Download Electron transport chains

Ch 9 Cellular Respiration
Harvesting Chemical Energy
Section 9.1
Catabolic pathways yield energy by
oxidizing organic fuels
Principles of Energy Harvest
Catabolic pathways:
• Fermentation
• Cellular Respiration
C6H12O6 + 6O2 ---> 6CO2 + 6H2O + E (ATP + heat)
Redox reactions
• Oxidation-reduction
• OIL RIG
(adding e- reduces + charge)
• Oxidation is e- loss; reduction is egain
• Reducing agent: e- donor
• Oxidizing agent: e- acceptor
Oxidizing agent in respiration
• NAD+ (nicotinamide adenine
dinucleotide)
• Removes electrons from food
(series of reactions)
• NAD + is reduced to NADH
• Enzyme action: dehydrogenase
• Oxygen is the eventual eacceptor
Electron transport chains
• Electron carrier molecules
(membrane proteins)
• Shuttles electrons that release
energy used to make ATP
• Sequence of reactions that
prevents energy release in 1
explosive step
• Electron route:
food---> NADH 
electron transport chain --->
oxygen
Cellular respiration
• Glycolysis: cytosol; degrades
glucose into pyruvate
• Kreb’s Cycle (Citric Acid
Cycle): mitochondrial matrix;
pyruvate into carbon dioxide
• Electron Transport Chain
(Oxidative Phophorylation):
inner membrane of
mitochondrion; electrons passed
to oxygen
Section 9.2
Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate
Glycolysis
•
1 Glucose ---> 2 pyruvate molecules
•
Energy investment phase: cell uses
ATP to phosphorylate fuel
•
Energy payoff phase: ATP is produced
by substrate-level phosphorylation and
NAD+ is reduced to NADH by food
oxidation
•
Net energy yield per glucose molecule:
2 ATP plus 2 NADH; no CO2 is
released; occurs aerobically or
anaerobically
Glycolysis
• Overview of Glycolysis
by the Khan Academy
Section 9.3
The Citric acid cycle completes the
energy-yielding oxidation of organic
molecules
• In the presence of O2, pyruvate enters the mitochondrion
• Enters via active transport , with the help of a transport
protein
• Before the citric acid cycle can begin, pyruvate must be
converted to acetyl CoA, which links the cycle to
glycolysis
• The citric acid cycle, also called the Krebs cycle, takes
place within the mitochondrial matrix
• The cycle oxidizes organic fuel derived from pyruvate,
generating 1 ATP, 3 NADH, and 1 FADH2 per turn
Kreb’s Cycle
•
Each pyruvate is converted into acetyl
CoA:
CO2 is released;
NAD+ ---> NADH;
coenzyme A, makes molecule very
reactive
•
From this point, each turn 2 C atoms enter
(pyruvate) and 2 exit (carbon dioxide)
•
Oxaloacetate is regenerated (the “cycle”)
•
For each pyruvate that enters:
3 NAD+ reduced to NADH
1 FAD+ reduced to FADH2
1 ATP molecule
• The citric acid cycle has eight steps, each catalyzed by a specific
enzyme
• The acetyl group of acetyl CoA joins the cycle by combining with
oxaloacetate, forming citrate
• The next seven steps decompose the citrate back to oxaloacetate,
making the process a cycle
• The NADH and FADH2 produced by the cycle relay electrons
extracted from food to the electron transport chain
Kreb’s Cycle
Kreb’s Cycle animation
Section 9.4
During oxidative phosphorylation, chemiosmosis
couples electron transport to ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and
FADH2 account for most of the energy extracted from food
• These two electron carriers donate electrons to the electron
transport chain, which powers ATP synthesis via oxidative
phosphorylation
• The electron transport chain is in the cristae of the mitochondrion
• Most of the chain’s components are proteins, which exist in multiprotein complexes
• The carriers alternate reduced and oxidized states as they accept and
donate electrons
• Electrons drop in free energy as they go down the chain and are finally
passed to O2, forming H2O
Electron transport chain
• Cytochromes carry electron
carrier molecules (NADH &
FADH2) down to oxygen
• Chemiosmosis:
energy coupling mechanism
• ATP synthase:
• produces ATP by using the H+
gradient (proton-motive force)
pumped into the inner
membrane space from the
electron transport chain; this
enzyme harnesses the flow of
H+ back into the matrix to
phosphorylate ADP to ATP
(oxidative phosphorylation)
• The electron transport chain generates no ATP
• The chain’s function is to break the large free-energy drop
from food to O2 into smaller steps that release energy in
manageable amounts
Fig. 9-16
H+
H+
H+
H+
Protein complex
of electron
carriers
Cyt c
V
Q


ATP
synthase

FADH2
NADH
2 H+ + 1/2O2
H2O
FAD
NAD+
ADP + P i
(carrying electrons
from food)
ATP
H+
1 Electron transport chain
Oxidative phosphorylation
2 Chemiosmosis
Electron Transport
• Cellular respiration video
Review: Cellular Respiration
•
•
Glycolysis:
2 ATP (substrate-level
phosphorylation)
•
Kreb’s Cycle:
2 ATP (substrate-level
phosphorylation)
•
•
•
•
•
Electron transport & oxidative
phosphorylation:
2 NADH (glycolysis) = 6ATP
2 NADH (acetyl CoA) = 6ATP
6 NADH (Kreb’s) = 18 ATP
2 FADH2 (Kreb’s) = 4 ATP
•
38 TOTAL ATP/glucose
Section 9.5
Fermentation and anaerobic respiration enable cells to produce
ATP without the use of oxygen
• Most cellular respiration requires O2 to produce ATP
• Glycolysis can produce ATP with or without O2 (in
aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with fermentation
or anaerobic respiration to produce ATP
• Anaerobic respiration uses an electron transport chain with
an electron acceptor other than O2, for example sulfate
• Fermentation uses phosphorylation instead of an electron
transport chain to generate ATP
• Fermentation consists of glycolysis plus reactions that
regenerate NAD+, which can be reused by glycolysis
• Two types:
1. alcohol fermentation
2. lactic acid fermentation
Alcohol Fermentation
• pyruvate is converted to
ethanol
• Occurs in 2 steps, first
step releases CO2
• Alcohol fermentation by
yeast is used in brewing,
winemaking, and baking
2 ADP + 2 Pi
2 ATP
Fig. 9-18
Glucose
Glycolysis
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
Lactic Acid Fermentation
• pyruvate is reduced to NADH, forming lactate as an end
product, with no release of CO2
• Lactic acid fermentation by some fungi and bacteria is
used to make cheese and yogurt
• Human muscle cells use lactic acid fermentation to
generate ATP when O2 is scarce
Fermentation and Aerobic Respiration Compared
• Both use glycolysis to oxidize glucose and other organic
fuels to pyruvate
• They have different final electron acceptors: an organic
molecule (such as pyruvate or acetaldehyde) in
fermentation and O2 in cellular respiration
• Cellular respiration produces 38 ATP per glucose
molecule; fermentation produces 2 ATP per glucose
molecule
Obligate anaerobes
• carry out fermentation or anaerobic respiration and cannot
survive in the presence of O2
Facultative anaerobes
• Yeast and many bacteria
• they can survive using either fermentation or cellular
respiration
Section 9.6
Glycolysis and the citric acid cycle connect to many other
metabolic pathways