Download 18 Pyruvate to Acetyl-CoA to Krebs Cycle A/P

18
Pyruvate to Acetyl-CoA to Krebs Cycle A/P Biology
Pages 162-177
Name: ______________________ Date: _________________ Period: ____________
Pyruvate is one of most important intermediate forms of the breakdown of high
energy macromolecules (such as sugar, lipid, and protein). Pyruvate can be converted to
lactic acid in bacteria and man when oxygen is in short supply or not available. The build
up of lactic acid causes some interesting effects.
1.) Lactic acid build up is a way to temporarily store a high-energy hydrogen
proton and electron in lactic acid by oxidizing NADH
NAD+. This allows the use
of this “energy source” (lactic acid) at another time. THIS ALSO allows for NAD+ to be
used rapidly in glycolysis to provide SOME “instant” ATP molecules from available
glucose breakdown! (See handout #17 – Glycolysis page- notice that NAD+ is needed to
allow glyceraldehydes 3 phosphate to be made into pyruvate, yielding ATP along the
way!)
2.) Lactic acid build up MAY cause a toxic environment for bacteria at some
concentration. In other words its waste product becomes toxic due to its ability to reduce
proteins and enzymes it comes in contact with (lowers the pH). So some bacteria may
produce this as a way to reduce bacterial or fungal competition for food. This “could”
play a role in infections in multi-cellular animals (look this up in the Internet if you desire
more information on this topic.)
3.) Lactic acid in multi-cellular organisms is an anaerobic reaction (happens in the
absence of oxygen) and in itself cause red blood cells to release oxygen more rapidly
(due to its acid nature) and thus relieve the anaerobic condition. Lactic acid can induce
pain and cramps due to the dual effect of high acid environment and anaerobic condition.
The most recent papers have show that this MAY be a secondary effect on calcium
channel proteins (transmembrane proteins) in the muscle. If you interrupt the calcium
flow in the muscle you could induce pain and cramps. When oxygen is more plentiful and
pyruvate is being utilized by the mitochondria, lactic acid can be used as a source of
energy, creating NADH and more pyruvate (again- see Handout #17).
In Yeast and Bacteria, pyruvate can also be converted to acetaldehyde and
eventually alcohol (see page 166). High concentrations of alcohol can also limit growth
of competing organisms. (Remember those old Westerns where the “Doc” pours whiskey
on a wound to “heal” the wound and “kill bacteria”?). The production of lactic acid
and/or ethanol creates carbohydrates (both alcohol and lactic acid are high energy
carbohydrates) that are still packed with high-energy bonds that can be utilized to create
more energy. The fact that bacteria and yeast are unable to utilize the energy from these
products perhaps arrests their ability to evolve into complex multi-cellular organisms.
They cannot use these high energy compounds because they do not have the necessary
enzymes to break them down! This gives these one-celled organisms little energy for the
amount of matter they have to consume. BUT, their job as decomposers and their sheer
numbers give them the ability to be “successful” and certainly useful in the cycling of
nutrients, water, and carbon in the ecosystem.
Millions of years ago heterotrophs (single celled and primitive multi-celled
organisms could only capture about 3.5% of the energy in the food it consumed.
Therefore, other heterotrophs that consumed them would only capture 2% of the
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remaining energy, or a total of 0.12% of the energy available in the original autotrophs.
This would mean that a VERY large base (amount) of these autotrophs would be
necessary to support all the heterotrophs that lived off them. Consider the magnitude of
this statement in terms of the geometric progression of each trophic level feeding off the
trophic level beneath them. (We will go more into trophic levels when we talk about
Ecology.) This could have led to the extinction of the dinosaurs! So what happened
many millions of years ago?
Millions of years ago, there must have been the introduction of a symbiotic
organism into a prokaryotic cell (prokaryotes are bacteria or bacteria-like organisms).
The introduction of mitochondria into the prokaryotic cells allowed them to utilize energy
more efficiently. In fact the mitochondria “fit” into the cells metabolic architecture by
picking up where Glycolysis left off. The glycolytic cycle produces (NET):
1.) two molecules of pyruvate
2.) two molecules of ATP (substrate level production)
3.) two molecules of NADH
The above Glycolytic reactions CAN OCCUR in the presence or absence of oxygen!
Now the pyruvate moves into the mitochondria, where its complete oxidation
to CO2 and H2O yields most of the ATP derived from glucose breakdown. The very
next step occurs JUST
INSIDE the
mitochondria. The
3,4
reactions you see to the
right shows the first
carbon removed from
pyruvate as an oxidative
decarboxylation of
pyruvate to acetyl-CoA.
The reaction is
basically: Pyruvate to
Acetyl-CoA. There is a
loss of carbon dioxide
and the Acetyl-CoA
then combines with
OAA to form Citrate in
the Citric Acid Cycle.
