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Cellular Respiration
Cell Respiration
Step 1 :Krebs Cycle
Step 2: Electron Transport Chain
(oxidative phosphorylation)
*Much more energy is released than in fermentation so
if an organism has a choice- will use cell respiration
*Note: your book separates Krebs into 2 steps: “Pyruvate
Processing” and “Krebs”. My lecture combines them as “Krebs”
in attempt to simplify the steps.
Cellular Respiration
Recall: Glycolysis left us with pyruvate
If O2 is present, pyruvate is converted to acetyl Co-A
which enters the Krebs cycle
CO2 + NADH +
Krebs
Consequences of this oxidation rxn:
1 of the 3 C atoms in pyruvate converted to CO2
The other 2C atoms are converted to Acetyl CoA
Energy released from this rxn used to reduce NAD+ to NADH
Cellular Respiration
acetyl Co-A
(higher energy)
Acetyl CoA enters the Krebs Cycle:
The Big Picture
•Successive oxidation
of 8 Carbon molecules
•Enough energy
released from this
to generate ATP,
NADH and FADH
(similar role as NADH)
(lower energy)
Cellular Respiration
Krebs Cycle: The Big Picture
6C
5C
4C
Fig 9.19
CO2 also produced as a result of oxidizing C molecules
(5C sugar 4 C sugar + CO2)
Cellular Respiration Krebs Cycle: The Big Picture
(e-carriers/donors)
Acetyl Co-A  CO2 + NADH + FADH + ATP
ee-
ee-
exhaled/
eused for
ephotosynthesis
O2
Can immediately
be used for
energy
(ATPADP+
energy)H O
2
(e-acceptor)
This happens in next step- electron
transport chain
Cellular Respiration
Where is this happening in the cell?
Glycolysis and
fermentation:
cytosol
Prokaryotes
Respiration:
mitochondria
Eukaryotes
Glycolysis,
Fermentation, and
respiration:
cytosol
Cellular Respiration
After glycolyis, pyruvate is
transported into matrix of
the mitochondria where
the Krebs cycle occurs
Where is this happening in the cell?
Cellular Respiration
What happens to the Krebs Cycle Products?
CO2 gas released
from cell (passes
through mitochondria
and cell membranes)
Electron carriers (NADH &
FADH) carry their
electrons to the inner
membrane where many molecules
that make up the electron transport chain are embedded.
inner membrane
outer
membrane
inner
membrane
space
Electron Transport Chain Big Picture
(NADH or FADH)
“chain” refers to a series of
molecules that receive and
then pass off electrons.
With each transfer, energy is
released
The final e- acceptor in the chain
in O2. The diff. in potential
energy between NADH and O2 is
large. The difference is what is
released.
When O2 receives
(O2) this is why
e- it gets reduced to
water. This is water
O2 needed
H2O
is a byproduct of
respiration
Electron Transport and Chemiosmosis
• Most of the ETC molecules are proteins containing
chemical groups that facilitate redox reactions. All but
one of these proteins are embedded in the inner
mitochondrial membrane.
– In contrast, the lipid-soluble ubiquinone (Q) can move
throughout the membrane.
• During electron transport, NADH donates electrons to a
flavin-containing protein at the top of the chain, but
FADH2 donates electrons to an iron-sulfur protein that
passes electrons directly
to Q.
Unlike in Glycolysis and Krebs where ATP is directly generated from the
change in free energy, there is another step between release of energy
and ATP in the ETC: Oxidative Phosphorylation
ATP
*Released energy from ETC used to pump H+ into inner memb space
(against gradient) where high [H+] accumulates (much potential energy
in this space). As those H+ move down gradient through ATP synthase,
the energy is release and that energy is used
to make ATP from ADP
Glycolysis, Krebs Cycle and ETC
(Cell Respiration)
C6H12O6 +
6O2 +
CO2 +
6 H20
+
ATP
Gets converted to pyruvate (after many intermediate C molecules),
then acetyl Co-A, then a series of more oxidized C molecules
and eventually all the carbons in glucose become C in CO2 (oxidized)
used as the final electron acceptor in the electron transport
chain, without it-- fermentation
produced in Krebs cycle
from reduction
of O2 in ETC
26 net/
glucose
ENERGY!
Feedback Inhibition
• Feedback inhibition occurs when an enzyme
in a pathway is inhibited by the product of that
pathway.
– Cells that are able to stop glycolytic reactions
when ATP is abundant can conserve their stores of
glucose for times when ATP is scarce.
Regulation of Glycolysis
•
•
•
•
ATP = product of glycolysis
ATP inhibits glycolysis
So if there is enough ATP, no glycolysis
Remember, glycolysis does require some
energy input.
• Natural selection favors organisms that can
conserve glucose when ATP is not needed.
Feedback Inhibition Regulates
Glycolysis
• During glycolysis, high levels of ATP inhibit the enzyme
phosphofructokinase, which catalyzes one of the early
reactions.
• Phosphofructokinase has two binding sites for ATP:
1. The active site, where ATP phosphorylates fructose-6phosphate, resulting in the synthesis of fructose-1,6bisphosphate
2. A regulatory site
High ATP concentrations cause ATP to bind at the
regulatory site, changing the enzyme’s shape and
dramatically decreasing the reaction rate at the active site.
Regulation of Krebs cycle
• The enzyme that converts acetyl CoA to the
first product in the Krebs cycle is inhibited
directly by ATP.
• There are other places where NADH or ATP
directly inhibits the Krebs cycle.
Regulation of pyruvate (before
entering Kreb (citric acid) cycle
• High levels of ATP, acetyl CoA, and NADH all
inhibit PDH (pyruvate dehydrogenase).
• High levels of NAD+, CoA, or AMP (aka low
ATP) speeds rxn
The cell can use other carbon
compounds in these processes
• Carbohydrates first, then fats, then proteins
Cellular Respiration
What if there is an over-flow of pyruvate: (more made by glycolysis
than is needed for cell respiration)?
Converted to an stored as:
Fats
What if there is an over-flow of glucose (more glucose in the blood
from glycolysis than is needed for cell respiration)?
Converted to an stored as:
glycogen
Anabolic Pathways Synthesize Key
Molecules
• Molecules found in carbohydrate metabolism
are used to synthesize macromolecules such as
RNA, DNA, glycogen or starch, amino acids,
fatty acids, and other cell components.