campbell & reece chapter 9. metabolic pathways that released stored nrg by breaking down...
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CELLULAR RESPIRATION &
FERMENTATION
CAMPBELL & REECECHAPTER 9
CATABOLIC PATHWAYS
metabolic pathways that released stored nrg by breaking down complex molecules
Fermentation
a catabolic pathway partial degradation of sugars or other
organic fuel anaerobic not as efficient as aerobic respiration
Cellular Respiration
generally means aerobic cells mostly use glucose as fuel
energy released: ATP + heat (so is exergonic)
Cellular Respiration
nrg released: G = -686 kcal/mol [2870kJ]Δ
How does degradation of glucose yield energy?
answer based on transfer of e- during chemical reactions
moving e- releases nrg stored in organic molecules which is ultimately used to synthesize ATP
Redox Reactions
Redox Reactions
Redox Reactions
Redox Reactions are Always Coupled
Redox Reactions
substance giving away e- is called the reducing agent
substance taking e- is called the oxidizing agent
Redox Reactions
some do not involve complete transfer of e- (as in forming ions)
Redox Reactions during Cellular Respiration: Glucose is Oxidized & O2 is
Reduced
What Organic Molecules Make Great Sources of Fuel?
*generally, organic molecules that have lots of hydrogen make excellent fuels because their bonds are source of “hilltop” e- whose nrg will be released as the e- “fall” down nrg gradient when transferred to O2
Cellular Respiration
H is transferred from glucose O2
as e- transferred nrg state of e- is lowered
that released nrg is available for ATP synthesis
Activation Energy
without EA barrier, glucose or other foods would spontaneously combine with O2 in air
body temperature not high enough to initiate combustion of glucose, enzymes required to lower EA
Oxidation Mini-Steps Release nrg slowly
glucose & other molecules are broken down in series of steps (each w/own enzyme)
@ key steps e- are stripped from glucose
each oxidation step involves e- traveling with H atom NAD+ NADH
oxidized reduced
state state
NAD+ / NADH
Nicotinamide Adenine Dinucleotide derivative of niacin
NAD+ / NADH
enzymes called dehydrogenases remove a pair of H atoms (with 2 e-) from substrate (glucose) thereby oxidizing it.
dehydrogenase then delivers the 2 e- along with 1 H (1 proton) to its coenzyme NAD+
2nd H+ is released to surroundings
NAD+ / NADH
by receiving 2 e- & 1 H+, NAD+ loses its (+) charge
NAD+ most versatile e- acceptor in cellular respiration (used in several redox reactions)
NAD+ / NADH
When e- passed from glucose NAD+ they lose very little of their nrg
cellular respiration uses e- transport chain to break fall of e- O2 into several nrg-releasing steps
Electron Transport Chain
consists of a # of molecules (proteins mostly) in inner membrane of mitochondria & plasma membrane of those prokaryotes that have aerobic respiration
@ “top” of chain NADH carries higher nrg e- removed from glucose “bottom” of chain lower nrg e- passed to O2
Electron Transport Chain
e- transfer from NADH O2 is exergonic reaction with a free energy change of : -53 kcal/mol (-222 kJ/mol)
instead of releasing all that nrg in 1 explosive step, e- cascade down the chain from 1 carrier molecule to next in series of redox reactions
each carrier is more electronegative than previous molecule
Electron Transport Chain
O2 is final e- acceptor because it is the most electronegative
can think of it as O2 pulling e- down the chain in nrg-yielding tumble
Electron Transport Chain
3 Stages of Cellular Respiration
1. Glycolysis2. Pyruvate Oxidation & Citric Acid Cycle3. Oxidative Phosphorylation
