Download aerobic respiration

How Cells Harvest Chemical
Energy
Chapter 6
Overview
• Photosynthesis
• Aerobic respiration
• Anaerobic respiration
• Alternate sources of energy
Components of a Reaction
Reactants
Intermediates
Products
A
B
C
Endergonic vs. Exergonic Reactions
Endergonic
= Energy-requiring
Exergonic
= Energy-releasing
Redox Reactions
One molecule gives up electrons = oxidized
One molecule gains electrons = reduced
H+ atoms released simultaneously
(are attracted to negative charge of electrons)
Coenzymes pick up e-s & H+ from substrates &
deliver to e- transfer chains
Electron Transfer Chains
Membrane-bound groups of enzymes/molecules
Accept & give up e-s in sequence
E-s enter chain at higher energy level than when they
leave it
(lose energy at each descending step of chain)
e-s
Substrate-Level Phosphorylation
Formation of ATP by direct transfer of Pi
group to ADP from intermediate
NAD+ & FAD
Coenzymes
1. Accept e-s & H+ from intermediates that
form during glucose catabolism
Become reduced = NADH & FADH2
2. NADH and FADH2 give up e-s & H+ to etransfer chains during final stages of
aerobic respiration
Become oxidized = NAD+ & FAD
Autotrophs
“Self-nourishing”
Synthesize own food
Obtain energy & organic compounds
(e.g. C) from the physical environment
Chemoautotrophs
Have no enzymes to allow for complex
metabolic reactions
Obtain energy & C from simple inorganic &
organic compounds e.g. CH4, H2S
Photoautotrophs
Contain light-sensitive molecules
Can split H2O & use electrons
Process releases lots of oxygen, which
reacts rapidly with metals & creates toxic
free radicals
Early photoautotrophs existed when there
was lots of Fe & metals everywhere
Released O2 oxidized these metals & rusted
them out
O2 could then be released freely
Over a few thousand years, O2 levels in
sea & atmosphere increased
Survival of the fittest:
Most anaerobes died out because couldn’t
neutralize toxic O2 radicals
Chemoautotrophs with little or no O2 tolerance
restricted to extreme & anoxic environments
As O2 accumulated in the atmosphere, O
atoms combined to form O3
= ozone layer
(protects against lethal UV radiation from sun)
Life was able to move out from the “darks” &
live under open sky
= diversification
= evolution
Photosynthesis
The process by which photoautotrophs use
light energy from the sun to make glucose,
which can then be converted into ATP
12H2O + 6CO2  6O2 + C6H12O6 + 6H2O
Respiration
= breathing
Cellular respiration
= getting energy from food
Organisms need usable energy in order to
survive
Obtained energy is converted into ATP
chemical bond energy
Can be used to do work e.g. metabolism
Anaerobes
Can’t tolerate O2
Make ATP via fermentation
1 glucose → 2 ATP
Clostridium difficile
e.g. first organisms, some
prokaryotes & eukaryotes
Aerobes
Require O2
Make ATP via aerobic respiration
(many also use anaerobic pathways)
1 glucose → 36+ ATP
(vital for survival of large organisms)
e.g. most eukaryotes, some prokaryotes
Facultative Anaerobes
Normally use aerobic pathways
(i.e. use O2)
Entamoeba histolytica
Can switch to anaerobic pathways
when O2 levels are low
Mitochondria
Membrane-bound organelles in most eukaryotic cells
(# differs depending on cell type)
Power source of cells
– Production of ATP in presence of O2
– Convert NADH and FADH2 into ATP energy via oxidative
phosphorylation
Allow cell to produce lots of ATP simultaneously
– Without mitochondria, complex animals wouldn’t exist
Mitochondrion Structure
Outer membrane
Selectively permeable
Inner membrane
Highly impermeable
Contains ATP synthase
Has membrane potential
Cristae
↑ surface area of inner membrane,
which ↑ capacity to generate ATP
Matrix
Contains 100s of enzymes which
oxidize pyruvate and fatty acids, and
control the Krebs cycle
Cellular Respiration
The oxidation of food molecules (e.g. glucose)
into CO2 & H2O
Energy released is captured as ATP
Used for all endergonic activities of cell
Enzymes catalyze each step
Intermediates formed at one step become
substrates for enzyme at next step
2 phases of cellular respiration:
Glycolysis:
Glucose → 2 pyruvates
Occurs in all cells
Oxidation of pyruvate into
CO2 and H2O:
Energy-releasing pathways
differ depending on cell & its
needs
40% of energy from glucose is harvested
Rest (60%) is lost as heat
A working muscle uses 10 million ATP per
second!
