How Cells Harvest Energy
Chapter 7
Respiration
• Organisms can be classified based on how
they obtain energy:
• autotrophs: are able to produce their own
organic molecules through photosynthesis
• heterotrophs: live on organic compounds
produced by other organisms
• All organisms use cellular respiration to
extract energy from organic molecules.
2
Respiration
• Respiration is a metabolic pathway of
Redox Reactions
• Respiration typically oxidizes
carbohydrates
• The type of molecule that is reduced
determines the type of respiration
• The energy produced is in the form of ATP
3
Respiration
• Cellular respiration a metabolic pathway of
redox reactions:
-oxidation – loss of electrons
– dehydrogenations: loss of hydrogen e-’s
-reduction - gain of electrons
– gain on hydrogen e-’s
• Oxidized molecules actually loose a
hydrogen atom (1 electron, 1 proton)
• Both the protons and electrons are used
by
4
Respiration
• During respiration, high energy electrons
are passed along chains of molecules Electron Transport Chains
• Energy is released as molecules in the
electron transport chains are oxidized
• The energy released is used to power the
production of ATP
5
6
Three Types of Respiration
• Respiration type is determined by the final
electron acceptors:
1. Aerobic Respiration: final electron
receptor is oxygen (O2)
2. Anaerobic Respiration: final electron
acceptor is an inorganic molecule other than
O2
3. Fermentation: final electron acceptor is
an organic molecule
7
Aerobic Respiration
• Glucose contains chemical energy that
can be transferred and stored as ATP
• Aerobic Respiration is a metabolic
pathway that oxidizes glucose and
transfers the energy to produce ATP
• Oxygen is the final electron acceptor:
• Recall:
C6H12O6 + 6 O2
Glucose
Oxygen
6 H2O + 6 CO2 + Energy
Water
Carbon Dioxide
• The Energy is in the form of ATP
Aerobic Respiration
C6H12O6 + 6 O2
6 H2O + 6 CO2 + Energy
-Now-
C6H12O6 + 6O2 + 38 ADP + 38 P
6 H2O + 6CO2 + 38 ATP
Aerobic Respiration
• Aerobic Respiration is a three stage
process:
Stage 1: Glycolysis
Stage 2: The Krebs Cycle
Stage 3: Oxidative Phosphorylation
• Each of these stages produce ATP
• At the end of all three stages, there is a
net gain of 38 ATP molecules (profit)
– recall: cells are very efficient because of
Stage 1: Glycolysis
• Glycolysis is a 10 step metabolic pathway
that cleaves glucose
• Glyo-lysis = “splitting glucose”
• Glycolysis occurs in the cell’s cytoplasm
– that’s where the enzymes for glycolysis are
located
Stage 1: Glycolysis
Glycolysis converts glucose to pyruvate
(pyruvic acid).
- a 10-step biochemical pathway
- occurs in the cytoplasm
- 2 molecules of pyruvate are formed from
each glucose
- net production of 2 ATP molecules
-2 NADH produced by reduction of 2 NAD+
–recall NADH just is an electron carrier
–(see ch. 6)
13
Stage 1: Glycolysis
• During Glycolysis, glucose (a 6 carbon molecule) is
chopped up into 2 Pyruvates (each pyruvate is a 3
carbon molecule)
• As glucose is cleaved, it is also being
oxidized - loosing electrons (hydrogens)
Figure 6_07
• Glucose is cut up
into 2 Pyruvates in 10
steps
Figure 6_07
• In step 1 ATP is
used to phosphorylate
glucose to make
G-6-P
• Phosphorylation
• 2 ATP must be
invested during
first two steps of
glycolysis
destabilizes the glucose
molecule so it can be cleaved
• Phosphorylation reactions
are carried out by enzymes
known as Kinases - see
chapter 6 and slide 38
Figure 6_07
• In step 2, G-6-P is
converted to F-6-P
• This step is carried out by
• 2 ATP must be
invested during
first two steps of
glycolysis
an isomerase enzyme
• recall isomers from ch. 3
and slide 32
Figure 6_07
•In step 3, 1 ATP is
• 2 ATP must be
invested during
first two steps of
glycolysis
used to phosphorylate
F-6-P to become F1,6-bP
• this step is carried out by
another kinase
Figure 6_07
• In steps 4 and 5, the
six-carbon molecule,
F-1,3-bP is cleaved
into 2 three-carbon
molecules
• G-3-P and
Dihydroxyacetone phosphate
(DHAP)
• Dihydroxyacetone
phosphate is immediately
converted into another G-3-P
Figure 6_07
Very Important!
