High School Biology Textbooks: A Benchmarks-Based Evaluation

Biology: A Community Context. South-Western Educational Publishing, 1998

Cell Structure and Function: Summary Content Analysis

This report summarizes the content analysis findings for the Cell Structure and Function topic across all nine high school biology textbooks evaluated.

Topic Maps: Cell Structure and Function

The Cell Structure and Function topic maps contrast the coherent set of ideas that the reviewers looked for with a composite of the treatment of those ideas found in all nine evaluated textbooks. The maps display the topic’s key ideas, prerequisite ideas, and related ideas (in boxes) and the connections among those ideas (lines and arrows):

You may find it helpful to print out these maps and refer to them as you read the rest of the Summary Content Analysis.

Treatment of Key Ideas

Idea a: Every cell is covered by a membrane that controls what can enter and leave the cell.

Typical treatment: Except for slight variations in wording, all textbooks were found to have a content match to this key idea. Textbooks present both parts of this idea, that a cell membrane defines the boundary of the cell and that the membrane controls what can enter and exit the cell, in statements such as:

All cells are separated from their surroundings by a cell membrane. The cell membrane regulates what enters and leaves the cell and also aids in the protection and support of the cell.

Biology by Miller and Levine, p. 90s

These statements, while clear and concise, are seldom further explained. Furthermore, these singular statements may be buried within too much detail about the membrane architecture or methods of transport across the membrane.

Textbooks typically include information about the cell membrane in a chapter about cells and provide additional detail about the processes of moving materials across the membrane (such as active and passive transport) in a separate chapter or section. In addition, textbooks include statements that explain what types of substances the cell membrane allows to enter and exit, such as food or other needed materials and waste products. Texts typically include brief explanations of the control function of the cell membrane, as well as figures and questions. Several textbooks provide analogies (such as a security guard, a window with a screen, the hull of a submarine, or the walls of a house) to clarify the role of the cell membrane.

Atypical treatment: Less commonly, textbooks provide statements about this key idea that are not as explicit as the one above. For example, the use of the terms “barrier” and “active transport” in the statement below obscures the basic idea of a cell membrane controlling what can enter and leave the cell:

The cell membrane acts not only as a barrier, but also as the site for active transport of nutrients into the cell and waste products out of the cell.

Biology: A Community Context, p. 207s

One textbook includes information about the cell membrane only in a broader context of homeostasis instead of a specific chapter about the cell. Also, a few textbooks include laboratory investigations that study diffusion through dialysis tubing. Unfortunately, diffusion through dialysis tubing is passive, and thus, not an entirely accurate model for the cell membrane. Since the active role of the cell membrane in controlling the entry of substances is not modeled, textbooks should explain (or involve the students in critiquing) the usefulness and limitations of models. However, this is not a feature of any of the textbooks.

Beyond: In addition to the basic information, most textbooks present 2–11 extra pages of information that goes beyond this key idea. Thus, statements that explain this key idea may be lost. For example, texts typically include details of membrane architecture (such as phospholipids and fluid mosaic model) and processes of selective transport (such as diffusion, osmosis, facilitated diffusion, proton pumps, active transport, endocytosis, and exocytosis). Similarly, textbooks often include diagrams representing the molecular structure of the lipid bilayer membrane and the proteins within it.

Idea b: Within the cell are specialized parts for the transport of materials, energy capture and release, protein building, waste disposal, passing information, and even movement.

Typical treatment: There is a content match to this key idea in most textbooks. Textbooks typically provide a general statement about this key idea as well as some specific examples of specialized cell parts and their functions. For example:

…[C]ells contain a variety of internal structures called organelles. An organelle is a cell component that performs specific functions for the cell.

Modern Biology, p. 71s

The powerhouses of the cell are the mitochondria….They not only help to release energy from food molecules but also help to synthesize building blocks of larger molecules.

BSCS Biology: An Ecological Approach, p. 106s

Information about cell parts and their functions are typically found in a single chapter or section about the cell. Most textbooks include a laboratory activity that has students examine different types of cells and identify the cell parts. Furthermore, most textbooks include diagrams and representations of specialized cell parts. However, laboratory activities and diagrams focus on identifying and labeling cell parts, rather than understanding function. Most textbooks also include questions for this key idea, but the questions typically involve rote recall of the names and functions of cell parts.

