Chapter 9: Cellular Respiration
AP Biology Notes:
I.
Chapter 9.1: Catabolic pathways yield energy by oxidizing organic fuels:
a. Fermentation: degradation of sugars that occurs without oxygen
b. Cellular respiration: oxygen is consumed as a reactant along with the
organic fuel
i. Degradation of sugar:
ii. C6H12O6 + O2  CO2 + H2O + Energy
iii. Exergonic reaction
1. ΔG = -686kcal/mole
2. Catabolic
3. Increase in entropy
c. Redox Reactions: Oxidation and Reduction
i. Transfer of electrons from one reactant to another =
oxidation/reduction reactions
1. Oxidation = the loss of electrons
2. Reduction = the gaining of electrons
3. Reducing agent = the electron donor- that which gets
oxidized
4. Oxidizing agent = the electron acceptor- that which gets
reduced
ii. Not all redox reactions involve the complete transfer of
electrons
1. Some change the degree of electron sharing
d. Oxidation of Organic Fuel Molecules During Cellular Respiration:
i. Respiration: oxidation of glucose
ii. C6H12O6 + 6O2  6CO2 + 6H2O + Energy
1. Glucose is oxidized
2. Oxygen is reduced
a. Electrons lose potential energy along the way
b. Energy is released
c. ΔG is negative
d. Downhill reaction
e. Energy becomes available for ATP synthesis
e. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain:
i. Energy cannot be harvested all at once:
1. Example: gasoline energy- if all the energy is harvested
at once an explosion ensues = no more energy for work
2. All of the energy from glucose is not harvest at the same
time during cellular respiration
a. Instead electrons are stripped from glucose at
key steps
i. the electron travels with a proton which
is a hydrogen atom
1. electrons lose very little potential
energy when they are transferred
ii. the electron travels to oxygen via a
coenzyme called: nicotinamide adenine
dinucleotide (NAD +)
1. NAD+ is a derivative of the vitamin
niacin
2. NAD+ is a oxidizer (electron
acceptor)
3. NAD+ is the most versatile
electron acceptor in cellular
respiration
3. How does NAD+ work:
a. Enzymes called dehydrogenases remove a part
of hydrogen atoms (two electrons and two
protons) from a substrate (glucose or some
other organic molecule) oxidizing it
b. The enzyme then delivers the two electrons and
one proton to NAD+
i. The other proton is released as H+ to the
surroundings
ii. NAD+ then becomes neutralized to NADH
1. NADH represents stored energy
that can be used to make ATP
when the electron is transferred
from NADH to oxygen
4. How do electrons stored by NADH reach oxygen:
a. Electron transport chain- electrons cascading
down a chain from one carrier molecule to the
next, losing small amounts of energy with each
step until they finally reach oxygen.
i. Breaks the fall of electrons from NADH to
oxygen into several steps instead of one
explosive reaction
ii. Located on the inner mitochondrial
memebrane
iii. Involves several proteins and other
molecules
b. NADH shuttles the electron to the top of the
electron chain (high energy end) and oxygen
captures electrons at the bottom of the electron
chain(low energy end)
c. Exergonic reaction
i. ΔG = -53kcal/mole
ii. Downhill reaction
II.
