Download CELLULAR RESPIRATION Teacher`s Guide

CELLULAR
RESPIRATION
Teacher's Guide
Cellular Respiration
Teacher's Guide
This teacher's guide is designed for use with the
CellularRespiration series of programs produced
by TVOntario, the television service of the Ontario Educational Communications Authority. The
series is available on videotape to educational
institutions and nonprofit organizations.
The Series
Producer/Director .
Project Officers.
Writers:
Consultant.
David Chamberlain
John Amadio, David Way
Susan Perry, David Way
Robert Whitney
The Guide
Writers:
Graphic Designer:
Randee Crisp,
George Laundry,
Robert Whitney
Roswita Busskamp
Copyright 1990 by The Ontario Educational
Communications Authority.
All rijahts reserved.
Printed in Canada.
3626/90
Introduction
1
1.
The Cell and Energy
3
2.
Glycolysis 1
6
3.
Glycolysis 2
9
4.
The Krebs Cycle
12
5.
Oxidative Phosphorylation
14
6.
Metabolism and Nutrition
17
Glossary
19
Bibliography
21
This series of six 10-minute programs illustrates
the complex world of biological respiration, at
both macro and molecular levels. Beginning with
a historical perspective and progressing to modern research and theories, the programs examine
enzymes and coenzymes, phosphorylation, biosynthesis, glycolysis, and the Krebs cycle.
Together, the Cellular Respiration video series
and teacher's guide:
•
describe the evolution of cellular respiration
that presaged the development of presentday life forms;
•
investigate the structure and function of the
mitochondrion organelle as the prime locus
for the biosynthesis of adenosine triphosphate
(ATP);
1
•
discuss glucose as the principal fuel of cellular
respiration and the involvement of ATP as the.
energy shuttle;
•
develop, in step-by-step fashion, the metabolism of glucose through the processes of
glycolysis, the Krebs or citric acid cycle, and
oxidative phosphorylation;
•
elucidate the role of oxygen in the controlled
combustion of glucose with the concomitant
production of the respiratory waste product
carbon dioxide; and
•
explain the relationships of the three food
groups-proteins, fats, and carbohydratesin nutrition.
After viewing this program and completing the
suggested activities, students should be able to:
•
name three major classes of molecules that
living things use to store energy, and designate
carbohydrates as those most frequently employed;
•
explain the meanings of the following terms:
cytosol, mitochondrion, matrix, cristae, adenosine triphosphate (ATP), high-energy bond,
phosphoryl group, adenosine diphosphate
(ADP), phosphorylation;
•
describe the appearance of a mitochondrion as
seen through the transmission electron microscope;
Regardless of its source, energy for living things
must be readily available at all times. Since inputs
are irregular and unreliable, constant availability
necessitates some form of energy storage. A brief
overview of the mechanism of energy storage and
release is the subject of this introductory program.
The digestive system extracts from an animal's food
the three major groups of macromolecules: proteins, fats, and carbohydrates.The most immediately available energy has been stored in carbohydrates. This series assumes that most energy is
provided to the cell in the form of glucose molecules. The release of chemical energy, to a form
useful to living things, is called cellular respiration.
•
account for the theory that both mitochondria
and chloroplasts evolved from independent
organisms;
•
describe the structure of an ATP molecule and
locate, within this structure, high-energy bonds;
•
explain the role of ATP in cell metabolism;
•
name three interconnected phases of cellular
respiration.
Crista
3
Cellular respiration is a complex series of chemical
reactions that occur in both the cytosol and the
mitochondria of a cell. A mitochondrion consists of
a pair of membranes surrounding an amorphous
interior, the matrix. The innermost membrane
forms many inward-facing folds, the cristae, which
greatly increase the amount of membrane that can
be packed within the mitochondrion. The similarity of a mitochondrion to a tiny cell suggests that the
mitochondria, like the chloroplasts, may have
evolved from independent beings that invaded
larger cells as parasites. Over millions of years, they
became tolerated by, then vital to, their hosts. As a
consequence, there are many similarities between
cellular respiration and photosynthesis. In fact, in
many ways, cellular respiration can be considered
the reverse of photosynthesis.
The reactions of cellular respiration, which provide
the ATP needed to drive life processes, are subdivided into three phases: glycolysis, the Krebs cycle,
and oxidative pbospborylation. All three phases
will be covered, in turn, by this series.
Cellular respiration transfers most of the glucose
molecules' energy into smaller "packages" of potential energy in molecules of adenosine
triphosphate (ATP). ATP molecules contain enough
energy to drive typical metabolic reactions.
BEFORE VIEWING
Some students may have little exposure to chemistry. A short lesson on (or review of) the concepts
of element, compound, atom, molecule, and covalent bond should precede the program. Emphasize
that a detailed knowledge of the structures of
respiratory intermediates is not necessary. Instead,
the student should appreciate that molecules have
unique and predictable shapes, and that cells
possess specialized agents (enzymes) that are able
to select one type of molecule from among the
multitude of other molecules present in the cell. A
quick review of a typical food chain and the place
of autotrophs and heterotrophs within it could also
be useful.
