High School Biology Textbooks: A Benchmarks-Based Evaluation

Project 2061 Analysis Procedure

Key Science Ideas Used in Evaluating the Textbooks' Content Alignment

In evaluating the content of the high school biology textbooks in this study, the review teams examined how completely each textbook's content aligns with specific key ideas in four topics: Cell Structure and Function, Matter and Energy Transformations, Molecular Basis of Heredity, and Natural Selection and Evolution. The ideas were drawn from Science for All Americans (American Association for the Advancement of Science [AAAS], 1989), Benchmarks for Science Literacy (AAAS, 1993), and the National Science Education Standards (National Research Council, 1996).

The key ideas for the Cell Structure and Function and Matter and Energy Transformations topics have been annotated below to provide users with additional guidance about what the reviewers looked for and how they judged the textbooks' content alignment.

Cell Structure and Function
(See what the reviewers looked for)

Idea a: Every cell is covered by a membrane that controls what can enter and leave the cell.
This idea has two components: (1) that every cell is covered by a membrane, which distinguishes the cell from its surroundings, and (2) that the cell membrane exercises control over what can enter and exit the cell. A full content match means that a textbook addresses both components. Details of the cell membrane’s architecture, such as the lipid bilayer structure of the membrane, structural features of transport proteins, the fluid mosaic model, and processes of selective transport across the cell membrane go beyond the expected level of understanding of this idea.

Idea b: Within the cell are specialized parts for the transportation of materials, energy capture and release, protein building, waste disposal, passing information, and even movement.
This idea presents the generalization that the cell has specialized parts that perform specific functions and gives examples of functions that the parts carry out. For a complete content match for this key idea, a textbook must both state the generalization and describe specific functions, such as energy capture and release. The names of the organelles are not needed and wrongly focus students and teachers on the names of the parts rather than their functions.

Idea c: The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. This idea has three components: (1) that cells make molecules, mostly proteins, (2) that proteins do the work of the cell, and (3) examples of cell work that is carried out by proteins. A complete content match for this key idea means that a textbook presents all three components. Specific examples of cell work by proteins, such as catalyzing cellular reactions and helping to move molecules across membranes, may help make this idea less abstract. However, details about protein synthesis go beyond the expected level of understanding for this idea.

Idea d: Most cells function best within a narrow range of temperature and acidity. At very low temperatures, reaction rates are too slow. High temperatures and/or extremes of acidity can irreversibly change the structure of most protein molecules. Even small changes in acidity can alter molecules and how they interact. Both single cells and multicellular organisms have molecules that help to keep the cells’ acidity within a narrow range.
This idea has three components: (1) that cells function best within a narrow range of temperature and acidity, (2) that these conditions are needed for the proper structure, and hence the function, of a cell’s proteins, and (3) that cells have molecules that help to maintain these conditions. For a complete content match, a textbook must present all three of these components and provide examples (and possibly counter-examples) of cells functioning in different conditions. Details of how particular amino acids are affected by changes in pH go beyond the expected level of understanding of this idea.

Idea e: Cell functions are regulated. Complex interactions among the different kinds of molecules in the cell cause distinct cycles of activities, such as growth and division. Cell behavior can also be affected by molecules from other parts of the organism or even other organisms.
This idea focuses on the regulation of cell functions, both internally and externally. For a complete content match, a textbook needs to both state the generalization and give examples of regulated activities, such as growth, division, hormones, and neurotransmitters. However, the details of the phases of the cell cycle (such as cytokinesis, G1, S, G2, interphase, and the stages of mitosis) and details about gene regulation (such as operon, lac operon, operator, repressor, and inducer) are beyond the expected level of understanding of this idea.

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Matter and Energy Transformations
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At the high school level, the Matter and Energy Transformations topic is to be treated at the molecular level. Ideas about matter transformation go beyond substances changing into new substances—textbooks must be explicit about atoms combining in new combinations during the transformations. Energy transformations are, as in grades 6–8, still to be viewed in terms of changing energy from one form to another. But now, chemical energy is defined in terms of the configurations of atoms: changing from lower energy configurations to higher energy configurations requires input of energy, whereas energy is released when higher energy configurations are changed into lower energy configurations.

