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CHAPTER 9
energy flows &
matter cycles
within an
ecosystem
Catabolic Pathways
that produce ATP

anaerobic = fermentation

aerobic = cellular respiration
 exergonic: G = -686 kcal/mol
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy*
*ATP & heat
C.R. is a series of
Redox Reactions

redox reactions involve a relocation of
electrons which releases energy that can
ultimately be used to synthesize ATP

redox stands for oxidation-reduction

oxidation = loss of electrons from a
substance

reduction = addition of electrons to a
substance
Example 1:
Na + Cl  Na+ + Cl

Na loses an electron  oxidized

Na donates its electron to Cl  reducing
agent

Cl gains an electron  reduced

Cl accepts an electron from Na 
oxidizing agent
Example 2:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy

glucose is oxidized

oxygen is reduced
 as glucose is broken down, electrons are
temporarily transferred to intermediate
“electron carriers” NAD+ & FAD (coenzymes)
which are reduced to NADH & FADH2
 NADH & FADH2 eventually transfer the
electrons to the electron transport chain which
uses the energy to drive the production of ATP
 O2 is the final electron acceptor (b/c high
electronegativity) in the electron transport
chain where it combines with H+ ions to form
H2O
*if you are confused, revisit this slide later
+
NAD
reduction
Stages of C.R.
glycolysis
1.


“splitting of sugar”
begins degradation of glucose
citric acid cycle
2.


aka: Krebs cycle
completes degradation of glucose
oxidative phosphorylation:
3.


2 parts: electron transport chain &
chemiosmosis
uses energy from stages 1 & 2 to produce ATP
Overview
Glycolysis

glucose is split into 2 three-carbon
sugars (G3P) that are rearranged to form
2 pyruvate

2 ATP are used to split glucose into the
two G3P but 4 ATP are made while
rearranging them into pyruvate; therefore,
glycolysis has a net production of 2 ATP

the oxidation of the two G3P produces 2
NADH, which carry the electrons to the
electron transport chain
*glycolysis
occurs in
the
cytoplasm
pyruvate is actively transported into the
mitochondrion where it is loses a carboxyl group
(CO2) & combines with coenzyme A forming
acetyl-CoA
 this is a redox reaction  1 NADH is formed for
each pyruvate that is converted to acetyl-CoA

Citric Acid Cycle




acetyl-CoA enters the citric acid cycle by
combining with oxaloacetate to form citrate
the next 7 steps convert the citrate back to
oxaloacetate
during the regeneration of oxaloacetate, 2
CO2, 3 NADH, 1 FADH2, & 1 ATP are made
the cycle turns twice for each glucose broken
down because 2 pyruvate are converted to 2
acetyl-CoA, each of which enters the citric
acid cycle; therefore, this step produces a
total of 4 CO2, 6 NADH, 2 FADH2, & 2 ATP
*the citric acid
cycle occurs in
the matrix of the
mitochondrion
Substrate-Level
Phosphorylation
the ATP made during glycolysis & the citric
acid cycle are made by substrate-level
phosphorylation
 in this process, an enzyme transfers a
phosphate group from a substrate
molecule to ADP

Oxidative Phosphorylation: Electron
Transport Chain
NADH & FADH2 produced during glycolysis
& the citric acid cycle transfer their
electrons to protein complexes in the
inner membrane of the mitochondrion
 as the electrons are passed down the
chain, energy is released that drives the
transport of H+ ions into the intermembrane space
 the final electron acceptor in the ETC is
O2, which is very electronegative; as the
O atoms accept electrons, they also
combine with H+ ions to form H2O

*ETC is located
in the inner
membrane of the
mitochondrion
Oxidative Phosphorylation:
Chemiosmosis
chemiosmosis = the process by which the
energy stored in a H+ ion gradient is used
to drive cellular work (in this case, ATP
synthesis)
 the H+ gradient was set-up by the ETC
 H+ diffuse back into the matrix (down
their concentration gradient) through a
channel protein called ATP Synthase
 the energy released from the exergonic
flow of H+ ions activates the ATP Synthase
(an enzyme) which catalyzes the
phosphorylation of ADP

ATP Synthase
Overview of Oxidative
Phosphorylation
How much ATP?
glycolysis & the citric acid cycle produce a
total of 4 ATP by substrate-level
phosphorylation
 each NADH & FADH2 that delivers electrons
to the ETC can drive the synthesis of 3 ATP
and 2 ATP, respectively (NOTE: these #s
are approximate)
 therefore, 10 NADH & 2 FADH2 can
theoretically drive the synthesis of 34 ATP
 GRAND TOTAL = 38 ATP

Efficiency of C.R.

about 40% of the energy stored in glucose
is transferred to ATP

the remaining 60% is lost as heat, some of
which is used to maintain our body temp.

this is actually quite efficient: only about
25% of the energy stored in gasoline is
converted to usable energy for moving a
car
Anaerobic Respiration

if O2 is not present, the ETC will back up and
oxidative phosphorylation ceases

in the absence of O2, it is still important for
the cell to produce some ATP

in anaerobic respiration, the only source of
ATP is glycolysis, which produces 2 ATP (it’s
not 38 but it’s better than none!)

to ensure that glycolysis continues, it is
followed by one of two types of fermentation
that recycle NADH to NAD+

facultative anaerobes – organisms that can
do either aerobic or anaerobic respiration
Alcoholic Fermentation

occurs in bacteria &
yeast

products are ethanol
& CO2

is used by humans in
for making bread
dough rise & alcoholic
beverages
Lactic Acid Fermentation
occurs in fungi and
bacteria used to make
cheese & yogurt
 occurs in human
muscle cells during
strenuous exercise
 product is lactic acid
 lactic acid build-up in
muscle cells causes
stiffness & soreness
 lactic acid is
eventually removed
by the blood & taken
to the liver

Glycolysis has
Evolutionary Significance

doesn’t require O2 – oldest known bacteria
lived in anaerobic conditions (before O2
accumulated on Earth)

it’s a widespread metabolic pathway
suggesting it evolved early in the history
of life

occurs in the cytoplasm (doesn’t require
membrane-bound organelles)
all organic molecules in food can be
used by cellular respiration

glycolysis & the citric
acid cycle are major
intersections of various
catabolic pathways
(see diagram)

monomers obtained by
the digestion of food
are used in anabolic
pathways that create
molecules needed by
cells that can’t be
obtained directly from
food
Cellular Respiration is Regulated by
Feedback Inhibition



a decrease in ATP will
speed up C.R. whereas
an excess of ATP will
slow it down
one enzyme that is
controlled this way is
phosphofructokinase
this enzyme is
controlled by allosteric
regulation: AMP
stimulates & ATP
inhibits