7 1 Risky Business Early life on Earth lived in anaerobic oxygenfree environments Some organisms lived and thrived in aerobic O 2 containing environments Produced ID: 784716
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Slide1
Chapter 7
How Cells Release Chemical Energy
Slide27
.1 Risky Business
Early life on Earth lived in anaerobic (oxygen-free) environmentsSome organisms lived and thrived in aerobic (O2-containing) environmentsProduced antioxidants that detoxify or prevent free radical formationOver time, aerobic respiration evolvedProduces ATP very efficientlyOccurs inside mitochondria
An
antioxidant
is a molecule that inhibits the oxidation of other molecules. Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may
damage cells
.
Antioxidants
such as
thiols
or ascorbic acid (vitamin C) terminate these chain reactions
.
Slide3Slide4Mitochondrial Malfunction
Aerobic respiration still produces oxygen free radicals
Failure to remove free radicals results in oxidative stressOxidative stress is linked to illnesses such as Alzheimer’s and Parkinson’s diseasesNerve cells are particularly sensitive: have a need for a large number of mitochondria and high ATP
Slide57.2
Overview
of Carbohydrate Breakdown PathwaysPhotosynthetic organisms capture solar energy and store it in sugarsEnergy stored in
sugars
must be converted to molecular energy (in the form of
ATP
) for use in energy-requiring reactions
Most eukaryotes and some bacteria breakdown carbohydrates via
aerobic respiration
Slide6Cycle of Photosynthesis and Aerobic Respiration
energy
A
E
R
O
B
I
C
R
E
S
P
I
R
A
T
I
O
N
P
H
O
T
O
S
Y
N
T
H
E
S
I
S
C
O
2
H
2
O
sugar
O
2
energy
Slide7Overview of Aerobic Respiration
C
6H12O6 (glucose) + O2 (oxygen) →
CO
2
(
carbon dioxide
) + H
2
O (
water
)
Three stages
Glycolysis
Acetyl-CoA formation and Krebs cycleElectron transfer phosphorylation (ATP formation)Coenzymes NADH and FADH2 carry electrons and hydrogen
Slide8Aerobic Respiration vs. Anaerobic Fermentation
Aerobic respiration and fermentation
both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvateAfter glycolysis, the two pathways divergeFermentation is completed in the cytoplasm, yielding 2 ATP per glucose moleculeAerobic respiration is completed in mitochondria, yielding 36 ATP per glucose molecule
Glycolysis
Fermentation
No O
2
Aerobic Resp.
O
2
Present
Slide9Comparison of
Aerobic Respiration
and Fermentation
AT
P
A
TP
A
TP
A
TP
A
TP
A
TP
additional
r
eactions
in cytoplasm
cytoplasm
cytoplasm
mitochondrion
ELECTRO
N
T
RANSFE
R
PHOSPHORYLATIO
N
KREB
S
CYCL
E
GLYCOLYSI
S
GLYCOLYSI
S
A
Aerobic respiration
.
B
Fermentation
Slide107.3 Glycolysis—Sugar Breakdown Begins
Glycolysis begins the
sugar breakdown pathway in both aerobic respiration and fermentationOccurs in cytoplasm of cellsRequires initial investment of 2 ATP moleculesProduces 4 ATP total, for a net yield of 2 ATP
Slide11Glycolysis
Stage 1
: Energy Investment StageCell uses ATP to phosphorylate compounds of glucoseStage 2: Energy Payoff StageTwo 3-C compounds oxidizedFor each glucose molecule:2 Net ATP produced by substrate-level phosphorylation2 molecules of NAD+NADH
Slide12Glycolysis – ATP-Requiring Steps
1
. An enzyme transfers
a
phosphate
g
r
oup
f
r
om
A
TP to glucose, forming glucose-6-phosphate (hexokinase)
A phosphate group from a second ATP
is transferr
ed to
the glucose-6-phosphate. The
resulting molecule
is
unstable, and it
splits into two
three-carbon
molecules.
The molecules
are
interconvertible, so
we will
call them
both PGAL
(phosphoglyceraldehyde).
Two
ATP
have now
been invested in
the
reactions.
ATP-Requirin
g Steps
An
enzyme
attaches
a
phosphate to
each
PGAL,
so two PGA (phospho- glycerate) form. T
wo
electrons
and a hyd
rogen ion (not shown) fr
om each PGAL are
accepted
by
NA
D
+
,
so
two NADH
form.
