VII. CHAPTER 7 – CELLULAR RESPIRATION
A. Section 1 – Glycolysis And Fermentation
1. Harvesting Chemical Energy
a. Cellular Respiration – a series of redox reactions in which cells
make ATP by breaking down organic compounds.
b. Glycolysis – a biochemical pathway that begins cellular
respiration and yields a small amount of ATP. The other
products of glycolysis can follow one of two pathways:
1. Anaerobic Pathway – yields additional ATP from
oxygen lacking products.
2. If oxygen is present, products enter aerobic
respiration. Aerobic respiration produces a much
larger amount of ATP than glycolysis alone.
2. Glycolysis
a. Glycolysis is a pathway in which one six-carbon molecule of
glucose is oxidized to produce two three-carbon molecules of
pyruvic acid.
b. Reactions of glycolysis take place in the cytosol and are
catalyzed by enzymes.
c. Step 1 (Energy Investment): Two phosphate groups are
attached to glucose, forming a new six-carbon compound. The
phosphate groups are supplied by two molecules of ATP,
which are converted into molecules of ADP in the process.
d. Step 2 (Cleavage of Sugar): The six-carbon compound
formed in Step 1 is split into two three-carbon molecules of
PGAL.
e. Step 3: The two PGAL molecules are oxidized, and each
receives a free-floating phosphate group. This produces two
molecules of a new three-carbon compound. Two molecules of
NAD+ are reduced to NADH after accepting the electrons from
the oxidation reaction.
f. Step 4 (Energy Generation): The phosphate groups added in
Steps 1 and 3 are removed from the three-carbon compounds.
This produces two molecules of pyruvic acid. Each of the four
phosphate groups combine with ADP to make four molecules
of ATP.
g. Because 2 ATP molecules are required to transport NADH into
the mitochondria, the net product of glycolysis is 2 ATP.
3. Fermentation – the combination of glycolysis with additional pathways
to convert pyruvic acid into other compounds in the lack of oxygen.
a. These additional pathways do not produce ATP. Instead, they
regenerate NAD+ which can be used to power glycolysis.
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b. Lactic Acid Fermentation – the process by which an enzyme
converts pyruvic acid into another three-carbon compound,
called lactic acid. NADH is oxidized back to NAD+. It occurs
in muscle cells.
1. When oxygen is once again available, the liver can
convert lactic acid back into pyruvate.
c. Alcoholic Fermentation – the process by which cells convert
pyruvic acid into ethyl alcohol. Yeast uses this pathway; and
hence they form the basis for the wine and beer industry.
1. Organisms can only use alcoholic fermentation at
concentration of alcohol at or below 14%. Otherwise,
the organism could die.
4. Energy Yield
a. Kilocalorie (kcal) – a value used to obtain the efficiency of
anaerobic pathways.
b. The complete oxidation of a standard amount of glucose
released 686 kcal.
c. The efficiency of glycolysis can be calculated using this
formula:
Energy required making ATP
Energy released by oxidation of glucose
B. Section 2 – Aerobic Respiration
1. Overview Of Aerobic Respiration
a. Mitochondrial Matrix – a space inside the inner membrane of a
mitochondrion. The pyruvic acid that is produced in glycolysis
diffuses across the mitochondrion’s double membrane into the
matrix.
b. Acetyl CoA (acetyl coenzyme A) – the product of a reaction
involving pyruvic acid and coenzyme A.
c. One carbon atom is lost during the reaction, which combines
with two oxygen molecules to form CO2. The reaction also
reduces a molecule of NAD+ to NADH.
2. The Krebs Cycle – a biochemical pathway that breaks down acetyl
CoA, producing hydrogen atoms and ATP.
a. The reactions that comprise the cycle were discovered in 1937
by Hans Adolf Krebs.
b. Step 1: A two-carbon molecule of acetyl CoA combines with a
four-carbon compound, oxaloacetic acid, to produce a six
carbon compound, citric acid.
1. This reaction regenerates coenzyme A, which moves
back out to carry and change more pyruvic acid.
c. Step 2: Citric acid releases a CO2 molecule and a hydrogen
atom to form a five-carbon compound. By losing a hydrogen
atom with its electron, citric acid is oxidized. The hydrogen is
transferred to NAD+, reducing it to NADH.
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d. Step 3: The five-carbon compound formed in Step 2 also
releases a CO2 molecule and a hydrogen atom, forming a fourcarbon
compound. Again, NAD+ is reduces to NADH. A
molecule of ATP is also synthesized from ADP.
e. Step 4: The four-carbon compound formed in Step 3 releases a
hydrogen atom to form another four-carbon compound. This
time, the hydrogen atom is used to reduce FAD to FADH2.
1. Like NAD+, FAD accepts electrons during redox
reactions.
f. Step 5: The four-carbon compound formed in Step 4 releases a
hydrogen atom to regenerate oxaloacetic acid, which keeps the
Krebs cycle operating. They hydrogen atom reduces NAD+ to
NADH.
g. Molecules used for each turn (one molecule of glucose causes
two turns of the Krebs cycle, as one glucose molecule can form
two molecules of acetyl CoA):
1. 6 NADH
2. 2 FADH2
3. 4 CO2;
4. 2 ATP. Hence, both the Krebs cycle and Glycolysis
produce two ATP molecules each.
3. Electron Transport Chain – constitutes the second stage of aerobic
respiration.
a. The electrons in the hydrogen atoms from NADH and FADH2
are in a high energy level. These electrons pass through a series
of molecules, and lose a portion of their energy in the process.
b. Energy shed off of electrons is used to pump protons from the
mitochondrial matrix to the other side of the inner membrane.
The concentration of protons becomes greater on the outside
the matrix.
c. The concentration gradient drives chemiosmosis via the ATP
synthase, located in the inner membrane. Energy harnessed
from protons moving back into the matrix synthesizes ATP
from ADP.
d. The last molecule in the chain transfers electrons to oxygen
atoms, which combine with four protons supplied by NADH
and FADH2 to form two molecule of water.
O2 + 4e- + 4H+
2H2O
4. Energy Yield
a. In total, the sum of all chemical reactions in aerobic respiration
generate the following amounts of NADH, FADH2, and ATP:
1. 10 NADH – each molecule that supplies to the electron
transport chain can make up to 3 ATP.
2. 2 FADH2 – each FADH2 molecule can generate 2 ATP.
3. The Krebs cycle and Glycolysis generate a product 4
ATP.
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4. 38 ATP molecules are produced. However, the actual
number of ATP molecules made can vary from cell to
cell.
5. Summarizing Cellular Respiration
a. The oxidation of glucose in aerobic respiration can be
summarized using the following equation:
C6H12O6 + 6O2
6CO2 + 6H2O + energy
b. The equation above is the general opposite of photosynthesis.
However, aerobic respiration is not the exact reverse of
photosynthesis.
c. Cellular respiration provides the ATP that all cells need to
support the activities of life.