CHAPTER 8: ENERGY AND METABOLISM

advertisement
CHAPTER 7: ENERGY AND METABOLISM
WHERE DOES IT ALL FIT IN?
Chapter 7 builds upon the cell anatomy coverage of Chapter 4 and provides students with details of
cellular energy concepts needed understand the metabolic pathways that run a cell. It is a critical step
into Chapter 8. By this point of the books students have been given many terms and concepts the
must now be put together to give a complete picture of the cell and cell interactions that build the
higher hierarchical levels. It is important to briefly highlight the information in the previous chapters
for students to gain a better understanding of the concepts in Chapter 7. Chapter 7 is critical for
students to understand other concepts that integrate cell function to homeostasis and development.
SYNOPSIS
Living organisms transform potential energy into kinetic energy to survive, grow, and reproduce.
The energy that the earth receives from the sun is transformed into heat energy as it warms the
continents and the oceans. Various kinds of photosynthetic organisms also absorb this energy
and convert it to potential energy in the form of chemical bonds.
Oxidation-reduction reactions are a class of reactions that pass electrons from one molecule to
another. A molecule that is oxidized loses an electron; one that is reduced gains an electron.
Oxygen is the most common electron acceptor in biological systems. Since the transfer of
electrons is accompanied by a transfer of protons in the form of H+ ions, oxidation generally
involves the removal of hydrogen atoms and reduction involves the addition of hydrogen atoms.
In biological systems, oxidation-reduction reactions are coupled to one another. In
photosynthesis, carbon dioxide is reduced to form glucose, storing energy. In cellular respiration,
the oxidation of glucose releases energy.
The First Law of Thermodynamics states that energy can be transformed from one state to
another, but cannot be created or destroyed. The Second Law of Thermodynamics states that
objects tend to move from a state of greater order to one of lesser order. Thus entropy, the
measure of disorder in a system, is constantly increasing. The amount of free energy available to
form chemical bonds is equal to the energy within a cell that is available to do work (enthalpy)
minus the product of temperature and entropy. A reaction proceeds spontaneously when its
change in free energy is a negative number.
The products of exergonic reactions contain less free energy than the reactants. Such reactions
proceed spontaneously and release the excess usable free energy. The products of endergonic
reactions have more free energy than the reactants, do not occur spontaneously, and require an
input of energy to proceed. Fortunately, even exergonic reactions require an input of a small
amount of activation energy to get started. Otherwise all combustible materials would have
burned up long ago. This activation energy is required to destabilize the existing chemical bonds;
something that occurs more readily in the presence of a catalyst.
Enzymes are biological catalyzing agents generally in the form of proteins and having names
ending in -ase. An enzyme brings two substances together in the proper orientation and stresses
57
certain bonds. It does not force a reaction to occur in a single direction but enhances the reaction
in both directions. Although many reactions involve discrete enzymes, many complex pathways
depend on multienzyme complexes to efficiently carry out their sequential reactions. Certain
RNA reactions possess unique RNA catalysts called ribozymes, giving strength to the argument
that RNA evolved prior to proteins. Enzyme activity is altered by several factors including
temperature and hydrogen ion concentration (pH). A competitive inhibitor binds at the same site
as the substrate, effectively inhibiting the reaction. Noncompetitive inhibitors and activators bind
to the allosteric site to alter reaction rates. Various cofactors are associated with most enzymes
and may be in the form of metal ions or nonprotein, organic molecules called coenzymes. One of
the more important coenzymes is nicotinamide adenine dinucleotide (NAD+), a hydrogen
acceptor that, when reduced, becomes NADH. This molecule is responsible for carrying the
energy of an electron and a hydrogen throughout the cell.
The chief energy currency of cells is the molecule adenosine triphosphate, ATP. This molecule is
composed of a five-carbon backbone to which an nitrogenous adenine base and a chain of three
phosphate groups are attached. The covalent bonds linking the phosphate groups are high energy
bonds that are readily broken to release 7.3 kcal/mole of energy. All cells use ATP to drive their
endergonic reactions. Cells do not store large amounts of ATP but possess a pool of ADP and
phosphates so that they can make ATP whenever it is needed.
Living organisms organize their metabolic activities in reaction chains called biochemical pathways.
The first metabolic pathways were anaerobic since oxygen was not present in the early atmosphere of
the earth. The product of one reaction becomes the substrate for the next. The step-wise nature of a
biochemical pathway reflects its evolution. Organisms rarely evolve new processes completely
independent of other processes; rather they utilize the machinery that already exists and add to it or
alter it slightly. The addition of new processes generally occurs at the beginning of the pathway; such
a pathway evolves backwards. The final reactions evolved first, the beginning reaction is the most
recent adaptation. The stepwise progression of pathways allows for more precise regulation.
LEARNING OUTCOMES











