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Powerful Misconceptions about Energy in Biology
««« By Dr. Maurice DiGiuseppe and Doug Fraser
Dr. Maurice DiGiuseppe is an assistant professor of science education at the
University of Ontario Institute of Technology (UOIT) in Oshawa, Ontario. His primary
areas of interest include science curriculum, science teacher education and professional
development, and science textbook and digital resources development. Dr. DiGiuseppe
was previously a science teacher and administrator in the Toronto area and a course
director in the Science, Mathematics, and Technology program at York University.
Dr. DiGiuseppe has authored numerous elementary and secondary school science
text- books and has conducted research in teacher professional development, nature of science, scientific inquiry, and digital
learning resources in science education. Dr. DiGiuseppe is a former president of STAO and a recipient of the Prime Minister’s
Award for Teaching Excellence in Science, Technology, and Mathematics.
Doug Fraser is a teacher at Timiskaming District Secondary School, in New Liskeard, Ontario, where he has been teaching
Biology and Science for over 25 years. Doug has co-authored a number of high school science and biology textbooks with
Nelson Education including Nelson’s grade 11 and 12 university and college prep biology textbooks. Doug is a regular
speaker at STAO’s annual conference having spoken on a wide range of topics from evolution and creationism, to computer
modeling of macromolecules and pseudoscience. Doug was the recipient of a 2007 Outstanding Biology Teacher Award
from the NABT and received the 2009 Premier’s Lifetime Achievement Award in Teaching Excellence.
Curriculum Connection: Senior Biology, Matter and Energy, Grade 11/12.
This is the second in a series of articles on misconceptions in science education. In this instalment we discuss two
misconceptions in biology on the concepts of matter and energy. In the first case, we address the question of what
happens to the body’s mass when a person loses weight, and in the second, we explore the meaning behind the common
claim that adenosine triphosphate (ATP) releases energy when it “splits” into adenosine monophosphate (ADP) and
inorganic phosphate (Pi ) according to the following equation: ATP➔ADP + Pi + Energy. In each case, we begin with a
diagnostic question (which we would like you to answer before reading the discussion), and then revisit the question at
the end of the discussion. We hope you enjoy this exercise.
Question 1: How can we account for the difference in mass when an organism “loses weight”?
Discussion: When an organism “loses weight”, its mass decreases. In Figure 1, it is obvious that the mouse contains less
matter in its body in the “after” picture than it does in the “before” picture. So, where has all of the excess matter gone?
➔
Figure 1:
This mouse has obviously lost a lot of weight.
before
after
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When organisms lose weight (on account of dieting, disease, exercise, etc.), their bodies experience a net loss of tissue
mass. When asked to account for this loss, students (and others) often provide one of the following explanations:
(a) Matter is converted into energy and is transferred to the external environment in the form of “heat”.
(b) Matter is lost to the external environment as water (increased perspiration, urination, etc.).
(c) A combination of explanations (a) and (b).
These naïve explanations probably develop because people (a) notice that they (or others) lose weight if they exercise
a lot, (b) notice that they feel hot and sweaty during bouts of strenuous exercise, and (c) assume that tissue matter is
converted into energy and water and leaves the body in the form of “heat” and perspiration. How did you answer the
diagnostic question?
Beginning in Grade 10, students learn that most organisms obtain most of their energy through the process of cellular
respiration. In this process, carbohydrates (and indirectly, also proteins, lipids, and nucleic acids) are eventually
converted into carbon dioxide and water (and energy) according to the following overall (exergonic) chemical equation:
C6H12O6 + 6O2 ➔ 6CO2 + 6H2O + Energy
This is a balanced chemical equation in which the total mass of reactants equals the total mass of products (i.e., the
energy output is not caused by the direct conversion of mass into energy). Thus, mass can only be “lost” if one or both
of the material products (CO2 and/or H2O) leaves the organism’s body. As we know, organisms excrete most of the CO2
produced in cellular respiration as a waste product, resulting in a net loss of mass and weight. Some of the water produced
in the reaction (metabolic water) is also excreted in expired breath and perspiration, and an even smaller quantity of mass
is excreted in the form of nitrogenous wastes in urine. Therefore, when an organism loses “weight”, the loss is primarily
caused by the excretion of carbon dioxide and metabolic water, not by the direct conversion of mass into energy (“heat”)
or solely by the excretion of ingested (exogenous) water.
