campbell & reece chapter 9. metabolic pathways that released stored nrg by breaking down...

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

Pyruvate Enters Mitochondria

Citric Acid Cycle

aka: Krebs Cycle tricarboxylic acid cycle functions as metabolic furnace that

oxidizes organic fuel derived from pyruvate

Summary of Krebs Cycle

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)

Brown Fat

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