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Unit 1: Metabolic Processes
Chapter 2: Cellular Respiration
2.1 Cellular Respiration: The Big Picture
Photoautroph
heterotroph
chemoautotroph
Overview of Cellular Respiration
Process of Cellular Respiration
1. Glycolysis – 10 steps breaking down glucose to
pyruvate (in cytoplasm)
2. Pyruvate Oxidation – 1 step occurring in the
mitochondria matrix
3. Krebs Cycle (tricarboxylic acid cycle, the TCA
cycle, or the citric acid cycle) – 8 steps
occuring in the mitochondria matrix
4. Electron transport and chemiosmosis
(oxidative phosphorylation) – many steps
occurring in inner mitochondrial membrane
Mitochondria: convert the potential energy of food
molecules into ATP.
• an outer mitochondrial membrane encloses the entire
structure
• an inner mitochondrial membrane encloses a fluid-filled
matrix
• between the two is the intermembrane space
• the inner membrane is elaborately folded with shelflike
cristae projecting into the matrix.
• The outer membrane contains many integral membrane
proteins that form channels through which a variety of
molecules and ions move in and out of the mitochondrion.
• The inner membrane contains complexes of 5 integral
membrane proteins that form the electron transport chain
• The matrix contains a mixture of soluble enzymes that
catalyze the breakdown of pyruvate. This series of
enzymatic reactions is the Kreb's cycle.
OVERALL CHEMICAL EQUATION:
1. Many enzymes, co-enzymes, and intermediate
chemicals are involved.
2. It is not a one-step process. Many reactions occur
to release energy in small amounts.
Oxidation-Reductions reactions:
• Glucose is broken down in a series of chemical steps
during cellular respiration. Each reaction requires a
specific enzyme
• At several points in this biochemical pathway,
oxidation-reduction reactions occur. One compound
will be oxidized (lose electrons/hydrogens) and
another will be reduced (gain electrons/hydrogens)
• Co-enzymes such as NAD and FAD acts as
electron/hydrogen acceptors. They will shuttle the
energy of the electrons to another part of the
process.
Nicotinamide Adenine Dinucleotide
(NAD+)
In being reduced, NAD can accept two electrons, but
only one proton. The other proton goes into solution as
a hydrogen ion.
Flavin Adenine Dinucleotide (FAD)
• The coenzymes gain energy when they
gain electrons (are reduced).
• This is a temporary state. In another series
of reactions, the coenzymes give up the
electrons (and thus the energy) and return
to their oxidized state.
• The energy they transfer is used to make
ATP.
Methods of forming ATP
• Substrate-Level Phosphorylation - the direct
transfer of a phosphate group from a substrate to
ADP to make ATP
• Oxidative phosphorylation - the production of
ATP using energy derived from the transfer of
electrons in an electron transport system. This is an
indirect method and occurs by chemiosmosis.
• Chemiosmosis - the production of ATP utilizing the
kinetic energy released when H+ flow through the
ATP synthase complex
Substrate level
phosphorylation
-requires a
substrate
-enzyme
-direct transfer of
a Pi
GLYCOLYSIS
• IN CYTOSOL
• ONE OF THE
OLDEST
PATHWAYS: all life
on earth performs
glycolysis
• DOES NOT
REQUIRE OXYGEN
(ANAEROBIC)
Glucose Activation:
In the first step, a
phosphate group
from ATP is attached
to glucose. This
increase the energy
level of glucose
This step is an
isomerization
This is the second
phosphorylation.
At this point, 2 ATP
molecules have been
USED.
In this step, the
glucose molecule
is split into two
three carbon
molecules.
DHAP undergoes
isomerization to
G3P. Why???
In this reaction, G3P
is phosphorylated by
inorganic phosphate
groups in the cytosol.
It is also oxidized: a
hydrogen and 2
electrons are used to
reduce NAD to
NADH.
Substrate level phosphorylation!!!
Formation of 2 ATP.
2
Isomerization
2
2
Phosphoenol pyruvate
is also known as PEP
2
Substrate level
phosphorylation:
formation of two
ATPs
Pyruvate is called
Pyruvic acid when it
is written in the
COOH form. The
terms are used
interchangeably.
Glycolysis Balance sheet:
• For each pyruvate molecule produced by
glycolysis, 2 ATP are formed  a total of 4
ATP from one glucose molecule.
• Since 2 ATP are used to energize the glucose
in the first step, there is a net output of 2 ATP
molecules.
• Some energy is bound in 2 molecules of
NADH + H+ and will be released in the
electron transport chain to form ATP.
What raw materials are necessary for a cell
to produce a molecule of ATP by substratelevel phosphorylation?
