9.1 How
Cells Acquire ATP
A. Cellular Respiration
1. Cellular respiration includes the various metabolic pathways
that break down
carbohydrates and other metabolites and build up ATP.
2. Cellular respiration requires oxygen
and gives off CO2.
3. Aerobic respiration usually
breaks down glucose into CO2 and H2O.
4. Overall equation for complete
breakdown of glucose requires oxygen (is aerobic):
C6H12O6 + 6O2 6 CO2 + 6 H2O +
energy
5. Glucose is high-energy molecule; CO2 and H2O are low-energy molecules;
process is
exergonic
and releases energy.
6. Electrons are removed from substrates
and received by oxygen, combines with H+ to
become
water.
7. Glucose is oxidized and O2 is
reduced.
8. Buildup of ATP is an endergonic
reaction that requires energy.
9. Pathways of aerobic respiration
allow energy in glucose to be released slowly; ATP is
produced
gradually.
10. Rapid breakdown of glucose would
lose most energy as non-usable heat.
11. Breakdown of glucose yields
synthesis of 36 or 38 ATP; this preserves 39% of energy
available in
glucose.
B. NAD+ and FAD
1. Each metabolic reaction in cellular respiration is catalyzed by its own
enzyme.
2. As a metabolite is oxidized, NAD+
accepts two electrons and a hydrogen ion (H+);
results in
NADH + H+.
3. Electrons received by NAD+ and
FAD are high-energy electrons and are usually carried
to the
electron transport system.
4. NAD+ is a coenzyme of
oxidation-reduction since it both accepts and gives up electrons.
5. Only a small amount of NAD+ is
needed in cells; each NAD+ molecule is used over and over.
6. FAD coenzyme of
oxidation-reduction can replace NAD+; FAD accepts two electrons,
becomes
FADH2.
C. Phases of Complete Glucose Breakdown
1. Aerobic respiration includes metabolic pathways and one
individual reaction:
a. Glycolysis
is the breakdown of glucose to two molecules of pyruvate.
1) Enough energy is released for immediate buildup of two ATP.
2) Glycolysis takes place outside the mitochondria and does not
utilize oxygen.
b. The
transition reaction: pyruvate is oxidized to an acetyl group and CO2 is
removed.
c. The
Krebs cycle:
1) This series of reactions gives off CO2 and produces ATP.
2) Produces two immediate ATP molecules per glucose molecule.
d. The
electron transport system:
1) Series of carriers accepts electrons from glucose; electrons are passed from
carrier
to carrier until received by oxygen.
2) Electrons pass from higher to lower energy states, energy is released and
stored
for ATP production.
3) System accounts for 32 or 34 ATP depending on the cell.
2. Pyruvate is a
pivotal metabolite in cellular respiration:
a. If O2 is
not available to the cell, fermentation, an aerobic process, occur.
b. During fermentation,
glucose is incompletely metabolized to lactate or CO2 and alcohol.
c. Fermentation
results in a net gain of only two ATP per glucose molecule.
9.2 Outside
the Mitochondria: Glycolysis
A. Glycolysis
1. Occurs in the cytosol outside the mitochondria.
2. Is the breakdown of glucose to
two pyruvate molecules.
3. Is universal in organisms;
therefore, most likely evolved before Krebs cycle and electron
transport
system.
B. Energy Investment Steps
1. Glycolysis begins with addition of two phosphate groups
activating glucose to react.
2. Two separate reactions use two
ATP.
3. Glucose, a C6 molecule, splits
into two C3 molecules, each with a phosphate group.
C. Energy Harvesting Steps
1. Two electrons and one hydrogen ion are accepted by NAD+ and result in two
NADH.
2. Enough energy is released from
breakdown of glucose to generate four ATP molecules.
3. Two to four ATP molecules produced
are required to replace two ATP molecules used in the
phosphorylation
of glucose.
4. There is a net gain of two ATP
from glycolysis.
5. Pyruvate enters mitochondria if
oxygen is available and aerobic respiration follows.
6. If oxygen is not available,
glycolysis becomes a part of fermentation.
9.3 Inside
the Mitochondria
A. Aerobic Respiration
1. Involves the transition reaction, the Krebs cycle, and the electron
transport system.
2. Is process in which pyruvate from
glycolysis is broken down completely to CO2 and H2O.
3. Takes place inside mitochondria.
B. Mitochondria
1. A mitochondrion has a double membrane with an intermembrane space between
the outer
and inner membrane.
