Cellular Respiration

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Chapter 9 Cellular Respiration Harvesting
Chemical Energy
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Metabolism includes all of an organisms chemical
reactions.  Metabolism is the sum of all anabolic
pathways and catabolic pathways.               -
Anabolic pathways result in the formation of
complex compounds from simpler ones.  For
example, the metabolic pathways that result in
growth are anabolic pathways.               -
Catabolic pathways release energy by breaking
down complex compounds into simpler ones.  The
energy stored in organic molecules is made
available to do work.  Thus, the metabolic
pathways that create the energy for growth are
catabolic pathways.
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A.  Principles of Energy Harvest   The most
common form of energy for humans is an organic
molecule, glucose (starches in pasta, potatoes,
bread, etc. are merely long chains of glucose
molecules).   Glucose (and other organic
molecules) is produced by plants as an outcome of
photosynthesis.     Plants, animals, fungi, and
many bacteria use glucose as the primary organic
molecule for energy.  See Fig. 9.2 for the
overall concept of energy production and
harvest.  Note that CO2 and H2O are used to make
glucose and are the products of glucose
catabolism.
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There are two types of Energy-Yielding
(catabolic) pathways  respiration and
fermentation.               1.  Fermentation is
a partial degradation of organic fuel (glucose)
without using oxygen.  Partial degradation
results in the creation of acids and alcohols
(hence the term "fermentation").  When you
exercise and your muscles hurt, this is because
your body was not able to get enough oxygen to
the muscles and glucose was fermented into acids
that cause the pain.
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•   2.  During respiration, energy is gained from
  the consumption of oxygen and organic fuel (This
  is the process denoted in Fig. 9.2).  The process
  can be summarized as
•                        
•                         Organic compound Oxygen
  ? Carbon dioxide Water Energy
•                        
•                         Example C6H12O6 6
  O2? 6 CO2 6 H2O Energy (ATP Heat)
•  
• C6H12O6 is the chemical formula for glucose.

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Note the following                           a. 
The energy gained from this reaction is used to
make ATP (useful to the cell) and some heat is
given off (wasted energy).                        
                        b.  ATP (Adenosine
triphosphate) is the energy source for cellular
work.  The main objective of cellular respiration
and fermentation is to produce ATP.  This ATP is
then later used to drive other cellular
reactions.  (e.g. when you shake when you are
cold, ATP is being broken down to create heat for
your body.)
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c.  In order to create ATP from food molecules,
cells need to catabolize these organic
molecules.  Catabolism usually is a process that
transfers electrons from organic molecules to
O2.  The result is the production of H2O.  
During this process, energy stored in the
electrons is used to create ATP. Processes in
which electrons are transferred from one molecule
(called the reductant) to another molecule
(called the oxidant) are termed redox reactions
(short for reduction-oxidation).  Note that
during catabolism of glucose, glucose is the
reductant because it has energetic electrons and
that oxygen is the oxidant to which these
electrons are passed. 
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Thus,   - Oxidation is the loss of electrons
from a substance.     - Reduction is the gain of
electrons to a substance.             Generalizati
on  Xe- Y ? X Ye-             Example  Na
Cl ? Na Cl-                                    
              In this example, sodium (Na) was
oxidized (lost electrons) chlorine (Cl) was
reduced (gained electrons). Which is the
oxidant? Which is the reductant?
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• Remember that
• - Transfer of electrons in redox reactions
  releases energy that can be used to produce ATP.
•  
• Transfer of electrons from glucose to oxygen is
  an example of a redox reaction.  During cellular
  respiration, electrons move away from an organic
  compound toward oxygen. 
• C6H12O6 6 O2? 6 CO2 6 H2O Energy (ATP
  Heat)
•  
•  d.  NAD serves as an oxidant (gains electrons)
  and as an electron carrier during cellular
  respiration.  Upon gaining two electrons, NAD
  becomes NADH and is electrically neutral.
•  
• - NADH acts as an electron escort.  It carries
  electrons from an organic compound (food) to the
  electron transport chain, where the electrons can
  be used to produce ATP.