ATP is a complicated molecule consisting of portions of a number of simpler, and more familiar,
molecules linked by covalent bonds. The simple
"building blocks" are a nitrogen-containing base
(adenine), a five-carbon sugar (ribose), and three
molecules of phosphoric acid. The energy resides
at one of two higb-energy bonds between the
remanants of phosphoric acid molecules
(phosphoryl groups). When an ATP molecule
provides energy to a reactant, it transfers one of its
"high-energy bonds" to the reactant. Of course,
some atoms of the ATP are also transferred.
Typically, the end phosphoryl group is transferred
to the reactant, and adenosine dipbospbate (ADP)
is left over. The reactant is now said to be
"phosphorylated" and the process of transferring a
phosphoryl group to the reactant is called
phospborylation. Phosphorylation reactions are
often employed in metabolism as a step in an
energy-consuming reaction.
AFTER VIEWING
Activity l:
How Carbohydrates Got Their
Name
Apparatus
sugar cubes
concentrated sulphuric acid
(Caution: highly corrosive)
crucible
mortar and pestle
protective cover for desktop
safety goggles
laboratory coat or apron
Note: This activity maybe
performed as a demonstration.
Method
1.
Grind a sugar cube to a powder
using a mortar and pestle.
2. Transfer the powdered sugar to a
crucible which has been placed
on a protective cover to prevent
damage to the desktop.
FIGURE 1.1 Structure of ATP
4
3, Be sure you are wearing safety goggles. Add
just enough concentrated sulphuric acid to the
crucible to cover the sugar.
4. Note the color, odor, and appearance of the
material left in the crucible. What do you think
it is?
hydroxyl groups (-OH) are in the correct
positions above or below the ring. Use as few
shifts of atoms and/or bonds as possible. In
your notes, record the steps you followed in
this conversion. Also record the number of
times you had to rotate a part of the molecule
without shifting bonds or atoms. Compare
your results with those of other students in the
class. Have your model evaluated by your
instructor before proceeding. Be sure to make
any alterations suggested by the instructor
before continuing.
Discussion
Concentrated sulphuric acid is a powerful dehydrating agent which will withdraw water from
other compounds, Assume that this will happen in
this experiment. In terms of elements, what appears to be the composition of the sugar, based on
the color of the resultant residue? Why, then, are
this and other sugars referred to as "carbohydrates"?
5. Evaluate the flexibility of the model. Is the
positioning of a hydroxyl group (-OH) on the
top or bottom of the formula significant?
Comment in your notes.
Activity 2:
Visualizing Molecules
Discussion
1. The formula of glucose is given in textbooks as
C6H1z06. To which of the structures in Figure
1.2 does it apply? Research the meaning of
isomer and isomerization and explain how
these terms relate to this activity.
Apparatus
molecular model kit
Method
2. Cellobiose is a disaccharide formed during the
digestion of cellulose, and maltose is a disaccharide formed during the digestion of starch.
Research the structures of these two sugars and
relate them to this activity. Can enzymes distinguish between these two disaccharides?
1. Examine the contents of the molecular model
kit. Note that there are wood spheres of various colors. These represent atoms of the
elements. You will be using only carbon (black),
hydrogen (white), and oxygen (red) in this
exercise.
2. Construct a model of a glucose molecule. Use
the structural formula on the left of Figure 1.2
for guidance. When the model has been
completed to your satisfaction, take it to your
instructor for evaluation. Make any alterations
suggested by your instructor before you continue.
3. Evaluate the flexibility of the model. Is the
positioning of a hydroxyl group (-OH) on the
right or left of the formula significant? Comment in your notes.
4. Now, attempt to convert your model into the
ring form depicted on the right of Figure 1.2,
Consider the ring to be perpendicular to the
page and be sure that the hydrogens (-H) and
FIGURE 1.2 Two Glucose Formulae (Ring Structure)
5
In the first half of glycolysis, the 6-carbon sugar
glucose, is broken into two 3-carbon molecules of
phosphoglyceraldehyde (PGAL). This requires
the addition to the original glucose molecule of
chemical potential energy supplied at ATP.
After viewing this program and completing the
suggested activities, students should be able to:
Glucose arises principally from the hydrolysis of
glycogen, a polysaccharide stored in the liver and
muscles. From the liver, glucose may be carried
by the circulatory system to target cells which it
enters easily by membrane. Upon arrival in the
cytosol, the glucose is phosphorylated by ATP in
an enzyme-catalyzed reaction. Its potential energy is thereby increased; it also acquires a
negative charge which prevents its escape from
the cell. The glucose phosphate is isomerized by
an enzyme to fructose phosphate which then
acquires a second phosphate group reaction with
a second ATP.The fructose diphosphate is then
split into two parts: dihydroxyacetone phosphate
(DHAP) and phosphoglyceraldehyde (PGAL). The
DHAP quickly undergoes isomerization to a second PGAL. Thus, a single 6-carbon glucose molecule has generated two 3-carbon PGAL molecules. Two ATP molecules have been sacrificed,
but the two PGAL molecules have a higher
potential energy than the original glucose molecule.
identify the major compound in which animals store energy;
outline the steps by which the first half of
glycolysis occurs;
describe the change in chemical potential energy accompanying the preparatory steps of
glycolysis;
discuss the "coupling" of energy-consuming
and energy-releasing reactions;
identify the cytosol as the site of glycolysis.
The energy that enters a cell in an energy-rich fuel
such as glucose must be used to synthesize ATP
molecules in order to be used effectively. The
process begins with a series of reactions known
collectively as glycolysis These reactions must
have evolved a very long time ago since they
exist, in identical form, in all living things.
