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Chapter 6
How cells harvest chemical energy
Musebio101
summer 2010
Photosynthesis and cellular
respiration provide energy for life
• Energy is necessary for life processes
– These include growth, transport, manufacture,
movement, reproduction, and others
– Energy that supports life on Earth is captured from
sun rays reaching Earth through plant, algae, protist,
and bacterial photosynthesis
Sunlight energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2
Glucose
+
+
H2O
O2
Cellular respiration
in mitochondria
ATP
(for cellular work)
Heat energy
Cellular respiration banks energy in
ATP molecules
• Cellular respiration is an exergonic process that
transfers energy from the bonds in glucose to
ATP
– Cellular respiration produces 38 ATP molecules from
each glucose molecule
– Other foods (organic molecules) can be used as a
source of energy as well
Introduction to Metabolism
• Cells break down organic molecules to
obtain energy
– Used to generate ATP
• Most energy production takes place in
mitochondria
Metabolism
• Metabolism – refers to all chemical reaction
occurring in body
– Catabolism – break down complex molecules
• Exergonic – produce more energy than they
consume
– Anabolism – combine simple molecules into
complex ones
• Endergonic – consume more energy than they
produce
• Adenosine triphosphate (ATP)
– “energy currency”
– ADP + P + energy ↔ ATP
Potential energy of molecules
Reactants
Amount of
energy
released
Energy released
Products
Metabolism
• Catabolism
– Is the breakdown of organic substrates
– Releases energy used to synthesize high-energy compounds
(e.g., ATP)
• Anabolism
– Is the synthesis of new organic molecules
Cellular respiration is a catabolic reaction
C6H12O6
Glucose
+ 6
O2
Oxygen
6 CO2
Carbon
dioxide
+ 6
H2O
Water
+
ATPs
Energy
Fuel
Energy conversion
Waste products
Heat
Glucose
Cellular respiration
Oxygen
Carbon dioxide
Water
Energy for cellular work
Energy conversion in a cell
Energy from
exergonic
reactions
Energy for
endergonic
reactions
Role of ATP in linking anabolic and
catabolic reactions
Adenosine
Triphosphate (ATP)
Phosphate
group
Adenine
Ribose
Hydrolysis
+
Adenosine
Diphosphate (ADP)
Carbohydrate Metabolism
• Mitochondrial Membranes
– Outer membrane
• Contains large-diameter pores
• Permeable to ions and small organic molecules (pyruvic acid)
– Inner membrane
• Contains carrier protein
• Moves pyruvic acid into mitochondrial matrix
– Intermembrane space
• Separates outer and inner membranes
Chemical energy (high-energy electrons)
Chemical energy
Glycolysis
Glucose Pyruvic
acid
Cytosol
Krebs
cycle
Mitochondrial
cristae
Via substrate-level
phosphorylation
1uring glycolysis,
D
each glucose
molecule is broken
down into two
molecules of pyruvic
acid in the cytosol.
Electron transport
chain and oxidative
phosphorylation
Mitochondrion
Via oxidative
phosphorylation
2 The pyruvic acid then enters
3
Energy-rich electrons picked up
the mitochondrial matrix, where bycoenzymes are transferred to the
the Krebs cycle decomposes it electron transport chain, built into
to CO2. During glycolysis and the cristae membrane. The electron
the Krebs cycle, small amounts transport chain carries out
of ATP are formed by substrate- oxidative phosphorylation,
which accounts for most of
the ATP
Figure
24.5
level phosphorylation.
Cellular respiration begins with
glycolysis
Carbohydrate Metabolism
• Glucose Breakdown
– Occurs in small steps
• Which release energy to convert ADP to ATP
– One molecule of glucose nets 36 molecules of ATP
– Glycolysis
• Breaks down glucose in cytosol into smaller molecules used by
mitochondria
• Does not require oxygen: anaerobic reaction
– Aerobic Reactions
• Also called aerobic metabolism or cellular respiration
• Occur in mitochondria, consume oxygen, and produce ATP
Catalysis
Enzyme
Enzyme
(a) Substrate-level phosphorylation
ENERGY INVESTMENT
PHASE
Glucose
ATP
Steps 1 – 3 A fuel molecule is energized,
using ATP.
