High School Biology Textbooks: A Benchmarks-Based Evaluation

Project 2061 Analysis Procedure

Connections Among Ideas Used in Evaluating the Textbooks' Content Coherence

In addition to evaluating how completely each textbook's content aligns with specific key ideas, the content analysis examined the connections made among ideas as an indicator of coherence. The reviewers examined separately three kinds of connections: (1) connections among key ideas, (2) connections between key ideas and their prerequisites, and (3) connections between key ideas and other, related ideas. To receive credit for a connection, a textbook needed to state both ideas being connected and make an explicit connection between them or between parts of them. Hence, any connections that were made were found on the pages where the key, prerequisite, or related ideas themselves were treated.

The following clarifications of the connections used to examine content coherence for the Cell Structure and Function and Matter and Energy Transformations topics provided reviewers with guidelines and examples to inform their judgments. To view maps for all four topics that contrast the connections that reviewers looked for with a composite of the connections reviewers found in all nine evaluated textbooks, see View Maps Summarizing Content Analysis Findings.

Cell Structure and Function
  1. Connections among key ideas
  2. Connections between key ideas and their prerequisites
  3. Connections between key ideas and related ideas

1. Connections among key ideas

Many of the key ideas about cell structure and function come directly from the section Cells in Science for All Americans. Ideas a and b are found in the first paragraph, the other key ideas are found in the second, third, and last paragraphs. The first paragraph relates the work of the cell to the molecules and structures that compose it. The second paragraph relates molecular functioning to a specific intracellular environment that the cell must maintain. The third paragraph relates the function of proteins to their structure. The final paragraph explains how molecules interact to regulate cellular activities and how these activities can be influenced by molecules from outside the cell.

All living cells have similar types of complex molecules that are involved in these basic activities of life. These molecules interact in a soup, about 2/3 water, surrounded by a membrane that controls what can enter and leave. In more complex cells, some of the common types of molecules are organized into structures that perform the same basic functions more efficiently. In particular, a nucleus encloses the DNA and a protein skeleton helps to organize operations. In addition to the basic cellular functions common to all cells, most cells in multicelled organisms perform some special functions that others do not. For example, gland cells secrete hormones, muscle cells contract, and nerve cells conduct electrical signals.

Cell molecules are composed of atoms of a small number of elements—mainly carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. Carbon atoms, because of their small size and four available bonding electrons, can join to other carbon atoms in chains and rings to form large and complex molecules. Most of the molecular interactions in cells occur in water solution and require a fairly narrow range of temperature and acidity. At low temperatures the reactions go too slowly, whereas high temperatures or extremes of acidity can irreversibly damge the structure of protein molecules. Even small changes in acidity can alter the molecules and how they interact. Both single cells and multicellular ogranisms have molecules that help to keep the cells’ acidity within the necessary range.

The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. Protein molecules are long, usually folded chains made from 20 different kinds of amino acid molecules. The function of each protein depends on its specific sequence of amino acids and the shape the chain takes as a consequence of attractions between the chain’s parts. Some of the assembled molecules assist in replicating genetic information, repairing cell structures, helping other molecules to get in or out of the cell, and generally in catalyzing and regulating molecular interactions. In specialized cells, other proteins molecules may carry oxygen, effect contraction, respond to outside stimuli, or provide material for hair, nails, and other body structures. In still other cells, assembled molecules may be exported to serve as hormones, antibodies, or digestive enzymes.

…Complex interactions among the myriad kinds of molecules in the cell may give rise to distinct cycles of activities, such as growth and division. Control of cell processes comes also from without: Cell behavior may be influenced by molecules from other parts of the organism or from other organisms (for example, hormones and neurotransmitters) that attach to or pass throught the cell membrane and affect the rates of reaction among cell constituents.

AAAS, 1989, pp. 63–64

Science for All Americans provides coherence by relating ideas about cell structure and function to ideas about the molecules of which cells are made. From the standpoint of a student textbook, we'd like to see a little more. (Actually, we'd like to see a lot more for students to actually appreciate the connection between ideas. However, the quality of connections is analyzed in the instructional analysis under criterion IV.5. For content coherence, a mere statement within the text is considered sufficient for credit.)

