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Chapter 7 – Cellular Pathways that Harvest Chemical Energy
Oxidation and Reduction Reactions
A.e
B
A
B.e
An oxidation reaction involves the transfer of an
electron from one compound to another. In this
case, A loses an electron to B (small arrow indicates
electron path). Thus, A is oxidized (loses electron)
while B is reduced (gains electron). The molecule
that accepts an electron is called the oxidizing agent
while the molecule that donates the electron is
called the reducing agent.
The oxidation of a molecule releases energy while the reduction of a molecule stores
energy. Since these reactions are coupled, the energy released in the oxidation is
transferred to and stored by the reduced molecule.
In metabolism, NAD+ and FAD are the oxidizing agents; then later NADH and FADH2
become the reducing agents.
Stages of Respiration
1. glycolysis – 6C glucose converted to two 3C pyruvates
2. pyruvate converted to two CO2 molecules and two 2C sugars
3. citric acid cycle - 2C sugars added to oxaloacteate to form two 6C sugars,
converted to 4 CO2
4. electron transport chain – electron carriers pass electrons to electron transport
chain to make ATP
Glycolysis
The first five steps are:
G3P
ATP
G
DAP + G3P
FBP
ATP
F6P
G6P
Glucose
Time
The first 5 steps of glycolysis require the input of energy in the form of 2 ATPs. Thus,
these steps are endergonic and have a positive ∆G.
The next five steps of glycolysis are exergonic. These steps produce 4 ATP, 2 NADH, 2
H+, and 2 H2O molecules per molecule of glucose. These steps also require the input of
NAD+, Pi, and ADP. If any of these molecules are in short supply, the process stops.
2NAD+ + 2Pi
2 ADP
2 G3P
2 BPG
G
2 3PG
2ADP
2 2PG
2 PEP
2NADH + 2H+
2 Pyruvate
2ATP
2H2O
Time
2ATP
The net production of ATP by glycolysis is two: 4 ATP are produced and 2 ATP are
used up. If cells are short on oxygen, and thus cannot go through the electron transport
chain, they will speed up glycolysis in order to utilize the ATP it can produce. However,
this is not very efficient because most of the energy in the glucose molecule is not
harvested.
Glycolysis occurs in the cytosol of the cell. The citric acid cycle and the electron
transport chain occur in the mitochondria. In order to convert the NADH produced in
glycolysis to ATP, the NADH needs to be transported into the mitochondria. This
transport requires energy, so 1 ATP molecule is used up per NADH transported. This is
energetically efficient because each NADH generates 3 ATP. Thus, for the two NADHs
transported, a net of 2 ATP are produced.
2 NAD+
2 Coenzyme A
2 Pyruvate
Acetyl-CoA
2 NADH
2 CO2
The pyruvate produced in glycolysis is transported into the mitochondria matrix. Each
pyruvate is oxidized to form one NADH and then is joined to coenzyme A to form acetylCoA.
Depending on the energy needs of the cell, acetyl-CoA enters the citric acid cycle or is
converted into fat for energy storage.
Citric Acid Cycle
The citric acid cycle occurs in the mitochondrial matrix. The two carbon acetyl-CoA is
added to the four carbon oxaloactate to form the six carbon molecule citric acid. In the
series of reactions that follow, the compound loses two carbon atoms to CO2, forms three
NADH molecules and one FADH2 molecule, as well as one molecule of GTP.
(Remember this is per pyruvate molecule. You need to double this to get the number per
glucose!) The remainder of the cycle is dedicated to rearranging the atoms to regenerate
oxaloacetate to allow the cycle to continue. Note that the two carbons that entered the
cycle in acetyl-CoA are not the two carbons that form CO2. The carbons exiting the cyle
came from the oxaloacetate molecule.
(multiply each quantity by 2)
Acetyl-CoA
NADH
OXAL
CIT
NAD+
MAL
ISO
H2O
NAD+
FUM
αKG
NADH + CO2
CoA
FADH2
NAD+
SUC
SCA
FAD
GTP
NADH + CO2
GTP + Pi
Electron Transport Chain
The electron transport chain occurs on the inner mitochondrial matrix. There, NADH
and FADH2 pass their electrons to the embedded carriers. The electron is then passed
down the series of cytochromes (yellow and blue), each one having a lower G than the
one before it. Because of this change in G, the energy produced is used to pump
hydrogen ions from the matrix to the intermembrane space. This creates an
electrochemical gradient across the inner mitochondrial membrane; with the
intermembrane space having a higher concentration of hydrogen ions. Oxygen accepts
the electron from the final cytochrome in the chain, and it is reduced to water. The
hydrogen ions leak back into the matrix through ATP synthase (purple), which couples
the energy produced by the ion movement to the formation of ATP from ADP and P.
This coupling of energy from electron movement to a chemical gradient is called
chemiosmosis.
The processes of proton pumping and ATP synthesis are coupled yet separate. Review
experiment on page 142 to see how this was determined.
Outer mitochondrial membrane
H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+
e-
e-
Intermembrane
space
Inner
mitochondrial
membrane
e-
eNADH
NAD+
H+
H+
ATP
H+
O2
ADP + P
H20
Q.
Could ATP be made if the one of the mitochondrial membranes was not intact?
Metabolism
We tend to focus on metabolism as the formation of ATP from glucose. This is really
only half of the process because metabolism consists of the sum of the catabolic and
anabolic reactions. Catabolic reactions are those that breakdown simple molecules to
form ATP. Anabolic reactions are those that consume ATP to produce larger molecules.
Q.
Why do NADH and FADH2 not pass their electrons and the energy they carry
directly to O2?
Q. Where are each of the ATP, NADH, FADH2, and GTP molecules produced (where
in cell)? Be sure to account for all of the molecules.