Unfortunately the reaction is not quite that
simple: It looks something like this:
In fact it is even more complex:
The take home message is that pyruvate enters
the mitochondria and loses carbon dioxide and
forms an intermediate (Acetyl-CoA) which then
enters the Krebs Cycle. The Krebs Cycle is
only found in the mitochondria (not found in
cytoplasm of any cell).
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There are in fact many contributors to the production of Acetyl-CoA.
1.) Protein metabolism
2.) Carbohydrate metabolism (glycolysis)
3.) Lipid metabolism
When Acetyl-CoA combines with Oxaloacetic acid (OAA) it produces Citrate. See
below: The formation of Citrate is the reason the Krebs Cycle is also called the
Citric Acid Cycle!
As always, the actual mechanics is really not this straightforward…but this gives us a
nice overview and shows the logical production of NADH and FADH2 and the release of
CO2. For each SINGLE cycle of the Citric Acid Cycle, there are two molecules of CO2
produced. The two carbons are replaced by the addition of Acetyl-CoA. For the entire
molecule of Glucose to be completely oxidized to CO2, the citric acid cycle would have
to go through its cycle two times. The NET production of ATP would be 36 ATP.
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Answer the first 15 questions here and on your scan-tron.
_____ 1.) Why can’t bacteria and yeast breakdown glucose beyond lactic acid/ethanol?
a.) they do not have the enzymes
b.) they don’t have mitochondria
c.) it may be advantageous for them to produce lactic acid/ethanol
d.) all of the above are correct
_____ 2.) What might be a reason that bacteria or yeast would want to produce lactic
acid or ethanol?
a.) lactic acid/ethanol in high concentrations are actually good for the bacteria and
yeast
b.) lactic acid/ethanol reflect UV-light and x-ray radiation
c.) lactic acid/ethanol is converted to sugar by sun light
d.) lactic acid/ethanol can be used as food later by bacteria and yeast
e.) lactic acid/ethanol may limit the growth of competing organisms around them
_____ 3.) All of the following are broken down into pyruvate by the cell EXCEPT for:
a.) lipids b.) protein c.) sugar d.) water e.) amino acids
_____ 4.) In breaking down glucose by Eukaryotic cells- all the carbons of glucose are:
a.) re-incorporated into sugar molecules
b.) are made into lactic acid under aerobic conditions
c.) are lost as CO2
d.) are made into ethanol under anaerobic conditions
e.) none of the above are correct
_____ 5.) Acetyl-CoA contains how many carbons from the original glucose molecule?
a.) 1 b.) 2 c.) 3 d.) 5 e.) 6
_____ 6.) Glycolysis- breakdown of glucose to 2 molecules of pyruvate is an:
a.) anaerobic reaction b.) aerobic reaction c.) neither of these
_____ 7.) Why would it be advantageous for a single celled organism to incorporate
mitochondria into its cellular structure?
a.) to be able to extract more energy from each molecule of glucose
b.) to be able to become more complex (evolve)
c.) to be able to isolate functions (become a multi-cellular organism)
d.) all of the above are correct
_____ 8.) In the reaction from pyruvate to acetyl CoA, which numbered carbons are lost
as CO2?
a.) 1,2 b.) 3,4 c.) 5,6 d.) 2,5 e.) 1,6
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_____ 9.) In yeast/bacteria- where does most of the energy of the break down of glucose
go (except for energy lost as heat)?
a.) glucose is completely broken down to CO2 and ATP
b.) glucose is re-constituted at the end of the glycolytic cycle-so energy is not lost
c.) some of the energy is captured by ATP/NADH and the rest is left with lactic
acid/ethanol
d.) all of the energy is left with the lactic acid/ethanol
_____ 10.) Where does glycolysis take place and why?
a.) takes place in the mitochondria because that is where the glucose must go in the
cell
b.) takes place in the cytoplasm because that is where the glycolytic enzymes are
c.) takes place in the nucleus, because that is where the glycolytic enzymes are
d.) takes place outside of the cell so the glycolytic enzymes do not breakdown the
cellular protein
e.) all of the above are correct
_____ 11.) What is “substrate-level phosphorylation”?
a.) this is when an inorganic phosphate is added to ADP by using an enzyme
b.) this is when the ATP is used as a substrate
c.) this is when inorganic phosphate is added to ADP through ATP synthetase
d.) this is when there is no phosphate available and the enzymes make some
e.) none of the above are correct
_____ 12.) In the Citric Acid Cycle- how many turns does it take to completely oxidize
one molecule of glucose?
a.) 5 turns b.) 4 turns c.) 3 turns d.) 2 turns e.) 1 turn
_____ 13.) When acetyl-CoA is joined with Oxaloacetic acid (OAA) what is formed and
how many carbons is in this product?
a.) Aconitate (6 carbons)
b.) Alpha-ketoglutarate (5 carbons)
c.) Succinate (4 carbons)
d.) Citrate (6 carbons)
e.) Malate (4 carbons)
_____ 14.) There are bacteria that can produce glucose in the presence of sunlight (bluegreen bacteria). This means that:
a.) they do not have to break down glucose for energy
b.) they use glucose directly to power enzymatic reactions in the cell
c.) they can break down glucose to lactic acid/ethanol
d.) they can break down glucose to CO2 plus water plus ATP
e.) all of the above are correct
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_____ 15.) Bacteria are able to break down glucose in the presence or absence of oxygen,
this means…
a.) they must take in air to help break down glucose
b.) bacteria in the soil cannot break down glucose at all
c.) bacteria does not need glucose as an energy source
d.) bacteria can partially break down glucose using glycolytic enzymes only
e.) none of the above are correct
Answer the following questions on this handout only!