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B.  The Process of Cellular Respiration           
    Cellular respiration involves three metabolic
stages               1.  Glycolysis            
2.  The Krebs Cycle (Citric Acid
Cycle)             3.  Electron Transport Chain
and Oxidative Phosphorylation              
Figure 9.6 (p. 164) An overview of cellular
respiration. Note that the organelle in which
2 and 3 occur is the mitochondrion.
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1.  Glycolysis.  Glucose is brought across the
cell's plasma membrane as the first step in its
catabolism.  Next, it is cleaved in two, a
process that is called Glycolysis (glyco
glucose, lysis cleave, hence the term
glycolysis), during which glucose is converted
into two molecules of pyruvate. This occurs in
the cytosol. Note The cell uses two molecules
of ATP to start the process of glycolysis.  But
that four ATP are produced, thus the cell gains
two ATP (Fig. 9.8). Note There are also two
molecules of NADH produced that will be used
later to make more ATP.
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2.  The Krebs Cycle breaks down pyruvate into
carbon dioxide (CO2).   - Note, this part of the
catabolism of glucose occurs in the
mitochondria.    Figure 9.11 (p. 168) A summary
of the Krebs cycle.
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The main events in the Kreb's Cycle to know
are a.  If oxygen is present, the pyruvate from
glycolysis enters a mitochondrion. b.  Pyruvate
is converted to acetyl coenzyme A (acetyl
CoA). c.  Acetyl CoA enters the Kreb's Cycle.
The cycle results in the production of carbon
dioxide (CO2) as exhaust.  This CO2 is produced
from the original molecule of glucose. d. 
During the cycle, a total of six (6) electrons
are transferred to NAD (forming NADH), two (2)
electrons are transferred to another electron
carrier, FAD (forming FADH2), and one molecule of
ATP is formed from each pyruvate. e.  Two (2)
molecules of CO2 are released from the system. 
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3.  Most of the energy from the original glucose
molecule is stored in the molecule's electrons. 
After the Kreb's Cycle is done, these electrons
are now present on the electron carriers, NADH
and FADH2.  The cell's next big concern is
converting this energy into more ATP.  How does
it do it?  By using the Electron Transport Chain
(ETP).  This process is also in the
mitochondrion the chain is in the mitochondrial
membrane as we will see below.   The electrons
can be thought of as possessing energy that can
be used to create ATP.  This energy is an
electro-potential, pretty much like the flow of
electrons that is used to light bulbs and used
for other processes in the home.    
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The ETP consists of multiple molecules and the
electrons pass from one to another, much like an
electric current.  During this passage, the
electrons give off energy into multiple
energy-releasing steps.  The beauty of the ETP
is shown in Figure 9.5 (p. 163) An introduction
to electron transport chains.  Note that if one
mixes H2 and O2 the energy is given off as an
explosion.  But if the electrons are passed
through the ETP, the energy can be captured in
smaller, controlled quantities to create ATP.
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Note the following                            
a.  NADH and FADH2 escort electrons from the
Krebs cycle to the first protein in the electron
transport chain.                         
                        b.  Electrons are passed
from one protein to the next until they reach an
oxygen molecule.  Oxygen then accepts the
electrons and bonds with two (2) hydrogen ions to
form water.
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c.  So how does ATP get made?  By passing
electrons to the outside of the membrane, we have
set up a hydrogen ion gradient across a membrane
that can be used to perform work in this case,
the gradient is across the mitochondrial membrane
and the work is the synthesis of ATP.  The work
is carried out by allowing the hydrogen ions to
flow back into the cell, much like water will
flow over a dam or through a water mill, during
which the flow can be used to do useful work. 
The gradient that results is known as the
proton-motive force.
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d.  Relatively high concentrations of H in the
intermembrane space lead to the flow of H ions
into the mitochondrial matrix through a channel
this channel is formed by a special enzyme called
an ATP synthase (i.e. synthesizer of ATP).    e. 
ATP synthase, the enzyme that makes ATP, is
located on the inner membrane of the
mitochondrion.  This process of making ATP is
known as oxidative phosphorylation. The entire
process of using the proton motive force to make
ATP is called chemiosmosis.   Figure 9.14 (p.
167) ATP synthase, a molecular mill.
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C.  A review of cellular respiration            
            Figure 9.16 (p. 173) Review  how
each molecule of glucose yields many ATP
molecules during cellular respiration.
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            Glucose ? NADH ? electron transport
chain ? proton-motive force ? ATP              
1.  Glycolysis                                   
  2 ATP               2.  Krebs Cycle 
                               2
ATP               3.  Electron Transport Chain 
         34 ATP                                  
     Total Gain                 38
ATP               - Energy stored in glucose that
is not consumed during cellular respiration is
lost as heat.  We use that to maintain a high
body temperature excess is dissipated by
sweating, etc.
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