Chapter 9 Cellular Respiration: Harvesting Chemical Energy

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Chapter 9 Cellular Respiration Harvesting
Chemical Energy

• Because order is intrinsically unstable, a cell
  must work just to maintain its complex structure

• Work involves organizing and building molecules,
  pumping substances into and out of the cell,
  growth and reproduction, and movement

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Even good-looking science teachers must have
energy to do work. Believe it or not, even
Bomber works!
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Principles of Energy Harvest

• In harvesting energy, cells employ complex
  chemical pathways which usually involve many steps

• Respiration involves the degradation (breakdown)
  of complex food molecules into simple waste
  products

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• Chemical reactions which occur in the cell, will
  be of two types

ANABOLIC
CATABOLIC
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• Anabolic pathways require the input of energy and
  are generally reactions which build molecules for
  the cell

Like this workman, anabolic reactions require the
input of energy
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• Catabolic reactions release stored energy by
  breaking down complex molecules. The energy then
  can be used to do work, or else is released as
  heat

In what ways is the burning of this candle like a
catabolic reaction?
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• Two important catabolic pathways we will be
  studying in this chapter are

FERMENTATION
CELLULAR RESPIRATION
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Fermentation and Respiration

• Fermentation is the partial degradation of sugars
  that occurs without the help of oxygen

• Cellular respiration requires oxygen and takes
  place within the mitochondria of eukaryotic cells

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What do you suppose is the purpose of the
mitochondrions many folds?
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Overall Equation For Respiration
Organic compounds
oxygen

AIR
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products
Energy
Carbon Dioxide
Water


AIR
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• Although many different molecules of
  carbohydrate, fat, and even protein may be used
  as fuel for the cell, glucose is typically used
  in cellular respiration

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ATP

• Stands for molecule known as adenosine
  tri-phosphate

• ATP is the standard energy source for all cell
  work

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• The energy for cellular work is obtained by
  cleaving phosphate groups from the tail of the
  ATP molecule

• Special enzymes remove the groups and add them to
  other molecules in a process called
  phosphorylation

• ATP then becomes ADP and the phosphorylated
  molecule is then primed to undergo some
  reaction in which the phosphate group will once
  again be removed

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This atomic diagram shows the structure of the
ATP molecule, including the phosphate tail above
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AD
The ATP cycle
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• A working muscle cell will recycle ATP molecules
  at the rate of about 10 million molecules per
  second!!!!!

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Redox Reactions

• Chemical reactions in which one or more electrons
  are transferred from one reactant to another are
  called oxidation-reduction reactions, or redox
  reactions for short

oxidation
The loss of an electron is called
reduction
The gain of an electron is called
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• When sodium (Na) and chlorine (Cl) combine to
  form table salt (NaCl)

Na

Cl
Na
Cl-
and

• Sodium loses an electron and is thus oxidized

• Chlorine picks up an electron and is said to be
  reduced

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• The attractive forces between the positively-
  charged sodium and the negatively-charged
  chlorine forms a new substance, table salt

NaCl

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• In a redox reaction

• The substance oxidized is called the

reducing agent

• The substance reduced is the

oxidizing agent
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• How do these terms apply to the redox reaction
  involving sodium and chlorine?

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• As we discuss energy transfers, keep in mind that
  oxygen is one of the most powerful oxidizing
  agents known. It readily steals electrons from
  substances that have them to give. Is oxygen
  oxidized, or reduced?

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• The relocation of electrons releases energy
  stored in food molecules, and this energy is used
  to synthesize ATP

food
ATP
ENERGY
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• Cellular respiration is a redox reaction
  involving the oxidation of glucose

• Organic materials containing large numbers of
  hydrogen atoms are excellent materials for
  oxidation. These substances usually include

fats
carbohydrates
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These are just a few of the foods containing
large amounts of various carbohydrate molecules
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Other foods contain large quantities of fat
molecules which may be used as fuel when
carbohydrates are in short supply
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• The release of energy during respiration is
  somewhat like the burning of fuel

• However, if the energy in food were released all
  at once, the cell would be destroyed by the heat
  released during the reactions

• Consequently, the energy from electron removal is
  transferred to a series of molecules which serve
  as carriers in an electron transport chain

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• Nicotinamide adenine dinucleotide, or NAD
  functions as the primary carrier of electrons as
  they are removed by enzymes called dehydrogenases

H---C---H

NAD
NAD
NADH
NADH
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• To summarize, the electrons removed by enzymes
  from food travel a downhill route of step by
  step reactions until they finally combine with
  oxygen

food
e-
NAD
e-
Electron Transport chain
e-
oxygen
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The Process Of Cellular Respiration