Some materials in this study present considerable detail about metabolism that goes well beyond the level of sophistication of the key ideas. This did not affect judgments about alignment: as long as a key idea is explicitly stated somewhere (even if it is camouflaged in needless detail), a material should receive credit for alignment to a key idea. However, the inclusion of more sophisticated ideas that camouflage the key ideas is noted in the "Beyond Literacy" section of the Matter and Energy Transformations Content Analysis reports.

Another issue is whether the aligned material will be seen by students. For example, will students encounter suggested answers provided in the teacher's guide to questions in the student text? (We gave credit here but noted the basis for the credit in the report.) Will students encounter background information in the teacher's guide that is not accompanied by suggestions to convey the information to students? (We did not give credit here.) Related to this is the question of the author's intent. What happens if alignment occurs only in a stated objective or an assessment task but is not explicitly presented in text or activities? (We gave credit but noted what it was based on in the report.)

Most key ideas state generalizations. While it is certainly important for materials to provide examples to support the generalizations, a material must either point out that these examples are instances of the generalization or state the generalization in order to receive credit for alignment. For example, if a material describes digestion and cellular respiration in humans without making the generalization to other organisms, it was given an incomplete alignment to key Idea c1 or c2. Or if a material presents instances of matter cycles—such as the carbon cycle or the nitrogen cycle—but does not make the point that all elements cycle, it received only incomplete credit for alignment to key Idea d1. These and other issues are described below in the clarifications of the key ideas that were used by the reviewers.

Key ideas about matter transformation.

The essence of all these ideas is transformation (as opposed to just naming the reactants and products). It must be explicit that something is being transformed (or converted, made, changed, etc.) into something else. Furthermore, at the grades 9–12 level transformations of matter are dealt with at the molecular level (as opposed to substances changing into other substances).

Idea a1: Plants make sugar molecules from carbon dioxide (in the air) and water.
This key idea specifies a molecular treatment of the overall reaction for photosynthesis, not just starting and final substances. At the molecular level, the carbon, hydrogen, and oxygen atoms in carbon dioxide and water are rearranged to form sugars. Presenting the equation with chemical formulas of reactants and products gets credit, though it would be better if the accompanying text used such expressions as "atoms of carbon dioxide and water are rearranged to form sugar molecules" or "the carbon atoms in sugar molecules come from carbon dioxide and the hydrogen atoms come from water." However, the balanced equation is not necessary; using molecular models to illustrate how atoms are recombined to make sugar out of carbon dioxide and water would suffice.

Idea b1: Plants break down the sugar molecules that they have synthesized into carbon dioxide and water, use them as building materials, or store them for later use.
This idea describes the three possible fates of the sugar molecules plants make: plants can transform some of the sugar molecules back into carbon dioxide and water molecules, they can assemble them to make body structures, or they can store them for later use. A material gets credit for an incomplete match for covering fewer than three of these fates. Look to see there is something explicit about transforming molecules or about atoms combining or rearranging. And look to be sure that the material is explicit about what the sugar molecules will be used for (as opposed to being just used or used up). The material must also be explicit about plants. Although credit is given for a statement that "all living things" carry out this process, note in the report if this is the case.

Idea c1: Other organisms break down the stored sugars or the body structures of the plants they eat (or animals they eat) into simpler substances, reassemble them into their own body structures, including some energy stores.
This idea describes the three possible fates of consumed food in organisms that don't make their own food: food is broken down (during digestion) into simpler substances (e.g., sugars) and further transformed into carbon dioxide and water; the simpler substances can be reassembled into the body structures of the consumers; some of the reassembled structures (e.g., fat) can serve as storage forms for later use. As with Idea b1, credit for an incomplete match is given if all fates are not listed. Also, note if the material truly deals with "other organisms" or just with humans; though every part doesn't need to be treated in multiple organisms, at least one should be. The respiration equation gets credit for the breakdown part.