• ADP
• Pi (or a phosphate-containing
intermediate from glucose)
• A substrate enzyme
A) In eukaryotic cells, where does
glycolysis occur?
B) What does glycolysis mean?
A) In the cytoplasm.
B) The breaking of the glucose molecule into
two pyruvate molecules
4.
• List the final products of glycolysis.
• What two products of glycolysis may be
transported into mitochondria for further
processing?
A) 2 pyruvate, 4 ATP, 2 NADH, 2H+, and 2
ADP
B) Pyruvate and NADH
#6. How do ATP and ADP differ in
structure and free energy content?
• ADP has 2
inorganic phosphate
groups attached to
an adenosine
molecule, whereas,
ATP has 3.
• ATP has 31 kJ/mol
more potential
energy than ADP
PYRUVATE OXIDATION
• The PYRUVATE molecules
produced by glycolysis enter the
mitochondria by active transport.
• Pyruvate oxidation occurs in the
matrix (inner membrane?) of the
mitochondria.
Pyruvate dehydrogenase complex
• Pyruvate oxidation is carried out by a very large
enzyme complex, the pyruvate dehydrogenase
complex, which is located in the mitochondrial
matrix.
• The complex is comprised of three separate
enzymes involved in the actual catalytic process,
and uses a total of five different cofactors.
• This reaction is irreversible, and is tightly
regulated
PYRUVATE OXIDATION
• The process of converting pyruvate to acetylCoA is an oxidative decarboxylation.
• First, the pyruvate is oxidized (it goes from 3C
to 2C acetyl.) CO2 is released as a result).
• Secondly, NAD+ is reduced to NADH + H+
• Thirdly, the 2-carbon acetyl group combines
with coenzyme A to form acetyl-CoA.
• This acetyl-CoA enters the Kreb's cycle.
Krebs Cycle
Sir Hans Krebs,
who won a
Nobel Prize for
its discovery,
preferred the
term
“Tricarboxylic
Acid Cycle”
(TCA cycle)
Stage 3: The Krebs Cycle
• A 2-carbon acetyl-CoA molecule is combined with a
4-C compound called oxaloacetate to produce a 6-C
citrate molecule.
• These citrate molecules are then oxidized to a 5-C
-ketoglutarate. Carbon dioxide and NADH are
produced.
• -ketoglutarate molecules are then further oxidized
to a 4-C succinyl Co-A compound. Carbon dioxide
and NADH are produced.
• The 4-C succinyl Co-A is then modified to
succinate. GTP is produced by substrate level
phosphorylation. It is converted to ATP.
• The 4 carbon succinate molecule is
oxidized to fumarate. FADH2 is
produced.
• Fumarate is hydrated to malate.
• Malate is oxidized to oxaloacetate.
NADH is produced.
• And the cycle starts again.
Note:
• Each of the 3 carbon atoms present in the
pyruvate that entered the mitochondrion
leaves as a molecule of carbon dioxide
(CO2)
• At 3 steps in the cycle, a pair of electrons
(2e-) is removed and transferred to NAD+
reducing it to NADH + H+
• At one step, a pair of electrons is removed
from succinate and reduces FAD to
FADH2
Summary of Kreb's cycle:
•
•
•
•
•
•
2 carbon dioxide molecules are released,
3 NADH are produced,
1 FADH2 is produced
1 molecule of ATP is formed
1 molecule of water is used
molecule of oxaloacetate is left to start
the cycle all over again.
• Remember, there are 2 molecules of
pyruvate formed from each molecule of
glucose, therefore the cycle runs twice for
each glucose molecule.
• Almost all the chemical energy extracted
from the pyruvate is carried by the
hydrogen and temporarily transferred to
the reduced coenzymes.
Describe the function of NAD+ and FAD in
cellular respiration.
• They act as coenzymes that harvest energy
from the reactions of glycolysis, pyruvate
oxidation, and the Krebs cycle and carry it to
power ATP synthesis by oxidative
phosphorylation.
• NAD+ is used to shuttle electrons to the first
component of the ETC.
• During oxidative phosphorylation, NAD+
removes 2 hydrogen atoms from a part of the
original glucose molecule.
• Two electrons and one proton attach to NAD+,
reducing it to NADH (NAD+ is the oxidized
form of NADH).
• This reduction occurs during glycolysis,
phruvate oxidation, and the Krebs cycle.
• FAD functions in a similar manner to NAD+.
• FAD is reduced by two hydrogen atoms from
the original glucose molecule to FADH2.
• This is done during the Krebs cycle.
• These reductions are energy harvesting and
will transfer their free energy to ATP
molecules.