Differentiate between kinetic and potential energy.
Describe how oxidation and reduction are interrelated in chemical reactions.
Understand The First Law of Thermodynamics and its impact on the amount of energy in the
universe.
Understand The Second Law of Thermodynamics and its relation to entropy and enthalpy.
Explain the energy requirements of endergonic and exergonic reactions.
Describe the importance of activation energy and how it can be altered.
Explain the nature of enzymes and multi enzyme complexes, how they affect reactions, and the
factors that affect their performance.
Define competitive inhibition, noncompetitive inhibition, and activation. Know how each
relates to the allosteric site.
Understand the unique catalytic nature of and properties associated with ribozymes.
Understand the structure of ATP and how it is able to drive biological reactions.
Describe how a general biochemical pathway evolves over time
58


Catabolism is the opposite of anabolism
The rate of an enzymatic reaction decreases with time as enzyme molecules are used up
during the reaction
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 7 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.

















Students believe respiration is the same as breathing
Students believe energy flow is one-way, rather than cyclical or two-way in a system
Students believe all chemical reactions release heat
Students believe energy comes from bonds
Students believe food is anything that goes into the organism including minerals, water,
carbon dioxide
Glycolysis is the same as fermentation
ATP gives energy to a cell
ATP is produced in the body and not in the cell
Only carbohydrates are involved in ATP production
Glucose is the only molecule that fuels ATP
ATP production increases with calories taken in the diet
Lipids are solely consumed during weight loss
All enzymes are killed by heating
Enzymes are so highly selective that they only bind to one type of substrate
Enzymes cannot bind to products of their reaction
Organisms dot no obey the laws of thermodynamics
Dietary calories (kilocalories) are the same as the standard calorie unit
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
As seen in figure 8.5, the condition of one’s bedroom, office, or desk is related to the Second
Law of Thermodynamics. There is a tendency for each of them to become more disorganized
(increased entropy). Cleanup and organization requires the input of energy.
It is easier to see how reactions with negative free energy occur spontaneously if the equation is
presented as DG = DH + (–TDS). For DG to be negative, disordering influences (–TDS) must be
larger than ordering influences (DH).
In relation to oxidation/reduction reactions, think of the reverse of what is expected. In reduction,
an electron is GAINED. When oxygen does what it is best at, accepting electrons, it is
REDUCED.
59
The lock and key analogy to enzyme action can be extended to include a master key system.
Sometimes a key opens only one specific lock. A master or submaster key may be able to open
several locks in a specific series. In addition, some high security doors may require two or more
locks to be unlocked to gain access. This is similar to how cofactors help control enzyme
activity. A multienzyme complex can be likened to a separate key ring used to open a series of
rooms in a certain building. I personally keep my work, home, and car keys on separate rings. It
makes any entry less cumbersome and prevents me from losing every thing all at once should I
leave a set of keys somewhere!
On a social comparison, a catalyst is like a good party host/hostess (or a romantic match maker).
He/she introduces two individuals that otherwise might not meet. The host/ess is not “used up” in
the process, but if there are too many unfamiliar individuals, the host/ess can become “saturated”
trying to pair up the guests.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 7.
Application
Analysis
Synthesis

Have students explain why an animal benefits from eating foods high in
free energy.

Have students explain why certain vitamins are known to regulate the rate
of certain metabolic pathways.