Idea d1: The chemical elements that make up the molecules of living things pass repeatedly through food webs and the environment, and are combined and recombined in different ways.
There are several parts to this idea: that chemical elements (as opposed to just examples of elements such as carbon or nitrogen) pass through food webs, that elements pass repeatedly through food webs (so more than one round in a cycle needs to be indicated), and that as the elements pass through food webs they are combined and recombined in different ways. To receive full credit for a content match, the material must treat all parts. The material gets incomplete credit for treating only some parts. Furthermore, full credit is given only if the three parts all deal with matter in terms of molecules (for an example of a text statement that deals with cycling of matter repeatedly but not at the molecular level, see page 13s of BSCS Biology: An Ecological Approach, which is cited in the Content Analysis report for that textbook). As with other ideas that state generalizations, describing instances without explicitly stating the generalization receives credit for an incomplete match. For example, giving examples of elements that recycle (such as in the carbon cycle or the nitrogen cycle) without making clear that all the elements that make up the molecules of living things recycle receives only incomplete credit for this part of the key idea (see, for example, the Content Analysis report for BSCS Biology: An Ecological Approach). "Repeatedly" means over and over again, so describing a single cycle of carbon through a food chain and back to carbon dioxide is insufficient for full credit. The material should indicate that plants can then reuse the carbon dioxide (that was released during respiration) to make sugars.

Key ideas about energy transformation.

The common feature of key ideas about energy transformation is that one form of energy is being converted into other forms—for example, light to chemical energy, chemical energy to chemical (or mechanical or electrical) energy and heat.

Idea a2: Plants transfer the energy from light to make "energy-rich" sugar molecules.
This idea makes explicit the transformation of light energy into chemical energy (or "energy-rich" sugar molecules). The material must go beyond stating that energy is used to make sugars or to convert carbon dioxide and water into sugars (because such statements could give the impression that energy is used up in the process). While some of the sun's energy is captured and only some of that is harnessed, some is both captured and stored in sugar molecules.

Idea b2: Plants get energy to grow and function by oxidizing the sugar molecules. Some of the energy is released as heat.
This idea describes the two fates in plants of the energy they stored in sugars: as plants oxidize (or burn or break down) sugars, plants can harness some of the energy stored in sugars (e.g., to make other molecules or body structures) and give some off as heat. Both fates are needed for a full match. The focus here is on the energy transformations; it does not matter whether matter is treated in terms of molecules or substances.

In addition, the material must convey the idea that the energy is used for something, such as growing, functioning, making new molecules, producing flowers, etc.

Although the term "oxidizing" appears in the key idea, the term is not required for a complete match. (It's not in Science for All Americans, though it appears in Benchmarks for Science Literacy and Atlas of Science Literacy.) However, some sort of active process must be conveyed—a kind of burning or even "breaking down"—to receive full credit. What isn't sufficient is the less sophisticated idea that organisms get energy from sugars; it must be clear that something is being done to the sugars.

Idea c2: Other organisms break down the consumed body structures to sugars and get energy to grow and function by oxidizing their food, releasing some of the energy as heat.
This idea describes the two steps that lead to the release of energy from sugars (break down or digestion to sugars and then oxidation of sugars), some of which is used to provide energy for growth and functioning and some of which is released as heat. As with Idea b2, the material can use "oxidizing," "burning," or even "breaking down to release energy" and still receive full credit. However, as with Idea b2, just indicating that organisms get energy from food is a less sophisticated idea (e.g., see AAAS, 1993, p. 136, 6C(3–5)/1). The material must also convey the idea that the energy is used for something, such as growing, functioning, making new molecules, replacing damaged tissue, etc.

Ideally, the material would make clear that other organisms (not just humans) do all these things; but reviewers should give credit for other organisms if at least one other organism was mentioned for at least one of the fates.