Addressing the weight-loss misconception in the classroom
The weight-loss misconception often develops because students (and their teachers) place too much emphasis on the
molecular and energy details of biological processes (e.g., reactants, products, energy), and not enough emphasis on the
effects of these processes at the organism level. It is a case of “not seeing the forest for the trees.” As teachers, we should
not assume that all students will make logical connections between, say, the process of cellular respiration (moleculelevel knowledge) and weight loss (organism-level knowledge) unless they are prompted to do so. Engaging students in
activities where they are encouraged to ask critical
The mouse lost
The mouse lost
questions, conduct research, and debate
mass mostly in
mass by excreting
Before After
competing claims may help them make
the form of water
carbon dioxide and
water
in
its
breath
inconspicuous connections between invisible
The mouse
lost mass in
(molecule-level)
and visible (organism-level)
the form of
heat
concepts and thereby avoid (or resolve)
misconceptions such as this. Concept cartoons
may assist in this regard. Figure 2 is a concept
cartoon that may be used in the classroom to
stimulate discussion and debate on the weight-loss
misconception.
Figure 2: Concept cartoon addressing weight loss.
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Question 2: What is the source of the ”Energy” in the equation: ATP ➔ ADP + Pi + Energy?
Discussion: Every student learns that ATP is the “energy currency” of the cell — that cells use ATP to provide energy for
cellular functions. This molecule is so important that we refer to mitochondria as “powerhouses” of the cell because
they produce ATP.
Of course all of this is true. ATP is a vitally important “energy-rich” molecule used to drive most of life’s energy-demanding
(endergonic) processes. But how does ATP actually supply this energy? How is this energy stored and released?
Before going any further, it may be useful to recall that the energy changes in a chemical reaction can be described
in terms of enthalpy change (∆H; endothermic/exothermic reactions) or in terms of changes in Gibb’s free energy (∆G;
endergonic/exergonic reactions). Gibb’s free energy calculations take entropy effects into consideration and quantify
the energy available to do work. While ATP is mentioned often in earlier grades, Gibb’s free energy is usually introduced
for the first time in SBI4U.
The ATP Cycle
A typical diagram of the “ATP cycle” (Figure 3), found in most biology texts and Internet websites, is intended to depict
the relationships between energy inputs and outputs, ATP, ADP and inorganic phosphate (Pi ). This simple diagram
provides no information on the actual source(s) of energy and shows only the splitting of a molecule of ATP into two
entities — ADP and Pi — with “energy” entering the process on the left and exiting the process on the right. As such,
the diagram appears to show a cycle of ATP decomposition and ATP synthesis. This diagram may elicit memories of
Grade 10 Science where such reactions are represented by simple AB ➔ A + B and A+B ➔ AB notations. As we will
note later, the synthesis and breakdown of ATP does not follow such a simple pattern.
Figure 3:
Typical ATP Cycle Diagram.
Given Figure 3, a student might reasonably assume that a matching word equation for the right side of the ATP cycle
would be:
ATP ➔ ADP + Pi + Energy
This ATP cycle diagram and the inferred word equation may give students the impression that the release of energy is
directly associated with the splitting of an ATP molecule.
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In addition to process diagrams and word equations, teaching-learning resources may also provide simplified diagrams
of the ATP molecule itself. In many resources, the bonds between the phosphate groups (phosphoanhydride bonds) are
labeled or depicted in a manner indicating that they are “high-energy bonds” (Figure 4).
Figure 4:
Typical representations
of “high-energy bonds”
in ATP molecules.
Now consider the following quotes from a senior biology textbook and from an Internet website:
“The ATP molecule is often referred to as a “coiled spring,” the phosphates straining to get away from
each other.” (Source: Raven and Johnson Biology – Fifth Edition)
“To trap energy released from exergonic catabolic chemical reactions, the cell uses some of that released
energy to attach an inorganic phosphate group on to adenosine diphosphate (ADP) to make adenosine
triphosphate (ATP). Thus, energy is trapped and stored in what are known as high-energy phosphate
bonds. To obtain energy to do cellular work during endergonic anabolic chemical reactions, the organism
enzymatically removes the third phosphate from ATP, thus releasing the stored energy and forming ADP
and inorganic phosphate once again.” (Source: http://faculty.ccbcmd.edu/biotutorials/energy/atp.html)
How might students interpret these descriptions? What connections are they likely to make between ATP, energy, and
chemical bonds?