Atypical treatment: A few textbooks provide an incomplete content match for this key idea. These textbooks do not include information about all of the functions specified in this key idea. For example, some do not mention the role of waste disposal.

A few textbooks use subtitles in the student text to hint at helpful analogies. For instance, the subtitles in some textbooks compare specialized cell parts to the specific tasks in a factory (e.g., the recycling center or powerhouse) or the rooms in a mansion (with different rooms for specialized purposes). Some textbooks include a model building activity that involves students in making various models of the cell and its specialized parts. One textbook provides a novel question about this idea that requires students to explain an analogy that compares a cell to a small country.

Beyond: Many textbooks include a lot of information that goes beyond this key idea. Specifically, textbooks include information about the details of the structure of organelles, including technical terms such as cristae, stroma, granum, thylokoid, actin, lumen, and gluey pectin layer. Much of the focus of these sections seems to be on labeling structures in diagrams (organelles and their particular structures), instead of focusing on the functions and interrelatedness of the structures.

Idea c: The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins.

Typical treatment: There is a content match to this key idea in most textbooks. Textbooks typically treat this key idea in a general statement, such as:

Proteins are essential to all life. They provide structures for tissues and organs and carry out cell metabolism.

Biology: The Dynamics of Life, p. 164s

Sometimes, a brief summary of the work of proteins is found in a separate chapter on gene expression, in a statement such as:

…[P]roteins have many different functions. Some proteins play a structural role. Others are enzymes that act as catalysts in chemical reactions.

Modern Biology, p. 203s

Unfortunately, statements like the ones above do not explain how proteins serve many functions within the cell. Terms and phrases such as “carry out cell metabolism” and “play a structural role” do not provide concrete instances of the work of proteins within cells. Often, textbooks provide specific instances of the work proteins do, such as serving as enzymes or channels in the cell membrane. Textbooks typically discuss the function of enzymes as catalysts for chemical reactions, but do not explain that these chemical reactions are occurring in the cell or accomplishing tasks for the cell.

Most of the information presented about this idea is at the molecular level, in a section on biochemistry, when textbooks discuss the structure of proteins or the function of enzymes. Thus, proteins are not explicitly related to the functioning of cells. Furthermore, no activities or questions focus on this key idea.

Atypical treatment: A few textbooks provide an incomplete content match for this idea. Specifically, only instances of some of the roles of proteins are provided, with no mention of accomplishing work for the cell or other general explanations. Other textbooks provide only vague statements. The following example uses complex terminology such as the phrase “metabolic activities,” but does not provide any concrete examples of metabolic activities. Nor does this statement explain that these activities are going on in the cell:

Proteins also play a vital role in the metabolic (chemical and physical) activities of all living things. Proteins called enzymes assist the chemical reactions of metabolism.

Biology: Principles & Explorations, p. 35s

In contrast, a few textbooks provide statements that are clear and provide both a general statement of the key idea and specific examples, such as:

Proteins are the main workhorses of the cell; they form important structural components of the cell and provide energy. In addition, enzymes, which are proteins, are responsible for facilitating all of the life processes you have been examining. The breakdown of complex molecules, the release and capture of energy in new chemical forms, and the synthesis of new biomolecules from the building blocks—all are carried out by proteins.

Insights in Biology: The Matter of Life, pp. 91–92s

One textbook provides a clear description of the importance and function of proteins in the context of why cells make proteins:

The nitrogenous bases in DNA contain information that directs protein synthesis. Why proteins and not other molecules? you might ask. The answer can be found in the diversity of things that proteins are capable of doing. Because most enzymes are proteins, proteins control biochemical pathways within the cell. Not only do proteins direct the synthesis of lipids, carbohydrates, and nucleotides, but they are also responsible for cell structure and cell movement.

Biology by Miller and Levine, p. 148s

Unfortunately this description is not found in the section on biochemical molecules or in the section on the cell, but appears several chapters later in the chapter on protein synthesis.