Idea d2: At each link in a food web, some energy is stored in newly made structures but much is dissipated into the environment as heat. Continual input of energy from sunlight keeps the process going.
This idea ties together the energy transformations involved in food webs and between food webs and the environment. Whereas Ideas a2, b2, and c2 deal with individual organisms, Idea d2 sums across organisms. The first part of the idea describes the two fates of energy in food webs-storage in the form of chemical energy (or in the bodies of organisms) and dissipation as heat. (It does not explicitly state that energy is also used for the growth and functioning of organisms. That is dealt with in Ideas b2 and c2. However, it's fine if materials make the connection.) The second part of the idea is the consequence of the first: since energy is continually dissipated away as heat (hence unable to be converted to chemical energy for use by organisms), energy must be continually supplied. For a material to receive full credit, it must state both fates of energy and indicate that continual input of energy is supplied by sunlight to keep the process going. The idea that the sun is the ultimate source of energy is insufficient and earns only incomplete credit.

It is important that materials convey that much of the energy is dissipated as heat—not just that some is. It is also important for the material to make it clear that not all of the energy is dissipated as heat, though the "10% rule" is not necessary. (In fact, biologists on our advisory group mentioned that the "10% rule" wasn't correct for most ecosystems.)

Finally, it is important for materials to make explicit that some of the energy actually goes somewhere else—for example, "used to make the organism's body tissue," "stored in organic material," or "used to make ATP energy." Stating only that energy is stored is insufficient and might convey the notion that "material-free" energy is stored.

Key ideas about matter and energy.

Idea e: 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.
This idea ties together the matter and energy stories at the ecosystem level, relates transformations within organisms and between organisms and the environment, and relates living organisms to the principles of transformation and conservation of matter and energy that apply to all natural systems. There are several parts to this key idea: (a) living organisms obey the same laws/principles of conservation and transformation of matter and energy as do physical systems, (b) matter and energy are transformed among living things and between them and the physical environment, (c) the total amount of matter and energy remains constant, and (d) the previous ideas hold even over long time spans. As with other key ideas, incomplete credit is given if a material treats only some of the parts.

To receive even incomplete credit, the material must talk about both matter and energy—conservation of matter alone and conservation of energy alone are grades 6–8 prerequisites. However, if the material treats conservation of matter (but not energy) and treats the idea that matter is conserved over long spans of time, it may be given incomplete credit.

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Molecular Basis of Heredity
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Idea a: The information [for specifying characteristics of an organism] passed from parents to offspring is coded in DNA molecules.

Idea b: DNA molecules are long chains linking just four kinds of smaller molecules, whose precise sequence encodes genetic information.

Idea c: Genes are segments of DNA molecules. Each DNA molecule contains thousands of discrete genes.

Idea d: The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires.

Idea e: A change in even a single atom in the DNA molecule can change the protein that is produced.

Idea f: Insertions, deletions, or substitutions in DNA can alter genes.

Idea g: A mutation of a DNA segment may not make much difference in the operation of the cell, may fatally disrupt it, or may change it in a significant way.

Idea h: An altered gene may be passed on to every cell that develops from it.

Idea i: When mutations occur in sex cells, they can be passed on to all cells in the resulting offspring; if mutations occur in other cells, they can be passed on to descendant cells only.

Idea j: Heritable characteristics ultimately produced in the development of an organism can be observed at molecular and whole-organism levels-in structure, chemistry, or behavior.

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Natural Selection and Evolution
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Idea a: The basic idea of biological evolution is that the Earth's present-day species developed (over many generations) from earlier, distinctly different species.

Idea b: Modern ideas about evolution (including natural selection and common descent) provide a scientific explanation for the history of life on Earth as depicted in the fossil record and in the similarities evident within the diversity of existing organisms.

Idea c: Natural selection provides the following mechanism for evolution: Some variation in heritable characteristics exists within every species, some of these characteristics give individuals an advantage over others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. The proportion of individuals that have advantageous characteristics will increase.

Idea d: Heritable characteristics strongly influence what capabilities an organism will have and how it will react, and therefore influence how likely it is to survive and reproduce.

Idea e: New heritable characteristics can result from new combinations of parents' genes or from mutations of genes in reproductive cells.

Idea f: When an environment changes (in this sense, other organisms are also part of the environment), the advantage or disadvantage of characteristics can change.

Idea g: Natural selection leads to organisms that are well suited for survival in particular environments.

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