• Steps 6-10 occur
twice for every
glucose that enters
glycolysis
• because there are now two
G-3P’s
Figure 6_07
• In step 6, G-3-P’s
are oxidized
• one NAD+ is reduced to
produce one NADH
• Also in step 6, G-3-P’s
are phosphorylated to
produce 1,3-BPG
Figure 6_07
• One ATP is
produced in step 7
• 1,3-BPG is
dephosphorylated to become
3-BPG
• ADP is phosphorylated to
ATP
Figure 6_07
• Steps 8 and 9 involve
structural changes of 3BPG to become
Phosphoenolpyruvate
Figure 6_07
• In step 10, one more
ATP is produced as
Phosphoenolpyruvate
is dephosphorylated to
become Pyruvic Acid
• another kinase
Fig. 7.6-1
Fig. 7.6-2
Fig. 7.6-3
Fig. 7.7-1
Fig. 7.7-2
Fig. 7.7-3
Glycolysis
Totals
Per Glucose
Costs
– 2 ATP
Yield
– 2 NADH
– 4 ATP
Net Gain from
Glycolysis
– 2 ATP
– 2 NADH
Glucose
2 ATP
4 ATP, 2 NADH
Pyruvate1
Pyruvate 2
NAD+, NADH
• During redox reactions, electrons carry
energy from one molecule to another
• NAD+ is an electron carrier
• NAD+ functions to carry electrons by
carrying Hydrogen atoms
- NAD+ accepts 2 electrons and 1 proton to
become NADH
- The reaction is reversible
- NAD+ + 2e-’s + 1p+
NADH
33
NAD+
NAD+ Reduced to NADH
36
37
FAD
• FAD is very similar to NAD+
• It has the same function of collecting and
carrying Hydrogen atoms from one
molecule to another
• FAD can carry 2 Hydrogen atoms
• FAD is Reduced to FADH2
Stage 2: The Krebs Cycle
• Also known as The Citric Acid Cycle
– citrate is the first molecule produced in this cycle
• The Krebs cycle is a metabolic pathway that
further cleaves and oxidizes pyruvate
• The Krebs Cycle occurs in the cell
membrane of Prokaryotic Cells and in the
mitochondria of Eukaryotic Cells
• In mitochondria, a multienzyme complex
called pyruvate dehydrogenase catalyzes
the reaction
Stage 2: The Krebs Cycle
• The Krebs cycle is fueled with pyruvates from glycolysis
– recall, there are 2 Pyruvates made from each Glucose, so there
are 2 Krebs Cycles for every glucose molecule
• Prep Step - before pyruvate enters the mitochondria for
the Krebs cycle it is cleaved, oxidized and converted to
become Acetyl-Coenzyme A (Acetyl-CoA)
Pyruvate
Acetyl-CoA
Krebs Cycle
CO2
Prep Step: Pyruvate Oxidation
• Pyruvates are oxidized to form AcetylCoA
– a CO2 moiety of pyruvate is exchanged for a
Coenzyme A(CoA) moiety
• The products of pyruvate oxidation
include:
-
1 CO2
1 NADH
1 acetyl-CoA which consists of 2 carbons
from pyruvate attached to coenzyme A
• Acetyl-CoA proceeds to the Krebs cycle
42
Prep Step: Pyruvate Oxidation
• Prep Step: Pyruvates are converted
to Acetyl-CoA with the release of CO2
in a preparation step
44
Stage 2: The Krebs Cycle
• The Krebs cycle further oxidizes the
acetyl group from pyruvate.