Step
1
ADP
P
Glucose-6-phosphate
P
Fructose-6-phosphate
P
Fructose-1,6-bisphosphate
2
ATP
3
ADP
P
Step 4 A six-carbon intermediate splits
Into two three-carbon intermediates.
4
P
Step 5 A redox reaction
generates NADH.
Glyceraldehyde-3-phosphate
(G3P)
P
NAD+
NAD+
5
P
NADH
5
NADH
+ H+
ENERGY PAYOFF PHASE
P
+ H+
P
P
ADP
P
P 1,3-Bisphosphoglycerate
ADP
6
6
ATP
ATP
P
P 3-Phosphoglycerate
7
Steps 6 – 9 ATP and pyruvate
are produced.
7
P
P
2-Phosphoglycerate
8
H2 O
P
P
ADP
Phosphoenolpyruvate
(PEP)
ADP
9
ATP
8
H2 O
9
ATP
Pyruvate
Substrate level phosphorylation
energy payout phase.
6
CH2OH
5
H H
4
O
H
1
H OH
HO OH
3
Dihydroxyacetone
phosphate
2
OH Glucose (1 molecule)
H
1
ATP
ADP
P OH2C
O
H H
HO OH
H
H OH
OH Glucose 6-phosphate
H
2
P OH2C6
O
1
5
CH2OH
2
H H
HO OH
4
3
OH
Phosphofructokinase 3
H Fructose 6-phosphate
ATP
ADP
P OH2C
O
H H
CH2O P
HO OH
OH
H
4
H
C O
HCOH
CH2O P
CH2O P
C O
CH2OH
Fructose 1, 6-bisphosphate
Glyceraldehyde
3-phosphate
2 NAD++ 22P
P
6
2 NADH+ 2H+
CH2O P
HCOH
C O P 1, 3-Bisphosphoglyceric acid
(2 molecules)
O
2 ADP
7
2 ATP
5
CH2O P
HCOH
COOH
8
CH2OH
HCO P
COOH
9
3-Phosphoglyceric acid
(2 molecules)
2-Phosphoglyceric acid
(2 molecules)
CH2
C O P Phosphoenolpyruvic acid
COOH (2 molecules)
2 ADP
10
2 ATP
CH3
C O
Pyruvic acid
COOH (2 molecules)
Fate of pyruvic acid
2 ADP
+2 P
2 ATP
GLYCOLYSIS
Glucose
2 NAD+
2 NADH
2 Pyruvate
2 NADH
2 CO2
released
2 NAD+
2 Ethanol
Alcohol fermentation
Glycolysis evolved early in the
history of life on Earth
• Glycolysis is the universal energy-harvesting
process of living organisms
– So, all cells can use glycolysis for the energy
necessary for viability
– The fact that glycolysis has such a widespread
distribution is good evidence for evolution
Carbohydrate Metabolism
• Oxidation and Reduction
– Oxidation (loss of electrons)
• Electron donor is oxidized
– Reduction (gain of electrons)
• Electron recipient is reduced
– The two reactions are always paired
Energy transfer
• Oxidation-reduction or redox reactions
– Oxidation – removal of electrons
• Decrease in potential energy
• Dehydrogenation – removal of hydrogens
• Liberated hydrogen transferred by coenzymes
– Nicotinamide adenine dinucleotide (NAD)
– Flavin adenine dinucleotide (FAD)
• Glucose is oxidized
– Reduction – addition of electrons
• Increase in potential energy
Carbohydrate Metabolism
• The TCA Cycle (citric acid cycle)
– The function of the citric acid cycle is
• To remove hydrogen atoms from organic molecules and transfer
them to coenzymes
– In the mitochondrion
• Pyruvic acid reacts with NAD and coenzyme A (CoA)
• Producing 1 CO2, 1 NADH, 1 acetyl-CoA
– Acetyl group transfers
