CELLULAR RESPIRATION
CHAPTER 7
WHERE IS THE ENERGY IN FOOD?
• Electrons pass from atoms or molecules to one
another as part of many energy reactions.
• Oxidation is when an atom or molecule loses an
electron.
• Reduction is when an atom or molecule gains an
electron.
• These reactions always occur together:
• Oxidation-reduction (redox) reactions
WHERE IS THE ENERGY IN FOOD?
• Redox reactions involve transfers of energy because the
electrons retain their potential energy.
• The reduced form of an organic molecule has a higher level of
energy than the oxidized form.
Loss of electron (oxidation)
o
A
o
+
Low energy
High energy
B
A
–
+
e–
B
A*
Gain of electron (reduction)
+
B*
WHERE IS THE ENERGY IN FOOD?
• The energy for living is obtained by breaking
down the organic molecules originally produced
in plants.
• The ATP energy and reducing power invested in
building the organic molecules are stripped away as
the chemical bonds are broken and used to make ATP.
• The oxidation of food stuffs to obtain energy is called
cellular respiration.
WHERE IS THE ENERGY IN FOOD?
• Cellular respiration is the harvesting of
energy from breakdown of organic
molecules produced by plants
• The overall process may be summarized as
C6H12O6
glucose
+
6 O2
6 CO2
oxygen
carbon
dioxide
+
6 H2O
water
+
energy
(heat or ATP)
CELLULAR RESPIRATION
• Cellular respiration takes place in two
stages:
– Glycolysis
• Occurs in the cytoplasm.
• Does not require O2 to generate ATP.
CELLULAR RESPIRATION
– Krebs cycle
• Occurs within the mitochondrion.
• Harvests energy-rich electrons through a cycle
of oxidation reactions.
• The electrons are passed to an electron
transport chain in order to power the
production of ATP.
Glucose
NADH
Cytoplasm
Glycolysis
ATP
Pyruvate
NADH
Pyruvate
oxidation
CO2
Intermembrane
space
AcetylCoA
Mitochondrial matrix
NADH
CO2
Krebs
cycle
ATP
FADH2
H2 O
ATP
NAD+ and ADF
e– Electron transport chain
Inner mitochondrial
membrane
Mitochondrion
USING COUPLED REACTIONS TO MAKE
ATP
• Glycolysis is a sequence of reactions that form
a biochemical pathway.
• In ten enzyme-catalyzed reactions, the six-carbon
sugar glucose is broken into two three-carbon
pyruvate molecules.
USING COUPLED REACTIONS TO MAKE
ATP
• The breaking of the bonds yields energy that is
used to phosphorylate ADP to ATP.
• This process is called substrate-level
phosphorylation.
• In addition, electrons and hydrogen atoms are
donated to NAD+ to form NADH.
http://www.youtube.com/watch?v=3GTjQTqUuOw&list=FL9N_Px072WuVorSwDfqf-9w&index=4&feature=plpp
Glucose
Glucose
1
Glycolysis
Pyruvate
oxidation
ATP
Phosphorylation of
glucose by ATP.
1
ADP
P
Glucose 6-phosphate
2–3
2
Rearrangement,
followed by a second
ATP phosphorylation.
P
Krebs
cycle
Fructose 6-phosphate
ATP
3
ADP
Electron transport
chain
P
4–5
The six-carbon molecule
is split into two three-carbon
molecules of G3P.
Fructose 1,6-bisphosphate
4,5
P
6
Oxidation followed by
phosphorylation produces
two NADH molecules and
gives two molecules of BPG,
each with one high-energy
phosphate bond.
P
Glyceraldehyde 3phosphate (G3P)
NAD+
Pi
P
Glyceraldehyde 3phosphate (G3P)
6
NAD+
Pi
NADH
NADH
P
P
1,3-bisphosphoglycerate (BPG)
P
P
1,3-bisphosphoglycerate (BPG) 11
7Removal of high-energy
phosphate by two ADP
molecules produces two
ATP molecules and gives
two 3PG molecules.
ADP
ADP
7
ATP
ATP
P
P
3-phosphoglycerate
(3PG)
3-phosphoglycerate
(3PG)
8
8–9
Removal of water gives
two PEP molecules, each
with a chemically reactive
phosphate bond.
P
P
2-phosphoglycerate
(2PG)
2-phosphoglycerate
(2PG)
9
P
10
Removal of high-energy
phosphate by two ADP
molecules produces two
ATP molecules and gives
two pyruvate molecules.