Aerobic Respiration
C6H1206 + 6O2 → 6CO2 + 6H2O
Breakdown of glucose in presence of O2
3 stages of reactions:
– Glycolysis
– Krebs Cycle
– Electron Transfer Phosphorylation
Glycolysis
– Glucose → 2 pyruvates
– Occurs in cytosol
Krebs Cycle
– Pyruvate → CO2 + H2O + e-s
– Occurs in mitochondria
Electron Transfer Phosphorylation
– Formation of lots of ATP
Stage I: Glycolysis
Glucose → 2 pyruvates
“Universal energy-harvesting process of life”
Initial energy-releasing mechanism for all cells
Occurs in cytosol
Coupled endergonic & exergonic reactions
Endergonic Steps of Glycolysis
Requires input of 2 ATP
ATP #1 phosphorylates glucose
Glucose → intermediate
ATP #2 transfers Pi to intermediate
Intermediate → PGAL + DHAP
DHAP converts into PGAL
= 2 PGAL enter next stage
Exergonic Steps of Glycolysis
Each PGAL gives 2 e-s + H+ to NAD+
2 NAD+ → 2 NADH
Intermediates each give Pi to ADP
2 ADP → 2 ATP
(substrate-level phosphorylation)
Pays back 2 ATP used in endergonic steps
Intermediates each release H+ + OH
2 intermediates → 2 PEP
Each PEP gives Pi to ADP
2 ADP → 2 ATP
(substrate-level phosphorylation)
2 PEP → 2 pyruvate
Sum Total of Glycolysis
Glucose → 2 pyruvate + 2 NADH + 2 ATP
From here, pyruvate can enter:
– Aerobic pathway (Krebs cycle)
– Anaerobic pathway (fermentation)
(depends on cell & environmental conditions)
Stage II: Krebs Cycle
Pyruvate → CO2 + H2O (+ e-s)
a.k.a. citric acid cycle
Occurs in mitochondria
Main function is to supply Stage III with e-s
(in order to reduce NAD+ & FAD in stage III)
Mitochondrial membrane proteins transport pyruvate
into inner compartment
Enzymes take 1 C from pyruvate
C + O2 → CO2
Intermediates + coenzyme A → acetyl-CoA
NAD+ is reduced into NADH
Acetyl-CoA enters Krebs cycle
Transfers 2 Cs to oxaloacetate → citrate
Rearrangement of intermediates occurs
2 C released → 2CO2
3 NAD+ + H+ + e-s → 3 NADH
ADP + Pi → ATP
FAD + H+ + e-s → FADH2
Oxaloacetate regenerates so that cycle can run again
In total, one turn of the cycle:
3 NADH + 1 FADH2 + 1 ATP
Cycle repeats again for 2nd pyruvate molecule
Remember 1 glucose → 2 pyruvates
After both pyruvates are broken down:
6 NADH + 2 FADH2 + 2 ATP
Sum Total of the Krebs Cycle
With 2 NADH from acetyl-CoA formation:
2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2
CO2 released into surroundings
NADH & FADH2 deliver e-s and H+ to 3rd stage
Stage III: Electron Transfer Phosphorylation
H+ + e-s → H2O + ATP
E-s delivered to electron transfer chains
(ETCs) in inner mitochondrial membrane
E- flow in ETCs drives phosphorylation of ADP
→ ATP (lots of it!)
a.k.a. oxidative phosphorylation
ATP formed by oxidation of NADH & FADH2
Responsible for high ATP yield
NADH & FADH2 give e-s to ETCs
Simultaneous release of H+
Energy released at each transfer of ETC
At 3 transfers, released energy pumps H+ across
mitochondrial membrane into outer compartment
Concentration & electric gradients result across
inner membrane
= membrane potential
H+ re-enters inner
compartment by flowing
down concentration
gradient through ATP
synthases
Causes reversible change in
shape of ATP synthases
ADP + Pi → ATP
(oxidative phosphorylation)
At end of ETCs, O2 picks up
e-s & H+→ H2O
Sum Total of ET Phosphorylation
H+ + e-s → H2O + 32 ATP
In liver, heart, and kidney:
– e-s from NADH delivered to different ETC
entry point
– H+ gradient makes 3 ATP (instead of 1)
– Results in 34 ATP total
Animation of ET Phosphorylation
• http://vcell.ndsu.nodak.edu/animations/etc/
movie.htm
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/
chapter9/animations.html#
In oxygen-starved cells, e-s have nowhere to go
so get gridlocked
No e- flow = no H+ gradients = no ATP forms
Results in cell death because not enough ATP
to sustain metabolic processes
3 different categories of poisons interfere
with cellular respiration:
• ETC Blockers
• Inhibitors
• Uncouplers
ETC Blockers
Block ETC at various
steps of chain
Starves cells of energy
by prohibiting ATP
synthesis
Inhibitors
Inhibit ATP synthase
No passage of H+
through ATP synthase
= no ATP generation
Uncouplers
Make mitochondrial
membrane leaky to H+
Electron transport & O2
consumption continue
but lack of H+ gradient
= no ATP synthesis
Sum Total of Aerobic Respiration
Glycolysis
Glucose → 2 pyruvate + 2 NADH + 2 ATP
Krebs Cycle
2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2
ET Phosphorylation
H+ + e-s (from coenzymes) → 32 ATP + 6H2O
Glucose + 6O2 → 6CO2 + 6H2O + 36 ATP
(38 ATP in liver, heart, kidney)
Cellular Respiration Video
• http://video.google.com/videoplay?docid=1
463788471587082686&q=respiration+ninj
a&total=2&start=0&num=10&so=0&type=s
earch&plindex=0
Anaerobic Respiration
Oxidation of molecules in absence of O2
Requires different electron acceptor at the
end of it
2 stages:
– Glycolysis
– Various Energy-Releasing Pathways:
• Alcoholic Fermentation
• Lactate Fermentation
• Anaerobic Electron Transfers
Stage I: Glycolysis
Glucose → 2 pyruvate + 2 NADH + 2 ATP
Glucose is not broken down any further into
CO2 or H2O
All ATP comes from glycolysis
(not enough energy to sustain large multicellular
organisms)
After Glycolysis
Final stages in fermentation pathways do
not generate more ATP
Instead, they regenerate NAD+ so that it
can act as an electron acceptor
Pyruvate not moved into mitochondria
(stays in cytosol & is converted into waste
products that can be transported out of cells)
Waste product depends on type of cell
e.g. ethanol in yeast
e.g. lactate in skeletal muscles, bacteria
Alcoholic Fermentation
2 pyruvates → 2 acetylaldehydes + 2CO2
NADH gives up e-s & H+ to acetaldehyde → ethanol
e.g. yeasts (Saccharomyces spp.) that ferment wine,
bread, etc.
