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Cellular Metabolism
Biology 105
Lecture 6
Chapter 3 (pages 56-61)
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Metabolism
Consists of all the
chemical reactions that
take place in a cell!
Cellular metabolism:
Aerobic cellular
respiration – requires
oxygen, produces
carbon dioxide
Anaerobic fermentation
– does not require
oxygen
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Summary of Cellular Respiration
Figure 3.27
Blood
vessel
Carrier
protein
Glucose
Oxygen
Extracellular fluid
Plasma
membrane
Glycolysis
glucose pyruvate
Mitochondrion
Citric
Acid
Cycle
Electron
Transport
Chain
Transition
Reaction
Electrons
transferred
by NADH
Cytoplasm
Electrons
transferred
by NADH
and FADH2
Electrons
transferred
by NADH
+32 ATP 36 ATP +2 ATP +2 ATP
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Aerobic Cellular Respiration
Aerobic cellular respiration:
Cells take sugar (glucose) and break it down into
carbon dioxide and water.
This requires oxygen!
This process produces energy in the form of
ATP.
C6H12O6 + 6O2 → 6CO2 +6H2O + energy
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Aerobic Cellular Respiration
There are four steps in aerobic cellular
respiration:
1. Glycolysis
2. Transition Reaction
3. Citric Acid Cycle (Krebs Cycle)
4. Electron Transport Chain
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NADH and FADH2 are important carriers of electrons
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Cellular Respiration
Phase 1: Glycolysis
Occurs in the cytoplasm
Splits 1 glucose into 2 pyruvate molecules
Generates a net gain of 2 ATP and 2 NADH
molecules
Does not require oxygen
Starts with glucose
Ends with 2 ATP, 2 NADH, 2 pyruvate
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Glycolysis
Figure 3.23
Cytoplasm
2 ADP
Glucose
Energy-
investment
phase
Glycolysis (in cytoplasm)
2 NADH
2 NAD+
4 ATP
4 ADP
Energy-
yielding
phase
2 ATP
The two molecules of
pyruvate then diffuse
from the cytoplasm into the inner compartment
of the mitochondrion,
where they pass through
a few preparatory steps
(the transition reaction) before entering the citric
acid cycle.
During the remaining
steps, four molecules
of ATP are produced.
During the first steps,
two molecules of ATP are
consumed in preparing glucose for splitting.
2 Pyruvate
Two molecules of nicotine
adenine dinucleotide
(NADH), a carrier of high-energy electrons,
also are produced.
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Cellular Respiration
Phase 2: Transition Reaction
Occurs within the mitochondria
Coenzyme A combines with pyruvate, and CO2
is removed from each pyruvate
Forms 2 acetyl CoA molecules
Produces 2 NADH
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Transition Reaction
Starts with:
2 pyruvate (which are 3-carbon molecules)
2 Coenzyme A
Ends with:
2 CO2
2 NADH
2 Acetyl CoA (which are 2-carbon molecules)
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Transition Reaction
Figure 3.24
NADH
(electron passes
to electron
transport chain)
NAD+
Pyruvate (from glycolysis)
Acetyl CoA
CoA
Coenzyme A
CO2
Transition Reaction (in mitochondrion)
Citric Acid Cycle
A molecule of NADH is
formed when NAD+
gains two electrons
and one proton.
One carbon (in the form
of CO2) is removed
from pyruvate.
The two-carbon
molecule, called
an acetyl group,
binds to
coenzyme A
(CoA), forming
acetyl CoA,
which enters the
citric acid cycle.
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Cellular Respiration
Phase 3: Citric Acid Cycle
Also known as the Krebs Cycle
Occurs in the mitochondria
Acetyl CoA enters the citric acid cycle
Releases 2 ATP, 2 FADH2, and 6 NADH
Generates 4 CO2 molecules
Requires oxygen
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Citric Acid Cycle
Starts with 2 Acetyl CoA
Ends with:
4 CO2
2 ATP
6 NADH and 2 FADH2
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Citric Acid Cycle
Figure 3.25
Acetyl CoA
CoA
Citrate
CO2
leaves
cycle
NAD+
NADH
-Ketoglutarate
CO2 leaves cycle
NADH
FAD
Succinate
FADH2
Malate
NAD+
ATP ADP
Citric Acid Cycle
NADH
Oxaloacetate
NAD+
Pi +
Acetyl CoA, the
two-carbon compound formed during the
transition reaction,
enters the citric acid cycle. The citric acid cycle also
yields several molecules of FADH2 and NADH, carriers of
high-energy electrons that
enter the electron transport chain.
The citric acid cycle yields
one ATP from each acetyl CoA that enters the cycle,
for a net gain of two ATP.
Citric Acid Cycle (in mitochondrion)
CoA
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Cellular Respiration
Phase 4: Electron Transport Chain
Electrons of FADH2 and NADH are transferred
from one protein to another, until they reach
oxygen
Releases energy that results in 32 ATP
Requires oxygen
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The Big Pay Off – Electron Transport Chain!