Such diagrams and written descriptions simplify the ATP cycle to the point of being erroneous and extremely misleading.
Many students, and indeed many teachers, accept these diagrams as complete and take the statements literally. In so
doing they conclude that the releasing of energy is a direct result of the breaking of chemical bonds. This is simply not
true. The energy released in the ATP cycle does not come from breaking phosphate-phosphate bonds in ATP, but rather
the splitting of ATP into ADP and Pi releases energy in spite of the fact that a “high-energy bond” is broken!
Bonds and Energy
By definition, a chemical bond refers to the forces of attraction between the electrons of one chemical entity and the
protons of another. When atoms form bonds, they are held together in a relatively stable arrangement and will remain
together unless energy is used to overcome these forces of attraction. A useful analogy is that of a nail “bonded” to a
magnet. The nail is held in position by forces of attraction between the magnet and the nail. The nail will remain bonded
to the magnet until enough energy is provided to overcome the forces of attraction. Energy must be “input” to pull the
nail away from the magnet, even a very weak magnet.
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Thus, it always takes energy to break a chemical bond. This is worth repeating: Breaking chemical bonds always requires
an input of energy — there are no exceptions! And, of course, the reverse is also true: Energy is always released when
new bonds form!
In the case of ATP, it takes energy to break the phosphate-phosphate bond. However, as in all other reactions, energy
is released when new bonds form. It is the formation of these new bonds that supplies much of the free energy released
in the ATP cycle.
Note that students often mistakenly associate a “strong bond” with high energy. They are told that ATP molecules have
“high-energy bonds” and that when these bonds break, energy is released. This is very misleading. It is therefore
important to remind students that a force of attraction is not energy. A stronger bond is more difficult to break than
a weaker bond and so requires a greater input of energy than a weaker bond. In the case of ATP, the phosphoanhydride
bonds are quite weak and require only a relatively small amount of energy to break them. The negative charges on the
phosphate groups of ATP cause these groups to repel each other, making the phosphoanhydride bonds weaker than they
otherwise would be. Some textbook descriptions of the repulsive charges on the phosphate groups imply that the ATP
molecule is spring loaded but again this is quite misleading. Imagine two powerful magnets with north or south poles
facing each other but also experiencing another slightly stronger force of attraction — just enough to hold the magnets
together. In order for the magnets to separate, this binding force, which is stronger than the repulsive magnetic forces,
must be overcome. Due to the magnitude of these forces of repulsion and attraction (analogous to our ATP phosphate
groups and “bond energy”), the breaking of the bond will require a net input of energy — the phosphate groups will not
act like a cocked spring and “fly apart”, releasing energy.
Thus, if breaking ATP’s phosphate bonds does not release energy, where does the released energy come from?
ATP contains high energy electrons, not high energy “bonds” — a force of attraction may not “have” energy — but
electrons do!
Unfortunately, as illustrated in the examples above, most simple diagrams and descriptions of the ATP cycle fail to
acknowledge any details of bonding arrangements, the formation of new bonds, or the actual details of the chemical
reactions themselves. Indeed, many, if not most, presentations of the cycle do not include one of the important
reactants, water! As biology teachers know, the “splitting” of ATP into ADP and Pi is in fact, a hydrolysis reaction and
therefore water is a reactant. A word equation for the reaction is:
ATP + H2O ➔ ADP + Pi + Energy
One might consider that water is also a reactant in photosynthesis but is never missing from the overall equation!
Why do we routinely omit it in the ATP hydrolysis reaction? Figure 5 shows the details of the ATP hydrolysis reaction.
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Figure 5:
Hydrolysis of ATP.
(Source: Nelson Biology 12 University Preparation,
2012, p.142)
Note that the reaction involves the breaking of two covalent bonds (an O-H bond in water and the P-O bond that holds
the terminal phosphate group) and the formation of one covalent bond (the P-O bond of the free Pi). This reaction also
results in the formation of two ions — the new O- ion on the ADP molecule and an H+ ion. These ions subsequently form
bonds with other ions, with each other, with other substrate molecules, and/or with water molecules. Under typical
cellular conditions the H+ ion often binds with water to form a hydronium ion (H3O+) as follows:
ATP + 2 H2O ➔ ADP + Pi + H3O+ + Energy
In all cases, the bonding of the new ions with other ions or molecules, and the process of dissolution results in a release
of energy! This energy is provided by the high-energy electrons contained within the original so-called “high-energy bonds.”
