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Chapter 6
How Cells Harvest Chemical Energy
BREATHING VERSUS RESPIRATION

BREATHING:
 Alternation
of inhalation and exhalation.
 Exchange of gases in which organisms obtain oxygen
from the air (or water) and release carbon dioxide.
 Exchange occurs in lungs (or gills).

CELLULAR RESPIRATION:
 Harvesting
of energy from food molecules by cells.
 Aerobic process (requires oxygen).
 Occurs inside cells (cytoplasm and mitochondria).
“Respiration” comes from Latin word for breathing.
Breathing and cellular respiration are closely
related, but not the same processes.
Breathing versus Cellular Respiration
CELLULAR RESPIRATION BANKS ATP
REACTION:
C6H12O6 + 6O2
(Glucose)
(Oxygen)
----> 6CO2 + 6H2O + ENERGY
(Carbon dioxide) (Water)
What happens to the energy in glucose or other
food molecules?
 Only about 40% of energy is turned into ATP
 The rest is lost as metabolic heat.
 One ATP molecule has about 1% of the
chemical energy found in glucose.
ENERGY CONVERSIONS ARE INEFFICIENT
Second Law of Thermodynamics
By Comparison Living Organisms Are Efficient
CATABOLISM:
Process of splitting larger molecules to smaller
ones. Catabolic reactions are exergonic and
release free energy.
THREE MAJOR CATABOLIC PATHWAYS
IN LIVING ORGANISMS
A. Aerobic (Cellular) respiration
B. Anaerobic respiration
C. Fermentation
MAJOR CATABOLIC PATHWAYS
A. Aerobic (Cellular) respiration:
 Requires
oxygen.
 Most commonly used catabolic pathway.
 Over 30 reactions. Used to extract energy from
glucose molecules.
 Final electron acceptor: Oxygen.
 Most efficient: 40% of glucose energy is
converted into ATP.
REACTION:
C6H12O6 + 6O2
---> 6CO2 + 6H2O + ENERGY
Glucose
Carbon dioxide Water
Oxygen
THREE MAJOR CATABOLIC
PATHWAYS
B. Anaerobic respiration:
 Does
not require oxygen.
 Used by bacteria that live in environments
without oxygen.
 Final electron acceptor: Inorganic molecule.
 Very inefficient: Only 2% of glucose energy is
converted into ATP.
 Final products: Carbon dioxide, water, and
other inorganic compounds.
THREE MAJOR CATABOLIC PATHWAYS
C. Fermentation:
 Does
not require oxygen.
 Used by yeast, bacteria, and other cells when
oxygen is not available.
 Final electron acceptor: Organic molecule.
 Very inefficient: Only 2% of glucose energy is
converted into ATP.
 Products depend on type of fermentation:
Lactic acid fermentation: Used to make cheese and
yogurt. Carried out by muscle cells if oxygen is low.
 Alcoholic fermentation: Used to make alcoholic
beverages. Produces alcohol and carbon dioxide.