P
Phosphoenolpyruvate Phosphoenolpyruvate
(PEP)
(PEP)
ADP
ADP
10
ATP
ATP
Pyruvate
Pyruvate
USING COUPLED REACTIONS TO MAKE
ATP
• Glycolysis yields only a small amount of ATP.
• Only two ATP are made for each molecule of glucose.
• This is the only way organisms can derive energy from
food in the absence of oxygen.
• All organisms are capable of carrying out glycolysis.
• This biochemical process was probably one of the
earliest to evolve.
HARVESTING ELECTRONS FROM
CHEMICAL BONDS
• In the presence of oxygen, the first step of
oxidative respiration in the mitochondrion is the
oxidation of pyruvate.
• Pyruvate still contains considerable stored energy at
the end of glycolysis.
• Pyruvate is oxidized to form acetyl-CoA.
ACETYL-COA
• When pyruvate is
oxidized, one of its
three carbons is
cleaved.
Glycolysis
Pyruvate
CO2
NAD+
• This carbon leaves as
CoenzymeA
NADH
part of a CO2 molecule.
Protein
Lipid
• In addition, a hydrogen
and electrons are
CoA–
Acetyl–CoA
removed from pyruvate
and donated to NAD+
ATP
Fat
to form NADH.
• The remaining two-carbon fragment of pyruvate is
joined to a cofactor called coenzyme A (CoA).
• The final compound is called acetyl-CoA.
KEY BIOLOGICAL PROCESS:
TRANSFER OF H ATOMS
• NADH and NAD+ are used by cells to carry
hydrogen atoms and energetic electrons.
1
2
Substrate
–
H +e
3
–
H +e
H + e–
NAD+
Product
NAD+
NAD+
H
NAD+
H
NAD+
Enzymes that harvest hydrogen atoms
have a binding site for NAD+ located
near the substrate binding site.
In an oxidation-reduction reaction, the
hydrogen atom and an electron are
transferred to NAD+, forming NADH.
NADH then diffuses away and is
available to donate the hydrogen to
other molecules.
HARVESTING ELECTRONS FROM
CHEMICAL BONDS
• The fate of acetyl-CoA depends on the
availability of ATP in the cell.
• If there is insufficient ATP, then the acetyl-CoA
heads to the Krebs cycle.
• If there is plentiful ATP, then the acetyl-CoA is
diverted to fat synthesis for energy storage.
KREBS CYCLE
• The second step of oxidative respiration is called
the Krebs cycle.
• The Krebs cycle is a series of 9 reactions that
can be broken down into three stages:
1. Acetyl-CoA enters the cycle and binds to a fourcarbon molecule, forming a 6-C molecule.
2. Two carbons are removed as CO2 and their electrons
donated to NAD+. In addition, an ATP is produced.
3. The four-carbon molecule is recycled and more
electrons are extracted, forming NADH and FADH2.
http://www.youtube.com/watch?v=-cDFYXc9Wko
THE KREBS CYCLE
Oxidation of pyruvate
Glucose
Pyruvate
Glycolysis
CO2
NAD+
Coenzyme A
Pyruvate
oxidation
NADH
CoA–
Acetyl-CoA
Krebs
cycle
Electron transport
chain
• Note: A single glucose molecule produces two turns
of the cycle, one for each of the two pyruvate
molecules generated by glycolysis.
1
The cycle begins when
a C2 unit reacts with a C4
molecule to give citrate
(C6).
Mitochondrial
membrane
Krebs cycle
CoA
2-4
1
(4 C) Oxaloacetate
Oxidative decarboxylation
produces NADH with the
release of CO2.
Citrate (6 C)
NADH
8-9
The dehydrogenation
2
9
NAD+
of malate produces a
third NADH, and the
cycle returns to its
starting point.
3
(4 C) Malate
Isocitrate (6 C)
NAD+
4
8
H2O
NADH
CO2
(4 C) Fumarate
-Ketoglutarate (5 C)
FADH2
7
NAD+
CO2
CoA
FAD
5
NADH
CoA-SH
S
(4 C) Succinate
Succinyl-CoA (4 C)
6
6-7
A molecule of ATP is
produced and the oxidation
of succinate produces FADH2.
CoA-SH
5
ATP
ADP
A second oxidative
decarboxylation produces
a second NADH with the
release of a second CO2.