Ask students explain why any environmental factor that slows down
diffusion would affect enzyme function.

Ask students to explain why reptiles move into the sunlight after eating a
large meal.

Ask students to explain may many plant seeds produce chemicals that
inhibit enzymes involved in starch digestion.

Ask students hypothesize the purpose of designing drugs that fit into the
allosteric site of certain types of enzymes.

Ask students to design an experiment that tests whether an enzymatic
reaction is being carried out by proteins or by nucleic acids.

Have students explain some researcher believe that metabolic pathways
existed before organisms had protein enzymes.

Ask students the value of a drug that blocks the enzymatic degradation of
60
fats.
Evaluation

Ask students to evaluate that statement that the heat produced during
metabolism is not wasted energy.

Ask students to come up with commercial uses of enzymes taken from
organisms that live in temperatures that exceed 200oC.

Have the students evaluate the claims that heating an injury increases the
healing ability of the body.
VISUAL RESOURCES
1. Any wooden or plastic interlocking puzzle can be used to show how an enzyme catalyzes
a reaction by stressing bonds and altering chemical shapes. The puzzle cannot be taken
apart until one knows the special twist or trick to it. Finding this may take hours. Once it
is known, the puzzle can be done very rapidly — like an enzyme catalyzing a reaction.
2. Pipe cleaners or modeling clay are excellent materials that can be molded to 3-D models
that help students visualize enzymatic interactions with substrates.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. “Hot and Cold” Reactions: Demonstrating Endothermy and Exothermy
Introduction
A fun visual demonstration is a proven way to reinforce learning of energy concepts. This
demonstration shows students the temperature changes associated with endothermic and
exothermic reactions.
-Endothermy
Materials







Approximately 64g of barium hydroxide octahydrate
Approximately 30g of ammonium nitrate
250 ml Erlenmeyer flask
Thermometer
Glass stirring rod
Distilled water
Well-vented area
Procedure & Inquiry
61
1.
2.
3.
4.
5.
6.
7.
8.
9.
Add 150 ml of water to the flask.
Add solid barium hydroxide octahydrate to the flask.
Place thermometer in the flask and tell the initial temperature to the class.
Add ammonium nitrate to Erlenmeyer flask and stir
Stir the two components together.
After 30 seconds ammonia vapors will exit the flask.
Read the temperature change to the class after this point.
Ask students what type of reaction took place.
Ask the students to think of possible ways endothermic reactions can be used in everyday
life.
10. Dispose of materials appropriately according to institutional chemical waste policies.
-Exothermy
Materials





Supersaturated sodium acetate
Thermometer
Sodium acetate crystals
250 mL Erlenmeyer flask
Glass stirring rod
Procedure & Inquiry
1.
2.
3.
4.
5.
6.
7.
Add 150ml of supersaturated sodium acetate to flask.
Add thermometer and announce temperature to class.
Add a large crystal of sodium acetate to the flask.
A long crystalline spike should form from the crystal.
Start announcing the temperature change to the class as heat is produced.
Ask students what type of reaction took place.
Ask the students to think of possible ways exothermic reactions can be used in everyday
life.
8. Dispose of materials appropriately according to institutional chemical waste policies.
B. Clay Enzymes with a Charge
Introduction
A quick demonstration of enzyme specificity can be demonstrated using clay and
magnets to demonstrate the induced fit model of enzyme function. This demonstration should be
done during a discussion of active site action.
Materials

