How Cells Release Stored Energy
How Cells Release Stored Energy
More than 100 mitochondrial disorders are known
Friedreich’s ataxia, caused by a mutant gene, results in loss of cordination, weak muscles, and visual problems
Animal, plants, fungus, and most protists depend on structurally sound mitochondria
Defective mitochondria can result in life threatening disorders
p.123
When Mitochondria Spin Their Wheels
Descendents of African honeybees that were
imported to Brazil in the 1950s
More aggressive, wider-ranging than other
honeybees
Africanized bee’s muscle cells have large
mitochondria
Photosynthesizers get energy from the sun Animals get energy second- or third-hand
from plants or other organisms Regardless, the energy is converted to the
chemical bond energy of ATP
Anaerobic pathways
Evolved first Don’t require
oxygen Start with glycolysis
in cytoplasm Completed in
cytoplasm
Aerobic pathways Evolved later Require oxygen Start with
glycolysis in cytoplasm
Completed in mitochondria
start (glycolysis) in cytoplasm
completed in mitochondrion
start (glycolysis) in cytoplasm
completed in cytoplasm
Aerobic Respiration
Anaerobic Energy-Releasing Pathways
Fig. 8-2, p.124
Main Types of Energy-Releasing Pathways
C6H1206 + 6O2 6CO2 + 6H20
glucose oxygen carbon
water
dioxide
CYTOPLASM
Glycolysis
Electron Transfer
Phosphorylation
Krebs CYCLE ATP
ATP
2 CO2
4 CO2
2
32
water
2 NADH
8 NADH
2 FADH2
2 NADH 2 pyruvate
e- + H+
e- + oxygen
(2 ATP net)
glucose
Typical Energy Yield: 36 ATP
e-
e- + H+
e- + H+
ATP
H+
e- + H+
ATP 2 4
Fig. 8-3, p. 135
NAD+ and FAD accept electrons and hydrogen
Become NADH and FADH2
Deliver electrons and hydrogen to the
electron transfer chain
A simple sugar
(C6H12O6)
Atoms held together by covalent bonds
In-text figure Page 126
Energy-requiring steps
◦ ATP energy activates glucose and its six-carbon
derivatives
Energy-releasing steps
◦ The products of the first part are split into
three-carbon pyruvate molecules
◦ ATP and NADH form
GLUCOSE
glucose
GYCOLYSIS
pyruvate
to second stage of aerobic respiration or to a different energy-releasing pathway
Fig. 8-4a, p.126
Glycolysis
ATP
ATP
2 ATP invested
ENERGY-REQUIRING STEPS OF GLYCOLYSIS
glucose
ADP
ADP
P
P
P
P
glucose–6–phosphate
fructose–6–phosphate
fructose–1,6–bisphosphate DHAP
Fig. 8-4b, p.127
Glycolysis
ATP ADP
ENERGY-RELEASING STEPS OF GLYCOLYSIS
NAD+
P
PGAL
1,3–bisphosphoglycerate
substrate-level phsphorylation
Pi
1,3–bisphosphoglycerate
ATP
NADH NADH
P
PGAL NAD+
Pi
P P P P
3–phosphoglycerate 3–phosphoglycerate
P P
2 ATP invested
ADP
Fig. 8-4c, p.127
Glycolysis
2 ATP produced
ATP ADP
P
substrate-level phsphorylation
2–phosphoglycerate
ATP
P
pyruvate pyruvate
ADP
P P
2–phosphoglycerate
H2O H2O
PEP PEP
Fig. 8-4d, p.127
Glycolysis
2 ATP invested
Energy-Requiring Steps of Glycolysis
glucose
PGAL PGAL
P P
ADP
P
ATP
glucose-6-phosphate
P fructose-6-phosphate
ATP
fructose1,6-bisphosphate
P P
ADP
Figure 8-4(2)
Page 127
ADP ATP
pyruvate
ADP ATP
pyruvate
H2
O P
PEP
H2
O P
PEP
P
2-phosphoglycerate
P
2-phosphoglycerate
ADP ATP
P 3-phosphoglycerate
ADP ATP
P 3-phosphoglycerate
NAD+ NADH Pi
1,3-bisphosphoglycerate P P
NAD+ NADH Pi
1,3-bisphosphoglycerate P P
PGAL P
PGAL P
Figure 8-4 Page 127
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH
Preparatory reactions ◦ Pyruvate is oxidized into two-carbon acetyl units
and carbon dioxide
◦ NAD+ is reduced
Krebs cycle ◦ The acetyl units are oxidized to carbon dioxide
◦ NAD+ and FAD are reduced
Fig. 8-5a, p.128
mitochondrion
mitochondrion
inner mitochondrial
membrane
outer mitochondrial
membrane
inner compartment
outer compartment
Fig. 8-6a, p.128
Second Stage Reactions
Two pyruvates cross the inner mitochondrial membrane.
outer mitochondrial compartment
NADH
NADH
FADH2
ATP
2
6
2
2
Krebs Cycle
6 CO2
inner mitochondrial compartment
Eight NADH, two FADH 2, and two ATP are the payoff from the complete break-down of two pyruvates in the second-stage reactions.