_____ 16.) When bacteria breakdown glucose other organisms can benefit…
a.) because bacteria produce high energy compounds that can be used by other
organisms
b.) bacteria can be used as food by other organisms
c.) bacteria can return nutrients to the soil (i.e. nitrogen fixation)
d.) bacteria still release CO2 and that can be used by plants
e.) all of the above are correct
_____ 17.) Where does pyruvate “go” to be further broken down in the eukaryotic cell?
a.) in the Golgi b.) in the mitochondria c.) in the nucleus d.) in the water vacuole
_____ 18.) When acetyl-CoA enters the Krebs Cycle (Citric Acid Cycle) – how many
carbon molecules are lost (as CO2) in ONE cycle of the Krebs? (see pg. 3 diagram)
a.) 1 b.) 2 c.) 3 d.) 4 e.) 6
_____ 19.) How many NADH are produce PER cycle of the Krebs?
a.) 1 b.) 2 c.) 3 d.) 4 e.) 6
_____ 20.) How many WATER molecules are lost during each cycle of the Krebs?
a.) 1 b.) 2 c.) 3 d.) 4 e.) 6
Answer the following questions on this sheet only.
21.) Consider the following…LARGE dinosaurs MAY have had a different set of
enzymes to break down sugar. If the process was only 10% efficient (that is they were
only able to extract about ten per cent of the energy from a molecule of glucose)
speculate why the dinosaurs might have become extinct? What if the enzymes needed to
breakdown the glucose efficiently (getting like 30-40% of the energy from glucose) were
temperature sensitive- that is they did not work if it was too hot or too cold?
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22.) In a few sentences, speculate on the importance of Glycolysis being an Anaerobic
reaction. Consider that millions of years ago there could have been a shortage of oxygen
due to low levels of producers. The reduction of producers might have been a result of
too many consumers trying to survive off of too few producers.
23.) Give some reasons why it was “necessary” for some organisms to incorporate
mitochondria into their metabolic structure.
24.) Give the general equation for Cellular respiration and indicate if plant cells use this,
animal cells use this, bacteria use this, and if yeast cells use this. See if you can find the
general equation for photosynthesis and write that equation under the equation for
cellular respiration. How are they the same? How are they different?
Date: ____________________________
Lesson Plan for Handout #18
A/P Biology
Objective: The learner will demonstrate the ability to explain why glycolysis can occur
in an anaerobic environment (that is what is the advantage to this physical fact) and what
conditions will allow pyruvate to go to lactic acid and/or ethanol. Moreover, the students
will be able to identify three sources of pyruvate and how pyruvate enters the
mitochondria and then the Kreb’s cycle. Students will be able to isolate those reactions
that contain substrate level phosphorylation and demonstrate how and when carbon
dioxide is released as pyruvate enters the mitochondria when given handout #18.
Content: Pyruvate/lactic acid/ethanol/aerobic/anaerobic reactions and the Krebs cycle.
Method: Power point, white board, discussion.
Homework: Handout #18
1.) In a few sentences, speculate on the importance of Glycolysis being an Anaerobic
reaction. Consider that millions of years ago there could have been a shortage of oxygen
due to low levels of producers. The reduction of producers might have been a result of
too many consumers trying to survive off of too few producers.
2.) Give some reasons why it was “necessary” for some organisms to incorporate
mitochondria into their metabolic structure.
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3.) In bacteria and yeast, where does all the energy go from the breakdown of glucose?
Is this efficient or inefficient and why? What are the positive aspects to this limited
oxidation of glucose? What are the negative aspects?
4.) Where does Glycolysis take place and where does pyruvate go once it is produced?
5.) Where is most of the ATP production derived from?
6.) The reaction from pyruvate to Acetyl-CoA releases which numbered carbon atoms as
CO2?
7.) What are the three contributors to the production of Acetyl-CoA in the human body?
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8.) In the Citric Acid Cycle, how many turns does it take to completely oxidize glucose?
9.) How are all the carbon atoms of glucose ultimately released? How is this related to
the following equation? What is the general name for the equation below? What is the
“opposite” reaction and what is it called? This “opposite” reaction is found in what type
of organism(s)?
C6H12O6 + 6 O2
glucose + oxygen
6 CO2 +
6 H2O
carbon dioxide + water + energy