2. Cristae are the
inner folds of membrane that jut into the matrix.
3. Matrix is the
innermost compartment of a mitochondrion and is filled with gel-like fluid.
4. Transition reaction and Krebs
cycle enzymes are in matrix; electrons transport system
is in
cristae.
5. Most ATP produced in cellular
respiration is produced in mitochondria.
C. Transition Reaction
1. Transition reaction connects glycolysis to the Krebs cycle.
2. In this reaction, pyruvate is
converted to a two-carbon acetyl group attached to coenzyme A.
3. This redox reaction removes
electrons from pyruvate by dehydrogenase using NAD+ as coenzyme.
4. Reaction occurs twice for each
original glucose molecule.
D. The Krebs Cycle
1. Krebs cycle reactions occur in matrix of mitochondria.
2. Cycle is named for Sir Hans
Krebs, who received Noel Prize for identifying these reactions.
3. Cycle begins by adding C2 acetyl
group to C4 molecule, forming citrate; also called
the citric
acid cycle.
4. The acetyl group is then oxidized
to two molecules of CO2.
5. During the oxidation process,
most electrons (e-) are accepted by NAD+ and NADH is formed.
6. In one instance, electrons are
taken by FAD, forming FADH2.
7. NADH and FADH2 carry these
electrons to electron transport system.
8. Some energy released is used to
synthesize ATP by substrate-level phosphorylation, as in glycolysis.
9. One high-energy metabolite
accepts a phosphate group and passes it on to convert ADP to ATP.
10. Krebs cycle turns twice for each
original glucose molecule.
11. Products of the Krebs cycle
per glucose molecule include 4 CO2, 2 ATP, 6 NADH and 2 FADH2
E. The Electron Transport System
1. Electron transport system is located in cristae of
mitochondria; consists of carriers
that pass
electrons.
2. Some protein carriers are cytochrome
molecules.
3. Electrons that enter the electron
transport system are carried by NADH and FADH2.
4. NADH gives up its electrons and
becomes NAD+; next carrier gains electrons and is reduced.
5. At each sequential oxidation-reduction
reaction, energy is released to form ATP molecules.
6. Oxygen serves as terminal
electron acceptor and combines with hydrogen ions to form water.
7. Because O2 must be present for
system to work, it is also called oxidative phosphorylation.
8. NADH delivers electrons to
system; by the time electrons are received by O2, three ATP are formed.
9. If FADH2 delivers electrons to
system, by the time electrons are received by O2, two ATP are formed.
10. Coenzymes and ATP recycle
a. Cell
needs a limited supply of coenzymes NAD+ and FAD because they constantly
recycle.
b. Once NADH
delivers electrons to electron transport system, it is free to pick up more
hydrogen.
c. Components
of ATP also recycle.
d.
Efficiency of recycling NAD+, FAD and ADP eliminates need to synthesize them
anew.
F. The Cristae of a Mitochondrion
1. Electron transport system consists of three protein complexes and two
protein mobile
carriers
that transport electrons between complexes.
2. NADH dehydrogenase complex,
cytochrome b-c complex and cytochrome oxidase
complex all
pump H+ ions into the intermembrane space.
3. Energy released from flow of
electrons down electron transport chain is used to
pump H+
ions, carried by NADH and FADH2, into intermembrane space.
4. Accumulation of H+ ions in this
intermembrane space creates a significant electrochemical gradient.
5. ATP synthase complexes
are channel proteins that also serve as enzymes for ATP synthesis.
6. As H+ ions flow from high to low
concentration, ATP synthase synthesizes ATP;
actual
mechanism is still unknown.
7. "Chemiosmosis"
term used since ATP production tied to electrochemical (H+) gradient
across a
membrane.
8. Once formed, ATP molecules
diffuse out of the mitochondrial matrix through channel proteins.
G. Energy Yield From Glucose Breakdown
1. Substrate-Level Phosphorylation
a. Per
glucose molecule, there is a net gain of two ATP from glycolysis in cytosol.
b. The Krebs
cycle in the matrix of the mitochondria produces two ATP per glucose.
c. Total of
four ATP are formed outside of the electron transport system.