Beyond: Most textbooks present information that goes beyond this key idea, such as the structure of proteins (including topics such as the amino group and dipeptide bonds), the formation of proteins, how channel proteins function, and how enzymes function.

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.

Typical treatment: Most textbooks provide an incomplete treatment of this key idea. Typically textbooks provide general explanations of how temperature and acidity affect enzymes or organisms but do not explain how these factors affect cells. For example:

Each enzyme functions best within a certain temperature range. When temperatures become too low or too high, reaction rates decrease sharply.

Biology: Visualizing Life, p. 81s

These explanations are usually found in a separate chapter or section on biochemistry and not well related to the sections on the cell. Specific instances of this key idea are not typically provided. Several textbooks provide a laboratory investigation in which students examine the effects of temperature and acidity on an enzyme. However, follow-up questions do not relate the investigation to the functioning of cells. Typically, no questions or representations focus on this key idea.

Atypical treatment: There is a wide range of treatment for this key idea. One textbook does not mention this key idea at all while another has a fairly complete description:

Two aspects of enzyme activity are very important to cells. Enzyme reactions are faster at high temperatures but only to a certain point. At higher temperatures, the enzymes may begin to lose their shape. Because fit is so important for proper enzyme action, enzymes that lose their shape no longer function. Enzyme activity also varies with the pH of the solution. Thus, the temperature and the pH must be at the right levels for enzymes to act effectively, as you observe in Investigation 4.3.

BSCS Biology: An Ecological Approach, p. 82s

Although this passage mentions that enzymes are important to cells, it does not make clear that cells will be affected by the malfunction of enzymes. A few other text explanations include a specific example of enzymes functioning best within a narrow range of temperature. The following example discusses enzyme malfunction and how it affects the organism (human being) but does not mention what happens to the cells.

For instance, many enzymes in your body shut down when you have a high fever. If the internal body temperature of a human being were to reach 44°C (112°F), many enzymes would be destroyed, and the individual would probably die.

Biology: Visualizing Life, p. 81s

Beyond: Very few textbooks present information that goes beyond this key idea or interrupts the flow of the presentation of this key 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.

Typical treatment: Most textbooks provide an incomplete treatment of this key idea. Typically textbooks provide a general statement about cell growth and division or protein synthesis being regulated, but do not further explain or provide specific examples. For example:

But cells do not always need to produce all of the proteins for which their genes contain instructions….Mechanisms to control gene expression have evolved to ensure that each protein is produced only when it is needed.

Modern Biology, p. 203s

Other statements address the latter part of this key idea, usually in a separate chapter or section on the endocrine system (however, the example below is taken from a section on proteins):

Proteins also function as hormones and neurotransmitters. In these functions, proteins serve as signaling devices that are involved in regulating the activities of the cells of organisms.

Biology: Principles & Explorations, p. 36s

Many textbooks discuss what happens when cells divide but do not mention the interactions of different molecules that cause the distinct cycle of cellular activities. Cancer is discussed as a consequence of the lack of regulation in cell division, likewise hormones are often mentioned as chemical signals between cells. This key idea often appears in a separate chapter or section about cell growth and division. Typically there are no questions, activities, investigations, or representations that focus on this key idea.

Atypical treatment: There is a wide range of treatment for this key idea. One textbook does not have a content match (has no mention of this key idea) while another has a fairly complete description:

Most biologists agree that a series of enzymes monitors a cell’s progress from phase to phase during the cell cycle. Certain enzymes are necessary to begin and drive the cell cycle, whereas other enzymes control the cycle through its phases. Occasionally, cells lose control of the cell cycle. This uncontrolled dividing of cells can result from the failure to produce certain enzymes, the overproduction of enzymes, or the production of other enzymes at the wrong time. Cancer is one result of uncontrolled cell division.

Biology: The Dynamics of Life, p. 217s

Another textbook includes a representation of regulated versus unregulated cell growth. It shows two Petri dishes of cells, one showing cells covering the bottom of the Petri dish, but have stopped dividing (regulated growth), and the other showing cells continuing to divide and pile up on one another (unregulated growth) (BSCS Biology: A Human Approach, p. E197).

