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SECTION MENU
Cellular Respiration
Introduction
Mitochondrion
Structure
Catabolic Pathways
Krebs Cycle #
Glycolysis Electron Transport
Chain
Fermentation
Oxidative
Phosphorylation
More Energy With
Oxygen Summary of
Cellular Respiration
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Introduction
Cellular Metabolism
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Notes:
• Photosynthesis is carried out by plants, cyanobacteria and
algae. Biochemical pathways in plant chloroplasts convert light
energy, H2O and CO2 into stored energy in the form of molecules
such as sugars. Oxygen is a waste product.
• Cellular respiration releases stored energy by breaking down
food molecules such as sugars. Oxygen is consumed.
• Photosynthesis and cellular respiration complement each other by
cycling CO2 and O2. Photosynthesis produces the O2 and food
molecules that cellular respiration uses. Cellular respiration produces CO2 that can be recycled into food by plants.
• At night, plants switch from photosynthesis to respiration to meet
their energy needs, but they usually burn up only a small portion
of their food reserves at this time.
Narration
Photosynthesis uses light energy to make molecules that fuel the processes of life. During
photosynthesis, raw materials—carbon dioxide
and water— are converted into energy-rich fuel
molecules such as sugars and fatty acids. To
harvest the energy stored in these fuel molecules,
cells convert them back into carbon dioxide and
water.
• CO2 is an important atmospheric gas. All organisms generate it,
but plants and micro-algae are mainly responsible for re-cycling
it. Along with the CO2 from our respiration, humans add large
quantities of CO2 to the atmosphere by burning fossil fuels, burning wastes, and so forth. This upsets the natural balance of the
CO2 cycle, as we are now observing with global climate change.
Obviously, maintaining forests and other long-term plant communities is an important factor for the global balance of CO2.
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Catabolic Pathways
Cellular Metabolism
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Notes:
• Fermentation and cellular respiration are the two catabolic
(‘breaking - down’) pathways used by organisms to take apart
organic molecules and capture energy stored in chemical bonds.
The cell’s main energy currency is ATP, adenosine triphosphate,
so catabolic pathways are needed to put ATP “in the bank”.
• To make ATP, a phosphate group is added to ADP, adenosine
diphosphate (an abundant molecule in the cell).
energy from
ADP + Phosphate +
= ATP
catabolic reaction
• When the cell needs energy, ATP provides it by donating an
energy-rich phosphate – re-forming the ADP molecule.
energy-rich
ATP =
+ ADP
Phosphate
Comparison of Catabolic Pathways
Energy-Harvesting
Processes
Uses Oxygen ?
Breakdown
Products
Effeciency of
ATP Production
Fermentation
Cellular
Respiration
Glycolysis
1 - Glycolysis
2 - Krebs Cycle
3 -Electron
Transport Chain
No
Usually
Ethanol or
lactic acid
H2O and CO2
Low - Only 2 ATP
per sugar molecule
High - Up to 18
times as much ATP
as fermentation
• Cells need a constant supply of ATP, so they continually recycle
ADP to ATP. For example, an active muscle cell recycles its ATP
at a rate of about 10 million molecules per second!
• Both fermentation and cellular respiration start with the same
process – glycolysis. To understand how cells get energy from
food, it is first essential to understand glycolysis.
• Fermentation occurs in the absence of oxygen. It only uses one
energy-harvesting process, glycolysis. Only a small portion of the
energy available in chemical bonds is collected. Cellular respiration combines glycolysis with two extra processes: the Krebs
cycle and the electron transport chain. These three processes
together extract much more energy than glycolysis alone.
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Glycolysis/
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Notes:
• Fermentation and celluar respiration both start with the same
energy-harvesting process – glycolysis. During glycolysis, a
sugar molecule, glucose, breaks down into two molecules of
pyruvate (the ion form of pyruvic acid).
• All cells use glycolysis to break down food molecules. It is one of
the most ancient biochemical pathways - it must have evolved
very early in protocells leading to the three domains of life.
• Glycolysis doesn’t need a special region of the cell or organelle
– it takes place right in the cell’s cytosol. Again, this indicates a
primitive biochemical pathway that evolved in prokaryotes.