Connections among key ideas about cell structure and function. The set of key ideas involves three levels of biological organization—cells, molecules, and to some extent organisms. Telling a coherent story involves relating processes that occur at these different levels of biological organization and relating them to examples from the real world. The central idea that cells assemble proteins to do their work (Idea c) and that cells carry out particular functions, such as transport of materials, energy capture and release, and protein building (Idea b) can be connected by demonstrating the role of proteins assembled by cells in these functions. For example, the text could illustrate the role of cell-assembled proteins in helping molecules get in or out of cells or in facilitating cellular reactions involved in energy capture and release. Describing the effects of changing temperature or acidity on the structure and function of cells and their proteins (Idea d) and on the cells’ ability to carry out particular work (Idea c) could help to make a connection between these ideas. These ideas could also be related to ideas about cell regulation (Idea e) by describing how a particular molecule from outside the cell affects a cell function by modifying the action of a particular protein. And a connection between a cell’s response to changing environmental conditions and the regulation of cell functions (Ideas d and e) could show how temperature affects the rate of protein functioning. This could be illustrated by proteins involved in cell division whose action is slowed down at colder temperatures, leading to slower cell division and hence a lower growth rate in colder weather of plants and cold-blooded animals.

[Connections for this topic]

2. Connections between key ideas and their prerequisites

For a material to receive credit for a connection, it must state both the prerequisite and the key idea and relate them in a relevant way. In other words, part of making a connection to a prerequisite is stating the prerequisite.

Within cells, many of the basic functions of organisms—such as extracting energy from food and getting rid of waste—are carried out. The way in which cells function is similar in all living organisms. (prerequisite to Idea b)

This grades 6–8 prerequisite explains that cells perform the basic functions of organisms and makes clear that cells function in essentially the same way in all living organisms. To establish a connection between this prerequisite and key Idea b (that cells have specialized parts for carrying out the necessary functions), a curriculum material could relate the functions of particular cell parts to the needs of the organism.

Various organs and tissues function to serve the needs of all cells for food, air, and waste removal. (prerequisite to Idea b)

This grades 6–8 prerequisite relates organ systems to the needs of cells. Students should understand that unlike organisms that consist of a single cell and are in direct contact with an external environment that supplies food and air and allows for rapid waste removal, cells in multicellular organisms are not in contact with an external environment. Hence, cells in multicellular organisms rely on organs and tissues to supply them with food and air and to remove wastes. A connection between this prerequisite and key Idea b (cells have specialized parts), can be made by providing simple examples of how organ systems serve the needs of cells, such as the role of the digestive system in breaking down macromolecules from food into subunits that can enter cells and the role of the circulatory system in transporting the subunits and oxygen to cells throughout the body and picking up waste for disposal.

Atoms may stick together in well-defined molecules or may be packed together in large arrays. (prerequisite to Idea c)

This grades 6–8 prerequisite introduces the idea that the basic building blocks of molecules are atoms, which can be combined in various ways. This prerequisite is needed for understanding that just as atoms can be put together to make molecules, molecules, such as amino acids, can be put together to make even larger molecules, such as proteins. A connection between this prerequisite and Idea c (that cells assemble proteins to do the work of the cell) could be made by demonstrating that protein molecules in cells are made up of the same atoms that are found in the food an organism eats. Dietary protein is broken down by the digestive system and the pieces are circulated to the cells. Once in the cells, the cell machinery assembles them into the proteins it needs. Thus, dietary protein and the protein molecules in cells are made from the same amino acids, just linked together in somewhat different ways. A more elaborate connection could be made by explaining that human cells are able to make only some of the amino acids needed to make their proteins. The amino acids that cannot be made within the cells must be supplied by the diet. Deficiencies of these particular amino acids could lead to the inability of cells to make all the proteins they need, either to carry out their own functions or for the functions of the organism.

The configuration of atoms in a molecule determines the molecule’s properties. Shapes are particularly important in how large molecules interact with others. (prerequisite to Ideas c and d)

This grades 9–12 prerequisite links the shape of molecules to their function. This prerequisite is salient to the understanding of key Idea d (most cells function best within a narrow range of temperature and acidity). Protein molecules provide a specific instance of both the prerequisite and Idea d and can be used to make a connection between them. Curriculum materials could illustrate both ideas with the same protein, showing how its shape is needed for its proper functioning (e.g., interacting with other molecules) and how both its shape and functioning are affected by changes in temperature and acidity.

