test your reflexes!!!jan 13, 2019 · electron transport chain are found in the extensive surface...
TRANSCRIPT
Test your reflexes!!!
• So all this stuff is about energy???
• Who’s got the energy to try this one???
• Gamers?
• Check it out.
• Living is work.
• To perform their many tasks,
cells require energy from
outside sources.
• In most ecosystems, energy enters as sunlight.
• Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them.
Introduction
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Fig. 9.1
• Organic molecules store energy in their arrangement
of atoms.
• Enzymes catalyze the systematic degradation of big
organic molecules that are rich in energy to smaller
waste products with less energy (catabolic,
exergonic, right?)
• Some of the released energy is used to do work and
the rest is dissipated as heat.
1. Cellular respiration and fermentation are
catabolic, energy-yielding pathways
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• Cellular respiration is similar to the combustion of gasoline
in an automobile engine.
• The overall process is:
• Organic compounds + O2 -> CO2 + H2O + Energy
• Carbohydrates, fats, and proteins can all be used as the fuel,
but it is traditional to start learning with glucose, as it is the
most common of these, and proteins are rarely used.
• C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy (ATP + heat)
• The catabolism of glucose is exergonic, delta G = - 686 kcal
per mole of glucose.
• Some of this energy is used to produce ATP that will perform cellular work.
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• Reactions that result in the transfer of one or more
electrons from one reactant to another are redox
reactions.
• Let’s see what you remember from your favorite
course last year:
• The loss of electrons is called ???????.
• The addition of electrons is called ???????.
3. Redox reactions
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Oxygen is one of the most potent oxidizing agents (which means that it gets __________?).
• An electron loses energy as it shifts from a less electronegative atom to a more electronegative one.
• A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work (like phosphorylate an ADP to make ATP).
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In cellular respiration, glucose and other fuel
molecules are oxidized, releasing energy.
• In the summary equation of cellular respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
• Glucose is oxidized, oxygen is reduced, and
electrons lose potential energy.
• Molecules with lots of hydrogen are excellent fuels
as their bonds are a source of energetic “hilltop”
electrons that “fall” down to oxygen, ending up in
water.
4. Electrons “fall” from organic molecules to
oxygen during cellular respiration
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Glucose and other fuels are broken down gradually in a pathway - a series of steps, each catalyzed by a specific enzyme.
• At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme like NAD+
(nicotinamide adenine dinucleotide).
• Part of the NAD molecule is niacin, one of the B vitamins listed on your cereal box.
5. The “fall” of electrons takes a whole
bunch of steps.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.4
• This changes the oxidized form, NAD+, to the
reduced form, NADH, or NADH2, or NADH + H+
• NAD + functions as the oxidizing agent in many of the redox
steps during the catabolism of glucose.
• The electrons carried by NADH lose very little of their potential energy in this process.
• This energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen.
• Again, electrons from molecules like glucose will end up being grabbed by oxygen – the oxygen you breathe!!
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Respiration occurs in three metabolic stages:
glycolysis, the Krebs cycle, and the electron
transport chain
involving oxidative
phosphorylation.
Overview of respiration
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Fig. 9.6
2.A.2.f.1. Explain the
connection between the
following in one sentence:
Glycolysis, glucose molecules,
free energy, ATP, ADP,
inorganic phosphate, pyruvate.
2.A.2.f.2. Where does pyruvate
go once it’s made?
• During glycolysis, glucose, a six carbon-sugar, is split into two, three-carbon sugars.
• These smaller sugars are oxidized and rearranged to form two molecules of pyruvate.
• Each of the ten steps in glycolysis is catalyzed by a specific enzyme.
• These steps can be divided into two phases: an energy investment phase and an energy payoff phase.
2. Glycolysis oxidizes glucose to pyruvate:
it occurs in the cytoplasm.
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• In the energy investment phase, ATP provides activation energy by phosphorylating glucose.
• This requires 2 ATP per glucose.