Narration
This module examines glycolysis, the first stage
in the breakdown of a sugar, a simple molecule,
into two molecules of pyruvic acid. Glycolysis
occurs in the cytosol, not in mitochondria where
the final steps of aerobic metabolism take place.
• After glycolysis has converted sugars (and other food molecules)
to pyruvate, two things can happen to the pyruvate.
1) In most anaerobic bacteria, and other cells lacking oxygen (such as our muscle cells during heavy exercise), the
pyruvate is converted to a simple 2- or 3- carbon compound
that is excreted out of the cell such as ethanol or lactic acid.
This is fermentation.
2) The pyruvate can enter mitochondria, and, in the presence of oxygen, be broken down further into H 2O and CO 2,
through two more processes – the Krebs cycle linked to the
electron transport chain . This is cellular respiration.
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Glycolysis Poster
Glycolysis/
Cellular Metabolism
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Notes:
• In glycolysis, a glucose molecule entering the cell is acted on by a
new enzyme in each of nine steps. Step one transfers a phosphate
group from ATP to the sugar, a process called phosphorylation.
The phosphorylated sugar has an electrical charge preventing it
from leaving the cell, due the impermeability of the cell membrane
to ions. In step three another molecule of ATP is used up, and
another phosphate group is added to the sugar. In step four the
molecule is split into two three-carbon sugars. These sugars are
isomers of each other, but only one, glyceraldehyde phosphate,
P-GAL, can be used in the steps that follow. Therefore, the second
sugar is converted to P-GAL by an enzyme called isomerase .
These first four steps are called the energy-investment phase.
Narration
The process (Glycolysis) occurs in a series of nine
steps mediated by enzymes, depicted here by
cylinders. In steps one through four, two ATPs are
used to split a glucose molecule into two molecules
of glyceraldehyde phosphate— P-GAL.
O
C
H
OH
H
C
C
H
H
O
P
Glyceraldehyde phosphate
Extensions:
• Step One: conversion of glucose to glucose-6-phosphate. This
phosphorylation is accomplished by the enzyme hexokinase.
• Step Two: glucose-6-phosphate is rearranged into its isomer, fructose-6-phosphate, by the enzyme phosphoglucoisomerase.
• Step Three: the enzyme phosphofructokinase adds a phosphate
group from a second ATP to create fructose 1,6-biphosphate.
• Step Four: cleavage of fructose 1,6-biphosphate into two isomer
sugars, dihydroxyacetone phosphate and glyceraldehyde phosphate
(P-GAL). The enzyme aldolase cleaves them, and the enzyme
isomerase changes all the sugars to the P-GAL form.
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• In step five, the sugar intermediate is oxidized. Electrons transfer to
NAD (in the presence of H+) forming NADH, one for each of the two
P-GAL’s. In step six glycolysis starts to produce some ATP, one for
each of the two P-GAL’s, so the net energy is now at zero.
• The final step produces 2 ATP. Therefore, for every single glucose
molecule entering gylcolysis, 2 ATP and 2 NADH result. The last
five steps of glycolysis are called the energy-yielding phase.
Extensions:
• Step Five: P-GAL molecules are oxidized by triose phosphate
dehydrogenase into 1,3-diphosphoglycerate. Two NADH (one from
each P-GAL) are formed.
Narration:
In the next steps (steps 5-9 out of 9), electrons
from P-GAL are transferred to molecules of
NAD+, converting them into molecules of NADH.
And two molecules of ATP are produced, paying
back the original investment of two ATPs. The
final reaction of glycolysis generates two more
ATPs and two molecules of pyruvic acid— an
important fuel molecule for mitochondria. So the
nine steps of glycolysis generate a net gain of two
ATPs and two NADHs— energy for cell use.
• Step Six: 1,3-diphosphoglycerate becomes 3-phosphoglycerate
through the action of the enzyme phosphoglycerokinase. Two ATP
(one from each P-GAL product) are harvested.
• Step Seven: 3-phosphoglycerate becomes 2-phosphoglycerate
through the action of phosphoglyceromutase.
• Step Eight: the enzyme enolase forms a double bond in the
substrate 2-phosphoglycerate, releasing water and phosphenolpyruvate.