The series devotes two programs to the description of glycolysis. This first program on the subject
shows that energy (ATP) must first be sacrificed in
order to prepare the way for its later extraction. It
also indicates the importance of thermodynamic
principles in explaining the progress of the reactions.
Figure 2, 1 Endergonic. This program traces the first 5
steps of glycolysis. In these steps, energy must be added
to the system (endergonic)
6
The reaction of ATP with fructose phosphate
illustrates the important concept of reaction coupling. The conversion of glucose phosphate to
fructose phosphate has a small positive free
energy change. Fructose phosphate tends to
spontaneously revert to glucose phosphate.
However, any fructose phosphate that forms is
phosphorylated to fructose diphosphate in a reaction with a large negative free energy change.
Thus, the net reaction changing glucose phosphate to fructose diphosphate (in two steps) has
a negative free energy change and proceeds
spontaneously. Many reactions in biochemistry
are driven against thermodynamic tendencies by
being coupled to a simultaneous reaction having
a large negative free energy change (often, the
hydrolysis of ATP).
Note: Numbers appearing in the following instructions are not given in the program. They indicate
the number of the carbon atom(s) bearing phosphate groups or to be included in a product. The
numbers are those appearing in Figure 1.1 (Program 1) in glucose or derived from these in a
chemical reaction.
Method
Use the model of glucose (non-ring form) constructed in Program 1, Activity 2, or construct
another following the instructions in that activity.
Using a single hole in an orange-colored atomic
model to represent phosphate, trace, with models, the conversion of glucose to glucose-6-phosphate (you must discard a hydrogen atom to make
room for the phosphate), glucose-6-phosphate to
fructose-6-phosphate (move the doubly bonded
oxygen from carbon atom #1 to carbon atom #2
but do not discard any atoms), and fructose-6phosphate to fructose- 1,6-diphosphate.
BEFORE VIEWING
Students should have some understanding of
thermodynamic principles. These can be developed to different levels, according to the needs
and expectations of the class. This could involve
a discussion of the First Law of Thermodynamics.
Discussion
1. The change of glucose-6-phosphate to fructose-6-phosphate is described as an isomerization. By what feature is a reaction identified
as an isomerization?
A qualitative understanding is most easily derived
from a consideration of the relative probabilities
of different distributions of energy and matter in
a chemical system (developed from a knowledge
of the probabilities of different numbers arising
when dice are rolled). This could be expanded
into a discussion of the Second Law of Thermodynamics. With some classes, you might continue to
discuss the chemical implications of the Third Law
of Thermodynamics (often called the Nernst Heat
Theorem).
2. You removed a hydrogen atom to make room
for a phosphate during phosphorylation of a
sugar. Hydrogen atoms cannot float about
freely in solution. Research and report on the
actual fate of the hydrogen you had to remove.
3. Splitting fructose-1,6-phosphate into dihydroxyacetone phosphate and phosphopglyceraldehyde requires the discarding of a car
bon-carbon bond (between carbon atoms 3
and 4 of the fructose-1,6-phosphate). No
atomic nuclei are lost or gained, but one or
more could change position. Using this information, predict the structural formula of dihydroxyacetone phosphate (from carbon atoms
1, 2, and 3 of fructose-1,6-phosphate) and
phosphoglyceraldehyde (from carbon atoms
4, 5, and 6 of fructose-1,6-phosphate).
AFTER VIEWING
ACtivity 1:
Modelling the Reactions of Glycolysis
Apparatus
molecular model kit
4. The enzyme converting dihydroxyacetone
phosphate into phosphoglyceraldehyde is
called "triose phosphate isomerase." Discuss
the appropriateness of this name.
7
3. is it possible to find a total measurement of the
heat given off.? (Hint: consider the heat energy
absorbed by the water.)
4. Is there a link between the molecular structure of a food substance and the energy
released? Discuss.
Activity 2:
Measuring the Relative Quantities
of Energy Released from Different
Foods
Apparatus
assorted foods (nuts, sucrose cube, marshmallow,
etc.)
calorimeters
needles
corks
water (at room temperature)
matches
petri dishes
balances
clay triangles
thermometers
Thermometer
Test tube
Method
1. Put 10 mL of water at room temperature into
a test tube. Fit the test tube into the top of the
calorimeter, as shown in the diagram.
2. Weigh and record the mass of the food.
3. Record the temperature of the water in the test
tube.
4. Drive a pin through the centre of a cork and
attach the food material to it at the top, as
shown in the diagram.
5. Light the food material with a match, and fit
the calorimeter over it so that the bottom of
the test tube is directly over the flame.
6. When the flame has gone out (after about 2
min of heating), record the final temperature
of the water.
7. Reweigh the food material and record its
mass.
8. Repeat this procedure for each of the other
food substances.
9. Calculate the temperature change caused by
each food substance per unit mass of that
substance burned.
Water
Soup can
Nut
Petri dish
Discussion
1. Compare each of the food materials used with
respect to their energy per unit mass. Which
food types contain the most energy per unit
mass?
2. Describe the possible sources of error in this
experiment.
8
respiration: the entire sequence of ten reactions
transfers only about two percent of the chemical
potential energy of a glucose molecule to the
production of ATP. The program shows how
simple organisms like yeast fulfill their energy
requirements from what little useful energy glycolysis produces by linking it to fermentation.