• From acetyl-CoA to oxaloacetic acid
• Produces citric acid
Acetyl CoA
CoA
CoA
CITRIC ACID CYCLE
2 CO2
3 NAD+
FADH2
3 NADH
FAD
3 H+
ATP
ADP + P
Glycolysis
Electron transKrebs
port chain
cycle
and oxidative
phosphorylation
Cytosol
Pyruvic acid from glycolysis
NAD+
NADH+H+
CO2
Transitional
phase
Acetyl CoA
Carbon atom
Mitochondrion
Inorganic phosphate
(matrix)
Oxaloacetic acid
Coenzyme A
Citric acid
NADH+H+ (pickup molecule) (initial reactant)
NAD+
Malic acid
Isocitric acid
NAD+
Krebs cycle
CO2
-Ketoglutaric acid
Fumaric acid
CO2
FADH2
FAD
NADH+H+
Succinic acid Succinyl-CoA
GTP
ADP
GDP +
NAD+
NADH+H+
Carbohydrate Metabolism
• The TCA Cycle
– CoA is released to bind another acetyl group
– One TCA cycle removes two carbon atoms
• Regenerating 4-carbon chain
–
–
–
–
Several steps involve more than one reaction or enzyme
H2O molecules are tied up in two steps
CO2 is a waste product
The product of one TCA cycle is
• One molecule of GTP (guanosine triphosphate)
The Krebs Cycle
CO2
CH3
CO
COOH
+
Pyruvic NAD
acid
CoA
CO
CH3
NADH+ H+ Acetyl
To electron
transport chain
coenzyme A
Oxaloacetic acid
NADH
+ H+
NAD+
To electron
transport
chain
COOH
HCOH
CH2
Malic acid COOH
7
H2 O
CoA
1
H2C COOH
HOCCOOH
H2CCOOHCitric acid
8
COOH
CH
Fumaric acid
HC
COOH
FADH2
COOH
CO
CH2
COOH H2O
2
H2C COOH
HC COOH
HOCCOOH
H Isocitric acid
KREBS
CYCLE
3
6
FAD
H2C COOH
H2C COOHCoA
Succinic acid
5
CO2
NAD+
NADH
+ H+
H2C COOH
CO2
4
HCH
H2C COOH
GDP CH2
O C COOH
ADP
Alpha-ketoglutaric
acid
O C S CoA
Succinyl
CoA
ATP
NAD+
GTP
NADH
+ H+
To electron
transport chain
Carbohydrate Metabolism
A Summary of the Energy Yield of Aerobic Metabolism.
Carbohydrate Metabolism
• Summary: The TCA Cycle (Krebs cycle)
CH3CO - CoA + 3NAD + FAD + GDP + Pi + 2 H2O 
CoA + 2 CO2 + 3NADH + FADH2 + 2 H+ + GTP
Carbohydrate Metabolism
• The Electron Transport System (ETS)
– Is the key reaction in oxidative phosphorylation
– Is in inner mitochondrial membrane
– Electrons carry chemical energy
• Within a series of integral and peripheral proteins
Carbohydrate Metabolism
• Coenzyme FAD
– Accepts two hydrogen atoms from TCA cycle:
• Gaining two electrons
• Coenzyme NAD
– Accepts two hydrogen atoms
– Gains two electrons
– Releases one proton
– Forms NADH + H+
1 Glucose
2
1 GLYCOLYSIS
ATP
2 NADH+ 2 H+
2 Pyruvic acid
2 FORMATION
2 CO2
OF ACETYL
COENZYME A
2 NADH+ 2 H+
2 Acetyl
coenzyme A
2 ATP
4 CO2
3
KREBS
CYCLE
6 NADH+ 6 H+
2 FADH2
4 ELECTRON
Electrons
TRANSPORT
CHAIN
32 or 34 ATP
e–
e–
e–
6 O2
6
H2 O
NADH
NAD+
+
ATP
2e–
Controlled
release of
energy for
synthesis
of ATP
H+
2e–
H+
H 2O
1

2
O2
Carbohydrate Metabolism
Oxidative Phosphorylation.