- Occurs in the matrix of the mitochondria
- Biochemical pathway of 5 steps
- First Step: Each Acetyl-CoA (2 carbon per
molecule) is bonded to an Oxaloacetate (a 4
carbon molecule) to produce Citrate
acetyl group + oxaloacetate
(2 carbons)
(4 carbons)
citrate
(6 carbons)
45
Stage 2: The Krebs Cycle
First Step of Krebs Cycle:
• Each Acetyl CoA (2 carbon per molecule)
is bonded to an Oxaloacetate (a 4 carbon
molecule)
• The new molecule made is Citrate (a 6
carbon molecule)
acetyl-CoA + oxaloacetate
(2 carbons)
(4 carbons)
citrate
(6 carbons)
Stage 2: The Krebs Cycle
• Citrate undergoes a five step cycle that
builds additional ATPS
• During the Krebs Cycle additional NADH’s
and FADH2’s are produced
• Citrate is eventually converted back
into oxaloacetate and the cycle
continues
48
Fig. 7.12-2
In step 1, acetyl-CoA
enters the mitochondria
and combines with
oxaloacetate
to form citrate
Fig. 7.12-2
Citrate is further oxidized
to produce 3 NADH and
and FADH2
Fig. 7.12-2
2 CO2’s are
produced as
Citrate is
converted back
into oxaloacetate
Fig. 7.12-2
The regenerated
oxaloacetate is
ready for another
Acetyl-CoA and
the Krebs cycle
continues
Stage 2: The Krebs Cycle
• The remaining steps of the Krebs cycle:
- release 2 molecules of CO2
- reduce 3 NAD+ to 3 NADH
- reduce 1 FAD (electron carrier) to
FADH2
- produce 1 ATP
- regenerate oxaloacetate
53
The Krebs Cycle Totals
(per pyruvate)
Totals
Costs
• 2 ATP
Yields
• 2 CO2
• 3 NADH
• 1 FADH2
• 1 GTP (immediately converted to 1 ATP)
Glucose
2 ATP
4 ATP, 2 NADH
Pyruvate1
Pyruvate 2
Pyruate converted to Acetyl Co-A
2 ATP
1 ATP, 2 CO2
3 NADH, 1 FADH2
Aerobic Respiration Review
• After glycolysis, pyruvate oxidation, and
the Krebs cycle, one glucose has been
completely cleaved and oxidized to produce:
- 6 CO2
- 4 ATP
- 10 NADH
- 2 FADH2
These electron carriers proceed
to the electron transport chain
for stage 3 of aerobic
respiration
56
Glucose
2 ATP
4 ATP, 2 NADH
Pyruvate1
Pyruvate 2
Pyruate converted to Acetyl Co-A
2 ATP
2 ATP, CO2
8 NADH, 2 FADH2
34 ATP
Electron Transport Chain
NADH, FADH2
Stage 3: Electron Transport Chain
• The electron transport chain (ETC) is a
series of membrane-bound electron carrier
molecules called cytochromes
- embedded in the mitochondrial inner
membrane
- electrons from NADH and FADH2 are
transferred to cytochromes of the ETC
- each cytochrome transfers the electrons to
the next cytochrome in the chain
58
Fig. 7.13a
Stage 3: Electron Transport Chain
Energy from the Electrons
• As the electrons are transferred, some
electron energy is released with each
transfer
• This energy is used by the cytochromes to
pump protons (H+) across the membrane
from the matrix to the inner membrane space
61
62
Stage 3: Electron Transport Chain
Energy from the Protons
• Electron energy is used by the
cytochromes to pump protons (H+) across
the membrane from the matrix to the inner
membrane space
• A proton gradient is established
– There are more protons on the
63
Stage 3: Electron Transport Chain
• The cytochromes are channel proteins
and use the electron energy to pump
protons (H+) across the inner mitochondrial
membrane
– Now there are more protons on the inside of
the membrane than the outside
• A proton gradient is established
• This proton gradient is potential energy
that can be utilized to make more ATP’s
– Recall diffusion: The protons want to equalize
their number on both sides of the membrane
Stage 3: Electron Transport Chain
Stage 3: Electron Transport Chain
• There are other channel proteins in the
membrane known as ATP synthases
• ATP synthases provide a channel for the
protons to diffuse through
• The rushing protons provides the energy
for ATP synthase to phosphorylate ADP to
ATP
68
69