Lactate Fermentation
NADH gives e-s and H+ to pyruvate → lactate
e.g. bacteria (Lactobacillus spp. and others) in
cheese, yoghurt, etc.
Certain organisms can couple aerobic &
anaerobic respiration or switch from one
mode to the other
e.g. skeletal muscles associated with bones
Variety of cell types within muscle fibres
Slow-Twitch Muscle Fibres
Lots of mitochondria & myoglobin
Make ATP via aerobic respiration
Used for steady, prolonged
activity
e.g. long-distance running,
migration, etc.
e.g. “dark meat” of birds
Fast-Twitch Muscle Fibres
Few mitochondria & no
myoglobin
Make ATP via lactate
fermentation
Used for short bursts of intense
activity
e.g. sprints, weight-training
e.g. “white meat” of birds
Anaerobic Energy Transfers
Pathway of some archaeans & bacteria
Inorganic compounds used as final eacceptors rather than O2
Aids in cycling of elements through biosphere
Energy yield varies but is small
Alternative Energy Sources
• Glycogen Stores
• Lipids
• Proteins
The Fate of Glucose
When food is ingested, glucose is absorbed
into the bloodstream
Pancreas secretes insulin to make cells take up
glucose faster
Cells convert glucose to glucose-6-phosphate
(intermediate of glycolysis)
Can’t leave cell once phosphorylated
Glycogen Stores
When more glucose than necessary is taken
in, is biosynthesized into glycogen
(stored in liver and muscles)
Only 1% of total energy stores
Glycogen stores used up within 12 hours if
regular meals aren’t eaten
When blood glucose drops, pancreas
secretes glucagon
Liver cells convert stored glycogen →
glucose & send back to blood
Can then enter glycolysis pathway
If excess carbs are eaten:
Glucose → pyruvate → acetyl-CoA
(aerobic respiration)
Acetyl-CoA doesn’t enter Krebs cycle if excess
glucose
Enters lipid biosynthesis pathway instead
↑↑↑ carbs = fat
Fat Stores
Fat stored as triglycerides in adipose cells
Triglyceride = glycerol + 3 fatty acid tails
Between meals or during sustained exercise,
fatty acids yield half of ATP needed by
muscle, liver, kidney cells
When blood glucose ↓, enzymes in adipose
cells separate glycerol & fatty acids and
release into blood
Glycerol → PGAL
Used in glycolysis
Fatty acids → acetyl-CoA
Used in Krebs cycle
Protein Stores
When proteins ingested, are broken down into
amino acids
Absorbed into bloodstream and taken up by cells
to make more proteins, etc.
If excess protein eaten, amino acids broken
down
Form acetyl-CoA, pyruvate, or intermediates of
Krebs cycle
“Reverse” pathways
also exist
Food molecules used
for biosynthesis
Requires ATP
Summary
Energy is required by all living
organisms to sustain life
Because energy flow is onedirectional, constant energy
inputs are needed
Interpreting Data
This graph illustrates the free energy
relative to oxygen of the electron
transport chain. The solid blue circles
are electron carrier molecules, and the
light blue ovals represent protein
complexes. From an energy standpoint,
are these reactions endergonic or
exergonic?
a.
Endergonic
b.
Exergonic
c.
Some are exergonic and others are
endergonic.
d.
There is not enough information.
Interpreting Data
What would happen to the flow of
electrons if oxygen were not present?
a.
The flow of electrons would
continue but at a slower rate.
b.
The flow would cease and ATP
production would stop.
c.
The presence of oxygen would
have no effect.