While the hydrolysis of ATP is useful in describing the ATP cycle, and as a basis for discussing energy changes, it is not
truly representative of the cellular interactions that actually utilize ATP energy. In cells, the release of ATP energy
invariably involves the interaction of ATP with a molecule other than water; for example, glucose or fructose during
glycolysis, or the membrane-bound sodium pump protein complex.
Entropy factors
Unfortunately, any discussion of entropy in relation to chemical reactions adds complexity that may not be well understood
by students. It is important not to confuse changes in entropy and free energy with bond energies. While it is true that
free energy values for a reaction may depart significantly from enthalpy values, this does not change the energy relationships associated with bonds.
The increase in entropy that occurs when ATP forms ADP and Pi is complex (depending on what the ATP actually reacts
with in the process) and can be a distraction from the simpler fundamentals. As a comparison — the melting of ice at
room temperature is a good example of a spontaneous process that is driven by entropy changes. However, this does
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not change the fact that the melting of ice is endothermic (∆H is negative) since the ice absorbs thermal energy from the
environment as it melts. In this process, the bonds between water molecules in the ice lattice are being broken and there
is an overall net decrease in free energy (i.e., the Gibb’s Free energy value is negative and the process is exergonic). The
entropy effects overcome the endothermic bond energy changes to make this process spontaneous. This is not the case
with the ATP cycle in which the hydrolysis of ATP is both exergonic and exothermic.
There are also equilibrium effects that influence the reaction dynamics. ATP concentrations are typically maintained within
a cell such that the forward reaction ATP + H2O ➔ ADP + Pi is strongly favoured by equilibrium dynamics.
It is important for SBI4U students to appreciate that the energy values for the ATP cycle, and indeed, virtually all
bio-chemical processes within living cells, are known with only modest precision — free energy changes are influenced
by, and are very sensitive to, a variety of factors including concentration and pH, and these are very difficult to measure
in real time as they are occurring within complex living systems.
Addressing the ATP misconception in the classroom
It is not easy to address or prevent misconceptions regarding the ATP cycle since oversimplified diagrams and explanations
abound. It may be beneficial to begin with a diagnostic assessment of what students already know about chemical bonding.
For example, by Grade 12, they should know that electrons are attracted to protons and that it takes energy to pull electrons
away from protons/nuclei; and that the breaking of bonds cannot be a “source” of energy. Make use of the magnet and
nail analogy to reinforce this concept, possibly with a hands-on demonstration.
Highlight and reinforce the idea that in chemical reactions, (a) existing bonds are always broken and new bonds are formed,
(b) all bond-breaking events require energy and all bond-forming events release energy, and (c) net energy changes
(i.e., endothermic/exothermic) are largely determined by the differences between energy inputs for bond breaking and
energy outputs during new bond formation. Also remind students that bonds forming between ions and water molecules
during the dissolution of ions is an additional source of energy. In the specific case of ATP — emphasize that energy is
released when the ATP reacts with water or other compounds to form ADP and Pi, each of which may be “free” in
solution or bound to other molecules.
Further Reading
Weight-loss Misconception
Food for Thought. Available at: http://doyle-scienceteach.blogspot.ca/2011/07/food-for-thought.html
When we lose weight, where does the fat go? Available at: http://sciencefocus.com/qa/when-we-lose-weight-wheredoes-fat-go
Where does all the weight I lost go? Available at: http://www.health.com/health/article/0,,20480476,00.html
ATP Misconception
Galley, W.C. (2004). Exothermic Bond Breaking: A Persistent Misconception. Journal of Chemical Education, 81(4), 523-526.
Novic, S. (1976). No energy storage in chemical bonds. Journal of Biological Education, 10(3), 116-118.
Storey, R.D. (1992). Textbook Errors & Misconceptions in Biology: Cell Energetics. The American Biology Teacher, 54(3),
161-166.
EXBAN: The Exothermic Bond-breaking Abolition Network. Available at: http://exban-group.mcgill.ca/introduction.htm
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