Fermentation converts pyruvate to acetaldehyde
then to ethanol and in the process regenerates the
NAD molecule. The NAD then cycles back into
glycolysis and maintains the production of ATP.
The discussion of fermentation provides an important example of biofeedback mechanisms.
After viewing this program and completing the
suggested activities, students should be able to:
•
trace the steps in the conversion of
phosphoglyceraldehyde to pyruvic acid;
•
account for the net gain of ATP during glycolysis;
•
explain the difficulties encountered if a cell
reduces its entire complement of NAD;
•
describe the importance of anaerobic fermentation in ensuring the continued production
of ATP as long as it is required and glucose is
available;
•
account for differences in identity and quantities of products of cellular respiration under
aerobic and anaerobic conditions.
While glycolysis is able to meet the demands of
simple organisms, more complex organisms need
additional reactions to harness the energy contained in the pyruvate and NADH molecules. The
program concludes by introducing the next stage
of cellular respiration-the Krebs Cycle-where
pyruvate is used to make additional ATP molecules.
BEFORE VIEWING
Review the concepts of oxidation and reduction:
students should be able to identify these processes by monitoring the transfers of electrons
and/or hydrogen atoms during organic chemical
reactions.
This third program of the series completes the
discussion of glycolysis by tracing the sequence of
reactions from PGAL to the final product, pyruvate.
The program illustrates the use of PGAL's potential energy to synthesize ATP molecules and to
reduce nicotinamide andenine dinucleotide (NAD)
to form the intermediate energy carrier molecule
NADH.
It is useful to establish a series of 1-carbon
molecules arranged in order of decreased reduction status or increased oxidation state based
upon the hydrogen to oxygen ratio. The series
should include methane, methanol, formaldehyde, formic acid, and carbon dioxide molecules.
The relative positions of fats, carbohydrates, and
a few amino acids in the reduction scale should be
investigated.
The net energy production of glycolysis demonstrates the inefficiency of this phase of cellular
9
Since texts vary in their naming of intermediates
of cellular respiration, students should be made
aware that acids may be named as if they were not
ionized (e.g., pyruvic acid) or as their anions (e.g.,
pyruvate). The latter represents the form found at
physiological pH, but the former makes it easier
to follow the fate of hydrogen atoms and the
formation of water during biochemical reactions.
Glass U-tube
One-hole stopper
Two-hole stopper
AFTER VIEWING
Activity 1:
To Detect the Waste Products of
Fermentation
(Anaerobic Respiration)
Apparatus
yeast packets (enough for number of pairs of
students in class)
sucrose solution
bromthymol blue solution
Benedict's solution
Erylenmeyer flasks (125 mL)
one-hole and two-hole rubber stoppers to fit
flasks
graduated cylinders (50 mL)
glass U-tubes
test tubes
stirring rods
marking pens
test tube holders
test tube racks
Bunsen burners
rings
ring stands
wire gauze
beakers
beaker tongs
flints
safety goggles
Method
1. Pour 15 mL of warm water into a 125 mL
Erylenmeyer flask.
2. Add a packet of yeast to the water and mix
with a stirring rod. Add 50 mL of sucrose
solution to the yeast mixture and mix well
with a stirring rod.
3. Put on safety goggles.
FIGURE 3.1 Apparatus to Detect the waste products of
fermentation
4.
Set up a hot water bath using wire gauze and
a ring on a ring stand above a Bunsen burner.
Heat 200 mL of water in a beaker until it comes
to a slow boil.
5. Add 5 drops of Benedict's solution and 5 mL
of sucrose solution to a test tube, and label this
test tube A.
6. Add 5 drops of Benedict's solution and 5 mL
of yeast and sucrose solution to a test tube,
and label this test tube B.
7. Heat test tubes A and B for 5 minutes in the
water bath, and record any changes.
8. Put 50 mL of bromthymol solution into a
second Erylenmeyer flask.
9. Insert one end of a glass U tube into a onehole rubber stopper and into a two-hole
rubber stopper at the other end. Insert the
one-hole stopper into the flask with the yeast
mixture, and the two-hole stopper into the
flask containing the bromthymol blue solution (see Figure 3.1). Note: The longer end of
the U-tube should be below the level of
bromthymol blue solution. If necessary, use
glycerin as a lubricant. If the end of the tube
is not below the level of the solution, call your
teacher for assistance. DO NOT adjust the
tubing yourself.
10. Leave the apparatus setup in a warm place
overnight.
10
11. After 24 hours, replace the flask containing
50 mL of bromthymol blue solution with
another flask containing 50 mL of fresh
bromthymol blue solution. Record any observations.
12. Record any observations after 48 hours.
Discussion
1. Discuss the reasons for the change or lack of
change in:
• test tubes A and B after heating
• the bromthymol blue after 24 hours
• the bromthymol blue after 48 hours
2. Discuss the difference in products formed
between anaerobic respiration which takes
place in yeast cells (fermentation) and anaerobic respiration which takes place in animal muscle cells.
Chemical reaction
FIGURE 3. 2 Exergonic. In the second series of events in
glycolysis, excess energy is released (exergonic)
Activity 2:
Early Anaerobic Biochemistry
Organisms that emerged during the first billion
years of the development of life on earth used no
atmospheric oxygen to fuel their activities. They
could fuel their metabolism only by ATP generated by glycolysis, which is thought to have been
one of the earliest of all biochemical processes to
have evolved.