HARVESTING ELECTRONS FROM
CHEMICAL BONDS
• In the process of cellular respiration, the glucose
is entirely consumed.
• The energy from its chemical bonds has been
transformed into:
• 4 ATP molecules.
• 10 NADH electron carriers.
• 2 FADH2 electron carriers.
USING THE ELECTRONS TO
MAKE ATP
• NADH and FADH2 transfer their electrons to a
series of membrane-associated molecules
called the electron transport chain.
• Some protein complexes in the electron transport
chain act as proton pumps.
• The last transport protein donates the electrons to
hydrogen and oxygen in order to form water.
• The supply of oxygen able to accept electrons makes
oxidative respiration possible.
http://www.youtube.com/watch?v=kN5MtqAB_Yc&list=FL9N_Px072WuVorSwDfqf-9w&index=2&feature=plpp
THE ELECTRON TRANSPORT CHAIN
Glucose
Intermembrane space
Glycolysis
H+
H+
H+
e–
e–
Inner
mitochondrial
membrane
Pyruvate
oxidation
Krebs
cycle
Electron transport
chain
e–
FADH2
NADH
+ H+
1
2H+ + –2 O2
NAD+
Protein
complex I
Krebs
Mitochondrial matrix
Protein
complex II
Protein
complex III
H2O
USING THE ELECTRONS TO
MAKE ATP
• Chemiosmosis is integrated with electron
transport.
• Electrons harvested from reduced carriers (NADH and
FADH2) are used to drive proton pumps and
concentrate protons in the intermembrane space.
• The re-entry of the protons into the matrix across ATP
synthase drives the synthesis of ATP by
chemiosmosis.
Pyruvate from
cytoplasm
H+
Inner
mitochondrial
membrane
H+
Intermembrane
space
Electron
transport
chain
e–
NADH
H+
2
Electrons provide
energy to pump protons
across the membrane.
1
Electrons are harvested and
carried to the transport chain.
e–
Acetyl-CoA
Krebs
cycle
H 2O
e–
NADH
e–
FADH2
3
Oxygen joins with
protons and electrons
to form water.
1
–
O2
2
+
O2
2 H+
CO2
2
H+
ATP
4
Protons diffuse back in down
their concentration gradient,
driving the synthesis of ATP.
Mitochondrial
matrix
H+
34
ATP
ATP
synthase
26
CELLS CAN METABOLIZE FOOD
WITHOUT OXYGEN
• In the absence of oxygen, organisms must rely
exclusively on glycolysis to produce ATP.
• In a process called fermentation, the hydrogen
atoms from the NADH generated by glycolysis are
donated to organic molecules, and NAD+ is
regenerated.
• With the recycling of NAD+, glycolysis is allowed to
continue.
FERMENTATION
• Bacteria can perform more than a dozen
different kinds of fermentation.
• Eukaryotic cells are only capable of a few
types of fermentation.
FERMENTATION
• In yeasts (single-celled fungi), pyruvate is converted
into acetaldehyde, which then accepts a hydrogen
from NADH, producing NAD+ and ethanol.
• In animals, such as ourselves, pyruvate accepts a
hydrogen atom from NADH, producing NAD+ and
lactate.
Ethanol fermentation in yeast
Lactic acid fermentation in muscle cells
H
Glucose
G
L
Y
C
O
L
Y
S
I
S
2ADP
2ATP
O–
C
O
C
O 2 Pyruvate
CH3
H
C
OH
2 ADP
CH3
2NAD+
2 Ethanol
2 ATP
2 NADH
H
C
CO2
O–
Glucose
O
CH3
2 Acetadehyde
O–
C
O
C
O
CH3
G
L
Y
C
O
L
Y
S
I
S
2 Pyruvate
H
C
O
C
OH
CH3
2NAD+
2 NADH
2 Lactate
GLUCOSE IS NOT THE ONLY
FOOD MOLECULE
• Cells also get energy from foods other than
sugars.
• These complex molecules are first digested into
simpler subunits, which are then chemically
modified into intermediates.
• These intermediates enter cellular respiration at
different steps.
Macromolecule
degradation
Cell
building
blocks
Nucleic
acids
Proteins
Polysaccharides
Nucleotides
Amino acids
Sugars
Deamination
Glycolysis
Lipids
and fats
Fatty acids
β-oxidation
Pyruvate
Oxidative
respiration
Acetyl-CoA
Krebs
cycle
Ultimate
metabolic
products
NH3
H2O
CO2