2
NAD
+
2
coenzymes
r
educed
ADP
2
PGAL
ADP
G
L
Y
C
O
L
Y
SI
S
glucose
glucose-6-phosphate
fructose-1,6-bisphosphate
ATP Building Steps
Slide13Glycolysis – ATP-Requiring Steps
to
second
stage of aerobic respiration or fermentation
An
enzyme
transfers
a
phosphate
g
r
oup
f
r
om
each
PEP
to
AD
P
,
forming two
mo
r
e
A
TP
and
two
molecules of pyruvate.
Summing
up, glycolysis yields
two NADH,
two
A
TP
(
net
),
and
two
pyruvate for
each
glucose molecule.
Depending
on
the type of cell
and
envi
r
onmental
conditions, the pyruvate
may
enter the
second
stage of
ae
r
obic
r
espiration or
it
may
be
used
in other ways,
such
as
in fermentation.
An
enzyme
transfers
a
phosphate
g
r
oup
f
r
om
each
PGA
to
AD
P
, forming two
A
TP
and
two
intermediate molecules (PEP).
The
original energy investment of
two
A
TP
has
now
been
r
ecove
r
ed
.
2
ADP
2
A
TP
p
r
oduced
by substrate-level phosphorylation
2
pyruvate
2
PGA
Net
2
A
TP
+
2
NADH
2
A
TP
p
r
oduced
by substrate-level phosphorylation
2 ADP
Slide14Glycolysis
Takes place in all cells of the body
Yields 2 Net ATPRecycles NADH (2)Creates 2 Pyruvic AcidRBC’s rely solely on glycolysis for energy…WHY?Pyruvic Acid has 1 of 3 fates:Aerobic Respiration (in mitochondria)Fermentation (Lactic Acid or Alcoholic)
Slide157.4 Second Stage of Aerobic Respiration
The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis
Occurs in mitochondriaIncludes two sets of reactions: acetyl-CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)
Slide16Second Stage of Aerobic Respiration – In the
Matrix
cytoplasm
outer membrane
inner membrane
The breakdown of 2 pyruvate to 6 CO
2
yields 2 ATP and 10 reduced coenzymes (
8 NADH, 2 FADH
2
). The
coenzymes
will carry their cargo of electrons and hydrogen ions to the third stage of aerobic respiration.
matrix
Slide17Acetyl-CoA Formation
In the inner compartment of the mitochondrion, enzymes split pyruvate, forming acetyl-CoA and CO
2 (which diffuses out of the cell) NADH is formed
Slide18The Krebs Cycle
A sequence of enzyme-mediated reactions that break down 1 acetyl-CoA into 2 CO
2Oxaloacetate is used and regenerated3 NADH and 1 FADH2 are formed1 ATP is formed
Slide19Acetyl–CoA Formation and the Krebs Cycle
Krebs Cycle
The final steps of the Krebs cycle regenerate
oxaloacetate.
8
NAD
+
combines with hydrogen ions and electrons,
forming NADH.
7
The coenzyme FAD combines with hydrogen
ions and electrons, forming FADH
2
.
6
One ATP forms by substrate-level phosphorylation.
5
An enzyme splits a pyruvate coenzyme A NAD
+
molecule into a two-carbon acetyl group and CO
2
. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons,
forming NADH.
1
The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme
A is regenerated.
2
A carbon atom is removed from an intermediate and leaves the cell as CO
2
. NAD
+
combines with released hydrogen ions and electrons, forming NADH.
3
A carbon atom is removed from another intermediate and leaves the cell as CO
2
, and another NADH forms.
Pyruvate’s three carbon atoms have now exited the cell, in CO
2
.
4
Stepped Art
Slide20Acetyl CoA & Krebs Cycle
End of the road for sugar…..
