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Cellular Respiration
Harvesting energy at the cellular level
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Cellular Respiration vs. Breathing
„
Breathing
‰
„
Cellular Respiration
‰
„
Moving air into and out of
lungs…gas exchange
Breaking down food
(glucose) to release the
potential energy.
How are they related?
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Types of Metabolism
‰
‰
Aerobic metabolism
„ Occurs when enough
oxygen reaches cells
to support energy
needs.
Anaerobic metabolism
„ Occurs when the
demand for oxygen is
greater than the
body’s ability to
deliver it.
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Main Types of
Energy-Releasing Pathways
Anaerobic pathways
„
„
„
„
Evolved first
Don’t require oxygen
Start with glycolysis in
cytoplasm
Completed in
cytoplasm
Aerobic pathways
„
„
„
„
Evolved later
Require oxygen
Start with glycolysis in
cytoplasm
Completed in
mitochondria
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Chemical Cycling between
Photosynthesis and Cellular Respiration
Recall…
‰
The ingredients for photosynthesis are carbon dioxide and
water.
„
„
‰
CO2 is obtained from the air by a plant’s leaves.
H2O is obtained from the damp soil by a plant’s roots.
Chloroplasts rearrange the atoms of these ingredients to
produce sugars (glucose) and other organic molecules.
„
Oxygen gas is a by-product of photosynthesis.
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Remember…
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ATP Is Universal
Energy Source
„
Photosynthesizers get energy from
the sun
„
Animals get energy second- or third-hand
from plants or other organisms
„
Regardless, the energy is converted to the
chemical bond energy of ATP
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Cellular Respiration Equation
„
Function:
‰
„
Location:
‰
„
Convert energy stored in glucose into ATP
Cytoplasm and mitochondria
Its sort of looks like the photosynthesis reaction…only in reverse.
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Cellular Respiration Overview
Glycolysis
Kreb’s
Cycle
Electron Transport
Chain
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The Role of Coenzymes
„
NAD+ and FAD accept electrons and hydrogen
„
Become NADH and FADH2 (When they are reduced)
„
Deliver electrons and hydrogen to the electron transfer
chains
‰
Think of them as “delivery molecules”…carrying their
cargo of electrons and hydrogen ions
‰
They pick up their cargo in one part of the cell, and
deliver it to another “drop off” site in the cell.
„
Function during REDOX reactions – reactions that
involve the movement of electrons between reactants.
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A closer look at REDOX reactions:
„
Comprised of two subreactions
‰
Reduction – when an electron is gained
‰
Oxidation – when an electron is lost
„
“LEO the lion says GER”
„
Carbon atoms in food molecules will be oxidized
(stripped of their electrons), producing CO2. The
electrons will eventually be gained by oxygen (in a
reduction step) to produce METABOLIC H2O.
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Glucose
„
A simple sugar
(C6H12O6)
„
Atoms held
together by
covalent bonds
Contains Potential
energy
„
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Cellular Respiration Overview
Glycolysis
Kreb’s
Cycle
Electron Transport
Chain
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Glycolysis is the first part of cellular
respiration.
„
It is an ancient pathway for metabolism
„
OCCURS WHETHER OXYGEN IS PRESENT OR
ABSENT!
„
Nearly all life, both prokaryotic and eukaryotic, do
glycolysis the same way
„
The enzymes of glycolysis are located in a cellular
space common to both prokaryotes and
eukaryotes….which is….?
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Glycolysis Occurs in Two parts
„
Energy-requiring steps
‰
ATP energy activates glucose and its six-carbon
derivatives
„
Energy-releasing steps
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The products of the first part are split into three-carbon
pyruvate molecules
‰
ATP and NADH form
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Energy-Requiring Steps of Glycolysis
Energy-Requiring
2 ATP invested
Steps
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
P
fructose1,6-bisphosphate
P
PGAL
P
PGAL
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P
NAD+
Pi
PGAL
NADH
P
P
1,3-bisphosphoglycerate
ADP
ATP
P
3-phosphoglycerate
P
2-phosphoglycerate
H2O
P
Energy-Releasing Steps
Step #6 is crucial
For ATP production!
NAD+ is required
And NADH is formed
PEP
ADP
ATP
pyruvate
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Glycolysis: Net Energy Yield
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed, 2 pyruvates
Net yield is 2 ATP, 2 NADH and 2
pyruvate molecules
This is NOT enough ATP energy to
Support higher life…the process continues
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Mitochondrial Anatomy
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Cellular Respiration Overview
Glycolysis
Kreb’s
Cycle
Electron Transport
Chain
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Second Stage Reactions
„
Preparatory reactions
‰
‰
„
Pyruvate is oxidized into two-carbon acetyl units
and carbon dioxide
NAD+ is reduced to form NADH
Krebs cycle (the citric acid cycle)
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‰
The acetyl units are oxidized to carbon dioxide
NAD+ and FAD are reduced to form NADH and
FADH2
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Preparatory Reactions
pyruvate
NAD+
coenzyme A (CoA)
NADH
O
O carbon dioxide
CoA
acetyl-CoA
PYRUVATE must be converted into
Acetyl CoA before it can enter the
Krebs cycle.
