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Chapter 9: Cellular Respiration
Chemical Energy and Food
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Food = chemical building
blocks living things need to
grow and reproduce
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Broken down gradually
Capture little bits of
energy at key steps
Use energy stored in
chemical bonds of foods
to produce ATP which
powers the activities of
the cell
Overview of Cellular Respiration:

It is the process that releases energy by breaking down food
molecules in the presence of oxygen
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6O2 + C6H12O6  6CO2 + 6H2O + energy (ATP)
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oxygen + glucose  carbon dioxide + water + energy (ATP)
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A cell needs to trap those little pieces of energy to make ATP
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Takes 3 main steps
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Glycolysis
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Krebs Cycle (Citric Acid Cycle)
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Electron Transport System
Step 1: Glycolysis = “sugar breaking”
Glucose  Pyruvate
(pyruvic acid)
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The beginning of the energy pathway in
respiration
Takes place in the CYTOPLASM
One molecule of glucose is broken in
half, producing 2 molecules of pyruvic
acid (a 3-carbon compound)

Uses 2 ATP molecules to get the
process started

In the end 4 molecules will have
been produced

Net gain = 2 molecules of ATP (a
relatively small amount)
Step 1: Glycolysis
Glucose  Pyruvate
Produces
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Advantages
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2 molecules of pyruvate
2 molecules of ATP
Happens quickly – can produce 1000s of
ATPs in a few milliseconds
Does not require oxygen
If oxygen is present leads to 2 other
pathways that release lots of energy =
aerobic respiration
If no oxygen, a different pathway is
followed = anaerobic respiration
Aerobic Respiration
(oxygen present)
Step 2: Krebs Cycle
(Citric Acid Cycle)


Occurs in the mitochondria
Pyruvic acid is broken down
into carbon dioxide in a
series of energy-extracting
reactions

1st step – citric acid
production

Pyruvic acid passes
through the
mitochondrial
membrane into the
matrix

Pyruvic acid – CO2 =
Acetyl CoA

Acetyl CoA + 4-C
compound = Citric Acid
Step 2: Krebs Cycle
(Citric Acid Cycle)

2nd step – stepwise energy
extraction, release of CO2
at each step

4-C molecule
replenished at end,
cycle can continue
Cha-Ching –O- Meter
ATP
NADH FADH2
Step 3: Electron
Transport Chain
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Cha-Ching –O- Meter
ATP
NADH FADH2
Occurs in the MITCHONDRIAL MEMBRANE
High energy electrons arrive in NADH and FADH2 from Krebs
and Gylcolysis
Movement of electrons down the ETC moves H into the
mitochondrial membrane, establishes a concentration gradient
Hydrogen moves back out through ATP synthase, spinning
the turbine and
creating ATP (massive
amounts of ATP!!!)
Requires O2; oxygen is
the final electron
acceptor at the end
of the ETC
Approximately 32-34 ATPs are generated for every glucose
Overview of Cellular Respiration

It is the process that releases energy by breaking
down food molecules in the presence of oxygen

6O2 + C6H12O6
6CO2 + 6H2O + Energy

oxygen + glucose
carbon dioxide + water +
energy

A cell needs to trap those little pieces of energy to
make ATP

Takes 3 main steps – glycolysis, krebs cycle, electron
transport system
Step 1: Glycolysis

The beginning of the energy pathway in respiration

The process in which one molecule of glucose is broken in half, producing
2 molecules of pyruvic acid (a 3-carbon compound)





Uses 2 ATP molecules to get the process started
In the end 4 molecules will have been produced
Net gain = 2 molecules of ATP (a relatively small amount)
If oxygen is present leads to 2 other pathways that release lots of energy =
aerobic respiration
If no oxygen, a different pathway is followed = anaerobic respiration
Aerobic Respiration –
oxygen present
Step 2: Krebs Cycle/Citric Acid Cycle

90% of the chemical
energy available in
glucose is still unused at
the end of glycolysis

It is locked in the pyruvic
acid

Pyruvic acid is broken
down into carbon dioxide
in a series of energyextracting reactions

1st step – citric acid
production
2nd step – energy
extraction

Step 3: Electron Transport Chain
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Uses the high energy electrons
from the Krebs cycle to convert
ADP into ATP
Don’t worry about all the steps
Produces 34 more ATP molecules
per glucose
Summary of ATP totals
Why do we need all this ATP?
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Cells need energy to work
Primary fuel is ATP (adenosine
triphosphate)
ATP = nitrogen containing compound
(adenine), 5-carbon sugar (ribose), and 3
phosphate groups
ADP = same as ATP but with 2 phosphate
groups
The 3rd phosphate is the key to how cells
store energy – when the cell has extra
energy available it adds a phosphate to
make 3 and when it needs energy it can
take a phosphate away
Releasing Energy from ATP

Energy is released from ATP when ATP is
changed into ADP + phosphate

ATP = compressed spring (unstable)

ADP = relaxed spring (stable)
Using Biochemical Energy
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Active transport
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Powers movement of organelles within the
cell
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ATP and Glucose
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Cells only contain a small store of ATP
because it is not efficient

Sugar can store up to 90 times the energy
than ATP

Cells use the energy in carbohydrates to
generate ATP form ADP as needed
Anaerobic Respiration –
Alcoholic Fermentation

Used by yeasts and a few other
microorganisms
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Pyruvic acid + NADH
alcohol + CO2 + NAD+
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Cause bread dough to rise
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When the yeast runs out of oxygen ,
it begins to ferment, giving off carbon
dioxide
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Makes the bread rise by forming
small air pockets in the bread
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Alcohol evaporates when the bread
is baked
Lactic Acid Fermentation
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If pyruvic acid accumulates it can be
converted to lactic acid
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Pyruvic acid + NADH
lactic acid + NAD+
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Lactic acid is produced in your
muscles when you exercise when
the body can’t supply enough
oxygen to the tissues
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Your muscles start to produce ATP
by lactic acid fermentation and lactic
acid builds up in your muscles
causing a painful burn
no oxygen present
Photosynthesis vs. Respiration
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Almost opposite reactions
Photosynthesis deposits energy
Respiration withdraws energy
The products of photosynthesis
are the reactants for respiration
Respiration takes place in all
eukaryotes and some prokaryotes
Photosynthesis takes place in
plants, algae, and some bacteria