II. Hydrogen carriers shuttle electrons in
REDOX reactions
Oxidation:
 Partial or complete loss of electrons or H atoms.
 When a molecule is oxidized it loses energy.
Reduction:
 Partial or complete gain of electrons or H atoms.
 When a molecule is reduced it gains energy.
REDOX Reactions:
 Reactions in which both oxidation and reduction
occur.
 Characteristic of many cell processes, including
aerobic respiration and photosynthesis.
Cellullar Respiration is a Redox Process:
Involves Both Oxidation and Reduction
Glucose is oxidized to carbon dioxide.
Oxygen is reduced to water.
III. Redox reactions in living organisms
Cellular Respiration: Macromolecules oxidized to release
energy (686 kcal/mole) which is used to synthesize ATP
C6H12O6 + 6O2
Glucose
Oxygen
---> 6CO2 + 6H2O + ENERGY
(oxidized) (reduced)
Photosynthesis: CO2 is reduced ; requires energy to drive
the reaction forward
6CO2 + 6H2O + ENERGY ---> C6H12O6 + 6O2
Carbon
Dioxide
Water
(reduced)
(oxidized)
High Energy
IV. Hydrogen carriers shuttle electrons in redox
reactions
1. Dehydrogenase: Removes hydrogen atoms (with their
electrons) from organic molecules and transfers them
to an electron carrier.
2. Electron Carrier Molecules:
 NAD+:
(Nicotinamide adenine dinucleotide)
Coenzyme that accepts and transfers most H and the
high energy electrons released by redox reactions
Reduction
NAD+ + 2H ------------> NADH + H+
(2H+ & 2e-)
 FADH2 (Flavin
adenine dinucleotide): Secondary H
carrier, related to NADH.
Dehydrogenase and Hydrogen Carriers
Shuttle Electrons in Redox Reactions
IV. Electrons “fall” from Hydrogen Carriers to
Oxygen in the Electron Transport Chain
 NADH:
(Nicotinamide andenine dinucleotide) :
Delivers H and the high energy electrons released by
redox reactions to electron carrier molecule of chain.
 Electron
transport chain: Proteins on inner
mitochondrial membrane that accept H and use high
energy electrons to produce ATP.
As Electrons “Fall” From Hydrogen Carriers
to Oxygen, Energy is Released
Two different means of ATP production:
1. Substrate-level phosphorylation:
 Generates
a small amount of ATP during cellular
respiration.
 Simple
process, does not require membranes.
 Phosphate
group is directly transferred from an
organic molecule to ADP to make ATP.
 Occurs
in first two stages of aerobic respiration:

Glycolysis

Kreb’s cycle
Two different means of ATP production:
2. Oxidative phosphorylation (Chemiosmosis):
 Generates
most of ATP made during cellular
respiration
 Complex
 Energy
process, requires mitochondrial membranes.
released from exergonic reactions of electron
transport is used to pump H+ ions across membrane,
creating a concentration gradient (potential energy).
 Chemiosmosis:
ATP is made by ATP synthase on
mitochondrial membranes, as H+ flow down
concentration gradient.
 Occurs
in last stage of aerobic respiration.
 Requires
the presence of OXYGEN
Two Mechanisms of ATP Synthesis:
Oxidative and Substrate Level Phosphorylation
V. Three Stages of Cellular Respiration
A. Glycolysis
B. Kreb’s Cycle
C. Electron Transport Chain & Chemiosmosis
Three Stages of Aerobic Respiration
A. Glycolysis: “Splitting sugar”
 Occurs
in the cytoplasm of the cell
 Does not require oxygen
 9 chemical reactions
 Net result: Glucose molecule (6 carbons each) is split
into two pyruvic acid molecules of 3 carbons each.
 Yield per glucose molecule:
2 ATP ( Substrate-level phosphorylation)
2 NADH + 2 H+
(2 ATP are “invested” to get 4 ATP back)
 Pyruvic acid diffuses into mitochondrial matrix where
all subsequent reactions take place.
Glycolysis: “Splitting” of Glucose into
Two Molecules of Pyruvic Acid
Conversion of Pyruvate to Acetyl CoA
 Before
entering the next stage, pyruvic acid (3C) must
be converted to Acetyl CoA (2 C).
 A carbon atom is lost as CO2.
 Yield per glucose molecule: 2 NADH + 2 H+
B. Kreb’s Cycle
 Occurs
in the matrix of the mitochondrion
 A cycle of 8 reactions
Reaction 1: Acetyl CoA (2C) joins with 4C molecule
(oxaloacetic acid) to produce citric acid (6C).
 Reactions 2 & 3: Citric acid loses 2C atoms as CO2.
 Reactions 4 & 5: REDOX reactions produce NADH
and FADH2.
 Reactions 6-8: Oxaloacetic acid is regenerated.