• Step Nine: pyruvate kinase catalyses the production of pyruvate,
and creates two molecules of ATP at the same time.
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Glycolysis Poster
Fermentation
Cellular Metabolism
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Notes:
• Fermentation (like cellular respiration) uses glycolysis to produce pyruvate plus 2 ATP and 2 NADH in the cytosol of the cell.
However, in fermentation, no more ATP is produced because
there is no oxygen present. Without oxygen, the energy still stored
in the chemical bonds of pyruvate is not available to the cell.
• Fermentation includes glycolysis plus reactions that transfer
electrons from NADH to pyruvate or a molecule derived from
pyruvate. The two main types of fermentation are: alcohol
fermentation and lactic acid fermentation.
• Alcohol fermentation converts pyruvate to ethanol (ethyl alcohol)
in two steps. These release CO2 and regenerate NAD+ (from
NADH, as shown in the poster image and video clip). Alcohol
fermentation by yeasts is used in brewing and wine making.
Narration
In microbes that carry out fermentation exclusively all of their energy carriers are generated by
glycolysis. Fermentation is an ancient method of
anaerobic metabolism evolving long before
oxygen appeared in the atmosphere.
• Lactic acid fermentation converts pyruvate to lactate with no
release of CO2. Lactic acid fermentation by bacteria or fungi is
used to make yogurt and cheese. Humans experience lactic acid
fermentation in muscle cells when oxygen is in low supply. The
accumulation of lactic acid as a waste product in our muscles may
cause cramps.
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More Energy With Oxygen/'
Cellular Metabolism
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Notes:
• In the anaerobic environment of early Earth (a reducing atmosphere), oxygen gas was extremely reactive. When the first cyanobacteria began releasing oxygen, they created habitats toxic to the
leagues of archea and bacteria that had evolved without oxygen.
• Natural selection favored bacteria with mutations that could deal
with the oxygen, including those that could USE oxygen to harvest
energy from food molecules. The advantages were huge. As much
as 18 times more ATP could be produced by catabolic reactions
when oxygen is present .
• Oxygen is highly electronegative – it has a strong affinity for
electrons. This chemical power is harnessed by cellular respiration
and used to extract the energy remaining in pyruvate formed by
glycolysis. Pyruvate is broken down (oxidized) all the way to CO 2
and H2O.
Narration
When cyanobacteria began liberating oxygen
into the atmosphere, around two billion years
ago they drastically changed the evolutionary
course of all life to follow. Oxygen’s powerful
attraction for electrons made it possible to break
down the end product of glycolysis— pyruvic
acid— to carbon dioxide and water.
Extensions:
• Cyanobacteria flourished in the early environment because they
made food from sunlight, water and CO2. Today, cyanobacteria are
still abundant in: lakes, ponds and rivers; surface waters of the
ocean; scums that form on wet surfaces; in wet soil, as symbiotic
organisms in lichens, sponges, corals, etc.; and in other habitats.
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Notes:
• The number of ATP molecules produced by cellular respiration is
up to 38 total. This is a vast improvement over the 2 molecules
produced by fermentation . The chemistry behind this increase
results from the power of oxygen to attract electrons, also known
as the electromotive power, or electronegativity.
• Only 2 of the potential 32-38 ATPs produced during cellular respiration actually come from glycolysis.
Extensions:
• Glycolysis is the only metabolic pathway common to all living
organisms, suggesting that glycolysis evolved at the beginning of
life, some 3.5+ billion years ago – a time when oxygen was not
available on earth.
Narration
Aerobic respiration is an efficient way to
extract energy from fuel molecules. Without
oxygen, a molecule of glucose produces two
ATPs through glycolysis. The same molecule
produces up to 38 ATPs when metabolized using
oxygen. As aerobic respiration became established in protist-like microbes and oxygen
became more abundant, the pace of evolution
picked up, leading to multicellular organisms
and the evolutionary explosion of the major
animal phyla.
• Early cells on earth (3.5 b.y.a) were probably bathed in their food
– a rich organic soup:
- continuous secretion of organic acids lowered the pH . Proteins
that acted as proton pumps to keep the pH inside the cell neutral
were favored. ATP synthase may have its roots here.