iii. Oxygen = final electron acceptor because
it has a high affinity for electrons(highly
electronegative)
f. The Stages of Cellular Respiration:
i. Three metabolic stages:
1. Glycolysis: occurs in the cytosol
2. The citric acid cycle: takes place in the mitochondrial
matrix(composed mostly of (50%) proteins)
a. Also called the tricarboxylic acid cycle and the
Krebs cycle(after Hans Krebs- German biologist1930’s)
3. Oxidative phosphorylation: occurs on the inner
mitochondrial membrane
Overview of Cellular Respiration:
a. Step one:
i. Each glucose is broken down into two pyruvate molecules
1. ATP is generated
2. Substrate level phosphorylation: ATP synthesis occurs
when an enzyme transfers a phosphate group from a
substrate molecule to ADP
a. Substrate molecule refers to an organic molecule
generated during the catabolism of glucose
3. Energy Investment phase: 2 ATP are used to provide
energy for a enzymatically controlled pathway breaking
down a 6 carbon glucose molecule to two 3- carbon
sugars
a. Net energy input = 2 ATP
4. Energy Payoff Phase: Enzymatic pathway where each 3carbon sugar is converted into a pyruvate molecule
a. 2 NAD+ molecules are oxidized to NADH
b. Net energy created 4 ATP
5. Overall Net energy: 2 ATP
6. Electron carriers are reduced(catalyzed by
dehydrogenase)
a. 2 NAD+ + 4e- + 4H+ yields 2 NADH + 2 H+
7. less than a quarter of the chemical energy stored in
glucose is released during this stage
a. the rest of the energy is stored in the two
pyruvate molecules
b. Step Two: The two pyruvate molecules enter the mitochondria
i. They get converted to acetyl CoA- or acetyl coenzyme A (2carbon molecule)
1. This is an intermediate process that occurs before
entering the citric acid cycle
2. Multi- Enzymatic process
3. 1 molecule of CO2 is released
III.
ii. The two acetyl CoA’s get oxidized to carbon dioxide in the citric
acid cycle
1. Involves eight steps, each controlled by a particular
enzyme
a. All enzymes are located in the mitochondrial
matrix
2. Two more CO2’s are released
3. Completes the breakdown of glucose
4. Called a cycle because a molecule oxaloacetate is broken
down in step one only to be regenerated in step eight
and reenter the cycle back at step one with a new
acetyl- CoA molecule.
5. For each acetyl- CoA that enters the cycle 1 ATP is
generated
a. Electron carriers NAD+ and FAD+ are reduced
i. 3 NAD+ are reduced
ii. 1 FAD+ are reduced
c. Step Three: NADH and FADH2 transfer electrons from the pyruvates to
the electron transport chain located on the inner mitochondrial
membrane
i. Electrons are transferred from one molecule to another
ii. At the end of the chain the electrons are transferred to oxygen
and combined with hydrogen to form water
iii. Any energy released at each step is stored in the inner
mitochondrial membrane space
1. This energy is used to synthesize ATP in a process
called chemiosmosis
Chapter 9.4: During Oxidative Phosphorylation, Chemiosmosis couples
electron transport to ATP synthesis.
a. The Pathway of Electron Transport:
i. Folding of inner mitochondrial membrane called cristae,
increase surface area
1. Provides space for thousands of copies of electron
transport chain which is a collection of molecules
a. Most of these molecules are proteins
ii. During electron transport, electron carriers alternate between
reduced and oxidized states
1. Electrons move downhill
iii. NADH carries an electron to the first protein complex and
reduces FMN (flavin mononucleotide), NADH becomes
oxidized to NAD+
1. FMN will then become oxidized by reducing the Fe-S
protein
2. Fe-S protein then will reduce ubiquinone (Q) the only
non- protein molecule in the complex
3.
4.
5.
6.
a. Q- resides between protein complex I and
protein complex III
In protein complex II, FADH2 will reduce Fe-S protein,
returning to its oxidized state (FAD+)
a. Fe-S protein from complex II will also reduce Q
Protein complex III- Q will reduce cytochrome b, which
transfers its electron to another Fe-S protein which then
reduces cytochrome c1.
a. Cytochromes are electron carriers that have a
prosthetic group called a heme group.
i. The heme has an iron atom that accepts
and donated electrons
Cytochrome c1 will then transfer its electron to
cytochrome c which resides between protein complex
III and protein complex IV.
In protein complex IV, cytochrome a is reduced from
cyctochrome c.
a. Cytochrome a then reduces cytochrome a3
b. Cytochrome a3 is the last stop for the electron
before it is finally accepted by oxygen
i. Oxygen, due to its high electronegative
state, accepts the electron along with two
hydrogens from the aqueous solution to
form water