NADH and FADH2 are important electron carriers.
They donate electrons to the electron transport
chain.
At the end of the chain, oxygen accepts the
electrons.
The electron transport chain produces ATP using
the ATP synthase molecule (a protein).
The electron transport chain produces 32 ATP!
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Electron Transport Chain
Figure 3.26
The molecules of NADH and FADH2 produced by earlier phases of cellular respiration pass their electrons to a series of protein molecules embedded in the inner membrane of the mitochondrion.
As the electrons are transferred from one protein to the next, energy is released and used to make ATP. 2e–
2e–
2e–
2e–
2e–
NAD+
High
Low
Membrane
proteins
H2O
2 H+ + O2 Energy released is used
for synthesis of ATP
FADH2
NADH
Electron Transport Chain (inner membrane of mitochondrion)
FAD
1 2
Po
ten
tial e
nerg
y
Eventually, the electrons are passed to oxygen, which combines with two hydrogens to form water.
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How is ATP made using the ETC?
1. In the mitochondria, the NADH and FADH2 donate electrons to the electron transport chain (ETC).
2. Oxygen is the final electron acceptor from the ETC.
3. The ETC uses the energy from the electrons to transport H+ against its concentration gradient, transporting the H+ from the lumen of the mitochondria to the intermembrane space (from the inner compartment to the outer compartment).
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How is ATP made using the ETC?
4. The ATP synthase transports the H+ back to the lumen (inner compartment) of the mitochondria.
5. The H+ falling through the ATP synthase provides the energy for ATP synthase to catalyze the reaction of:
ADP + P → ATP
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Summary of Cellular Respiration
Table 3.5
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Summary of Cellular Respiration: The Players
One molecule of glucose is broken down and
36 ATP are generated.
Oxygen is used by the electron transport chain
– it accepts electrons from the ETC.
Carbon dioxide is produced by the Transition
Reaction and the Citric Acid Cycle.
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Glycolysis: kicks off the process by taking in
one glucose produces 2 ATP
Transition Reaction: produces CO2 and NADH
Citric Acid Cycle: produces 2 ATP, lots of
NADH and FADH2, and some CO2
Summary of Cellular Respiration Summary of Cellular Respiration: The Steps
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Electron Transport Chain
Takes electrons from NADH and FADH2 and
uses them to produce ATP using the ATP
synthase molecule.
Requires oxygen! Oxygen is the final electron
acceptor on the electron transport chain.
One glucose can produce a total of 36 ATP!!!
Summary of Cellular Respiration Summary of Cellular Respiration: The Steps
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Complex
carbohydrates must
first be broken down
into glucose before
entering glycolysis.
Fats and proteins
enter the process at
different steps.
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A few words about oxygen…
Cellular respiration requires oxygen – this is
why it is called aerobic cellular respiration.
Sometimes organisms, including humans, need
to produce energy without using oxygen.
When you need energy quickly, or if there is
not enough O2, then the cell will use only
glycolysis.
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Anaerobic Fermentation
Breakdown of glucose without oxygen.
Takes place entirely in the cytoplasm.
It is very inefficient and results in only two
ATP.
Anaerobic Fermentation: anaerobic pathway to
produce ATP from glycolysis, without the Citric
Acid Cycle and the ETC.
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Fermentation in Animals
When cells need energy quickly, they will use
this pathway for a short time.
2 pyruvic acid + 2 NADH → 2 lactate and 2
NAD+
End result = lactate and 2 ATP produced (from
glycolysis) and NAD+ is regenerated.
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Q: What is the starting molecule of glycolysis?
1. Acetyl CoA
2. Protein
3. Glucose
4. Pyruvate (pyruvic acid)
Q: Which stage produces CO2?
1. Glycolysis
2. Electron Transport Chain
3. Transition
4. Citric Acid Cycle
5. Both 3 and 4
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Q: Which stage uses O2?
1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
Q: Which stage produces the most NADHs?
1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
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Q: Which stage produces the most ATP?
1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
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Important Concepts
Read chapter 3, pages 56-61
What are aerobic cellular respiration and
anaerobic fermentation? What are the
differences between them?
What are the four steps of aerobic cellular
respiration, what happens in each step, what
are the starting molecules, what comes out of
each step, where in the cell does each step
occur, how many ATP and NADH/FADH2 are
produced in each step?
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Important Concepts
Describe in detail how ATP is made using the electron transport chain.
What are the roles of ATP synthase, H+, O2, NADH and FADH2, and the electron transport chain in ATP production?
Know the overall picture of cellular respiration (summary slides).
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Important Concepts
What is the role of oxygen in cellular respiration, and which steps produce carbon dioxide?
What is anaerobic fermentation, what steps are involved in fermentation, and what end products are produced in humans? Is oxygen required? If so, when is it used?