Cellular
Respiration
Chapter 9: Cellular Respiration
Cellular Respiration
Living cells require energy from
outside sources
Organisms use glucose (C6H12O6) as
their main energy source
Cellular respiration is the
process of breaking down food
molecules to release energy (as ATP)
Energy is released in the process of
respiration when the cells of plants
and animals convert sugar and
oxygen into carbon dioxide and
water
Respiration
 The breakdown of organic
molecules is exergonic
 Aerobic respiration
consumes organic molecules
and O2 and yields ATP
(oxygen required)
 Anaerobic respiration is
similar to aerobic respiration
but consumes compounds
other than O2 (no oxygen
required)
 Fermentation is a partial
degradation of sugars that
occurs without O2
Cellular Respiration
 Cellular respiration includes both aerobic
and anaerobic respiration but is often used to
refer to aerobic respiration
 Although carbohydrates, fats, and proteins
are all consumed as fuel, it is helpful to trace
cellular respiration with the sugar glucose:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
(ATP+heat)
Redox Reactions
 The transfer of electrons during chemical reactions
releases energy stored in organic molecules
 This released energy is used to make ATP
 Chemical reactions that transfer electrons between
reactants are called oxidation-reduction reactions, or
redox reactions
 In oxidation, a substance loses electrons, or is
oxidized
 In reduction, a substance gains electrons, or is
reduced (the amount of positive charge is reduced)
 In cellular respiration, the glucose is oxidized and O2
is reduced
NAD+
 In cellular respiration, glucose and
other organic molecules are broken
down in a series of steps
 Electrons from organic compounds
are usually first transferred to
NAD+ (nicotinamide adenine
dinucleotide), a coenzyme
 As an electron acceptor, NAD+
functions as an oxidizing agent
 Each NADH (the reduced form
of NAD+) represents stored
energy that is tapped to
synthesize ATP
 NADH passes the electrons to
the electron transport chain
Electron Transport Chain
 Unlike an uncontrolled
reaction, the electron
transport chain passes
electrons in a series of
steps instead of one
explosive reaction
 O2 pulls electrons
down the chain in an
energy-yielding
tumble
 The energy yielded is
used to regenerate
ATP
Stages of Cellular Respiration
1) Glycolysis - Anaerobic (breaks down glucose
into two molecules of pyruvate)
2) Citric Acid Cycle - Aerobic (Kreb’s Cycle completes the breakdown of glucose)
3) Oxidative phosphorylation - Aerobic (ETC accounts for most of the ATP synthesis)
Mitochondria
1) Glycolysis
 Cytoplasm
2) Citric Acid Cycle
 Matrix of
mitochondria
3) Oxidative
Phosphorylation (ETC)
 Cristae of
mitochondria
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Step 1: Glycolysis
 “Splitting of sugar”
 Breaks down glucose
(C6H12O6) into two
molecules of pyruvic acid
- AKA pyruvate (C3H4O3)
 Anaerobic
 Occurs in the cytoplasm
 NAD picks up H+ and
electrons to form NADH2
Glycolysis Summary
Location: Cytoplasm
Products
Reactants
 Glucose (6-C)
 2 NAD+
 2 ATP
 2 Pyruvates (3-C)
 2 NADH
 4 ATP total
 ** 2 ATP NET since
2 are used initially
Simple Summary
Summary total
Bridge Reaction
 In the presence of O2, pyruvate enters the mitochondrion
 Before the citric acid cycle can begin, pyruvate must be
converted to acetyl CoA, which links the cycle to glycolysis
 In the mitochondria matrix…
1) Pyruvic Acid loses a C to form acetic acid (2-C)
2) The lost carbon binds with O2 making CO2
3)Acetic acid binds with Coenzyme-A forming Acetyl Co-A
Step 2: The Kreb’s Cycle
(Citric Acid Cycle)
 Takes place within the mitochondrial
matrix
 There are 8 steps, each catalyzed by
a specific enzyme
 The acetyl group of acetyl CoA joins
the cycle by combining with
oxaloacetate (4-C molecule), forming
a 6-C molecule known as citric acid
(citrate)
 The next seven steps decompose the
citrate back to oxaloacetate, making
the process a cycle
Step 2: The Kreb’s Cycle
(Citric Acid Cycle)
 2 molecules of CO2 are released
 NAD+ and FAD (flavin adenine dinucleotide - another
ion carrier) pick up electrons and H+ becoming NADH
and FADH2
 The NADH and FADH2 produced by the cycle relay
electrons extracted from food to the electron transport
chain
 The cycle generates 1 ATP, 3 NADH, and 1 FADH2 per
turn
 Recall that two molecules of pyruvate are formed during
glycolysis resulting in two turns of the Kreb’s cycle for
each glucose molecule!