- the supply of ATP was limited. Membrane bound proteins that
could transport H+ using electron transport between molecules of
different redox potential without spending ATP were favored
- some prokaryotes evolved electron transport chains that harnessed more energy than they needed to maintain internal pH.
Those cells that could manufacture their own ATP must have had a
tremendous competitive advantage.
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Mitochondrion Structure
Cellular Metabolism
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Notes:
• The mitochondrion is made up of two membranes, the inner and
the outer membrane. The outer membrane is smooth but the
inner membrane is convoluted with infoldings called cristae that
increase surface area.
• In the matrix, many different enzymes are concentrated, and this
is where the Krebs cycle takes place. Proteins, such as the
enzyme that makes ATP, ATP synthase, are built into the inner
membrane.
Narration
Most of a eukaryotic cell’s ATP synthesis occurs in
mitochondria. A mitochondrion is a sack within a sack.
The inner sack is folded, increasing surface area for ATP
synthesis. Between the inner and outer membranes is the
inter-membrane space— a reservoir for hydrogen ions
used for synthesizing ATP from ADP. The inner chamber, known as the matrix, is a soup of enzymes that
dismantle fuel molecules in the Krebs cycle. The knobs
are where ATP is synthesized. Simply put, a mitochondrion is an energy transformer where energy and fuel
molecules, such as pyruvic acid, are transferred to ATP.
Carbon dioxide, the carbon end product of cellular
respiration, diffuses out leaving the cell through the
plasma membrane.
Extensions:
• According to the endosymbiotic theory, mitochondria represent ingested cells that evolved over time to become endosymbionts which evolved further to become integrated into the
eukaryotic host. One of the many pieces of evidence supporting
this is that the mitochondrion has two membranes – an inner one
corresponding to the endosymbiont’s original membrane, and an
outer membrane corresponding to the host’s food vacuole membrane. Another element of evidence is that mitochondria have
their own DNA, totally separate from the cell DNA. This is likely
vestigial DNA left over from the endosymbiont. A mitochondrialike symbiont would have conferred a great advantage to a host
cell, providing a method to rid the cell of toxic O 2 and sharing the
high yield of energy it harvested through aerobic respiration.
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Krebs Cycle/
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Notes:
• The Krebs cycle, also called the citric acid cycle , is the second phase in cellular respiration . It occurs in the matrix of the
mitochondria, and requires oxygen to proceed. The Krebs cycle
oxidizes (and breaks down) the product of glycolysis: pyruvate .
• Pyruvate enters the matrix through transport proteins in the
mitochondrial membranes. Inside, it reacts in a three-step
process. 1) Pyruvate releases CO 2 and becomes acetate , 2)
NAD+ is reduced to NADH and 3) coenzyme A (CoA) combines
with the acetate, creating acetyl coenzyme A ( acetyl CoA).
Narration
In eukaryotic cells, pyruvic acid enters the mitochondrion where in the matrix, it reacts with
coenzyme A to form an important intermediate:
acetyl coenzyme A. Acetyl Co A feeds a two-carbon
group into a series of reactions called the Krebs
cycle in which the carbon backbones are broken
down producing more energy-carrier molecules.
• The acetyl CoA is now ready to feed acetate into the Krebs
cycle to be further oxidized , meaning that the Krebs cycle
intermediates loose electrons. These electrons are first donated
to energy-carrier molecules , NADH and FADH 2 which, in turn,
pass the electrons to the electron transport chain .
Extensions:
• The Krebs cycle shows a net transfer of carbon atoms of zero.
Two carbons enter in the form of acetate and two different
carbons leave in the oxidized form of CO 2, as a waste product.
• The Krebs cycle depends on the regeneration of a compound
called oxaloacetate , which, when combined with acetyl CoA,
begins the cycle all over again.
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Notes:
• There are eight steps in the Krebs cycle.
• In the first step, acetyl CoA adds it’s acetate (2-carbon) to
oxaloacetate (4-carbon) producing citrate (6-Carbon). The
coenzyme, CoA, is once again available to be primed with an
acetate group (from pyruvate) or to be used in step four.
• In step two, water is added to citrate to produce isocitrate (6carbon).