Kreb’s Cycle: Two Carbons In, Two Carbons Out
Details of Kreb’s Cycle
B. Kreb’s Cycle
 Carbons
are released as CO2
 Yield per glucose molecule:
2 ATP (substrate-level phosphorylation)
6 NADH + 6 H+
2 FADH2
C. Electron Transport Chain & Chemiosmosis
 Most
ATP is produced at this stage
 Occurs on inner mitochondrial membrane
 Electrons from NADH and FADH2 are transferred to
electron acceptors, which produces a proton gradient
 Proton gradient used to drive synthesis of ATP.
 Chemiosmosis: ATP synthase allows H+ to flow across
inner mitochondrial membrane down concentration
gradient, which produces ATP.
 Ultimate acceptor of H+ and electrons is OXYGEN,
producing water.
Electron Transport & Chemiosmosis: Generates
Most ATP Produced During Cellular Respiration
NOTE: The electron transport chain ONLY
works when OXYGEN is available at the end
of the chain to accept the electrons and H+ to
form water.
C. Electron Transport Chain & Chemiosmosis
Yield of ATP through Chemiosmosis:
 Each
NADH produces 3 ATP
 Each FAHD2 produces 2 ATP
2 NADH (Glycolysis) x 3 ATP
= 6 ATP
2 NADH (Acetyl CoA) x 3 ATP
= 6 ATP
6 NADH (Kreb’s cycle) x 3 ATP
= 18 ATP
2 FADH2 (Kreb’s cycle) x 2 ATP
= 4 ATP
________________
32 - 34 ATP
These ATPs are made by oxidative phosphorylation
or chemiosmosis.
VIII. Total Energy from cellular respiration
Substrate
Phosphoryl
Process
Oxidative
e-Carrier Phosphoryl TOTAL
Glycolysis 2 ATP
2 NADH ---> 4 - 6 ATP
6-8 ATP
Acetyl CoA
Formation
2 NADH ---> 6 ATP
6 ATP
Kreb’s
6 NADH ---> 18 ATP
2 ATP
2 FADH2 ---> 6 ATP
Total yield per glucose :
24 ATP
__________
36-38 ATP
Many poisons interrupt cellular respiration
 Electron
transport chain blockers:

Rotenone: Pesticide

Cyanide

Carbon monoxide
 ATP

synthase inhibitors
Oligomycin: Antifungal drug. Used on skin.
 Uncouplers:
Make mitochondrial membrane
leaky to H+ ions. Abolishes H+ gradient, can’t
make ATP through chemiosmosis.

Dinitrophenol (DNP): Used in 1940s as weight loss
drug.
Effects of Various Poisons on the Electron
Transport Chain
FERMENTATION OCCURS WHEN OXYGEN
IS NOT AVAILABLE
 Yeasts normally use aerobic respiration to
process food.
 If oxygen is not available, they use fermentation,
which is less efficient.
 Types of fermentation:
Alcoholic fermentation
Glucose ----> 2 pyruvate ----> 2 Ethanol + 2 CO2
Lactic acid fermentation
Glucose ----> 2 pyruvate ----> 2 Lactic acids
Fermentation Occurs When Oxygen is Unavailable
Alcoholic and Lactic Acid Fermentation:
An Alternative to Aerobic Respiration
ORGANISMS CAN BE CLASSIFIED BASED
ON THEIR OXYGEN REQUIREMENTS
 Strict
aerobes: Require oxygen for survival.
All large organisms are aerobes.

Examples: Humans, dogs, insects.
 Strict
anaerobes: Grow only in the absence of
oxygen. Are poisoned by oxygen.

Examples: Bacteria that live in soil and animal
intestines.
 Facultative
anaerobes: Grow with/without
oxygen. Grow better with oxygen.

Examples: Yeast and many bacteria.
All Food Molecules are Fed into The Catabolic
Pathway of Aerobic Respiration