Download RESPIRATION: SYNTHESIS OF ATP

RESPIRATION: SYNTHESIS OF ATP
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Respiration is a series
of coupled reactions
• Carbon (in glucose) is
oxidized
• ATP is formed from
ADP plus phosphate
O2
ADP + Pi
CO2 + H2O
ATP
Synthesis of ATP
Anaerobic conditions (fermentation)
! Glycolysis depends on a supply of substrates:
glucose, ATP, ADP, Pi, NAD+
! NAD+, FAD present in only small amounts in cell,
and NAD+ and FAD are used up in glycolysis and
the citric acid cycle.
! Therefore, NAD+ must be regenerated from NADH
to allow continued glycolysis, citric acid cycle
operation.
! In air, the electron transport chain regenerates
NAD+ and FAD by passing electrons to O2.
! Without air, the electron transport chain cannot
oxidize NADH, FADH2; citric acid cycle stops.
! Without air, some cells regenerate NAD+ (from
glycolysis only) by passing e- (+ H+) to pyruvic acid
! Result: continued glycolysis, forming 2 ATP per
glucose
Muscle cells
Reduction of
pyruvate produces
lactic acid
Yeast cells
Reduction of
pyruvate produces
ethanol
Variations:
! Most plants make EtOH, but are
hurt by large amounts; some
plants make lactic or malic acid
and tolerate these better.
! Most animals make lactic acid,
but the acid hurts; goldfish make
EtOH and excrete it.
Synthesis of ATP
Aerobic conditions: electron transport chain
! Electron carriers (4 protein complexes)
positioned close together in the membranes of
the cristae; FAD, heme are associated with
proteins (enzymes) that facilitate transfer of
electrons; Q floats in lipid bilayer.
! Carriers have increasing affinity for electrons;
thus, electrons move from carrier to carrier in a
specific order.
Electron transport chain: electrons move from
carrier to carrier in a specific order
FAD
FAD
Succinic
acid
Fumaric
acid
Synthesis of ATP
Aerobic conditions: electron transport chain
! Electron carriers (4 protein complexes)
positioned close together in the membranes of
the cristae; FAD, heme are associated with
proteins (enzymes) that facilitate transfer of
electrons; Q floats in lipid bilayer.
! Carriers have increasing affinity for electrons;
thus, electrons move from carrier to carrier in a
specific order.
! Carriers are positioned in cristae so that H+
moves from inside to outside of membrane as
electrons move from NADH to O2.
! H+ moves back to the inside through an enzyme
--ATP synthetase--that forms ATP + H2O from
ADP + Pi.
Electron transport chain: H+ moves from inside to
outside of membrane
FAD
FAD
Succinic
acid
Fumaric
acid
ATP synthetase: H+ moves back to the inside
through an enzyme that forms ATP + H2O from
ADP + Pi.
ATP synthetase: Adding ATP to the enzyme pumps
H+ through the membrane (running backwards
relative to ATP synthesis). It also makes the center
protein rotate. The ATP synthetase is a rotary pump!
(Running forward, it is a turbine.)
Calculating the ATP yield of respiration of
one glucose molecule
(Fig. 9.13)
Glycolysis
+2 NADH
+2 ATP
Pyruvate oxidation
+2 NADH
Citric acid cycle
+2 ATP (GTP)
+6 NADH
+2 FADH2
Electron transport chain -2 NADH
+4 ATP
-8 NADH
+24 ATP
-2 FADH2
+4 ATP
+36 ATP
Rate control:
Should respiration run
at the same rate whatever
the demand for energy?
Homeostasis: rate of
respiration (fermentation) is
controlled by level of ATP
Allosteric enzyme:
Phophofructokinase is
inhibited by ATP and
Citrate (see Fig. 9.16)
Summary: how free energy flows through
the cell
! Cells get free energy in the form of glucose (or
other organic molecules)
! Oxidation of glucose releases free energy; much
is saved as reduced NADH and FADH2 (and a
little ATP) are formed in coupled reactions; the
rest lost as heat
! Oxidation of NADH and FADH2 releases free
+
energy; much is saved as electrochemical (H )
gradient; the rest lost as heat
+
! Reversal of electrochemical gradient (H
transport) releases free energy; much is saved as
ATP; the rest lost as heat
! Hydrolysis of ATP releases free energy; some is
saved (in energy of position, new chemical
gradients from transport of compounds across
membranes, synthesis of polymers, etc.); the rest
lost as heat.
How did primeval organisms respire?
Methanogens may have been early life forms
Methanogenesis
4H2 + CO2 --> CH4 + 2 H2O
∆Go = -131 kcal/mol
∆G depends also on concentrations. This reaction
works if H2 and CO2 concentrations are high.
H+
e-
H2
ATP
CO2
CH4
H2O
H+