• In the energy payoff phase, ATP is produced by substrate-level phosphorylationand NAD+ is reduced to NADH.
• 4 ATP (gross) and 2 NADH are produced per glucose.
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Fig. 9.8
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Fig. 9.9a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.9b
• The net yield from glycolysis is 2 ATP (made 4,
used 2) and 2 NADH per glucose.
• No CO2 is produced during glycolysis.
• Glycolysis occurs whether O2 is present or not,
therefore these reactions are said to be anaerobic.
• If enough dissolved O2 is present in the cell, pyruvate
moves into the mitochondrion to the Krebs cycle and
the energy stored in NADH can be converted to ATP by
the electron transport system and oxidative
phosphorylation.
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• More than three quarters of the original energy in the
glucose molecule we started with is still present in
two molecules of pyruvate.
• The rest is in the 2 ATP’s and 2 NADH’s produced, and
some has been lost as heat.
• If oxygen is present, pyruvate enters the
mitochondrion where enzymes of the Krebs cycle
complete its oxidation into carbon dioxide.
3. The Krebs cycle completes the energy-yielding
oxidation of organic molecules:
a closer look
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• As pyruvate enters the mitochondrion, a
multienzyme complex modifies it to acetyl CoA
which enters the Krebs cycle in the matrix.
• A carboxyl group is removed as CO2, causing this
reaction to be called oxidative decarboxylation.
• A pair of electrons is transferred from the remaining
two-carbon fragment to NAD+ to form NADH.
• The oxidized
fragment, acetate,
combines with
coenzyme A to
form acetyl CoA.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.10
• The Krebs cycle is named after Hans Krebs, who
was largely responsible for elucidating its
pathways in the 1930’s. Watch here
• This cycle begins when 2C acetate (vinegar) from
acetyl CoA combines with 4C oxaloacetate to form
6C citrate (citric acid, like in orange juice).
• Ultimately, after several reactions, the oxaloacetate is
recycled and the acetate is broken down to CO2.
• Each cycle produces one ATP by substrate-level
phosphorylation, three NADH, and one FADH2
(another electron carrier) per acetyl CoA.
• Since our original glucose gave rise to 2 acetyl CoA’s,
two Krebs cycle sets of reactions will result.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2.A.2.f.3. Explain the connection
between the following in one
sentence: Krebs cycle, carbon
dioxide, organic intermediates,
ATP, ADP, inorganic phosphate,
substrate level phosphorylation,
coenzymes.
2.A.2.f.4. Explain the role of
NADH and FADH2.
• The Krebs cycle
consists of eight
steps. These rxns
occur in the
matrix, the fluid
inside the inner
membrane. The
presence of the
enzymes
necessary for the
reactions in the
matrix is an
example of how
structure is
related to
function.
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Fig. 9.11
• The conversion of
pyruvate and the
Krebs cycle
produces large
quantities of
electron carriers.
Remember, the
electrons were
originally part of the
glucose molecule we
started with.
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Fig. 9.12
• Only 4 of 38 ATP ultimately produced by the
aerobic breakdown of one glucose molecule are
derived from substrate-level phosphorylation.
• The vast majority of the ATP comes from the energy
in the electrons carried by NADH (and FADH2).
• The energy in these electrons is used in the electron
transport system to power ATP synthesis.
4. The electron transport chain (ETC)
produces most of the ATP
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2.A.2.g. Briefly explain how the ETC establishes an
electrochemical gradient across membranes.
2.A.2.g.1. Identify where Electron transport chain
reactions occur.
2.A.2.g.2. Contrast the ETC in cellular respiration and
photosynthesis.
2.A.2.g.3. Describe the formation of a proton gradient
in prokaryotes and eukaryotes.
2.A.2.g.4. Explain what generates ATP from ADP and
inorganic phosphate.
2.A.2.g.5. In cellular respiration, explain how
decoupling oxidative phosphorylation from electron
transport is involved in thermoregulation. See Sci.