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=CoA
acetyl-CoA
Krebs
Cycle
CoA
oxaloacetate
citrate
NADH
H2O
NAD+
H2O
malate
NAD+
H2O
FADH2
isocitrate
NADH
fumarate
O
O
α-ketoglutarate
FAD
NAD+
NADH
CoA
O
O
succinate
succinyl-CoA
ATP
ADP +
phosphate group
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The Krebs Cycle (the citric acid cycle)
Overall Reactants
„
„
„
„
Acetyl-CoA
3 NAD+
FAD
ADP and Pi
Overall Products
„
„
„
„
„
Coenzyme A
2 CO2
3 NADH
FADH2
ATP
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Results of the Krebs Cycle
„
All of the carbon molecules in pyruvate end
up in carbon dioxide gas.
‰
„
„
„
What does the cell do with the CO2 ?
Coenzymes are reduced (they pick up
electrons and hydrogen)
One molecule of ATP is formed
Four-carbon oxaloacetate is regenerated so
that the “wheel” can turn once more!
(assuming there is another acetyl CoA being
delivered)
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Coenzyme Reductions During First
Two Stages
„
Glycolysis
Preparatory
reactions
Krebs cycle
2 NADH
2 FADH2 + 6 NADH
„
Total
2 FADH2 + 10 NADH
„
„
2 NADH
There is potential energy still
left in these Molecules!
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Cellular Respiration Overview
Glycolysis
Kreb’s
Cycle
Electron Transport
Chain
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Electron Transfer and Oxidative
Phosphorylation
„
„
Occurs in the mitochondria
Coenzymes deliver electrons to electron
transfer systems (electron cargo is delivered!)
‰
„
„
Oxidized coenzymes go back to glycolysis (step
#6) and Krebs cycle to allow them to continue!
Electron transfer through the chain sets up a
H+ ion gradient
Flow of H+ down gradient powers ATP
formation
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Creating an
+
H
Gradient
OUTER COMPARTMENT
NADH
e-
ee-
NAD+
(Back to
Earlier
Steps)
ee-
INNER COMPARTMENT
H2O
eO2 + e-
e-
Metabolic water (water made from metabolism)
1. As NADH is oxidized, the electrons leave NADH and enter the electron
transport chain. The electrons flow towards and eventually combine with
oxygen to form metabolic water
2. As the electrons flow through the chain, hydrogen ions are PUMPED across
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the membrane!
Importance of Oxygen
„
Electron transport and oxidative
phosphorylation requires the presence of
oxygen
„
Oxygen withdraws spent electrons from the
electron transport system, then combines
with H+ to form Metabolic water
What is the difference between water you
drink and metabolic water?
„
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Making ATP in the mitochondria
ATP
INNER
COMPARTMENT
ATP synthase
ADP
+
P
1. Hydrogen ion “flow” down their gradient back into the inner compartment
through ATP Synthase.
2. As they flow through the enzyme, it rotates (like a generator), and
combines ADP + P (a phosphate group) and forms ATP!
3. The SPEED of the flow, POWERS the “recharging” of the ATP “battery”!
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Summary of Energy Harvest
(per molecule of glucose)
„
Glycolysis
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„
Krebs cycle and preparatory reactions
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„
2 ATP formed by substrate-level phosphorylation
Electron transport phosphorylation
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2 ATP formed by substrate-level phosphorylation
28 - 30 ATP formed
ADDING IT ALL TOGETHER GIVES YOU AROUND
32 ATP MOLECULES PER GLUCOSE!
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Fermentation Pathways
„
Begin with glycolysis
„
Are anaerobic: don’t require oxygen
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Yield only the 2 ATP from glycolysis. Doesn’t
use mitochondria. Occurs in the cytoplasm
„
2 ATP/glucose only supports “higher” life for a
short time. Humans use this only transiently!
„
Steps in fermentation serve only to regenerate
NAD+ so that glycolysis can continue.
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Lactate Fermentation
HUMANS DO THIS
GLYCOLYSIS
C6H12O6
2
STEP #6
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
LACTATE
FORMATION
LDH enzyme
electrons, hydrogen
from NADH
2
lactate
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GLYCOLYSIS
Alcoholic
Fermentation
C6H12O6
2
ATP
energy input
STEP #6
2 NAD+
2 ADP
2
4
NADH
ATP
2 pyruvate
energy output
2 ATP net
ETHANOL
FORMATION
2 H2O
2 CO2
YEAST DO THIS
ADH enzyme
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
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Alternative Energy Sources
FOOD
fats
fatty
acids
glycerol
glycogen
complex
carbohydrates
proteins
simple sugars
amino acids
glucose-6-phosphate
NH3
GLYCOLYSIS
PGAL
carbon
backbones
urea
pyruvate
acetyl-CoA
KREBS
CYCLE
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Evolution of Metabolic Pathways
„
When life originated, atmosphere had little oxygen
„
Earliest organisms used anaerobic pathways
„
Later, the evolution of photosynthesis increased
atmospheric oxygen
„
Cells arose that used oxygen as final acceptor in
electron transport and energy harvesting became
more efficient!
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