1.
2.
Discuss the kind of organisms that would
have probably been alive at that time. Explain
and give the range of variability of possible
life forms.
Identify and discuss the kinds of organisms
that still survive today, using glycolytic reactions alone to produce the ATP needed to
carry on their metabolic activities.
1 1
After viewing this program and completing the
suggested activities, students should be able to:
•
appreciate that prehistoric life on land must
have been preceded by the emergence of a
glycolytic cycle and the accumulation of
atmospheric oxygen;
•
recognize that glycolysis does not result in
sufficient energy for energetic life forms;
•
trace and understand the sequences in the
Krebs cycle (citric acid cycle);
•
identify the end products of the cycle;
•
state the energy products or carriers resulting
from this cycle;
•
explain the fate of the glucose's carbon atoms.
Survival depends upon the availability of large
reserves of energy. Glycolysis, however, is an
inefficient source of energy and cannot supply
these large reserves; therefore other phases of
energy production are required. This program
examines the second phase of cellular respiration,
the Krebs cycle. The program follows the fate of
pyruvate from glycolysis as it is acted on as a
substrate by enzymes within the mitochondria to
generate ATP and intermediate energy carriers.
Pyruvate is a 3-carbon molecule. Through oxidative decarboxylation in the cytosol it is transformed into the 2-carbon molecule acetyl-CoA
which enters the mitochondrion. Once inside the
mitochondrial matrix, acetyl-CoA transfers its
energy into the Krebs cycle. The program follows
the ten reactions of the Krebs cycle, focusing on
the production of energy carriers.
A review of the Krebs Cycle shows that the energy
input from each acetyl-CoA creates one ATP
molecule, one FADH Z , and three NADH molecules. Since each glucose molecule from glycolysis results in two molecules of acetyl-CoA, the
cycle is considered to turn twice.
A summation of glycolysis, oxidative decarboxylation, and the Krebs cycle together gives the
total energy products from one glucose molecule
as: four ATP molecules, ten NADH molecules, and
two FADHZ molecules. The carbon atoms of the
glucose molecule have been expelled as six
molecules of waste carbon dioxide. Most of the
energy of glucose has been transferred to the
intermediate energy carriers NADH and FADH Z .
The program concludes by setting up the final
stage of cellular respiration, oxidative phosphorylation, where the intermediate energy carriers are
used to synthesize numerous ATP molecules.
The steps in the cycle-can be summarized as
follows:
1. Acetyl-CoA reacts with oxoaloacetate to form
citric acid.
2. Citric acid loses a molecule of water to become aconitate.
3. Aconitate adds water and is isomerized to
become isocitrate.
4. Isocitrate encounters NAD+, forming oxalosuccinate and NADH.
5. Oxalosuccinate loses a molecule of CO 2 to
become ketoglutarate.
12
6.
Ketoglutarate reacts with CoA to form succinyl-CoA and a NADH molecule.
7. Succinyl-CoA joins with ADP and a phosphate
to release CoA, an ATP molecule, and succinate.
8. Succinate joins with an FAD molecule to form
an FADH 2 molecule and fumarate.
9. Fumarate adds water to become malate.
10. Malate reacts with NAD+ to become oxaloacetate and form a NADH molecule.
A summary of total energy products can be given
as follows:
BEFORE VIEWING
Help the students to consolidate the previous
material by stressing the following points.
1.
Glycolysis is a very inefficient process: it
yields only about 2% of the available energy
of glucose. Glycolysis alone, therefore, could
not provide the energy needed to power
energetic organisms.
2.
Pyruvate formation was the end process of the
glycolytic pathway; this pyruvate will be the
starting point of the Krebs cycle.
AFTER VIEWING
1.
In total, this process utilizes approximately 40% of
the available energy, whereas glycolysis utilizes
only about 2%.
Divide the class into two main groups. One
group, which can be subdivided into several
research sections, is to write out the structural
formulae for all of the Krebs cycle intermediary compounds, including high-energy transfer compounds (NADH and FADH) and other
products such as CO 2.
The other group is to build the intermediaries
from atomic model kits, using the standard color
codes to represent different kinds of atoms.
Both groups should thoroughly brief their members with an eye to presenting a detailed account
of their results to the class.
2.
NADH
Figure 4. 1 Energy release from the Krebs Cycle (The
cycle can be considered to turn twice)
13
Discuss the following points.
a. It is evident that glycolysis does not produce enough ATP energy for higher life
forms to carry out their activities,
b. Why would glycolysis and the Krebs cycle
functioning together in tandem still not
provide enough energy to fuel complex
organisms?
c. What is the significance of the word cycle
in the term Krebs cycle? What substance is
regenerated at the end of the cycle and is
used at the beginning of the next one?
Why is this cycle gone through twice for
the complete respiration of each glucose
molecule?
After viewing this program and completing the
suggested activities, students should be able to:
•
trace how phase 1 and 2 of cellular respiration
lead into oxidative phosphorylation;
•
describe the process of the electron transport
chains;
•
understand the role of oxygen in siphoning
electrons from the electron transport chains;
•
explain how the energy gradient across the
intermitochondrial membrane is created, and
why this gradient is important;
•
follow the steps in ATP synthesis at the matrix
side of the membrane;
•
sum up the total production of ATP, NADH,
and FADH2 from a single glucose molecule;
•
state how many ATP molecules are produced
at any step.