Skills Exercise
• Because it is folded, thousands of copies of the
electron transport chain are found in the
extensive surface of the cristae, the inner
membrane of the mitochondrion. This is
another classic example of how structure is
related to function in the mitochondrion. Pay
attention for another in a few minutes.
• Most components of the chain are proteins
that are bound with prosthetic groups that can
alternate between reduced and oxidized states
as they accept and donate electrons. Many
are of a type called cytochromes.
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• Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein.
• The electrons continue along the chain, which includes several cytochrome proteins and one lipid carrier.
• The electrons carried by FADH2 have lower free energy and are added to a later point in the chain.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.13
• Electrons from NADH or FADH2 ultimately pass
to oxygen, the so-called final electron acceptor,
causing this to be called aerobic respiration.
• The electron transport chain generates no ATP
directly.
• The movement of electrons along the chain does
contribute to a process called chemiosmosis,
which leads to ATP synthesis by oxidative
phosphorylation (or ox-phos as it’s referred to in
small talk at biologists’ cocktail parties).
• Here’s how it works:
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• A proton gradient is produced by the movement of
electrons along the electron transport chain, because
several chain molecules can use the exergonic flow of
electrons to pump H+ from the matrix to the
intermembrane space.
• The gradient produced involves more protons in the
intermembrane space than in the matrix. Since this space
is small, it is easy to build up a steep gradient, another
example of which theme?
• This gradient represents a form of potential energy, very
similar to the one in a flashlight battery.
• This concentration of H+ is the proton-motive force.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.15
• A protein complex, ATP
synthase, in the cristae
actually makes ATP from
ADP and Pi.
• The energy of the
proton gradient is used
as the source of power to
do the work of ATP
synthesis.
• How about another look
at the animation?
• http://vcell.ndsu.nodak.edu/animations/
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.14
• The ATP synthase molecules are the only place that
will allow H+ to diffuse back to the matrix.
• This exergonic flow of H+ through the protein complex
is used by the enzyme to generate ATP.
• This coupling of the redox reactions of the electron
transport chain to ATP synthesis is called
chemiosmosis.
• The energy from glucose electrons was used to move
protons across a membrane (uphill, so to speak), and
when they passively flowed back across the membrane
(downhill), their energy was used to do the work of
making ATP. Cool, eh? Let’s rap it up.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Let’s review a theme thing before we go on to
something else…
• The mitochondrion is a classic example of how
structure is related to function on the cellular
level.
• Describe the three aspects of its structure that are
examples of this theme and explain how they
contribute to efficient function.
• How do prokaryotic cells accomplish this?
• And connect this to this endosymbiosis thing…
Use representations and models to analyze how
cooperative interactions within organisms
promote efficiency in the use of energy and
matter. [LO 4.18, SP 1.4]
Use representations to pose scientific questions
about what mechanisms and structural
features allow organisms to
capture, store, and use free energy. [LO 2.4, SP
1.4, SP 3.1]
Here’s how ADP and ATP get into and out
of mitochondria – ADP/ATP translocase
• During respiration, most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force (gradient) -> ATP.
• Considering the fate of carbon, one six-carbon glucose molecule is oxidized to six CO2 molecules.
• Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation involving the ETC.
5. Didn’t you always want to know how
many ATP’s the breakdown of one
glucose molecule generates?
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• Each NADH from the Krebs cycle and the conversion of
pyruvate contributes enough energy to generate a
maximum of 3 ATP’s.
• The NADH from glycolysis may also yield 3 ATP.
• Each FADH2 from the Krebs cycle can be used to generate
about 2ATP’s.
• In some eukaryotic cells, NADH produced in the cytosol
by glycolysis may be worth only 2 ATP.
• The electrons must be shuttled to the mitochondrion.
• In some shuttle systems, the electrons are passed to
NAD+, in others the electrons are passed to FAD.
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• Assuming the most energy-efficient shuttle of
NADH from glycolysis, a maximum yield of 34
ATP is produced by oxidative phosphorylation.
