetate
Malate
Citrate
Isocitrate
CO2
Citric
acid
cycle
H2O
NAD+
NADH
+ H+
Fumarate
a-Ketoglutarate
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
Net Products
Glycolysis:
2 pyruvate
2 NADH
2 ATP
Pyruvate Oxidation:
2 NADH
Krebs Cycle:
6 NADH
2 FADH2
2 ATP
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 cristae of the
mitochondrion
• The carriers alternate reduced and oxidized states
as they accept and donate electrons
• Electrons lose energy as they go down the chain
and are finally passed to O2, forming water
• The electron transport chain generates no ATP
LE 9-13
NADH
50
Free energy (G) relative to O2 (kcal/mol)
FADH2
40
FMN
I
Multiprotein
complexes
FAD
Fe•S II
Fe•S
Q
III
Cyt b
30
Fe•S
Cyt c1
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylation:
electron transport
and chemiosmosis
IV
Cyt c
Cyt a
Cyt a3
20
10
0
2 H+ + 1/2 O2
H2O
ATP
Chemiosmosis
• 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
LE 9-14
INTERMEMBRANE SPACE
H+
H+
H+
H+
H+
H+
A rotor within the
membrane spins
as shown when
H+ flows past
it down the H+
gradient.
H+
A stator anchored
in the membrane
holds the knob
stationary.
A rod (or “stalk”)
extending into
the knob also
spins, activating
catalytic sites in
the knob.
H+
ADP
+
P
ATP
i
MITOCHONDRAL MATRIX
Three catalytic
sites in the
stationary knob
join inorganic
phosphate to
ADP to make
ATP.
• 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
1 NADH = 3 ATP
1 FADH2 = 2 ATP
10 NADH = 30 ATP
2 FADH2 = 4 ATP
34 ATP
Aerobic: existing in
the presence of
oxygen
Anaerobic: existing
in the absence of
oxygen
Fermentation
• Cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
fermentation to produce ATP
Fermentation
• Anaerobic catabolism of organic nutrients
• After pyruvate is produced in glycolysis, it
is reduced, and NAD+ is regenerated
– Prevents cell from depleting the pool of
NAD+, needed in glycolysis
– No additional ATP is produced
Organisms Classified by Oxygen
Requirements
• Strict (obligate) aerobes —require O2 for
growth and metabolism
• Strict (obligate) anaerobes —only grow in
the absence of O2 (O2 is toxic)
• Facultative anaerobes —capable of
growing in either aerobic or anaerobic
conditions
1. NADH is
oxidized to
NAD+ and
pyruvate is
reduced to
lactate (lactic
acid)
•Commercially important products: cheese & yogurt
•Human muscle cells switch to lactic acid fermentation
when O2 is scarce. Lactate accumulates, slowly carried to
liver and converted back to pyruvate when O2 is available
1. Pyruvate loses
CO2 and is
converted to
the
acetylaldehyde
(2C)
2. NADH is
oxidized to
NAD+ and
acetylaldehyde
is reduced to
ethanol (EtOH)
•Many bacteria and yeast carry out alcohol
fermentation under anaerobic conditions
Alcohol Fermentation
Yeast during brewing process
http://www.langhambrewery.co.uk/content/ferme
ntation-yeast.jpg
Actively fermenting beer. The
yeast mass is converting the
sugars to alcohol and carbon
dioxide.
www.berkshirebrewingcompany.com/.../page4.h
tml
Versatility of Catabolism
• Starchglucose in
digestive tract
• Liver converts
glycogenglucose
• Excess amino
acidspyruvate, acetyl
CoA, and α-ketoglutarate
• Fatsglycerol + fatty acids
• Glycerolglyceraldehyde
phosphate
• Fatty acidsacetyl CoA
(beta oxidation)
Biosynthesis
• Some organic molecules of food provide
carbon skeletons or raw materials for
making new macromolecules
• Some organic molecules from digestion
used directly in anabolic pathways
• Some precursors come from glycolysis
and Krebs Cycle
• Anabolic pathways require ATP produced
from catabolic pathways
Feedback Mechanism
of Control
• Ratio of ATP:ADP and
AMP reflects energy
status and
phosphofructokinase is
sensitive to changes in
the ratio
• Citrate and ATP =
allosteric inhibitors
• ADP and AMP =
allosteric activators