• Reduced NAD+ and FAD move free energy
from one place to another and from one
molecule to another.
As a result of glycolysis, pyruvate oxidation, and the
Krebs cycle, only a small portion of the energy of
glucose has been converted to ATP. In what form is
the rest of the usable energy found at this stage of the
process?
• The rest of the usable energy is stored
as FADH2, and NADH.
• 2 FADH2 are produced during the
Krebs cycle.
• The free energy stored in these
molecules is released during
chemiosmosis and ETC.
Electron Transport Chain
and Chemiosmosis
Oxidative Phosphorylation
ETC animation!
• http://www.biologycorner.com/bio3/notesrespiration.html
The Electron Transport Chain
The Electron Transport System
• The reactions of the electron transport
chain take place within the inner
membrane of the mitochondrion.
• This is the portion of respiration which
yields the greatest amount of energy for
the cell (32 ATP).
• So far only 4 molecules (net) of ATP are
produced (by substrate-level
phosphorylation).
• Most of the energy is still carried by the
NADH + H+ or the FADH2 .
• In the electron transport chain, this energy
is used to form ATP.
• Hydrogen atoms are carried into the chain
by NADH + H+ and FADH2 .
• At the membranes, the hydrogen atoms
are separated into electrons (e-) and H+.
• The electrons from the hydrogen atoms
are passed along from one compound to
another in a series of redox reactions.
• At three sites along the chain, some of the free
energy released from the transfer of the
electrons is used to pump protons (H+)
against their concentration gradient from the
matrix of the mitochondrion into the
intermembrane space (an example of active
transport).
• There are now more H ions in the
intermembrane space than in the matrix.
• This results in a concentration gradient that is
utilized in the synthesis of ATP.
• NADH + H+ enters
at the first complex
and contributes to
the formation of 3
ATP.
• FADH2 enters at the
second complex and
contributes to the
formation of 2 ATP.
The integral membrane proteins (complexes)that make up the
respiratory chain accomplish the following:
• the stepwise transfer of electrons from NADH + H+ (and
FADH2) to oxygen atoms to form (with the aid of protons) water
molecules (H2O)
• harnessing the energy released by this transfer to the pumping of
protons (H+) from the matrix to the intermembrane space
• protons are pumped at 2 - 3 complexes
• protons are pumped out at each complex as electrons pass
through it.
• the gradient of protons formed across the inner membrane by
this process of active transport forms a concentration gradient
• the protons can flow back down this gradient, re-entering the
matrix, only through the ATP synthase complex.
Chemiosmosis
• A high concentration of H+ develops on the outer side of the
membrane.
• As their concentration increases, a strong diffusion gradient
is set up.
• The only exit for these protons is through the ATP synthase
complex.
• This special complex in the membrane permits H+ to pass
through the membrane, down a concentration gradient.
• The energy released as these protons flow down their
gradient is harnessed to the synthesis of ATP.
• As it does, enzymes use the kinetic energy of the moving H+
to join phosphate and ADP forming ATP.
• The process is called chemiosmosis and is an example of
facilitated diffusion.
• A total of 32 ATP molecules are formed from one
molecule of glucose.
• For each pair of H atoms picked up by the NAD+, 3
molecules of ATP are produced and for each pair
picked up by the FAD, 2 molecules of ATP are
produced.
• At the end of the chain, most of the energy has been
extracted from the electron pair - this electron pair is
then transferred to an oxygen atom to form water.
• Since 2 ATP (net) come directly from glycolysis
and 2 ATP from the cycle, a total of 36 ATP (net)
are formed from each molecule of glucose.
Sum up total production of NADH, FADH2,
ATP from a single glucose molecule
Compare substrate-level phosphorylation and
oxidative phosphorylation.
• S.L.P. generates ATP directly from an enzyme
catalyzed reaction, whereas O.P. generates ATP
indirectly by the chemiosmotic potential created
• The process is oxidative because, it involves several
sequential redox reactions, with oxygen being the
final electron acceptor.
• It is more complex than S.L.P., and it produces more
ATP.
Why is aerobic respiration a more efficient energyextracting process than glycolysis alone?
• Glycolysis only transfers about 2.1% of the free
energy available in 1 mol of glucose into ATP.
Most of the energy is trapped in 2 pyruvate and 2
NADH.
• Aerobic respiration further processes the pyruvate
and NADH during pyruvate oxidation, the Krebs
cycle, chemiosmosis, and ETC. By the end of
aerobic respiration, all the energy available in
glucose has been harnessed.
#14 a) What part of a glucose molecule
provides electrons in cellular respiration?