• Respiration is a function involving three
  metabolic phases

Glycolysis
the Kreb's cycle
electron transport chain
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• In glycolysis, which occurs in the cytosol,
  glucose is broken down into two molecules of a
  substance called pyruvate

• The Krebs cycle takes place in the matrix of the
  mitochondrion. It completes the breakdown of
  pyruvate into carbon dioxide

• In the third stage, which occurs on the surface
  of the mitochondrial membranes themselves, ATP is
  generated by a process known as oxidative
  phosphorylation

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• The ATP generated in both glycolysis and the
  Krebs cycle are formed by substrate-level
  phosphorylation. This simply means that a
  phosphate is transferred from another molecule to
  ADP

• The ATP formed in the last step accounts for 90
  of the ATP produced by the cell. In oxidative
  phosphorylation, the flow of electrons across the
  mitochondrial membranes generates the ATP

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For each molecule of
GLUCOSE
the cell can obtain a maximum of
38 ATP
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Glycolysis The Oxidation Of Glucose To Pyruvate

• Glycolysis means splitting of sugar

• In glycolysis, glucose (C6) is converted into two
  molecules of pyruvate (C3)

PYRUVATE
GLUCOSE
PYRUVATE
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• The entire process of glycolysis involves ten
  steps, each of which is catalyzed by a specific
  enzyme

• Each enzyme fits only with the product of the
  previous step

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• The ten steps of glycolysis can be divided into
  two phases

energy-investment phase
energy-payoff phase
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• In the energy-investment phase, the cell actually
  uses energy to phosphorylate the fuel molecules

• During the energy-payoff phase, these molecules
  are then taken apart in order to generate surplus
  ATP for the cell, plus 2 NADH

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• No oxygen is utilized during glycolysis, and no
  carbon dioxide is produced

• Net energy gain for the cell from glycolysis is 2
  ATP

glucose
-
2 ATP
pyruvate
4 ATP

pyruvate

2 NADH
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The Krebs Cycle

• Glycolysis releases only a small portion of the
  energy stored in glucose

• Most of the energy remains stored in the two
  molecules of pyruvate

• If oxygen is present inside the cell, the
  pyruvate enters the mitochondrion, where enzymes
  of the Krebs cycle complete the oxidation process

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Pyruvate entering the mitochondrion
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• Before entering the Krebs cycle, pyruvate is
  converted to a compound called acetyl CoA in a
  three step reaction

1. A CO2 molecule is removed and the pyruvate
becomes a 2-carbon molecule known as acetate
O CO CO CH3
CO CH3
acetate
CO2
pyruvate
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2. An enzyme transfers the removed electrons to a
molecule of NAD, forming the energy-rich
compound NADH
e-
NAD
NADH
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3. Finally, a sulfur containing compound derived
from vitamin B called coenzyme A is attached,
forming Acetyl CoA
S CoA

CO CH3
S CoA CO CH3

Coenzyme A
acetate
Acetyl CoA
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• Once formed, acetyl CoA is now ready to enter the
  Krebs cycle

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• The Krebs cycle was named for Hans Krebs who
  figured the steps of this pathway in the 1930s

• The pathway involves eight steps, each regulated
  by a specific enzyme located in the mitochondrial
  matrix

Where is the matrix of this mitochondrion?
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Steps Of The Krebs Cycle

• Acetyl CoA combines with a 4-carbon compound
  called oxaloacetate to form a 6-carbon citric acid

C-C
C-C-C-C (oxaloacetate)
C-C-C-C-C-C (Citric acid)
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2. Citric Acid loses a water molecule, and
another is added back, converting it to an isomer
known as Isocitrate
C-C-C-C-C-C
-


H2O
H2O
C-C-C-C-C-C (isocitrate)
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3. Isocitrate loses a CO2, becoming a 5-carbon
Ketoglutarate. Also NADH is formed by oxidation
-

C-C-C-C-C-C (isocitrate)
C
C-C-C-C-C (ketoglutarate)
NADH

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4. This 5-carbon compound loses another CO2 and
becomes a 4-carbon compound . Coenzyme A is
added, forming Succinyl CoA and another NADH is
formed

-
C-C-C-C-C
C
C-C-C-C
C-C-C-C-CoA (succinyl CoA)



NADH
CoA
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5. ATP is formed by substrate-level
phosphorylation. CoA is replaced by a phosphate
group which is then transferred to GDP to form
GTP. GTP then donates its phosphate to ADP,
forming ATP
-
C-C-C-C-CoA


CoA
P
GDP

GTP
C-C-C-C-P
ADP

ATP
C-C-C-C (Succinate)
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6. Two hydrogens are now removed by oxidation and
are transferred to FAD to form FADH2. The new
molecule formed is also a 4-carbon compound, but
is known as Fumarate
-
C-C-C-C (fumarate)
C-C-C-C