2. Oxidative Phosphorylation
a. Most ATP
is produced by the electron transport system.
b. Per
glucose, 10 NADH and two FADH2 molecules provide electrons and H+ ions to
electron transport system.
c. For each
NADH formed within the mitochondrion, three ATP are produced.
d. For each
FADH2 formed by Krebs cycle, two ATP result since FADH2 delivers
electrons after NADH.
e. For each
NADH formed outside mitochondria by glycolysis, two ATP are produced as
electrons are shuttled across mitochondrial membrane by an organic molecule and
delivered to FAD.
f. Heart and
liver cells, which have high metabolic rates are exception; NADH results in
production of three ATP.
g.
Prokaryotes lack mitochondria; each NADH produces three ATP for total of 38
ATP.
3. Efficiency of Complete Glucose
Breakdown
a. Energy
difference between total reactants (glucose and O2) and products (CO2 and H2O)
is 686 kcal.
b. ATP
phosphate bond has energy of 7.3 kcal; 36 to 38 are produced during glucose
breakdown for total of at least 263 kcal.
c.
Efficiency is 263/686 or 39% of available energy in glucose is transferred to
ATP.
9.4
Fermentation
A. Cellular Respiration Includes Fermentation
1. Fermentation consists of glycolysis plus reduction of pyruvate
to either lactate or alcohol and CO2.
2. NADH passes its electrons to
pyruvate instead of to an electron transport system; NAD+ is
then free to
return and pick up more electrons during earlier reactions of glycolysis.
3. Examples:
a. Anaerobic
bacteria produce lactic acid when we manufacture some cheeses.
b. Anaerobic
bacteria produce industrial chemicals: isopropanol, butyric acid, propionic
acid, and acetic acid.
c. Yeasts
use CO2 to make bread rise and produce ethyl alcohol in winemaking.
d. Animals reduce
pyruvate to lactate when it is produced faster than it can be oxidized by Krebs
cycle.
B. Advantage and Disadvantage of Fermentation
1. Despite low yield of two ATP molecules, fermentation provides quick burst of
ATP
energy for
muscular activity.
2. Disadvantage is that lactate is
toxic to cells.
a. When blood
cannot remove all lactate from muscles, lactate change pH and causes
muscles to fatigue.
b.
Individual is in oxygen debt because oxygen is still needed after exercising.
c. Recovery
occurs after lactate is sent to liver, converted into pyruvate; then respired
or
converted into glucose.
C. Efficiency of Fermentation
1. Two ATP produced per glucose molecule during fermentation is equivalent to
14.6 kcal.
2. Complete glucose breakdown to CO2
and H2O during cellular respiration results in 686 kcal of energy.
3. Efficiency of fermentation is
14.6/686 or about 2.1%; much less efficient than complete
breakdown of
glucose.
9.5
Metabolic Pool
A. Degradative and Synthetic Reactions
1. Degradative reactions participate in catabolism and break down molecules;
they tend to be exergonic.
2. Synthetic reactions participate
in anabolism and build molecules; they tend to be endergonic.
B. Catabolism
1. Just as glucose was broken down in cellular respiration, other molecules
undergo catabolism.
2. Fat breaks down into glycerol and
three fatty acids.
a. Glycerol
is converted to PGAL, a metabolite in glycolysis.
b. An
18-carbon fatty acid is converted to nine acetyl-CoA molecules that enter the
Krebs cycle.
c.
Respiration of fat products can produce 216 ATP molecules; fats are efficient
form of stored energy.
3. Amino acids break down into
carbon chains and amino groups.
a.
Hydrolysis of proteins results in amino acids.
b. R-group
size determines whether carbon chain is oxidized in glycolysis or the Krebs
cycle.
c. Carbon
chain is produced in liver by removal of the amino group.
d. Amino
group becomes ammonia (NH3), which enters urea cycle and becomes part of
excreted urea.
e. Length of
R-group determines number of carbons left after deamination.
C. Anabolism
1. ATP produced during catabolism drives anabolism.
2. Substrates making up pathways can
be used as starting materials for synthetic reactions.
3. Molecules used for biosynthesis
constitute metabolic pool.
4. Carbohydrates can result in fat
synthesis: PGAL converts to glycerol, acetyl groups join
to form
fatty acids.
5. Some metabolites can be converted
to amino acids by transamination, transfer of an amino
acid group
to an organic acid.
6. Plants synthesize all amino acids
they need; animal lack some enzymes needed to make
some amino
acids.
7. Humans synthesize 11 of 20 amino
acids; remaining 9 essential amino acids must be
provided by
diet.