Cellular respiration in its first phase, glycolysis,
produces only two molecules of ATP. Phase 2, the
Krebs cycle, produces only two more ATP.
However, phase 3, oxidative phosphorylation,
produces an energy payload.
This process takes place within the inner mitochondrial membrane. Embedded within this
membrane are four adjacent protein complexes
that make up the electron transport chain.Three
of these complexes act as proton (H+) pumps.
Their function is to remove energy from the
electrons as they move in pairs down an energy
gradient.
The process begins as NADH donates two electrons to the first complex. Two hydrogen ions
hitch a ride into the intermembrane space and the
two electrons transfer to the second complex and
return to the matrix side of the membrane. Two
more hydrogen ions are moved into the third
complex and are carried to the intermembrane
space. Two electrons return down the fourth
complex and two more hydrogen ions move into
the intermembrane space. (Six hydrogen ions
have now crossed.) Finally, an oxygen atom
picks up two electrons and two hydrogen ions
and forms water. (It is the primary role of the
oxygen to siphon the electrons from the electron
transfer chains.)
The other energy carrier produced by the Krebs
cycle, FADH 2, enters the chain and results in four
more hydrogen ions being transferred to the
intermembrane space. The concentration of H+ is
much higher in the intermembrane space than on
the matrix side. This concentration results in a
potential energy gradient, and this energy will be
used to synthesize ATP. Pairs of protons (H+) are
moved down special channels; these protons
activate an enzyme on the matrix side. This
enzyme catalyzes the reaction of ADP with a
phosphate group to synthesize ATP.
14
In summary, glycolysis results in two ATP molecules plus four more at the electron transport
chain, for a total of six ATP molecules. Oxidative
decarboxylation and the Krebs cycle produce two
ATP, eight NADH, and two FADHZ molecules. The
eight NADH energy carriers produce 24 ATP
molecules, and the two FADH Z produce another
four ATP molecules. The net result is 36 molecules
of ATP. Therefore, cellular respiration results in 36
ATP molecules from one glucose molecule; this
represents about 41% of the available energy from
the glucose molecule.
BEFORE VIEWING
1.
2.
AFTER VIEWING
Activity 1:
The Energy of Carbohydrates
The catabolic metabolism of glucose could be
expressed as follows:
Note that 36 molecules of ATP are ultimately
produced from 1 molecule of glucose.
Students should review the structure of the
mitochondria., and consider such terms as
cytosol, intermembrane space, cristae, matrix,
and electron transport chain. They should
review, as well, these processes: diffusion,
osmosis, and active transport.
1.
2.
Review the Krebs cycle in terms of where the
intermediate energy carriers NADH and FADH Z
are given off.
How many ATP molecules can be produced
from 1 mole of glucose? (Recall that 1 mole
contains approximately 6 x 10 23 molecules.)
Each mole of ATP represents a capture of 31
kJ. Calculate the total energy available for the
36 ATP molecules.
Cytoplasm
Mitochondrion
Figure 5.1 An overview of oxidative respiration
1 5
3. One mole of glucose represents about 2831 kj
(this value might differ slightly in different
textbooks). From your answer to question 2
above, calculate the overall efficiency.
4. Given that glucose has a formula of C6 H12 06,
calculate its molecular mass.
5. Suppose a candy bar contained 90 grams of
100% glucose. Theoretically, how much energy could it release in kilojoules? Theoreti-cally, how many molecules of ATP could be
produced?
Activity 2:
Mitochondria Morphology
1. Consult a suitable text containing large electron micrographs of mitochondria. Study
photographs from muscle tissue and from at
least two other types of tissue (e.g., liver,
pancreas, kidney, digestive tract, etc.) and
obtain clear photocopies of them.
2, Discuss the differences and similarities between mitochondria from the different tissues, and relate this to their tissue function.
3. Identify the outer and inner membranes,
cristae, and matrix of a mitochondrion.
4. Where are the respiratory proteins located?
What is the ultimate fate of the electrons at the
end of the electron transport chain? What
drives the protons across the inner membrane
and what is their ultimate fate? Discuss.
5. During fermentation (anaerobic respiration)
what is the fate of the electron generated
during the glycolysis of glucose?
16
After viewing this program and completing the
suggested activities, students should be able to:
•
recognize that much of our knowledge about
cells comes from the development of models;
•
appreciate the immense turnover of ATP in
the human body in a normal day;
•
understand the basic operation of a muscle;
•
describe how cells respond to an oxygen
shortage caused by overexertion;
•
describe how an oversupply of ATP may be
stored eventually as "fat";
•
appreciate that the complexity and collective
behavior of cells is a reaffirmation of life itself.
Scientists frequently develop models to explain
the complexity of cellular respiration. These
models, though, are often schematic diagrams
and do not come close to revealing the magnificence of the collective power of cells. Our bodies
use and recycle about 40 kg of ATP each day, and
strenuous activity may cause them to use as much
as 0.5 kg per minute. For all body movements, it
is ATP which provides the driving energy. This
program examines the ability of cellular respiration to adjust to different conditions in the human
body.