Hydrogen atoms
B) Describe how E.T.C. set up a proton gradient in
response to electron flow.
•
•
•
•
•
The ETC passes protons from the mitochondrial matrix to
the intermediate space.
NADH gives up the two electrons it carries to NADH
hydrogenase.
Electron carriers, ubiquinone and cytochrome c, shuttle
electrons from NADH hydrogenase to cytochrone b-c1
complex to cytochrome oxidase complex.
Free energy is lost from the electrons during each step in
this process, and this energy is used to pump H+ from the
matrix into the intermembrane space.
The final step in the electron transport chain sees oxygen
accept 2 electrons from cytochrome oxidase complex, and
it consumes protons to form water.
c) How is the energy used to drive the synthesis of ATP?
• The protons that accumulate in the intermembrane space
create an electrochemical gradient.
• The gradient has 2 components: electrical caused by a
higher positive charge in the intermembrane space than in
the matrix, and a chemical gradient created by a higher
concentration of protons in the intermembrane space.
• The electrochemical gradient stores free energy; the protonmotive force (PMF).
• The mitochondrial membrane is almost impermeable to
protons, so the protons are forced to pass through ATP
synthase, reducing the energy of the gradient.
• The energy is used by the enzyme ATP synthase to create
the 3rd phosphate-ester bond forming ATP.
d) What is the name of this
process?
• Chemiosmosis (oxidative phosphorylation)
e) Who discovered the
mechanism?
• Chemiosmosis was discovered by Peter
Mitchell in 1961.
A) Distinguish between an electron carrier and a terminal
electron acceptor.
B) What is the final electron acceptor in aerobic respiration?
a) An electron carrier is first oxidized and
then reduced by a more electronegative
molecule. A terminal electron acceptor is
only reduced (it is at the end of the ETC)
b) oxygen
Explain how the overall equation for cellular
respiration is misleading.
• It does not include the numerous enzymes,
coenzymes, and intermediate chemicals involved in
the process.
• It also shows the conversion of glucose and oxygen
to carbon dioxide and water as a simple, one-step
process, where it is actually much more involved
than that.
Difficulties and Misconceptions
• The following is a list of items students find
deceiving.
Sometimes 36 ATP are produced and sometimes 38 ATP are
produced.
• Since the inner mitochondrial membrane is
impermeable to NADH (from glycolysis), it has 2
shuttle systems that pass electrons from cytosolic
NADH in the inter-membrane space to the matrix.
• Glycerol-phosphate shuttle transfers the electrons
from cystolic NADH to FAD to produce FADH2
(resulting in the synthesis of 2ATP)
• Aspartate shuttle (less common) transfers electrons
to NAD+ instead of FAD, forming NADH (resulting
in the synthesis of 3 ATP)
DAY 14
2.3 Related Pathways p. 117-124
QUIZ ON CHAPTER 2
tomorrow
Fermentation occurs in
the ABSENCE OF
OXYGEN.
LACTIC ACID
FERMENTATION
or
ALCOHOLIC
FERMENTATION.
Aerobic
respiration
Fermentation
• Yields 36
ATP/glucose
• Yields 2
ATP/gluocose
• Produces CO2
and water
• Produces ethanol
or lactic acid
Protein Catabolism
• Proteins undergo deamination
(removing an amino group from
amino acids
• They are then converted into
ammonia and excreted
deamination
Lipid Catabolism
• In beta-oxidation, fatty acids are
sequentially degraded into 2carbon acetyl portions that are
converted into acetyl-CoA and
respired through the Krebs cycle,
ETC, and chemiosmosis.
•Fat cannot be used directly to produce energy
for a cell.
•First, fat must by hydrolyzed into glycerol and
fatty acids. The glycerol can enter glycolysis
after either being converted to glucose (via
gluconeogenesis) or changed into
dihydroxyacetonephosphate (DHAP).
-The fatty acids are broken down to two-carbon
units (acetyl-CoA) in a process called boxidation, which can be fed directly into Krebs
cycle.
Anaerobic Pathways
• When oxygen is not available
• Eukaryotes still carry out glycolysis by
transferring the H atoms in NADH to
pyruvate
• The NAD+ molecules formed allow
glycolysis to continue
Ethanol (Alcohol) Fermentation
Occurs in yeast cells and is used in wine, beer, and
bread making
Ethanol (Alcohol) Fermentation
• A molecule of CO2 is removed from
pyruvate, forming a molecule of
acetaldehyde
• The acetaldehyde is converted to ethanol by
attaching H from NADH
• FINAL PRODUCTS: ATP, CO2, ethanol
A particular organism releases carbon dioxide
and alcohol as its end products. The organism is
most likely which of the following?
a.
b.
c.
d.
e.
an animal
an alga
a green plant
a yeast
a virus
d.
a yeast
Anaerobic and aerobic respiration are similar in all but
one of the following ways. Which one is the
exception?