Three different colors of modeling clay
62

Small cylindrical magnets marked with white correction fluid to show the positive and
negative poles.
Procedure & Inquiry
1. Mold a large wad of clay into a virtual enzyme with a square indentation representing the
active site. Place five magnets into the active site. One magnet should be placed in each
wall with the negative poles facing out.
2. Tell the class that this now represents an enzyme active site.
3. Then tell the class you are going to make to potential substrates.
4. Form one substrate using another color of clay. Make the clay into a square that would fit
into the active site. Add five magnets to the clay block. The magnets should be placed
with the positive poles facing out.
5. Place the “substrate” into the “active site” showing how shape and charge create a best
fit.
6. Next, form the other substrate using the remaining color of clay. Make the clay into a
square that should fit into the active site. Add five magnets to the clay block. The
magnets should be placed with the negative poles facing out.
7. Place the “substrate” into the “active site” showing how it is almost impossible to get a
good charge fit.
8. Ask the class to explain the cause of the charges in the active site.
9. Ask the class to discuss other factors besides shape and charge that may affect the
interaction between the active site and substrate.
USEFUL INTERNET RESOURCES
1. Case studies are excellent for reinforcing scientific concepts. The University of Buffalo
produced a study using the principles of the drug development process as means of
teaching about modifying metabolic pathways. This case study can be done in class or be
given as a take-home. The case study can be found at
http://ublib.buffalo.edu/libraries/projects/cases/FENPHEN.html.
2. Thermodynamics is often obscure to many students. The teaching is thermodynamics can
be reinforced with an entertaining case study called A Fridge in Space:
A Case Study in Thermodynamics. This case study shows the interrelationship between
biology and physics principles. It can be found at
http://ublib.buffalo.edu/libraries/projects/cases/fridge_in_space.html.
3. A good foundation in chemistry is essential for applying the principles covered in
Chapter 7. A website called Chemistry Biologists produced by the Royal Society of
Chemistry provides valuable teaching resources relevant to Chapter 7. The website can
be found at http://www.chemsoc.org/networks/LearnNet/cfb/index.htm.
4. Introducing students to the commercial uses of enzymes is an interesting way to give
everyday applications of the information in Chapter 7. Maps biotechnology company in
India provides a history of commercial enzymes web page that shows the commercial
value of exploiting cellular functions. The website is available at
http://www.mapsenzymes.com/History_of_Enzymes.asp.
63
LABORATORY IDEAS
Commercial Applications of Enzymes
a. Have students investigate one commercial use of enzymes using a long-term
project that they can monitor for a period of time.
b. Tell the class that they will be using enzymes to as a way of recycling paper into
animal feed. This process is used in many countries where animal feed is
expensive and waste disposal is prohibitive because of a lack of landfill space.
c. Students should be able to make a setup that degrades the paper into slurry useful
as an animal nutrient.
d. Provide students with the following materials:
i. Amylase powder or liquid
ii. Cellulase powder or liquid
iii. Protease powder or liquid
iv. Pectinase powder or liquid
v. Phosphate buffer
1. Monosodium phosphate, monohydrate 58.4
2. Disodium phosphate, heptahydrate 154.7
3. Mixed together in 1 liter of distilled water
vi. Test tubes
vii. Test tube rack
viii. 1cm2 pieces of newspaper
e. Ask students to test the effectiveness of particular types of enzymes on degrading
waste paper into animal feed.
f. Have them research the chemistry of newspaper and the activity of each enzyme
supplied for the study.
g. Then ask the students to select and enzyme or enzyme mixture the completely
degrades the paper into a solution that be turned into animal feed.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
Students who have successfully mastered the content of Chapter 7 can apply their knowledge for
service learning activities in the following ways:
1. Have students talk to youth sports groups about misconceptions of energy supplements
and athletic peformance.
2. Have students design prepare a talk for seniors groups about the effectiveness, benefits,
and risks of nutritional supplements that supply “energy”.
3. Have students tutor middle school or high school biology students studying cell energy
concepts.
64
4. Have students judge science fair projects related to cell function.
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S.
Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and
does not discriminate on the following basis:

against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political
affiliation or belief; and

against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on
the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or
his or her participation in any WIA Title I-financially assisted program or activity.
This workforce solution was funded by a grant awarded under the President’s CommunityBased Job Training Grants as implemented by the U.S. Department of Labor’s Employment
and Training Administration. The solution was created by the grantee and does not
necessarily reflect the official position of the U.S. Department of Labor. The Department of
Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with
respect to such information, including any information on linked sites and including, but not
limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy,
continued availability, or ownership. This solution is copyrighted by the institution that
created it. Internal use by an organization and/or personal use by an individual for noncommercial purposes is permissible. All other uses require the prior authorization of the
copyright owner.
65
Related documents
Download