The six carbon atoms from two pyruvates diffuse out of the mitochondrion, then out of the cell, in six CO
Fig. 8-6b, p.128
pyruvate
NAD+
NADH
coenzyme A (CoA)
O O carbon dioxide
CoA acetyl-CoA
Acetyl-CoA Formation
acetyl-CoA
(CO2)
pyruvate
coenzyme A NAD+
NADH
CoA
Krebs Cycle CoA
NADH
FADH2
NADH
NADH
ATP ADP + phosphate group
NAD+
NAD+
NAD+ FAD
oxaloacetate citrate
Fig. 8-7a, p.129
Preparatory Reactions
glucose
GLYCOLYSIS
pyruvate
KREBS CYCLE
ELECTRON TRANSFER PHOSPHORYLATION
Fig. 8-7b, p.129
Preparatory Reactions
Overall Reactants
Acetyl-CoA
3 NAD+
FAD
ADP and Pi
Overall Products
Coenzyme A
2 CO2
3 NADH
FADH2
ATP
NAD+
NADH
=CoA acetyl-CoA
oxaloacetate citrate
CoA
H2
O
malate isocitrate
H2
O H2
O
FAD
FADH2 fumarate
succinate
ADP + phosphate
group ATP
succinyl-CoA
O O
CoA NAD+
NADH
O O NAD+
NADH
a-ketoglutarate
Figure 8-6 Page 129
All of the carbon molecules in pyruvate end up in carbon dioxide
Coenzymes are reduced (they pick up electrons and hydrogen)
One molecule of ATP forms
Four-carbon oxaloacetate regenerates
Glycolysis 2 NADH Preparatory reactions 2 NADH Krebs cycle 2 FADH2 + 6 NADH
Total 2 FADH2 + 10 NADH
Occurs in the mitochondria
Coenzymes deliver electrons to electron transfer chains
Electron transfer sets up H+ ion gradients
Flow of H+ down gradients powers ATP formation
glucose
GLYCOLYSIS
pyruvate
KREBS CYCLE
ELECTRON TRANSFER PHOSPHORYLATION
Fig. 8-8a, p.130
Phosphorylation
Fig. 8-8b, p.130
OUTER COMPARTMENT
INNER COMPARTMENT
Electron Transfer Chain ATP Synthase
ATP
H+
H+ H+
H+
H+
H+ H+ H+
H+ H+
H+
H+ H+
H+
H+
H+
H+
H+ H+
H+
H+
H+
H+
H+
NADH + H+ NAD+ + 2H+ FAD + 2H+ FADH2 2H+ + 1/2 02
H2O ADP + Pi
e- e- e-
Phosphorylation
glucose
glycolysis
e–
KREBS CYCLE
electron transfer
phosphorylation
2 PGAL
2 pyruvate
2 NADH
2 CO2
ATP
ATP
2 FADH2
H+
2 NADH
6 NADH
2 FADH2
2 acetyl-CoA
ATP 2 Krebs Cycle
4 CO2 ATP
ATP
ATP
36
ADP + Pi
H+
H+
H+
H+
H+
H+ H+
H+
Fig. 8-9, p.131
Phosphorylation
NADH
OUTER COMPARTMENT
INNER COMPARTMENT
ATP
ADP + Pi
INNER COMPARTMENT
Electron transport phosphorylation requires the presence of oxygen
Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water
Glycolysis
◦ 2 ATP formed by substrate-level phosphorylation
Krebs cycle and preparatory reactions
◦ 2 ATP formed by substrate-level phosphorylation
Electron transport phosphorylation
◦ 32 ATP formed
NADH formed in cytoplasm cannot enter mitochondrion
It delivers electrons to mitochondrial membrane
Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion
Electrons given to FAD yield less ATP than those given to NAD+
686 kcal of energy are released
7.5 kcal are conserved in each ATP
When 36 ATP form, 270 kcal (36 X 7.5) are
captured in ATP
Efficiency is 270 / 686 X 100 = 39 percent
Most energy is lost as heat
Do not use oxygen
Produce less ATP than aerobic pathways
Two types
◦ Fermentation pathways
◦ Anaerobic electron transport
Begin with glycolysis
Do not break glucose down completely to
carbon dioxide and water
Yield only the 2 ATP from glycolysis
Steps that follow glycolysis serve only to
regenerate NAD+
C6H12O6
ATP
ATP NADH
2 acetaldehyde
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
glycolysis
ethanol formation
2 ATP net
2 ethanol
2 H2O
2 CO2
Fig. 8-10d, p.132
Alcoholic Fermentatio
n
C6H12O6
ATP
ATP
NADH
2 lactate
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
glycolysis
lactate fermentation
2 ATP net
Fig. 8-11, p.133
Lactate Fermentation
Fig. 8-12, p.133
Lactate Fermentation
Carried out by certain bacteria
Electron transfer chain is in bacterial plasma
membrane
Final electron acceptor is compound from
environment (such as nitrate), not oxygen
ATP yield is low
FOOD
complex carbohydrates
simple sugars
pyruvate
acetyl-CoA
glycogen fats proteins
amino acids
carbon backbones
fatty acids
glycerol
NH3
PGAL
glucose-6-phosphate
GLYCOLYSIS
KREBS CYCLE
urea
FOOD
fats glycogen complex
carbohydrates proteins
simple sugars (e.g., glucose)
amino acids
glucose-6-phosphate
carbon backbones
NH3
urea
ATP
(2 ATP net)
PGAL
glycolysis ATP 2
glycerol fatty acids
NADH pyruvate
acetyl-CoA
NADH CO2
Krebs Cycle
NADH, FADH2
CO2
ATP
ATP
ATP
many ATP
water H+
e– + oxygen
e–
4
ATP 2
Fig. 8-13b, p.135
electron transfer phosphorylation
Alternative Energy Sources
When life originated, atmosphere had little
oxygen
Earliest organisms used anaerobic pathways
Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
Cells arose that used oxygen as final
acceptor in electron transport
p.136b
Processes Are Linked