A) NAD+ is reduced
B) carbon dioxide is a product
C) ADP is combined with inorganic phosphate to
form ATP
D) acetaldehyde is converted into ethanol
E) both can release energy from glucose
D)
acetaldehyde is converted into ethanol
Lactate (lactic acid) fermentation
• Occurs in animal muscle cells
during strenuous exercise
• FINAL PRODUCTS: ATP, lactate
What happens to lactic acid after it is formed in a
muscle cell?
• Lactic acid travels in the bloodstream to the
liver, where it is oxidized back to pyruvate,
which then goes through the Krebs cycle
and oxidative phosphorylation.
• The presence of lactic acid in the muscle
tissues leads to stiffness, soreness, and
fatigue.
Oxygen debt
• Oxygen debt refers to the extra oxygen
required by the liver to oxidize lactic acid to
CO2 and water (through the aerobic
pathway)
• Panting “pays” for the oxygen debt
During active exercise, the supply of oxygen becomes
inadequate for the level of activity you are attempting to
maintain. How do the catabolic reactions of the cell continue?
• Glycolysis continues to supply small
amount of ATP, and the pyruvate that
normally would continue on the Krebs
cycle as acetyl-CoA is instead converted
to lactate to regenerate NAD+ to allow
glycolysis to continue.
VO2 max and the Lactate Threshold
• The maximum oxygen uptake (VO2 max) is the
maximum volume of oxygen that the cells of the
body can remove from the bloodstream in one
minute per kg of body mass while the body
experiences max. exertion.
• The lactate threshold (LT) is the value of exercise
intensity at which blood lactate concentration begins
to increase sharply.
This course has placed an emphasis on carbohydrates as an energy
source, yet our diets also contain fats and proteins. Explain the role of
fats and proteins in producing energy for an organism.
•
The emphasis on carbohydrates is justified, since they are the principle energy source for
humans, both in terms of consumption and biochemical preference. However, there are a few
other food sources that produce energy for us if the circumstances warrant.
•
In the case of fat, the body usually turns to this as a source of energy once carbohydrate
reserves are nearly depleted. Fat can be enzymatically broken down into glycerol and fatty
acids. It is the fatty acids that contain most of the energy from the fat. In a process known as
b-oxidation, the fatty acids are cleaved two carbon atoms at a time and joined to coenzyme-A
to form acetyl-CoA. (Note: Fatty acids must have an even number of carbons. Fatty acids
with an odd number of carbons should produce acetyl-CoA also, but the last unit will be
formyl-CoA, which is toxic!) This acetyl-CoA can then enter into the Krebs cycle and go on
to produce energy in the same manner as carbohydrates.
•
When most carbohydrate and fat has been exhausted, the body can turn to protein as an
energy source. First, the protein has to be broken down into its component amino acids and
deaminated. The deamination process leads to the production of ammonia, which is a waste
product. Depending on what amino acid we are talking about, it can enter at either the level of
pyruvate or a number of points in the Krebs cycle and produce energy in the same manner as
carbohydrates.
Overview of Cellular
Respiration which occurs in
STAGE 1: GLYCOLYSIS
STAGE 2: TWO
MAIN PATHWAYS,
DEPENDING ON
WHETHER THERE IS
OXYGEN IN THE CELL.
Aerobic Respiration
produces nearly 20 times
as much ATP as is
produced by Glycolysis
alone.
The co-enzymes NADH+ H+ and FADH2 must now be oxidized so they can continue to
transfer the hydrogen to the ETC. The components are arranged in order of increasing
electronegativity, with the weakest at the beginning and the strongest (oxygen) is at the
end. As the NADH+ H+ and FADH2 are oxidized, the electrons from the 2 hydrogen
atoms are passed along the components in a series of redox reactions. As the electrons
are passed along the complexes, energy is stripped from them. This "downhill" series
of electron transfers gradually lowers the level of energy in the electrons and when most
of the energy is spent, the electrons are accepted by oxygen. The energy, that is
stripped, is used to pump the H+ across the membrane from the matrix to the
intermembrane compartment
A high concentration of H+ now exists in the intermembrane compartment. The protons
can flow back down this gradient, re-entering the matrix, only through another complex
of integral proteins in the inner membrane, the ATP synthase complex. The energy
released, as these electrons flow down their gradient, is harnessed to the synthesis of
ATP. The process is called chemiosmosis and is an example of facilitated diffusion.