NADH+H+
Krebs Electron transcycle port chain
and oxidative
phosphorylation
Free energy relative to O2 (kcal/mol)
Glycolysis
FADH2
Enzyme
Complex I
Enzyme
Complex II
Enzyme
Complex III
Enzyme
Complex IV
The actions of the three proton pumps and ATP
synthase in the inner membrane of mitochondria
Space between outer
and inner mitochondrial
membranes
+
nner
mitochondrial
membrane
H+
+ +
+
e–
–
Mitochondrial –
matrix
+ H+ NAD
NADH
1
+
Cyt
c
e–
e–
Q
e–
H+
–
–
+
H+
+
e–
–
1 1/2 O2
H+
2
H+ channel
3
3 H2O
–
3
–
ADP P
+
ATP synthaseATP
NADH dehydrogenaseCytochrome b-c1
Cytochrome oxidase
complex: FMN and complex: cyt b, cyt c1, complex: cyt a,
five Fe-S centers
and an Fe-S center cyt a3,and two Cu
High H+ concentration in
intermembrane space
Membrane
Proton
pumps
(electron
transport
chain)
ATP
synthase
Energy
from food
ADP +
+
Low H concentration
in mitochondrial matrix
(b) Oxidative phosphorylation
Overview of metabolic processes
Stage 1 Digestion in
GI tract lumen to
PROTEINS CARBOHYDRATES
FATS
absorbable forms.
Transport via blood to
Amino acidsGlucose and other sugars
Glycerol Fatty acids
tissue cells.
Glucose Glycogen
Stage 2 Anabolism Proteins
Fats
(incorporation into
molecules) and
Pyruvic acid
catabolism of nutrientsNH3
to form intermediates
Acetyl CoA
within tissue cells.
Stage 3 Oxidative breakdown
Krebs
of products of stage 2 in
Infrequentcycle
CO2
mitochondria of tissue cells.
O2
CO2 is liberated, and H atoms
Oxidative
phosphorylation
removed are ultimately delivered
H2O
H
(in
electron
to molecular oxygen, forming
transport chain)
water. Some energy released is
used to form ATP.
Summary of cellular respiration
Chemical energy (high-energy electrons)
Chemical energy
Glycolysis
Glucose Pyruvic
acid
Cytosol
Mitochondrial
cristae
Via substrate-level
phosphorylation
Krebs
cycle
Electron transport
chain and oxidative
phosphorylation
Mitochondrion
Via oxidative
phosphorylation
1
2he pyruvic acid then enters 3 Energy-rich electrons picked up by
During
glycolysis,
T
each glucose
the mitochondrial matrix, where coenzymes are transferred to the electron transport chain, built into the cris
molecule is broken the Krebs cycle decomposes it
membrane. The electron transport cha
down into two
to CO2. During glycolysis and
molecules of pyruvic the Krebs cycle, small amounts carries out oxidative phosphorylation,
acid in the cytosol. of ATP are formed by substrate- which accounts for most of the ATP
generated by cellular respiration.
level phosphorylation.
Food, such as
peanuts
Carbohydrates
Fats
Glycerol
Sugars
Proteins
Fatty acids
Amino acids
Amino
groups
Glucose
G3P
Pyruvate
GLYCOLYSIS
Acetyl
CoA
ATP
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron Transport
and Chemiosmosis)
Review
• Energy is harvested from high energy
glucose by the mitochondria
• The process begins in the cytoplasm with
glycolysis where glucose is converted to
pyruvate
• The TCA cycle (Krebs cycle) harvests
energy from the pyruvate and stores it as
reduced electron carrier molecules
• The carrier molecules cash in these
electrons for ATP in the electron transport
chain if oxygen is available
Review (cont)
• 2 net ATP are made from each glucose in
glycolysis
• 34 additional ATP are made from each
glucose if oxygen is available to help run
the TCA cycle and electron transport chain