• This plus the 4 ATP from substrate-level
phosphorylation gives a bottom line of 38 ATP.
• This maximum figure does not consider other uses of
the proton-motive force, or the leaking of protons
through other gaps in the membrane, so, as always,
efficiency is not 100%, so 38 ATP’s per glucose would
be the absolute max.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.16
• How efficient is respiration in generating ATP?
• Complete oxidation of glucose releases 686 kcal per
mole.
• Formation of each ATP requires at least 7.3 kcal/mole.
• Efficiency of respiration is 7.3 kcal/mole x 38
ATP/glucose/686 kcal/mole glucose = 40%.
• The other approximately 60% is lost as heat.
• Cellular respiration is remarkably efficient in
energy conversion. Your car gets less than 40%
efficiency from the gas it burns.
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• Oxidation refers to the loss of electrons to any
electron acceptor, not just to oxygen.
• In glycolysis, glucose is oxidized to two pyruvate molecules with NAD+ as the oxidizing agent, not O2.
• Some energy from this oxidation produces 2 ATP (net).
• If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron transport chain.
• But glycolysis generates 2 ATP whether oxygen is
present (aerobic) or not (anaerobic).
Sometimes there is just not enough oxygen
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• Anaerobic catabolism of sugars can occur by fermentation.
• Glycolysis doesn’t need oxygen, but it does need NAD+.
• If the NAD+ pool is exhausted, glycolysis shuts down.
• Under aerobic conditions, NADH transfers its electrons to the electron transport chain, recycling NAD+ to keep glycolysis and the Krebs cycle going.
• Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD+ by transferring electrons from NADH to pyruvate.
• All fermentation occurs in the cytoplasm.
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• In alcohol fermentation, pyruvate is converted to
ethanol in two steps.
• First, pyruvate is converted to a two-carbon compound,
acetaldehyde, by the removal of CO2.
• Second, acetaldehyde is reduced by NADH to ethanol.
• Alcohol fermentation
by yeast is used in
baking and
winemaking.
• The acetaldehyde can
give you a headache,
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Fig. 9.17a
• During lactic acid fermentation, pyruvate is
reduced directly by NADH to form lactate ( lactic
acid).
• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt.
• Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce.
• The waste product, lactate, may cause muscle fatigue (or does it?) but ultimately it is converted back to pyruvate in the liver.
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Fig. 9.17b
• Some organisms (facultative anaerobes),
including yeast and many bacteria, can survive
using either fermentation or respiration.
• At a cellular level, human
muscle cells can behave
as facultative anaerobes,
but nerve cells cannot.
• For facultative anaerobes,
pyruvate is a fork in the
metabolic road that leads
to two alternative routes.
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Fig. 9.18
• The oldest bacterial fossils are over 3.7 billion
years old, appearing long before appreciable
quantities of O2 accumulated in the atmosphere.
• Therefore, the first prokaryotes may have
generated ATP exclusively from glycolysis.
• The fact that glycolysis is also the most
widespread metabolic pathway and occurs in the
cytosol without membrane-enclosed organelles,
suggests that glycolysis evolved early in the
history of life.
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Construct explanations of the mechanisms and
structural features of cells that allow organisms
to capture, store, or use free energy. [LO 2.5, SP
6.2]
Describe specific examples of conserved core
biological processes and features shared by all
domains or within one domain of life, and how
these shared, conserved core processes and
features support the concept of common
ancestry for all organisms. [LO 1.15, SP 7.2]
2.A.1.c. Explain how energy-related
pathways in biological systems are
sequential and may be entered at
multiple points in the pathway by
using one of the examples below:
Krebs cycle, Glycolysis, Calvin
cycle, or Fermentation
• Glycolysis can accept a wide range of carbohydrates.
• Polysaccharides, like starch or glycogen, can be hydrolyzed to glucose monomers that enter glycolysis.