• In step three, CO 2 is released producing alpha-ketoglutarate
(5-carbon). At this point one NAD+ is reduced (gains electrons)
to NADH.
Narration
Acetyl co A feeds a two-carbon group into a
series of reactions called the Krebs cycle in which
the carbon backbones are broken down producing
more energy-carrier molecules. Beginning the
cycle, a two-carbon group from acetyl co A, joins
a four-carbon molecule creating a six-carbon
intermediate. This molecule then reacts to give
up carbon dioxide creating one NADH energy
carrier. The next reaction yields another molecule
of carbon dioxide and provides enough energy to
charge another NADH and produce one ATP.
• In step four, another CO2 is lost and another NAD+ is reduced
(gains electrons) to NADH. Also, CoA attaches and forms
succinyl CoA (the succinyl group is 4-carbon).
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• Steps 5-8 (out of eight) of the Krebs cycle.
• In step five, the CoA of succinyl CoA is replaced by a phosphate
group, which is transferred immediately to GDP to make guanosine triphosphate (GTP) and the compound succinate (4-carbon). If GTP donates it’s phosphate group to ADP, ATP will result.
• In step six, electrons from succinate oxidize FAD to form
FADH 2.This forms fumarate (4-carbon). Like NADH, the FADH2
energy-carrier molecule will donate it’s electrons to the electron
transport chain.
• In step seven, water is added (hydrolysis) – fumarate becomes
malate (4-carbon) .
Narration
The four-carbon intermediate has sufficient energetic electrons to charge two more energy-carrier
molecules reforming the original four-carbon
molecule that will react with an incoming acetyl
Co A to complete a full cycle.
• In step eight, the final step, malate is oxidized producing another
molecule of NADH and regenerating oxaloacetate (4-carbon).
The cycle is ready to begin again.
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Krebs Cycle ) Summary
Cellular Metabolism
(Energy carriers
produced for each
Acetyl)CoA)
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Notes:
• In sum, this is what the Krebs cycle produces from one sugar:
2 ATP
6 NADH
2 FADH2
Since 2 ATP is the same amount of ATP produced by glycolysis,
it is very clear that the energy-carrier molecules, NADH and
FADH2 are needed to charge up more ADP to make ATP, before
they donate their energized electrons to oxygen.
• This is exactly what happens in the third process of cellular
respiration involving the electron transport chain. In this process, the energy carrier molecules work with proteins embedded
in the mitochondrial inner-membrane to make more ATPs.
Narration
So each acetyl Co A entering the Krebs cycle
produces enough energetic electrons to charge
several energy carriers.
Extensions:
• When ATP is produced in the first two stages of cellular respiration, (glycolysis and the Kreb’s cycle) the reactions are linked
to active, phosphorylated intermediates, or ‘substrates’. This
type of reaction is called substrate level phosphorylation .
• In the final stage of cellular respiration, the elctron transport
chain, coupled with ATP synthase, produces 34 ATP molecules
in reactions where oxygen forms water.This process of ATP
synthesis is called oxidative phosphorylation.
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Electron Transport Chain
Cellular Metabolism
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Notes:
• The electron transport chain is a series of molecules embedded in the inner mitochondrial membrane. The cristae provide
space for thousands of copies of the electron transport chain in
each mitochondrion. The chain is made up of proteins with
some non-protein prosthetic groups attached that are necessary for the catalytic functions of the enzymes. During electron
transfer, these prosthetic groups alter between a reduced and
an oxidized state as they accept and donate electrons.
• The function of the electron transport chain is to gradually
release the energy produced by glycolysis and the Krebs cycle
in manageable amounts. The overall energy drop for electrons
travelling from NADH to oxygen is 53 kcal/mol. However this
system makes it possible to successfully utilize the energy for
ATP synthesis.
Narration
The Krebs cycle produces the energy carriers that
transfer energy to proteins embedded in the inner
membrane of a mitochondrion. These electron
transport proteins pull hydrogen ions out of the
matrix, literally jamming them into the intermembrane space setting up conditions for ATP
synthesis.