The rate of reactions among atoms and molecules depends on how often they encounter one another, which is affected by the concentration, pressure, and temperature of the reacting materials. (prerequisite to Idea d)

This grades 9–12 prerequisite describes factors affecting reaction rates. This prerequisite may contribute to understanding how molecules within the cell, such as proteins, interact. A connection to the idea that cells and proteins function best within particular ranges of temperature and acidity (key Idea d) could be made by showing that decreasing temperature decreases reaction rates within cells. For example, cold temperatures decrease the growth rate of plants (and thus, we do not need to mow grass in the winter months) and the activity of cold-blooded animals.

[Connections for this topic]

3. Connections between key ideas and related ideas

As with key ideas and prerequisites, a material can only earn credit for a connection if it states both ideas in close proximity and relates them in some way. Since related ideas are often treated in different chapters, connections could be made either in these different chapters or in the chapters where the key ideas are treated. Both ideas need to be stated wherever the connection is made.

In addition to the basic cellular functions common to all cells, most cells in multicelled organisms perform some special functions that others do not. (related to Idea b)

Here is where a curriculum material has a chance to relate the common functions that all cells need to accomplish in order to survive to the special functions that certain cells perform for the organism. One way to make this connection would be to provide examples of cells that perform unique tasks for the organism, such as nerve cells, muscle cells, and gland cells, and explain that even though these cells have unique tasks, they also carry out all of the normal life processes, such as breaking down food or protein building, that are common to all cells.

Protein molecules are long, often elaborately folded chains made from 20 different kinds of smaller (amino acid) molecules. The function of each protein molecule depends on its shape. The shape depends on interactions among the amino acids and between them and their environment. (related to Ideas c and d)

This related idea provides salient information about the structure of proteins and how the function of each protein depends on its particular shape. This related idea can be important for understanding why cells function best with a narrow range of temperature and acidity (Idea d). As the shape of proteins is important for their correct functioning, changes in temperature and acidity can change the shape of the proteins. This related idea can also expand the understanding of the key idea that the work of the cell is carried out by proteins (Idea c). Proteins, as 3-dimensional molecules with particular shapes, become key cell structures and regulators of cell functions. Presenting the ideas in close proximity and stating that one helps to explain the other is a simple way to make the connection.

Some protein molecules assist in replicating genetic information, repairing cell structures, helping other molecules to get in or out of the cell, and generally in catalyzing and regulating molecular interactions. (related to Ideas c, d, and e)

This related idea lists some specific roles that proteins perform within the cell, thereby elaborating on the “work” that the proteins do within the cell (Idea c). Presenting the ideas in close proximity and making clear that all these functions are carried out by proteins that cells assemble (rather than by proteins consumed as food) is a simple way to make a connection. A connection to ideas about the regulation of cells and protein function (Ideas d and e) could be made by using the same protein to illustrate pairs of ideas.

Any system is usually connected to other systems, both internally and externally. Thus a system may be thought of as containing subsystems and as being a subsystem of a larger system. (related to Idea b)

This related idea is a generalization about systems and subsystems, of which the cell is an example. Cells are systems containing subsystems (organelles) and cells of particular organs are subsystems of a larger system (the organism). Because thinking about things as systems calls attention to what needs to be included among the parts to make sense of it, to how the parts interact with one another, and to how the system as a whole relates to other systems, seeing the cell as both a system of interacting parts and a subsystem of an organism can help students understand it better. A connection could be made to other kinds of systems and subsystems with which students are familiar, such as a classroom within school and a school within a district.

Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which can include material, energy, or information) can become the input to other parts. (related to Idea e)

This related idea is a generalization about systems, having to do with feedback and control. As with the previous related idea, understanding feedback and control in a simpler system, such as a thermostat (or a teacher in a classroom) could help students understand the principles of feedback and control and their applicability to cell regulation. A connection to the key idea that cell functions are regulated (Idea e) could be made by presenting simpler instances, showing that they are instances of a general property of systems, and then showing how this general principle applies to proteins and cells. For example, the text could show how the products of one series of reactions within a cell become the reactants for the next series, thus controlling their rate. Or the production of a protein product of one series of reactions could shut down the series of reactions when levels of that protein get too high.