• Other hexose sugars, like galactose and fructose, can also be modified to undergo glycolysis.
• Proteins and fats can also enter the same respiratory
pathways used by carbohydrates.
2. Glycolysis and the Krebs cycle connect to
many other metabolic pathways
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• Carbohydrates, fats,
and proteins can all
be catabolized
through the same
pathways.
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Fig. 9.19
• Proteins must first be digested to individual amino acids.
• Amino acids that will be catabolized must have their
amino groups removed via deamination.
• The nitrogenous waste is excreted as ammonia, urea, or
another waste product.
• The carbon skeletons are modified by enzymes and enter
as intermediaries into glycolysis or the Krebs cycle
depending on their structure.
• Usually, a cell will only use protein for energy if it is very
low on carbs and fats, such as in starvation situations.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The cell can also use fats to make ATP.
• Fats must be digested to glycerol and fatty acids.
• Glycerol can be converted to glyceraldehyde phosphate,
an intermediate of glycolysis.
• The rich energy of fatty acids is accessed as fatty acids
are split into two-carbon fragments of acetic acid.
• These molecules enter the Krebs cycle as acetyl CoA.
• In fact, a gram of fat will generate twice as much ATP as a
gram of carbohydrate via aerobic respiration, and the
greater percentage of the ATP’s produced by your cells at
rest come from the fats in your last meal.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The daily cycle looks like this:
• A meal is digested, lipid fragments and simple sugars get into cells and are used to make ATP.
• Extra glucose is packed into glycogen in liver and muscle cells for later use. When they are full, extra fats and sugars are converted to stored fats in fat cells.
• As you run low on ATP’s, your body first breaks down the glycogen to free up glucoses, then when those run low the stored fats start getting broken down. This usually starts occurring after 20 minutes or so of activity.
• Not all the organic molecules of food are
completely oxidized to make ATP.
• Intermediaries in glycolysis and the Krebs cycle
can be used to make needed compounds.
• For example, a human cell can synthesize
about half the 20 different amino acids by
modifying compounds from the Krebs cycle.
• Glucose can be synthesized from pyruvate
and fatty acids from acetyl CoA.
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• Basic principles of supply and demand
regulate the metabolic economy.
• It is an adaptive advantage if a cell only
make substances such as ATP when it
needs them.
• If ATP levels drop, catabolism speeds up
to produce more ATP; when it is high,
catabolism slows. Here’s how:
3. Feedback mechanisms control cellular
respiration
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• Control of catabolism is
based mainly on
regulating the activity of
enzymes at strategic
points in the catabolic
pathway.
• One strategic point occurs
in the third step of
glycolysis, catalyzed by
phosphofructokinase.
• See Sci. Skills Exercise
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 9.20
• Allosteric regulation of phosphofructokinase sets
the pace of respiration.
• This enzyme is allosterically inhibited by ATP and
stimulated by AMP (derived from ADP).
• It responds to shifts in balance between production
and degradation of ATP: ATP <--> ADP + Pi >
AMP + Pi.
• Thus, when ATP levels are high, inhibition of this
enzyme slows glycolysis.
• When ATP levels drop and ADP and AMP levels rise,
the enzyme is active again and glycolysis speeds up.
• This is a classic example of a negative feedback loop.
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• Citrate, the first product of the Krebs cycle, is also
an inhibitor of phosphofructokinase.
• This synchronizes the rate of glycolysis and the Krebs
cycle.
• Also, if intermediaries from the Krebs cycle are
diverted to other uses (e.g., amino acid synthesis),
glycolysis speeds up to replace these molecules.
• Metabolic balance is augmented by the control of
other enzymes at other key locations in glycolysis
and the Krebs cycle.
• Cells are thrifty, expedient, and responsive in their
metabolism.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Hollywood often has a tough time
with science
• Tell me what you think of this bit of testimony
from an expert witness.
• In case you haven’t seen the movie, here’s the
famous ending.
• Now how about that Q10 thing?