Stage 3: Electron Transport Chain
• In Aerobic Respiration, oxygen is the final
molecule to receive the hydrogens as they are
passed down the Electron Transport Chain
• The result is water: O2 + 4e- + 4H+
2H2O
• Oxygen is reduced to water
Oxygen is the Final Electron Acceptor in
Aerobic Respiration
The Electron Transport Chain
Energy Yield of Respiration
• The ETC is very efficient and produces most of the ATP
for cellular respiration (34 of the 38)
• Theoretical energy yields
– 38 ATP per glucose for bacteria
– 36 ATP per glucose for eukaryotes
• Actual energy yield
– 30 ATP per glucose for eukaryotes
– reduced yield is due to “leaky” inner membrane
– reduced yield also due to chemiosis - the use of the
proton gradient for purposes other than ATP synthesis
72
73
Glucose
2 ATP
4 ATP, 2 NADH
Pyruvate1
Pyruvate 2
Pyruate converted to Acetyl Co-A
2 ATP
2 ATP, CO2
8 NADH, 2 FADH2
30+ ATP
And H2O
Electron Transport Chain
NADH, FADH2
Respiration Without O2
• Respiration occurs without O2 via either:
1. Anaerobic Respiration
• use of inorganic molecules (other than O2)
as the final electron acceptor
2. Fermentation
• use of organic molecules as the final
electron acceptor
75
Oxidation Without O2
• Anaerobic respiration produces fewer
ATPs per glucose molecule compared to
Aerobic Respiration
• Anaerobic respiration is much less efficient
than aerobic respiration
• The exact amount of ATP production
depends on the organism and the final
electron acceptors that are used
Oxidation Without O2
• Anaerobic respiration is much less efficient
than aerobic respiration
– the ETC is bypassed
– not all molecules are as readily reduced as O2
– other final electron acceptors may be reduced
to produce harmful products
• fermentation of organic molecules produces acids
and alcohols
Oxidation Without O2
• Anaerobic respiration by methanogens
– methanogens reduce CO2 to regenerate NAD+
– CO2 is reduced to CH4 (methane)
• Anaerobic respiration by sulfur bacteria
– inorganic sulphate (SO4) is reduced to
hydrogen sulfide (H2S)
78
Oxidation Without O2
Fermentation reduces organic molecules in
order to regenerate NAD+
1. Ethanol fermentation occurs in yeast
2. Lactic acid fermentation occurs in
animal cells (especially muscles)
79
Oxidation Without O2
Fermentation reduces organic molecules in
order to regenerate NAD+
1. Ethanol fermentation
• NADH reduces acetaldehydes to produce
ethanol (an alcohol)
• CO2, ethanol, and NAD+ are produced
2. Lactic acid fermentation
• NADH reduces pyruvate to produce lactic
acid
80
Fig. 7.19-1
Fig. 7.19-2
Other Nutrients Serve as Energy Sources
• In addition to Glucose, many other molecules can
be used by cells to produce energy through
cellular respiration
• A variety of Carbohydrates, Lipids and Proteins
can be catabolized for energy
• All of these must go through preparatory steps
before they can enter into glycolysis
• For example:
– Amino acids must go through a deamination process
– Fatty Acids must go through beta oxidation
Catabolism of Protein & Fat
Catabolism of proteins:
• Amino acids undergo deamination to
remove the amino group (H2N-)
- amino group removed as urea in the urine
• Remainder of the amino acid is converted
to a molecule that enters glycolysis or the
Krebs cycle
- for example:
• amino acid alanine is converted to pyruvate to enter
Krebs cycle
• animo acid aspartate is converted to oxaloacetate84 to enter
Fig. 7.21
Catabolism of Protein & Fat
• Catabolism of fats:
– triglycerides are broken down to fatty acids
and glycerol
– fatty acids are converted to acetyl groups by
-oxidation
– acetyl groups can enter the Krebs cycle
• The respiration of a 6-carbon fatty acid
yields 20% more energy than glucose
86
87
Amino
Acids
Lipids
Preparatory Steps
89