[Connections for this topic]

Matter and Energy Transformations
  1. Connections among key ideas
  2. Connections between key ideas and their prerequisites
  3. Connections between key ideas and related ideas

1. Connections among key ideas

Many of the key ideas about matter and energy transformations come directly from the section Flow of Matter and Energy in Science for All Americans. Idea e is the first paragraph and the other key ideas are from the second and third paragraphs. The second paragraph relates key ideas about matter and energy transformations in organisms (Ideas a1, a2, b1, b2, c1, and c2) to one another in the context of a food web. The third paragraph (and the last sentence of the second) relates the matter transformations in food webs (Idea d1) to elements and molecules.

FLOW OF MATTER AND ENERGY
However complex the workings of living organisms, they share with all other natural systems the same physical principles of the conservation and transformation of matter and energy. Over long spans of time, matter and energy are transformed among living things, and between them and the physical environment. In these grand-scale cycles, the total amount of matter and energy remains constant, even though their form and location undergo continual change.

Almost all life on earth is ultimately maintained by transformations of energy from the sun. Plants capture the sun's energy and use it to synthesize complex, energy-rich molecules (chiefly sugars) from molecules of carbon dioxide and water. These synthesized molecules then serve, directly or indirectly, as the source of energy for the plants themselves and ultimately for all animals and decomposer organisms (such as bacteria and fungi). This is the food web: The organisms that consume the plants derive energy and materials from breaking down the plant molecules, use them to synthesize their own structures, and then are themselves consumed by other organisms. At each stage in the food web, some energy is stored in newly synthesized structures and some is dissipated into the environment as heat produced by the energy-releasing chemical processes in cells. A similar energy cycle begins in the oceans with the capture of the sun's energy by tiny, plant-like organisms. Each successive stage in a food web captures only a small fraction of the energy content of organisms it feeds on.

The elements that make up the molecules of living things are continually recycled. Chief among these elements are carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, calcium, sodium, potassium, and iron. These and other elements, mostly occurring in energy-rich molecules, are passed along the food web and eventually are recycled by decomposers back to mineral nutrients usable by plants. Although there often may be local excesses and deficits, the situation over the whole earth is that organisms are dying and decaying at about the same rate as that at which new life is being synthesized. That is, the total living biomass stays roughly constant, there is a cyclic flow of materials from old to new life, and there is an irreversible flow of energy from captured sunlight into dissipated heat.

AAAS, 1989, pp. 66–67

The Science for All Americans section on energy transformations relates chemical energy to the configurations of atoms in molecules and changes in energy to changes in configuration:

Different energy levels are associated with different configurations of atoms in molecules. Some changes in configuration require additional energy, whereas other changes release energy. For example...a chlorophyll molecule can be excited to a higher-energy configuration by sunlight; the chlorophyll in turn excites molecules of carbon dioxide and water so they can link, through several steps, into the higher-energy configuration of a molecule of sugar (plus some regenerated oxygen). Later, the sugar molecule may subsequently interact with oxygen to yield carbon dioxide and water molecules again, transferring the extra energy from sunlight to still other molecules.

p. 51

In Science for All Americans, presenting these ideas in the context of food webs provides coherence. From the standpoint of a student textbook, we'd like to see a little more. (Actually, we'd like to see a lot more for students to actually appreciate the connection. However, this is examined under the instructional criteria. For content coherence, a mere statement in text is considered sufficient.)

Connections among key ideas about transformations of matter. One way to make connections between ideas about matter transformations in individual organisms (Ideas a1, b1, and c1) and matter transformations in ecosystems (Idea d1) would be to state that the combination and recombination of molecules in food webs is the sum of all the combinations and recombinations of molecules that occur among the individual organisms that make up the food web. This also serves to make the link at the molecular level.

Here are some examples of legitimate connections (in bold), if the key ideas are treated in close proximity:

a1 to b1: Plants make sugar molecules from carbon dioxide (in the air) and water. Plants can then break down the sugar molecules that they have synthesized....

a1 to c1: Plants make sugar molecules from carbon dioxide (in the air) and water. Organisms that eat the plants break down the stored sugars or....

b1 to c1: Both plants and other organisms break down....

a1, b1, c1 to d1: Plants make sugar molecules.Plants and other organisms. As a result (of these matter transformations in individual organisms), the chemical elements that make up the molecules of living things....

