Transcript Pyruvic acid is chemically groomed for the Krebs cycle

STAGES OF CELLULAR RESPIRATION AND
FERMENTATION
Overview: Respiration occurs in three main stages
• Cellular respiration oxidizes sugar and
produces ATP in three main stages
– Glycolysis occurs in the cytoplasm
– The Krebs cycle and the electron transport chain
occur in the mitochondria
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• An overview of cellular respiration
High-energy electrons
carried by NADH
GLYCOLYSIS
Glucose
Pyruvic
acid
Cytoplasmic
fluid
Figure 6.8
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KREBS
CYCLE
ELECTRON
TRANSPORT CHAIN
AND CHEMIOSMOSIS
Mitochondrion
Glycolysis harvests chemical energy by oxidizing
glucose to pyruvic acid
Glucose
Figure 6.9A
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Pyruvic
acid
• Details of
glycolysis
Steps 1 – 3 A fuel
molecule is energized,
using ATP.
Glucose
Step
PREPARATORY
PHASE
(energy investment)
1
Glucose-6-phosphate
2
Fructose-6-phosphate
3
Fructose-1,6-diphosphate
Step 4 A six-carbon
intermediate splits into
two three-carbon
intermediates.
4
Glyceraldehyde-3-phosphate
(G3P)
ENERGY PAYOFF
PHASE
5
Step 5 A redox
reaction generates
NADH.
6
Steps 6 – 9 ATP
and pyruvic acid
are produced.
1,3-Diphosphoglyceric acid
(2 molecules)
7
3-Phosphoglyceric acid
(2 molecules)
8
2-Phosphoglyceric acid
(2 molecules)
2-Phosphoglyceric acid
(2 molecules)
9
Figure 6.9B
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Pyruvic acid
(2 molecules
per glucose molecule)
Pyruvic acid is chemically groomed for the Krebs
cycle
• Each pyruvic acid molecule is broken down to
form CO2 and a two-carbon acetyl group, which
enters the Krebs cycle
Pyruvic
acid
Acetyl CoA
(acetyl coenzyme A)
CO2
Figure 6.10
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The Krebs cycle completes the oxidation of organic
fuel, generating many NADH and FADH2
molecules
Acetyl CoA
• The Krebs cycle
is a series of
reactions in
which enzymes
strip away
electrons and
H+ from each
acetyl group
KREBS
CYCLE
Figure 6.11A
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2
CO2
2 carbons enter cycle
Oxaloacetic
acid
1
Citric acid
CO2 leaves cycle
5
KREBS
CYCLE
2
Malic
acid
4
Alpha-ketoglutaric acid
3
CO2 leaves cycle
Succinic
acid
Step 1
Acetyl CoA stokes
the furnace
Steps 2 and 3
NADH, ATP, and CO2 are generated
during redox reactions.
Figure 6.11B
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Steps 4 and 5
Redox reactions generate FADH2
and NADH.
Chemiosmosis powers most ATP production
• The electrons from NADH and FADH2 travel
down the electron transport chain to oxygen
• Energy released by the electrons is used to
pump H+ into the space between the
mitochondrial membranes
• In chemiosmosis, the H+ ions diffuse back
through the inner membrane through ATP
synthase complexes, which capture the energy
to make ATP
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• Chemiosmosis in the mitochondrion
Protein
complex
Intermembrane
space
Electron
carrier
Inner
mitochondrial
membrane
Electron
flow
Mitochondrial
matrix
ELECTRON TRANSPORT CHAIN
Figure 6.12
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ATP SYNTHASE
Connection: Certain poisons interrupt critical
events in cellular respiration
Rotenone
Cyanide,
carbon monoxide
ELECTRON TRANSPORT CHAIN
Figure 6.13
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Oligomycin
ATP SYNTHASE
Review: Each molecule of glucose yields many
molecules of ATP
• For each glucose molecule that enters cellular
respiration, chemiosmosis produces up to 38
ATP molecules
Cytoplasmic
fluid
Mitochondrion
Electron shuttle
across
membranes
GLYCOLYSIS
2
Glucose
Pyruvic
acid
by substrate-level
phosphorylation
2
Acetyl
CoA
used for shuttling electrons
from NADH made in glycolysis
KREBS
CYCLE
by substrate-level
phosphorylation
KREBS
CYCLE
ELECTRON
TRANSPORT CHAIN
AND CHEMIOSMOSIS
by chemiosmotic
phosphorylation
Maximum per glucose:
Figure 6.14
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Fermentation is an anaerobic alternative to
aerobic respiration
• Under anaerobic conditions, many kinds of
cells can use glycolysis alone to produce small
amounts of ATP
– But a cell must have a way of replenishing
NAD+
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• In alcoholic fermentation, pyruvic acid is
converted to CO2 and ethanol
– This recycles NAD+ to keep glycolysis working
released
GLYCOLYSIS
Glucose
2 Pyruvic
acid
Figure 6.15A
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2 Ethanol
Figure 6.15C
• In lactic acid fermentation, pyruvic acid is
converted to lactic acid
– As in alcoholic fermentation, NAD+ is recycled
• Lactic acid fermentation is used to make cheese
and yogurt
GLYCOLYSIS
Glucose
2 Pyruvic
acid
Figure 6.15B
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2 Lactic acid
INTERCONNECTIONS BETWEEN
MOLECULAR BREAKDOWN AND SYNTHESIS
Cells use many kinds of organic molecules as fuel
for cellular respiration
• Polysaccharides can be hydrolyzed to
monosaccharides and then converted to glucose
for glycolysis
• Proteins can be digested to amino acids, which
are chemically altered and then used in the
Krebs cycle
• Fats are broken up and fed into glycolysis and
the Krebs cycle
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• Pathways of molecular breakdown
Food, such as
peanuts
Polysaccharides
Fats
Proteins
Sugars
Glycerol Fatty acids
Amino acids
Amino
groups
Glucose
G3P
Pyruvic
acid
Acetyl
CoA
GLYCOLYSIS
Figure 6.16
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KREBS
CYCLE
ELECTRON
TRANSPORT CHAIN
AND CHEMIOSMOSIS
Food molecules provide raw materials for
biosynthesis
• In addition to energy, cells need raw materials
for growth and repair
– Some are obtained directly from food
– Others are made from intermediates in
glycolysis and the Krebs cycle
• Biosynthesis consumes ATP
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• Biosynthesis of macromolecules from
intermediates in cellular respiration
ATP needed to
drive biosynthesis
KREBS
CYCLE
GLUCOSE SYNTHESIS
Acetyl
CoA
Pyruvic
acid
G3P
Glucose
Amino
groups
Amino acids
Fatty acids Glycerol
Sugars
Proteins
Fats
Polyscaccharides
Cells, tissues, organisms
Figure 6.17
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The fuel for respiration ultimately comes from
photosynthesis
• All organisms have the
ability to harvest energy
from organic molecules
– Plants, but not animals,
can also make these
molecules from inorganic
sources by the process of
photosynthesis
Figure 6.18
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