e- transport chain chemiosmosis
GLYCOLYSIS
2 parts:1. Energy Investment Phase2. Energy Payoff Phase
Glycolysis
anaerobic in cytoplasm no CO2 released uses 2 ATP, makes 4 ATP 2 NAD+ + 4 e- + 4H+ 2 NADH + 2H+ glucose 2 pyruvate + 2 H2 O
Citric Acid Cycle
pyruvate mitochondria via active transport (eukaryotic cells)
pyruvate stays in cytoplasm in prokaryotes that perform aerobic respiration
Linking Glycolysis & Citric Acid Cycle
1. Pyruvate’s carboxyl group (already oxidized so has little chemical nrg) is removed as CO2
2. Remaining 2 C fragment is oxidized acetate (ionized form of acetic acid) with e- NAD+ NADH
3. CoA (derived from vit. B) attached via S atom to acetic acid acetyl CoA
Fate of Pyruvate in Mitochondria
Citric Acid Cycle
aka: Krebs Cycle tricarboxylic acid cycle functions as metabolic furnace that
oxidizes organic fuel derived from pyruvate
Energy-Rich Molecules Produced in Citric Acid Cycle
for each acetyl group entering cycle: 3 NAD+ 3NADH 1 FAD + 2 e- + 2H+ 1 FADH2
* 1 GDP + 1ATP 1GTP + 1ADP
* GTP made in many animal cell mitochondria: GTP similar to ATP in structure & function /example of substrate-level phosphorylation
Oxidative Phosphorylation
@ end of Citric Acid Cycle only have 4 ATP made (counting glycolysis)
also have NADH & FADH2 (hi nrg e- carriers) which accounts for most of nrg extracted form glucose
Electron Transport
collection of molecules embedded in inner membrane of mitochondria (prokaryotes have them embedded in their plasma membrane)
inner membrane has multiple folds allowing for multiple copies of e- transport chain to be working at same time
Electron Transport Chain
most of the molecules are proteins, rest are nonprotein components necessary for catalytic functions of certain enzymes
there is a drop in free nrg as e- move thru e- transport chain alternating reduced state oxidized state
Electron Transport Chain Animations
http://www.johnkyrk.com/mitochondrion.html
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__electron_transport_system_and_formation_of_atp__quiz_1_.html
http://www.science.smith.edu/departments/Biology/Bio231/etc.html
Electron Transport Chain Animations
http://www.dnatube.com/video/2354/Detailed-ElectronTransport-Chain
http://vcell.ndsu.nodak.edu/animations/etc/movie-flash.htm
What does e- transport chain accomplish?
e- transport chain makes no ATP directly it does break the fall of e- from food to O2
into a series of smaller steps that releases nrg in manageable amts
for every 4 e- 1 O2 + 4 H+ 2 H2 O (O2 is final e- acceptor)
Chemiosmosis
inner membrane protein ATP Synthase makes ADP + Pi ATP using the proton (H+) gradient as nrg source
chemiosmosis is the process in which nrg stored in H+ gradient across membrane is used to drive cellular work
(see animations)
ATP Production in Cellular Respiration
Efficiency of Cellular Respiration
% of chemical nrg in glucose ATP oxidation of 1 mol glucose under
standard conditions = 686 kcal/mol 1 ATP stores 7.3 kcal/mol efficiency of cellular respiration =
7.3kcal/mol x 32mol ATP/1 mol glucose÷ 686 kcal/mol = 0.34 34%
actually a little higher: under cell conditions ΔG is lower
66% of nrg from oxidation of glucose lost as heat
adaptation in hibernating animals: use brown fat: cells packed full of
mitochondria & that has a protein in inner membrane that allows H+ to flow down its concentration gradient w/out making ATP (so oxidation of stored fats generates heat w/out making ATP)
Hibernating Animal Adaptations
w/out this adaptation ATP would build up to point where cellular respiration would shut down
What if there is no oxygen?