Another way to make connections among ideas about matter transformation would be to make comparisons between organisms in terms of which processes they are capable of carrying out. For example, students might be asked to compare how plants and animals make carbon-containing body structures, with the expectation that they would note that both plants and animals make body structures from sugars but that plants can build sugars from CO2 whereas animals must start from C6H12O6.

Or students could be asked to trace a carbon atom through the organisms in a hypothetical food chain, indicating the different molecules in which it could appear.

Connections among key ideas about transformations of energy. As with connections among key ideas about matter, connections among key ideas about energy transformation can be made quite simply if the key ideas are presented in close proximity to one another. Consider the following examples:

a2 to b2: Plants cannot use sunlight as a direct source of energy for maintenance or growth. Instead, plants must first transform the energy from light into "energy-rich" sugar molecules. Plants can then get energy to grow and function by oxidizing the sugar molecules that they have synthesized, releasing some of the energy as heat.

a2 to c2: Plants transfer the energy from light into "energy-rich" sugar molecules. Organisms that eat the plants oxidize the sugar molecules, harnessing some of it for maintenance and growth and releasing some of the energy as heat.

b2 to c2: Both plants and other organisms oxidize the sugars....

Or the material might show how one idea is a consequence of another. For example, a material might state Ideas b2 and c2, indicate that because usable energy is lost to heat at each link in a food web, food webs need a continual input of energy, and then state that the sun fulfills this continual need for energy (part of Idea d2).

Connections between key ideas about matter and energy transformations. When the ideas are treated in close proximity to one another, making connections between them could be done simply. For example, a material would be given credit for the connection between Ideas a1 and a2, if, when describing the synthesis of sugars in photosynthesis, it made clear that the sugar molecules store both carbon atoms and energy (in the configuration of atoms) (see an example connection in the Content Analysis report for BSCS Biology: An Ecological Approach). Check to be sure that the material is explicit about both the fate of the matter and the fate of the energy.

In principle, a material can begin its presentation of matter and energy transformations at any level of biological organization and still make connections between the key ideas; it need not present all the ideas in the context of ecosystems. Most textbooks present key ideas about matter and energy transformations in separate chapters on cell metabolism, human digestion, and ecosystems, often starting at the molecular level and working their way up to ecosystems. Hence, the treatment of matter and energy transformations in organisms occurs several chapters after its treatment in cells; and its treatment in ecosystems occurs several chapters after its treatment in organisms. This does not mean that a connection can't be made, but the textbook will have to work harder to do so. For example, when the book gets to the organism level, it could remind students about the key idea(s) at the cells level and then indicate that what occurs in organisms is the sum of what goes on in all their cells. Similarly, the material could relate organisms to ecosystems by indicating that matter and energy transformations in ecosystems are simply the sum of what goes on in all its organisms.

[Connections for this topic]

2. Connections between key ideas and their prerequisites

For a material to receive credit for a connection it must state both the prerequisite and the key idea and relate them in a relevant way. In other words, part of making a connection to a prerequisite is stating the prerequisite.

Food provides the molecules that serve as fuel and building materials for all organisms. (prerequisite to Ideas b2, c2, b1, and c1)
The most fundamental prerequisite (a grades 6–8 idea) is the definition of food. It is an important starting place for the topic matter and energy transformations because food is what students are familiar with (rather than sugar molecules). The prerequisite goes beyond the idea that organisms need food to grow or for energy by explicitly stating that food is the "stuff" that provides both fuel and building materials. Whether a material starts with substances or molecules, it needs to end up at the molecular level. At the substance level, food is the source of the building materials that organisms need to increase in mass and the source of the fuel that provides energy needed to carry out life functions (e.g., add new cells, convert inputs to outputs). At the molecular level, food provides the molecular building blocks that are assembled into body structures or used as fuel.