Products:NADH/FADH2/H+ (these products advance to the ETC)
CO2
ATP
Slide217.5 Aerobic Respiration
’
s Big Energy Payoff Many ATP are formed during the third and final stage of aerobic respirationElectron transfer phosphorylation (also known as the Electron Transport Chain)Occurs in mitochondria (using christae)Results in attachment of phosphate to ADP to form ATP
Slide22Electron Transfer Phosphorylation
Coenzymes NADH and FADH
2 donate electrons and H+ to electron transfer chainsActive transport forms a H+ concentration gradient in the outer mitochondrial compartment H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP
…just like…
Finally, oxygen accepts electrons and combines with H
+
, forming
water
(it’s here where 4% of oxygen gets over reduced and becomes a free radical )
Slide23Slide24Slide25Slide26Summary: The Energy Harvest
Typically, the breakdown of one glucose molecule yields
36 ATPGlycolysis: 2 ATPAcetyl-CoA formation and Krebs cycle: 2 ATPElectron transfer phosphorylation: 32 ATP
Slide27Slide287.6 Fermentation
Glycolysis is the first stage of fermentation
Forms 2 pyruvate, 2 NADH, and 2 ATPPyruvate is converted to other molecules, but is not fully broken down to CO2 and waterRegenerates NAD+ but does not produce ATPProvides enough energy for some single-celled anaerobic species
Slide29Two Fermentation Pathways – Alcoholic Fermentation
Alcoholic fermentation
Pyruvate is split into acetaldehyde and CO2Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and ethanolCarried out by single-celled organisms such as fungi (yeast)Used to make beer, wine, and bread
Slide30Alcoholic Fermentation
Slide31Two Fermentation Pathways – Lactate Fermentation
Lactate fermentation
Pyruvate receives electrons and hydrogen directly from NADH, forming NAD+ and lactateCarried out by beneficial bacteriaUsed to make yogurtAlso carried out by animal white skeletal muscle cellsUseful for quick
,
strenuous
activities
Does not support prolonged
activity
3 min max
Slide32Lactate/ Lactic Acid Fermentation
Slide337.7 Alternative Energy Sources in Food
Aerobic respiration can produce ATP from the breakdown of complex carbohydrates, fats, and proteins
As in glucose metabolism, many coenzymes are reducedThe energy of the electrons the coenzymes carry ultimately drives the synthesis of ATP in electron transfer phosphorylation
Slide34Energy From Complex Carbohydrates
Enzymes break starch and other complex carbohydrates down to
monosaccharide subunitsMonosaccharides are taken up by cells and converted to glucose-6-phosphate, which continues in glycolysisA high concentration of ATP causes glucose-6-phosphate to be diverted from glycolysis and into a pathway that forms glycogen
Slide35Complex Carbohydrates are Broken Down into Monosaccharides
OH
star
c
h
(
a
compl
ex
c
a
r
bo
hy
d
rate) glucose (a simple sugar)
OH
CH
2
OH
HO
HO
O
Slide36Most carbohydrates enter cellular respiration during glycolysis. In some cases, entering the pathway simply involves breaking a glucose polymer down into individual glucose molecules.
For instance, the glucose polymer glycogen is made and stored in both liver and muscle cells in our bodies. If blood sugar levels drop, the glycogen will be broken down into phosphate-bearing glucose molecules, which can easily enter glycolysis.
Slide37Energy From Fats
Enzymes
cleave fats into glycerol and fatty acidsGlycerol products enter glycolysisFatty acids are converted to acetyl-CoA and enter the Krebs cycleFatty acid breakdown yields more ATP per carbon atom than carbohydratesWhen blood glucose level is high, acetyl-CoA is diverted from the Krebs cycle and into a pathway that makes fatty acids
Slide38Fats are Broken Down by Separating the Glycerol Head and Fatty Acid Tails
a
triglyc
er
i
d
e (
f
a
t
)
g
l
y
c
erol head
fatty acid tails
Slide39Energy From Proteins
“When
you eat proteins in food, your body has to break them down into amino acids before they can be used by your cells. Most of the time, amino acids are recycled and used to make new proteins, not oxidized for fuel.However, if there are more amino acids than the body needs, or if cells are starving, some amino acids will broken down for energy via cellular
respiration”—Khan Academy
Slide40Energy from Proteins
Enzymes split dietary proteins into amino acid subunits, which are used to build proteins or other
molecules (or energy?? WHEN?)First the amino group is removed and converted into ammonia (NH3), a waste product eliminated in urineOnce deaminated, different amino acids enter the cellular respiration pathways at different stages
Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle forms, depending on the amino acid
Slide41Slide42Amino Acids Are Converted
alan
i
n
e
(
a
n
am
i
n
o
a
cid
)
pyruvate
Slide43Slide44Points to Ponder
Why are so many diseases attributed to defective mitochondria?
What is “metabolic water”?What happens to lactate produced during periods of intense muscle activity? Why is it important that the lactate is broken down quickly?