Download Cellular Respiration

Chapter 9
p. 160-170
Principles of Energy Harvest
 Cells require energy to
perform many types of work
 By breaking down complex
organic molecules into simpler
products
 Some energy used for work; the
rest is released as heat
 Catabolic Pathway: releases
stored energy
Cell Respiration & Fermentation
are Catabolic
 Fermentation: does NOT need O2 to break down
sugars
 Cell Respiration: uses O2 to break down organic
molecules (glucose)
 Yields most amt ATP
 Occurs mostly in mitochondria
 ∆G = -686 kcal/mole
Cells Recycle ATP
 ATP releases energy when
phosphate group is removed
 P group transferred to another
molecule (“phosphorylated”)
 Causes molecule to perform
work
Redox Reactions Release Energy
 Rearrangement of e-’s during chemical rxns releases energy
stored in food
 Redox rxn: “reduction-oxidation”; involves transfer of e-’s
from one reactant to another
 Oxidation: loss of e- by reducing agent
 Reduction: gain of e- by oxidizing agent
 May also involve change in e- sharing
 As e- moves toward more electronegative atom, energy is
released
Electrons “fall” from Organic
Molecules to O2
 Cell respir. is a redox rxn; glucose is oxidized & O2 is
reduced
 C6H12O6 + 6O2 → 6CO2 + 6H2O +energy
 H+ transferred from glucose → O2
 H+ will lose its e-’s easily to O2
 Carbs & fats (high in H+) act as reservoirs of e-’s for cell
respiration
“Fall” of Electrons Occurs in Steps
 Glucose (org mol.) is broken down in a series of steps,
each w/ a catalyst
 H+ atoms from glucose are transferred to NAD+
(coenzyme) before O2
 Dehydrogenase: enzyme that removes pair of H atoms
from glucose (org. mol.)


Delivers 2 e- & 1 proton to NAD+
e-’s lose very little energy in this process
 To get e-’s from NADH to O2 requires
the use of the Electron Transport
Chain (ETC)
 Passes e-’s along in series of steps
 Involves proteins embedded in inner
membrane of mitochondria
 e-’s in NADH at “top”; O2 at “bottom”
 Each molecule along chain more
electroneg. than one before
 During cell respir., e-’s move from
food→NADH→e- transport
chain→O2
Overview of Cellular
Respiration
 Occurs in 3 stages:
 1) Glycolysis (glucose → 2




pyruvate)
2) Krebs Cycle (pyruvate
derivitive →CO2)
3) Electron Transport Chain
Glycolysis & Krebs break down
glucose (org. mol.)
ETC accepts e-’s from first 2
stages (via NADH)
ATP Synthesis during Cellular
Respiration
 Oxidative Phosphorylation: energy released in ETC
used for ATP synthesis in mitochondrion
 Substrate-Level Phosphorylation: produces small
amounts ATP during glycolysis & Krebs
 An enzyme transfers P group from organic molecule to
ADP
 Each molecule glucose yields ~38 ATP
Glycolysis Harvests Chemical
Energy
 Glycolysis = “splitting of
sugar”
 10-step process
 Steps 1-5 = spends 2 ATP
 Steps 6-10 = produces 4 ATP + 2
NADH
 Net Yield = 2 ATP, 2 NADH
 NADH creates more ATP in
ETC
 Energy in pyruvate extracted in
Krebs
Pyruvate → Acetyl CoA in
Mitochondria
 In presence of O2,
pyruvate enters
mitochondria
 In mitoch.: pyruvate
→ acetyl CoA →
Krebs
Krebs Cycle
 8-step cycle that takes in acetyl CoA and expels CO2
 Acetate + oxaloacetate → citrate
 Citrate → oxaloacetate
 Energy gets stored in 3 NADH & 1 FADH2, which give
e-s to ETC
 2 ATP produced (2 acetates enter cycle)
Chapter 9
p. 170-178
ETC Coupled to ATP Synthesis
 So far, most energy is stored in NADH & FADH2,
linking Krebs to ETC
 ETC does not make ATP directly; it allows e-s to fall
gradually to O2
 Mostly composed of proteins with non-protein
(prosthetic) groups attached

Groups undergo redox rxns as they move e-s
 Cytochrome: protein
w/ a heme group
 FADH2 adds e-s
further down; creates
1/3 less energy than
NADH
ATP Synthesized by Chemiosmosis
 ATP Synthase: protein in inner membrane of
mitochondria; converts ADP + P → ATP
 Uses energy from H+ conc. gradient across mitoch.
membrane (“Proton-Motive Force”)


Created by redox rxns in ETC (pump H+ out)
As H+ diffuses back in through ATP synthase, force “churns”
synthase, connecting ADP + P → ATP
 Chemiosmosis:
coupling of chemical
rxn to osmosis of H+
back into
mitochondria
 Also occurs in
chloroplasts for
photosynthesis and in
cell membrane of
prokaryotes
Cell Respiration: a review
 Energy flow: glucose → NADH → ETC → proton-motive
force → ATP
 ATP synthesis:
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
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

Glycolysis: 2 ATP
Krebs: 2 ATP
ETC: 34ATP
Total: ~38 ATP
# ATP estimated because:
 1) depends on whether NADH/FADH2 used
 2) some energy used for other work
 Usable energy yield ~40%; rest lost as heat, sweat, etc.
Related Metabolic Processes
 ATP yield from cell respiration depends on presence of
O2 (aerobic)
 When O2 absent, ATP generated by Fermentation
(anaerobic)
 Glycolysis produces 2 ATP regardless
 NAD+ is oxidizing agent
Fermentation Produces ATP w/out
O2
 Fermentation: generates 2 ATP by substrate-level
phosphorylation w/ NAD+ (glycolysis)
 e-’s from NADH → pyruvate (or derivitive) and NAD+
gets recycled
 Many types; differ in waste products made
Alcoholic Fermentation
 Pyruvate →
acetylaldehyde (releases
CO2)
 Acetylaldehyde →
ethanol (NADH is
oxidized)
 Performed by yeast
(brewing) and bacteria
Lactic Acid Fermentation
 Pyruvate → lactate (ionized
lactic acid)
 NO CO2 released
 NADH is oxidized
 Performed by fungi & bacteria to
make cheese & yogurt
 Also occurs in muscles early in
exercise, when O2 is low
 As lactate accumulates, muscle
fatigue & burning result
Fermentation/Respiration
Compared
 Both use glycolysis to break




down glucose (2 ATP)
Differ in how NADH is
oxidized to NAD+
Pyruvate leads to next step –
depends on presence of O2
Cell respiration includes Krebs
and ETC, producing ~19x’s
more ATP
Faculative Anaerobes: can
survive using either process
Evolutionary Significance
of Glycolysis
 Ancient prokaryotes
probably generated ATP
through glycolysis due to
low levels atmospheric O2
 Determined to have evolved
early because:
 Very widespread pathway
 Occurs in cytosol (not in
membrane-bound organelle)
Versatility of Catabolism
 Glycolysis can accept many types of organic molecules
 Carbs: starch, glycogen, disacch. break down to glucose →
glycolysis
 Proteins: excess amino acids remove amine group → glycolysis
 Fats: glycerol → glyceraldehyde phosphate → glycolysis
 Beta Oxidation: breaks down fatty acids into pieces that enter
Krebs
 Produces >2x’s ATP as carbs, but must work harder