Making the connection between the idea that food molecules are fuel and the processes of making food and oxidizing food can be done by making explicit that food is chemical energy. For example, consider how BSCS Biology: An Ecological Approach makes the connection to Idea c2:

Where do you get your energy? It may take some imagination to see energy in a hamburger and a pile of french fries. There is energy in this food, however. It is chemical energy. Chemical energy is found in the structure of the molecules that make up the meat and the potatoes. Other forms of energy include electrical, mechanical, heat, light, and nuclear energy. The most important form of energy for you is the chemical energy stored in the food you eat. You begin to release this energy as you digest your food. Most of the energy from your food is released within your cells in a complex series of chemical reactions. You use this energy to grow and to develop.

p. 10s

and then to Idea b2:

The sugars formed by photosynthesis provide food for the plant. The plant then can use the energy in the sugars to grow and reproduce. Energy that is not used may be stored in the form of starch to be used at a later time.

p. 11s

Arrangements of atoms have chemical energy. (prerequisite to Idea a2)
This grades 6–8 prerequisite establishes the possibility that molecules can be "energy-rich" and thus frames the way key Idea a2 is to be viewed: sugars are "energy-rich" because of the arrangements of their atoms. One way to make a connection between this key idea and its prerequisite is to restate the key idea using the language of the prerequisite: the synthesized sugars are "energy-rich" because of the arrangement of carbon, hydrogen, and oxygen atoms that make it up. While this may sound like hand waving, recall that we do not hold high school materials accountable for teaching grade 6–8 prerequisites, only that they state them. So if students already knew that arrangements of atoms had chemical energy (for example, the arrangements of atoms in methane, propane, octane and other fuels have energy in the arrangements of atoms that can be readily converted to heat when they are burned) then it may only take hand waving to make the connection to biological molecules.

Different amounts of energy are associated with different configurations of atoms and molecules. Some changes of configuration require an input of energy whereas others release energy. (prerequisite to Idea d2)
This grades 9–12 prerequisite is more sophisticated than the 6–8 idea that arrangements of atoms have chemical energy but flows logically from it: if arrangements of atoms have energy, then different arrangements probably have different energies and hence molecules that change configuration may either gain or lose energy. Viewing energy transformations in organisms from this perspective allows us to see Idea a2 from a new lens (and to make a connection between it and this prerequisite): the synthesized sugars are "energy-rich" because the arrangement of carbon, hydrogen, and oxygen atoms in the sugar molecules has more energy stored in it than was stored in the original carbon dioxide and water molecules. Similarly, to make the connection to the ideas that "Plants and other organisms get energy...by oxidizing their food" (Ideas b2 and c2) a material could state that as the sugar molecules are oxidized to carbon dioxide and water, the configuration of the atoms changes from high to low energy configuration.

[As in physical systems] Energy can only change from one form into another. (prerequisite to a2, b2, and c2)
This grades 6–8 prerequisite provides a basis for thinking about energy-related processes in cells, organisms, and ecosystems. They are all just instances in the biological world in which energy changes from one form to another (rather than arising out of nowhere or disappearing). Assuming the prerequisite is presented, a connection could be made by applying the language of energy forms and transformations among them to the key ideas. For example, a material could make the connection between this prerequisite and the idea that "Other organisms...get energy to grow and function by oxidizing their food, releasing some of the energy as heat" (Idea c2) by noting that the chemical energy in sugars was transformed into two different forms of energy—a less transportable but more immediately usable form of chemical energy (ATP) and heat. Or even better, the text could state that the chemical energy in sugars could be converted into some mechanical energy (such as in contracting muscle cells) and heat.

An especially important kind of reaction between substances involves combination of oxygen with something else-as in burning or rusting. (prerequisite to Ideas b2 and c2)
This grades 6–8 prerequisite is setting up a potentially helpful analogy to oxidation. To make the connection, a material could indicate that oxidation involves the combination of oxygen with the sugar molecules (as in burning) but that unlike burning oxidation in cells of organisms doesn't release all the energy as heat. Cells are also able to transform some of the chemical energy into a form that can do a cell's work—like contracting muscles, carrying messages down nerves, moving cilia, and so forth. BSCS Biology: An Ecological Approach relates oxidation to burning (p. 77s) but never explains that they both involve the combination of oxygen with something else. Since the book does not state the prerequisite, it doesn't get credit for a connection to it. However, if a material presents Ideas b2 and c2 in terms of "breaking down" (rather than oxidizing), the idea is not really needed.