Kreb’s Cycle Summary
Location: Mitochondrial Matrix
Products
Reactants
 2 Acetyl Co-A
 8 NADH (2 from
transition)
 2 FADH2
 2 ATP
 6 CO2 (2 from transition)
Kreb’s Summary
Kreb's Summary 2
Step 3: Electron Transport Chain
(ETC)
 Aerobic process
 Requires oxygen as the final
electron acceptor
 Takes place in the cristae of
the mitochondria
 A series of molecules that
excited electrons pass along,
to release energy as ATP
 Most of the chain’s
components are proteins,
which exist in multiprotein
complexes
Step 3: Electron Transport Chain
(ETC)
 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 carriers alternate reduced and oxidized states
as they accept and donate electrons
 Electrons drop in free energy as they go down the
chain
 They are finally passed to O2 (final electron
acceptor), forming H2O
NADH and FADH2
 Dump the electrons and protons they’ve gathered
throughout glycolysis and the citric acid cycle
 Again, oxygen is the final electron acceptor
 O2 + 2e- + 2H+  H2O
 Electrons are passed through a number of proteins
including cytochromes (each with an iron atom) to O2
 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
 ETC uses chemiosmosis to generate large amounts of ATP
Chemiosmosis
 Electron transfer in the ETC causes
proteins to pump H+ from the
mitochondrial matrix to the
intermembrane space
 H+ then moves back across the
membrane, passing through
channels in ATP synthase (enzyme
that acts like an ion pump)
 ATP synthase uses the exergonic
flow of H+ to drive phosphorylation
of ADP
 This is an example of
chemiosmosis, the use of energy
in a H+ gradient to drive cellular
work
 The H+ gradient is called the
proton-motive force
ETC Summary
ETC
ETC Summary
Location: Cristae of Mitochondria
Reactants
 10 NADH
 2 FADH2
Product
 34 ATP
 Each NADH makes 3
 Each FADH2 makes 2
The bulk of ATP is made in the ETC!!
Simpler ETC Summary
Best ETC Summary
Whole Respiration Process
Total Energy
~38 ATPs per 1 glucose broken down
Total ATP from 1 molecule of glucose in AEROBIC CONDITIONS
Stage
Glycolysis
ATP
+ 4 Total
+ 2 NET (b/c 2 are used in the first step)
CA Cycle
+2
ETC
+34
_________________
TOTAL + 38
During cellular respiration, most energy flows in this sequence:
Glucose -> NADH -> electron transport chain -> proton-motive
force -> ATP
About 40% of the energy in a glucose molecule is transferred to
ATP during cellular respiration, making about 38 ATP
Fermentation
 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
 Fermentation uses phosphorylation instead of an
electron transport chain to generate ATP
 2 Types:
 Lactic Acid Fermentation
 Alcohol Fermentation
Lactic Acid Fermentation
 In 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
Lactic Acid Fermentation
 Example: Burning
feeling in muscles
during a workout
• From oxygen debt
• Aerobic respiration
cannot occur
• Lactate builds up in
muscles leaks into
blood
Alcohol Fermentation
 In alcohol
fermentation,
pyruvate is converted
to ethanol (type of
alcohol) in two steps,
with the first releasing
CO2
 Bacteria and fungi
(yeast)
 Alcohol fermentation
by yeast is used in
brewing, winemaking,
and baking
Fermentation
 Obligate anaerobes
carry out fermentation
or anaerobic respiration
and cannot survive in
the presence of O2
 Yeast and many
bacteria are
facultative
anaerobes, meaning
that they can survive
using either
fermentation or cellular
respiration
Review
Role of Macromolecules
 Catabolic pathways funnel electrons from many kinds
of organic molecules into cellular respiration
 Glycolysis accepts a wide range of carbohydrates
 Proteins must be digested to amino acids
 Amino groups can feed glycolysis or the citric acid
cycle
 Fats are digested to glycerol (used in glycolysis) and
fatty acids (used in generating acetyl CoA)
 Fatty acids are broken down by beta oxidation and
yield acetyl CoA
 An oxidized gram of fat produces more than twice as
much ATP as an oxidized gram of carbohydrate
Regulation of Cell Respiration
 Feedback inhibition is the
most common mechanism
for control
 If ATP concentration begins
to drop, respiration speeds
up
 When there is plenty of ATP,
respiration slows down
 Control of catabolism is
based mainly on regulating
the activity of enzymes at
strategic points in the
catabolic pathway
Review Questions
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Define cellular respiration and state its importance as a life process.
Differentiate between aerobic respiration, anaerobic respiration, and
fermentation.
State and explain the chemical equation for cellular respiration.
Define oxidation and reduction and explain the idea of redox reactions.
Explain the use of NAD+ as a coenzyme.
Explain the electron transport chain (ETC).
Name the 3 major stages of cell respiration, along with their locations.
Explain glycolysis, stating the reactants, products, and major activities.
Explain the bridge reaction, stating the reactants, products, and major activities.
Explain the Kreb’s cycle, stating the reactants, products, and major activities.
Explain glycolysis, stating the reactants, products, and major activities.
Explain the ETC, stating the reactants, products, and major activities.
Explain the role of oxygen in the ETC.
Define chemiosmosis and explain its role in cellular respiration.
Differentiate between lactic acid fermentation and alcohol fermentation.
Differentiate between oblicate anaerobes and facultative anaerobes.
Explain the role of macromolecules in cellular respiration.
Explain how cell respiration is regulated.