The program begins with modelling the role of
ATP in the contraction of a muscle. The action of
ATP is shown on the two proteins in muscle cells
actin and myosin. In time of overexertion, the
body may suffer a temporary oxygen shortage as
the circulatory system cannot provide the oxygen
quickly enough. While glycolysis can provide a
small quantity of ATP, not enough is synthesized
and this results in an energy shortage.
The program describes how the process of cellular respiration takes steps to overcome this shortage. The pyruvate that normally heads off to the
Krebs cycle follows a different path when oxygen
is in short supply-a path that leads to the
synthesis of lactic acid. The steps in this sequence
ensure the continuous production of ATP. There
is, of course, a debt to pay: a burning sensation
within the muscles caused by the lactic acid
buildup. Fortunately, after a short rest, the return
of oxygen results in the metabolism of the lactic
acid.
Too much glucose intake, on the other hand, can
result in the production of too much ATP. This
surplus triggers a sequence of events whereby
acetyl-CoA produces fatty acids that are stored as
fat.
This process can be reversed by dieting, in which
the fat can be metabolized. This is done through
sequences that lead either to the glycolytic pathway or directly into the Krebs cycle.
Throughout this series the programs have depicted how resourceful cells are and how the
collective behavior of a cell is a reaffirmation of
the driving force of life itself.
17
FORE VIEWING
1.
2.
it could be advantageous for the student to
recall or to look up the general structure of a
muscle. Recognition of such things as the
protein layers of actin and myosin and the
mechanics of muscle contraction would be
helpful.
Review program 4 with special reference to
the section on the electron transport chain.
Review in particular the purpose of oxygen
and the role of NAD+ and its development.
AFTER VIEWING
Activity 1:
Muscle and Fat Energetics
Discuss each of the following:
1. What role does each of the following play
during the contraction of a muscle: pyruvate,
NADH, NAD+, lactic acid, ATP, ADP, glycogen, and oxygen?
2. Explain what happens when a muscle is
overexerted as during strenuous exercise and
how this condition is alleviated.
3. Discuss the conversion of energy during muscle
contraction.
4. What are some other uses of ATP by cells of
multicellular organisms?
5. Trace the catabolism of fatty acids through the
Krebs cycle. How does the ATP yield from a
6-carbon fatty acid compare with the ATP
yield from glucose? What problem occurs if fat
catabolism is excessive?
Activity 2:
Observation of Skeletal Muscle
Apparatus
beef toluidine-blue stain
prepared slides of skeletal muscle
prepared slides of cardiac muscle, if available
microscopes
microscope slides
cover slips
dissecting needles
medicine droppers
forceps
Method
1. Obtain a piece of beef from your teacher. Pull
the point of the dissecting needle across the
long grain of the muscle several times until a
small strand of tissue is removed. Caution:
Use the dissecting needle with care as it is very
sharp.
2. Using forceps, transfer the strand of beef to
the centre of a clean slide.
3. Put 2 drops of toluidine-blue on the tissue. Let
the stain remain for 2 minutes, then add 2
drops of water to the slide. Cover the tissue
with a cover slip.
4. Examine the tissue under the microscope at
low power. Focus on a portion that is thin and
lightly stained. Draw a portion of what you
see.
5. Switch to high power. Look for the striated
appearance of the muscle cells. Muscle cells
are made of microfilaments called myofil
aments, which are composed of the proteins
actin and myosin. The portion of muscle from
one stripe to the next is called a sarcomere.
The darkly stained structures are the nuclei of
the muscle cells. Locate the sarcomeres and
the nuclei, and draw a diagram labelling these
structures.
6. Use high power to examine prepared slides of
skeletal muscle and, if available, cardiac
muscle.
Discussion
1. From your observations, is a muscle fibre
composed of several small cells, or one long
cell containing many nuclei?
2. Discuss the role of myosin, ATP, and actin
during the contraction of a muscle cell.
3. What initiates contraction in vertebrate skeletal muscle? What other chemicals are involved?
18
decarboxylation the removal of the carboxyl
group (COON) from an organic molecule
acetylCoAthe main molecule of energy metabolism; contains a high energy bond
actin one of two proteins making up the microfilaments of muscle tissue
adenine an organic base consisting of two carbon-nitrogen rings
ADP adenosine diphosphate, a substance produced when ATP gives up energy through the loss
of a phosphate radical
anaerobic fermentation fermentation is the
extraction of energy from organic compounds;
anaerobic means that the process does not involve oxygen
ATP adenosine triphosphate, a nucleotide made
up of adenine, ribose sugar, and three phosphate
groups; this is the energy carrier in cell metabolism
carbohydrate a compound containing carbon,
hydrogen, and oxygen wherein the ratio of hydrogen to oxygen is 2:1; carbohydrates include sugar,
starch, etc.