Extensions:
• Electrons transferred from food during glycolysis and the Krebs
cycle are transferred by NADH to the first molecule of the electron transport chain, flavoprotein. In the next redox reaction,
the electron passes to an iron-sulfur protein. Next, the electron
passes to ubiquinone. This electron carrier is a lipid. All the
carrier molecules below this step are called cytochromes and
they contain a heme group. The last cytochrome of the group,
cyt a3 passes the electron to molecular oxygen (O 2). The oxygen also picks up a pair of hydrogen (H +) ions from the aqueous
solution to form water.
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Oxidative Phosphorylation
Cellular Metabolism
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Notes:
ATP synthase uses an ion gradient to drive oxidative phosphorylation. In this case the ion gradient is a proton or hydrogen ion
(H+) gradient. The energy for ATP synthesis comes from the difference in concentration of H+ on opposite sides of the inner mitochondrial membrane. This can also be considered a difference in pH
since pH is a measure of H+ concentration.
• A flow of electrons through the molecules of the electron transport
chain is used to pump H+ (protons) across the inner membrane from
the matrix to the intermembrane space. The H+ will diffuse back
across the membrane, but only through the ATP synthase proteins.
Narration
Electrochemical activities within a mitochondrion
synthesize most of a cell’s ATP. During the
process electron transport proteins pump hydrogen ions into the intermembrane space, creating
an imbalance in concentration and in electrical
charge. The imbalance initiates a backflow of
hydrogen ions through ATP synthase— the knobs
on the mitochondrion’s inner membrane. The
enzyme uses energy from the backflow of hydrogen ions to synthesize most of a cell’s ATP.
• The flow of H+ through the ATP synthase is used to drive the phosphorylation of ADP. Therefore, ATP production depends upon the
redox reactions that occur on the electron transport chain, which
creates the H+ gradient. This coupling is called chemiosmosis.
Chemiosmosis differs from osmosis because protons are being
pushed across the membrane, while water molecules freely diffuse
in osmosis
.
Extensions:
• The mechanism for creating a H+ gradient from the electron transport chain has been elucidated by scientists. In essence, only some
of the carrier molecules in the chain can accept or release H + along
with electrons. Therefore at certain stages of the chain, electron
transfers cause H+ to be collected and deposited into the
intermembrane space. The gradient that results is called a protonmotive force , and has the ability to do work. The mechanism for
ATP synthase to harness this down hill current of H+ has not yet
been discovered.
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Summary of Cellular Respiration
Cellular Metabolism
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Summary:
• Cellular Respiration involves three processes: 1) glycolysis,
2) Krebs cycle (citric acid cycle), and 3) electron transport.
• In the cytosol, glycolysis breaks down (oxidizes) sugars to pyruvate in nine steps. In this process 2 ATP molecules and 2 NADH
energy carrier molecules are created for each sugar.
• Pyruvate travels into the mitochondrion, where it will enter a cycle
of reactions called the Krebs cycle. In the Krebs cycle, carbons
are conserved – a 2-carbon reactant (acetyl-) enters the cycle, and
two oxidized carbons (2 CO 2) leave the cycle. The eight steps of
the Krebs cycle produce 2 ATP, 6 NADH, and 2 FADH 2 for every
sugar broken down.
•
•
•
•
•
Summary of Cellular Respiration
Glycolysis creates pyruvic acid which enters
the mitochondrion.
The Krebs cycle produces energy carrier
molecules that drive an electron transport
chain (ETC).
The ETC pumps protons into the intermembrane space.
Energy from the back-flow of protons through
ATP synthase converts ADP to ATP.
ATP exits the mitochondrion to the cytosol
where it energizes the chemical reactions of
the cell.
• The NADH and FADH 2 energy-carrier molecules travel to the inner
membrane of the mitochondrion where they donate electrons to a
series of embedded energy-carrier molecules called the electron
transport chain.The electrons are ultimately donated to oxygen in
the presence of hydrogen ions, and water is formed.
• Using energy from activated electrons transferred from the Krebs
cycle by NADH and FADH 2 , the ETC proteins (and one lipid) pump
hydrogen ions into the intermembrane space. A special transport
protein, ATP synthase, harvests energy as the proteins return to the
matrix, to create new ATP molecules from ADP and phosphate.
About 30-34 ATP are formed from one original sugar in this
oxidative phosphorylation.
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