Most of what goes on in the universe.involves some form of energy being transformed into another. Energy in the form of heat is almost always one of the products of an energy transformation. (prerequisite to Idea d2)
This grades 6–8 prerequisite states the generalization that nearly all energy transformations give off heat. (The first sentence in the idea provides context for the second but is not prerequisite to the key ideas.) The heat given off by biological systems is not always as directly observable as in physical systems, so appreciating the generalization in physical systems (plus specific instances in plants and other living organisms—Ideas b2 and c2) makes its application to food webs more plausible. A connection could be made between this prerequisite and the idea that "At each link in a food web...much of the energy is dissipated into the environment as heat" (part of key Idea d2) by prefacing the statement of the key idea with the phrase, "As in physical systems" and then giving (or having students recall) examples in physical systems in which heat was a product of an energy transformation.

Carbon and hydrogen are common elements of living matter. (prerequisite to Idea a1)
This grades 6–8 prerequisite provides the information needed to understand a few simple transformations of matter in organisms. The more elaborate list of elements is not needed until students need to understand the generalization of matter conservation across elements (Idea d1). A connection could be made by stating the prerequisite, asking how these elements get incorporated into living matter (or our bodies), indicating that it all starts with plants, and then stating Idea a1. BSCS Biology: An Ecological Approach uses a similar approach to make the connection:

Of the more than 100 different elements found on the earth, only about 30 are used in the makeup of organisms. Most of these elements, such as the hydrogen found in a water molecule and the carbon found in the carbon dioxide of the air, are very common. Figure 1.12 shows the proportions of elements in humans.

Plants and animals are made up of many different compounds, but the atoms used to make up these compounds occur all around you in the non-living world. For example, plants use the simple compounds of carbon dioxide and water in photosynthesis....Using light energy, they link together the atoms from carbon dioxide and water to make sugars.

p. 12s

No matter how substances within a closed system interact with one another, or how they combine or break apart, the total mass of the system remains the same. The idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same. (prerequisite to Idea e)
This grades 6–8 prerequisite states the principle/law of conservation of matter, first in terms of mass and then in terms of atoms. To make a connection between this prerequisite and Idea e, a material could state that this same principle applies to living systems, give examples, and then state Idea e.

[Connections for this topic]

3. Connections between key ideas and related ideas

As with key ideas and prerequisites, a material can only earn credit for a connection if it states both ideas and relates them in some way. Since related ideas are often treated in different chapters, connections could be made either in these different chapters or in the chapters where the key ideas are treated. Both ideas need to be stated wherever the connection is made.

Within cells are specialized parts for the capture and release of energy. (related to Ideas a2, b2, and c2)
Here is where a material has a chance to relate energy transformation in cells to energy transformation in organisms. This 6–8 idea that "within cells, many of the basic functions of organisms—such as extracting energy from food..." relates a cell's structure to its function. One way to make the connection between this related idea and the ideas that "plants and other organisms oxidize sugar molecules to get energy to grow and function" (Ideas b2 and c2) would be to relate cell functioning to the functions of organs or of the organism. For example, a text could point out that while all cells have energy needs, tissues with higher energy needs (such as muscle) tend to have more mitochondria—the cell parts where most energy is released—than cells with low energy needs (such as bone).

Carbon atoms can easily bond to several other carbon atoms in chains and rings to form large and complex molecules. (related to Idea a1)
This related idea—that describes what is possible for carbon atoms—can reinforce the idea that plants "assemble" sugar molecules from carbon dioxide and water (Idea a1). A connection could be made between the two ideas by reminding students that living things don't violate basic chemical principles:

Carbon atoms can easily bond to several other carbon atoms in chains and rings to form large and complex molecules. And in fact they do. Plants take simple molecules of carbon dioxide and water and put them together to form rings and put the rings together to form more complex structures. All of this is possible because carbon can bond to other carbon atoms in rings or chains.

The chief elements that make up the molecules of living things are carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, calcium, sodium, potassium, and iron. (related to Ideas a1 and d1)
This related idea—that lists the chief elements that make up the bodies of all living things—can reinforce the ideas that these elements are incorporated into the molecules that plants synthesize (Idea a1) and recycled (Idea d1). One way to make a connection between this related idea and Idea d1 would be to have students consider what would happen if one or more of the elements was somehow "locked up" in the bodies of dead organisms. For example, the external skeletons of marine organisms are made of polymers of carbon atoms (in the form of chitin). When these organisms die, they deposit several tons of chitin on the bottom of the sea. What would happen if decomposers didn't recycle the carbon?

[Connections for this topic]