cellular respiration, the production of energy
through the process of oxidation; the energy is
produced through the Krebs cycle and
phosphorylation
citric acid cycle see Krebs cycle
coenzyme a cofactor that is a nonprotein organic
molecule; a cofactor is an enzyme employing
metal ions to acquire electrons
Coenzyme A organic molecule involved in enzyme catalyzed process; this two-carbon molecule is the main molecule of energy metabolism
crista folded innner membrane of a mitochondrion; the folds or cristae produce a large surface
area in which are contained the electron transport
chains
DH" dihydroxyacetone phosphate, one of the
products of the splitting of fructose diphosphate
along with PGAL; the DHAP then undergoes
isomerization to become a second molecule of
PGAL
electron transport chain protein chain embedded within the mitochondrial membrane which
facilitates the passage of electrons; third stage of
respiration and principal site of ATP synthesis in
the cell
entropy refers to the unavailability of energy in
a system, and a measure of a system's randomness
or disorder; the basis of the Second Law of
Thermodynamics
enzyme a protein that speeds up or slows down
certain chemical reactions but does not, itself,
change
FAD+ the oxidized form of FADH 2
FADH2 flavin adenine dinucleotide; a carrier of
lower energy electrons
fatty acid an organic acid with a single carboxyl
radical along with other carbon and hydrogen
atoms
glycogen a polysaccharide in which starch is
stored in animal cells
glycolysis the process through which glucose is
broken down to synthesis ATP
Krebs cycle a cycle of oxidation and reduction
and the decarboxylation reactions from which a
cell can derive ATP; also called the citric acid cycle
since the cycle which begins with pyruvate later
forms citric acid which is oxidized to form CO 2
lipid organic compound insoluble in water but
soluble in certain organic liquids such as fats, oils,
water, phospholipids, etc.
matrix the inner compartment of a mitochondrion
19
mitochondrion cytoplasmic organelle; each one
represents a complete mechanism that produces
energy (plural: mitochondria)
myosin one of the muscle proteins
NAD nicotinamide andenine dinucleotide, a
coenzyme that acts as an electron acceptor; NAD+
is its oxidized form
NADP nicotinamide adenine dinucleotide phosphate, an electron acceptor in the process of
respiration
phosphoglyceraldehyde shortened to PGAL, a
three-carbon molecule; a six-carbon molecule of
glucose is broken into two molecules of PGAL
with the input of ATP
photosynthesis the formation of carbohydrates
from carbon dioxide and water in the presence of
light and chlorophyll
protein a chain of amino acids joined by peptide
bonds
pyruvate a three-carbon (3C) compound; the
end product of glycolysis and the material with
which the Krebs cycle begins
ribose a sugar of the five-carbon type
sarcomere the fundamental unit of contraction
in muscle tissue
20
Dobson, G. P. and Hochachka, P. W. Role of
glycolysis in adenylate depletion and repletion
during work and recovery in teleost white muscle.
The journal of Experimental Biology 129:125-40,
May 87.
Akeroyd, F. Michael. Teaching the Krebs cycle.
Journal o fBiological Education 17:245-56, fall 83.
Alterthum, Flavio; Dombek, K. M.; and Ingram,
L. O. Regulation of glycolytic flux and ethanol
production in saccharomyces cerevisiae: effects
of intracellular adenine nucleotide concentrations
on the in vitro activities of hexokinase,
phosphofructokinase, phosphoglycerate kinate,
and pyruvate kinase. Applied and Environmental
Microbiology 55:1312-14, May 89.
Erickson, R. P.; Harper, K. J.; and Hopkin, S. R.
Adenine nucleotides and other factors indicative
of glycolytic metabolism in murine spermatozoa.
The journal of Heredity 78:407-09, Nov-Dec 87.
Furth, Anna and Harding, John. Why sugar is bad
for you. New Scientist 123:44-7, S 23 89. A good
article for this series; deals with the evidence of
sugar-caused damage to long-lived proteins.
Bodner, George M. Metabolism: part 3. Lipids.
Journal of Chemical Education 63:772-75, Sep 86.
Milligan, L. P. and McBride, B. W. Energy costs of
ion pumping by animal tissues. The journal of
Nutrition 115:1374-82, Oct 85.
. Metabolism: glycolysis or the EmbdenMyerhoff pathway. Journal of Chemical Education 63:566-70, Jl 86. An excellent article for this
series; the steps are clearly laid out complete with
equations.
Poolman, Bert; Bosman, Boukje; and Kiers, Jan.
Control of glycolysis by glyceraldehyde-3-phosphate dehydrogenase in streptococcus cremoris
and streptococcus lactis. Journal of Bacteriology
169:5887-90, Dec 87.
. Metabolism: part 2. The tricarboxylic acid
(TCA), citric acid, or Krebs cycle. journal of
Chemical Education 63:673-77, Aug 86. Differen
tiates the tricarboxylic acid (TCA) from glycolysis,
and describes the connection between the two as
being the conversion of pyruvate into acetyl
coenzyme A.
Sherman, W. Mike. Carbohydrates, muscle glycogen, and improved performance. Physician and
Sports Medicine 15:157-61, Feb 87.
Brand, Martin D. and Murphy, Michael P. Control
of electron flux through the respiratory chain in
mitochondria and cells. Biological Reviews of the
Cambridge Philosophical Society 62:141-93,
May 87.
Simard, Clermont et al. Effects of carbohydrate
intake before and during an ice hockey game on
food and muscle energy substrates. Research
Quarterly for Exercise and Sport 59:144-47,
June 88.
Wright, Russell G. and Bottino, Paul J. Mitochondrial DNA. Science Teacher 53:27-31, Apr 86.
21
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Cellular Respiration
Videotapes
Program Title
The Cell and Energy
Glycolysis 1
Glycolysis 2
The Krebs Cycle
Oxidative Phosphorylation
Metabolism and Nutrition
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