without an adequate supply of O2 “pulling” e- thru transport chain oxidative phosphorylation eventually stops
2 things cells can do to get some ATP out of organic fuel w/out O2
1. Anaerobic respiration2. Fermentation
Anaerobic Respiration
uses e- transport chain (fermentation does not)
used in anaerobic bacteria: have e- transport chain but O2 is not the final
e- acceptor some marine prokaryotes use (SO4 -²) sulfate
ion as final e- acceptor H2 S
Fermentation
uses no O2 & no e- transport chain is extension of glycolysis in cytoplasm
that generates ATP by substrate-level phosphorylation of glycolysis & recycles NADH back to NAD+
Alcohol Fermentation
Alcohol pyruvate ethanol in 2
steps:1. 2 pyruvate 2 CO2 + 2
acetaldehyde2. 2 acetaldehyde + 2
NADH 2 ethanol + 2 NAD+
Lactic Acid Fermentation
Lactic Acid pyruvate is reduced
directly by NADH lactate (end product)
lactate is ionized form of lactic acid
used by fungi & bacteria to make cheese & yogurt
Comparing Anaerobic Respiration, Fermentation &
Aerobic Respiration all 3:1. produce ATP by harvesting chemical
nrg in food2. use glycolysis to oxidize glucose
pyruvate with a net production of 2 ATP by substrate-level phosphorylation
3. use NAD+ as oxidizing agent
Key Differences
methods of oxidizinf NADH NAD+1. Fermentation pyruvate or acetaldehyde2. Anaerobic Respiration e- transport chain atom less
electronegative than O like S H2S3. Aerobic Respiration e- transport chain O2 H2O
Unique to Aerobic Respiration
oxidative phosphorylation yields up to 16x more ATP/glucose molecule
Obligate Anaerobes
only carry out fermentation or anaerobic respiration
O2 is toxic to them
Faculative Anaerobes
yeasts & many bacteria make enough ATP to survive w/out
aerobic oxidation but if O2 available can go thru oxidative phosphorylation
muscle fibers (cells) can behave as faculative anaerobes
Faculative Anaerobes in the Lab
Identify
Evolutionary Significance of Glycolysis
ancient prokaryotes used glycolysis to make ATP b/4 O2 present in atmosphere
oldest prokaryotes: 3.5 billion yrs old 2.7 billion years ago O2 in atmosphere:
source: cyanobacteria thru photosynthesis
Glycolysis is a metabolic “heirloom” from early cells that continues to function in fermentation & as 1st stage in breakdown of organic molecules by respiration
Versatility of Catabolism
Glycolysis & Citric Acid Cycle lead to many other metabolic pathways
food we eat has very little glucose in it: glycolysis can accept other
carbohydrates glycogen breaks down to glucose disaccharides monosaccharides
Proteins & Lipids as Fuel
Proteins used as Fuel
1st broken down to their a.a. those not needed for protein synthesis
can be converted to intermediates of glycolysis & Citric Acid Cycle1st amino group removed (deamination)
Lipids used in Glycolysis
1st glycerol & fatty acids glycerol glyceraldehyde 3-phosphate
(intermediate in glycolysis) fatty acids beta oxidation 2-C
fragments Citric Acid Cycle as acetyl-CoA
beta oxidation process generate NADH & FADH2 e- transport chain (reason why lipids have more nrg stored than carbs)
Beta Oxidation
Anabolic Pathways
cpds formed as intermediaries in glycolysis & Citric Acid Cycle diverted to anabolic pathways as precursors cell uses to synthesize what it needs (using ATP in process) a.a. (can make ~12) pyruvate glucose acetyl CoA fatty acids
Anabolic Pathways
Feedback Mechanisms Control Cellular Respiration
cells use supply & demand principles (does not synthesize more cpds than it needs)
Feedback inhibition: end product of anabolic pathway inhibits enzyme(s) that catalyze early step of pathway
Control of Catabolism
if cell “working” harder will speed up rate of respiration
when plenty of ATP for work cell is doing production slows down
control achieved by regulating enzymes @ strategic places in pathway
Phosphofructokinase
enzyme in glycolysis that catalyzes addition of 2nd phosphate group which is 1st step that commits the substrate irreversibly to glycolytic pathway
allosteric enzyme: has receptor sites for specific inhibitors & activators
Phosphofructokinase
inhibitor is ATP activator: AMP is also sensitive to concentration of
citrate: when citrate builds up in mitochondria some diffuses into cytoplasm and acts as inhibitor
Words to Remember
The energy that keeps us alive is released, not produced, by cellular respiration