2H
FAD is a hydrogen-acceptor similar to NAD

FADH2
FAD
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7. The chemical bonds in Fumarate are now
rearranged by the addition of a water molecule.
This slight modification results in the formation
of Malate.
C-C-C-C (fumarate)

H2O

C-C-C-C (Malate)
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8. In the last step of Krebs, another hydrogen
is removed and added to NAD to form NADH. The
result is the regeneration of Oxaloacetate, which
then is ready to take another fragment of acetyl
CoA through the cycle
H
-

C-C-C-C
C-C-C-C (oxaloacetate) to step 1

NAD
NADH
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• Up to this point, we have produced only four
  molecules of ATP from a single glucose molecule,
  all by substrate-level phosphorylation.

2 ATP from glycolysis (one from each pyruvate)
2 ATP from the Krebs Cycle (one from each Acetyl
CoA)
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• The remaining ATP to be produced by the
  mitochondrion, will be produced by oxidative
  phosphorylation

• This process takes place along the inner
  mitochondrial membrane (cristae) and involves the
  oxidation of the NADH and FADH2 formed during the
  first two stages of respiration.

• As electrons are removed, they are passed from
  molecule to molecule along what is known as an
  electron transport chain

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If you follow the arrows, you can see the paths
taken by the electrons as they are removed from
NADH and FADH2
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• Nearly all of the components of the chain are
  proteins embedded in the inner membrane itself.

• The folding of the mitochondrial membrane allows
  for thousands of copies of the transport chain to
  be present in each mitochondrion

• Frequently, these electron carriers are referred
  to as cytochromes because of their common
  chemical structure

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This diagram shows a portion of the electron
transport chain. The arrows show the passage of
electrons from one carrier to another. Notice
that the hydrogens removed are pumped out of the
membrane
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• This mechanism by which the electron transport
  chain is coupled with the production of ATP is
  called chemiosmosis

• By pumping hydrogens into the intermembrane
  space, the proton pump creates a gradient
  (difference in concentrations) of protons on
  either side of the membrane

• These protons then move through the only channels
  available to them---------ATP synthase channels

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• ATP synthase will then use the energy from this
  flow to phosphorylate ADP into ATP

ADP

P
ATP
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In this simplified drawing, the electron
transport system removes ions from the matrix.
These ions then flow back into the matrix, thus
providing the energy for ATP production.
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After the hydrogens are pumped into the
intermembrane space, they will flow back across
the membrane through channels made of ATP synthase
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Intermembrane space
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• Each molecule of NADH will yield about 3 ATP.
  Each molecule of FADH2 yields only 2 ATP.

• The two NADH produced during glycolysis will
  result in about 6ATP

• The two NADH produced during the conversion of
  pyruvate to acetyl CoA will yield about 6 ATP

• Krebs produces a total of 6 NADH resulting in 18
  ATP, and 2 FADH2 which will generate 4 ATP

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• Coupled with the 4 ATP produced as a result of
  substrate-level phosphorylation, the total energy
  yield from a single molecule of glucose is 38 ATP

• 2 ATP from glycolysis

• 2 ATP from Krebs

• 34 ATP from electron transport

38 ATP
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• Finally, oxygen, which diffuses into the
  mitochondrion serves as the final hydrogen
  acceptor as H2O is formed. This water (as vapor),
  as well as the CO2 produced, leaves the
  mitochondrion and is exhaled.

Why cant I be the final hydrogen acceptor?
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Energy Flow During Respiration
glucose
NADH
Electron transport
Proton motive force
ATP
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• Respiration is only about 40 efficient

• The other 60 of the energy is lost as heat used
  to maintain our body temperature, or dissipated
  through sweating and other cooling mechanisms

• Although this may seem very inefficient, the most
  efficient automobiles only convert about 25 of
  the energy in gasoline to the movement of the car

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• Oxidative phosphorylation, which generates most
  of the cells ATP, is dependent upon an adequate
  supply of oxygen to the cell

• When oxygen is not available, fermentation
  provides a mechanism for obtaining ATP without
  the help of oxygen

How in the world can you get ATP without oxygen?
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• In glycolysis, the oxidizing agent is NAD, not
  oxygen

• If you remember, 2 ATP are produced from
  substrate-level phosphorylation, as well as 2
  NADH which carry over to electron transport

• Without oxygen, no electron transport chain takes
  place (oxygen must be the final acceptor), and
  all available NAD would soon be tied up as NADH

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Glycolysis Summarized

C6H12O6 (glucose)
2 NAD


2 C3 (pyruvate)
2 ATP
2 NADH

• If all NAD is tied up as NADH, then no more
  glucose can be oxidized and no more ATP
  generated. The cell will die

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• Fermentation allows cells devoid of oxygen a
  mechanism to regenerate NAD and thus continue to
  harvest ATP from glucose

• In the absence of oxygen, a cell will undergo one
  of two fermentation types

alcohol fermentation
lactic acid fermentation
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• In alcohol fermentation, pyruvate is converted to
  ethanol . NADH is used in the process, and CO2 is
  produced

C3 (pyruvate) NADH
C2 (ethanol) CO2 NAD
NAD now returns to glycolysis for the oxidation
of more glucose
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Many microorganisms, such as yeast, carry on
alcohol fermentation naturally in the absence of
oxygen. We have used these organisms for
centuries in the manufacture of various alcoholic
beverages, including wine (right)
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Some microbes use sugars such as sucrose,
fructose, and maltose in fermentation, resulting
in the same products
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• In lactic acid fermentation, pyruvate is reduced
  directly by NADH, with no release of CO2

C3 (pyruvate) NADH
C3 (lactic acid) NAD
As in alcohol fermentation, NAD will now be used
again in glycolysis to oxidize glucose
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Man has also used various microbes to convert
milk into cheese products and yogurt by lactic
acid fermentation
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Lactic acid fermentation is also responsible for
muscle cramps sometimes experienced during
exercise. The lactic acid will be converted back
to pyruvate as soon as the oxygen supply is
adequate
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DOC frequently experienced lactic acid buildup
in his days as a professional boxer
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Respiration And Fermentation Compared

• Both respiration and fermentation are methods of
  harvesting ATP from glucose

• Both respiration and fermentation use the
  glycolysis pathway to produce pyruvate and 2 ATP

• In both respiration and fermentation, NAD is the
  oxidizing agent of glycolysis

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GLUCOSE
Pyruvate is the key fuel for both respiration and
fermentation
cytosol
PYRUVATE
Without oxygen
With oxygen
Acetyl CoA
Ethanol or Lactic acid
Krebs cycle
mitochondrion
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Key Differences Between Respiration and
Fermentation

• Oxidative Respiration is 19 times more efficient
  than fermentation

2 ATP
fermentation
38 ATP
respiration
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• The primary function of fermentation is to
  regenerate NAD so that ATP can continue to be
  harvested from glycolysis

• Oxygen serves as the final electron acceptor for
  respiration

• Some organisms can carry on either process,
  depending upon oxygen availability. They are
  referred to as facultative anaerobes

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Other Catabolic Pathways

• Normally glucose is used as the primary source of
  fuel for the cell

• After glucose, other complex carbohydrates may be
  converted to glucose and used as fuel

• When carbohydrates are exhausted, the cell will
  then convert fats into glycerol and fatty acids
  which then enter glycolysis and Krebs,
  respectively.

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• And finally in an effort to stay alive, a cell
  when deprived of other fuel sources will actually
  digest protein into amino acids. These can then
  be converted to pyruvate or enter Krebs as
  acetyl-CoA.

• This last-resort metabolic pathway can be seen in
  video footage of starving children, whose bodies
  have been digesting themselves

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Proteins
Carbohydrates
Fats
Amino acids
sugars
glycerol
pyruvate
Fatty acids
NH3
Acetyl CoA
A
T
P
Krebs Cycle
Electron Transport
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Anabolic Pathways (Biosynthesis)

• Not all of the food we eat is converted into ATP

• Many food substances are needed as skeletons
  from which to build other molecules for the cell

• For example, amino acids are needed to make new
  protein, fatty acids and glycerol are needed to
  manufacture lipids for cell use, and many
  hormones and enzymes must also be manufactured

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• These processes do not produce energy, instead
  they use it. Such energy-using reactions are
  referred to as

ANABOLIC REACTIONS

• In addition, Krebs and glycolysis enable a cell
  to convert molecules into other molecules as
  needed

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Feedback Mechanisms

• The cell does not waste energy making more of a
  substance than it needs

• If there is a glut or backlog of a particular
  material, the metabolic pathway that synthesizes
  that material is switched off

• The most common mechanism for regulating these
  pathways is feedback inhibition (the presence of
  a material will prevent its synthesis)

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Too much of a good thing will prevent the cell
from synthesis of that material. Excess food,
even fat free, may be converted to fat by the cell
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• Likewise, when a cell is low on ATP, respiratory
  pathways are opened and respiration increases.

When Coach Young skis, more energy is required
not only for activity, but to keep his body
temperature at 98.6 degrees
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ATP
ATP
INHIBITION
NO INHIBITION
Decrease in energy production
Increase in energy production
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Thought to Remember
Living cells are efficient, thrifty, expedient,
and responsive in their metabolism
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If e mc2, and D mass divided by volume, and
all respiratory metabolic pathways have been
discussed then this chapter is over!!!