One textbook provides a useful analogy for cell regulation:

Every cell must also be able to regulate when particular genes are used. Imagine if every instrument in an orchestra played at full volume constantly. All you would hear is noise! No orchestra plays that way, because music is not noise—it is the controlled expression of sound. Similarly, every function that a living organism carries out is the controlled expression of genes, each used at the proper moment to achieve precise effects.

Biology: Principles & Explorations, p. 190s

Beyond: Many textbooks include information that goes beyond the scope of this key idea; specifically, many textbooks include detailed information about 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).

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Building a Case

None of the textbooks attempts to present an evidence-based case for any of the key ideas.

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Connections

Connections among key ideas.
Overall, textbooks do not make many connections (i.e., explain the relationship) among key ideas about cell structure and function. The most common connection made in high school biology textbooks explains the role of the cell membrane (Idea a) in the communication between cells (part of key Idea e). This is often done by explaining the how proteins in the cell membrane function to receive chemical messages (for example, hormones) from other parts of the organism that affect the cell’s behavior. For example:

The information that a cell receives from its neighbors allows it to determine its position in the body, to adjust its metabolism to suit its particular function, and to grow and divide at the proper time. Without communication, multicellular life would not be possible….Cells in your body are constantly bombarded with news sent by other cells….Some receptor proteins are located on the cell’s surface and function like tiny antennas, transmitting information from the world outside the cell to its interior.

Biology: Visualizing Life, pp. 63–64s

This type of connection often is mentioned in a chapter on the endocrine system, a chapter that is widely separated from and not specifically related to the cell membrane. Thus, it is not an effective connection between these two ideas.

However, most statements and passages in textbooks that could be used to relate these key ideas to one another are not effective because they are too far away from where the key ideas are presented. Four textbooks have statements that could link the idea that proteins do the work of the cell (Idea c) to the idea that cell functions are regulated (Idea e). For example:

…[R]egulatory proteins…ensure that the events of cell division occur in the proper sequence and at the correct rate.

Modern Biology, p. 212s

Proteins also function as hormones and neurotransmitters. In these functions, proteins serve as signaling devices that are involved in regulating the activities of the cells of organisms.

Biology: Principles & Explorations, p. 36s

The former example appears in a section on gene expression and does not establish that regulatory proteins are performing a function for the cell, and the latter example appears in a section on the structure of proteins. Neither establishes that proteins are doing the work of the cell and thus cannot provide an effective link between these two key ideas.

In rare instances, a single passage in the student book seems to connect parts of three key ideas together. For example, one book explains that:

The ribosome’s only job is to make proteins. Each protein made in the rough ER has a particular function; it may become the protein that forms a part of the plasma membrane, the protein released from the cell, or the protein transported to other organelles.

Biology: The Dynamics of Life, p. 188s

This passage is found in the section on organelles. It mentions that parts of the cell make proteins (part of Idea b—specialized parts for specific functions), thus that the cell is making proteins (Idea c) and that these built proteins go on to serve the cell as part of the cell membrane (Idea a). However, without more emphasis, connections as dense as this, involving three key ideas, could be easily missed by students. Concrete examples could help call students’ attention to important relationships explained so briefly in text passages.

Other connections between key ideas are much less common, appearing in one or two textbooks. For example, one book explains how the idea the cells do work (Idea c) is related to the idea that cells function best within a narrow range of temperature and acidity (Idea d). The book explains that:

Enzyme reactions are faster at high temperatures but only to a certain point. At higher temperatures, the enzymes may begin to lose their shape. Because fit is so important for proper enzyme action, enzymes that lose their shape no longer function. Enzyme activity also varies with the pH of the solution. Thus, the temperature and the pH must be at the right levels for enzymes to act effectively.…

BSCS Biology: An Ecological Approach, p. 82s

This passage provides an appropriate link between proteins doing the work of the cell and the need for the proper temperature and pH. This book explains that the work of enzymes depends on its fit with other molecules, thus if the shape is changed, its ability to do work is changed.

Connections between key ideas and their prerequisites.
Prerequisite ideas are presented occasionally in textbooks. Only one prerequisite idea is mentioned in all of the textbooks, namely that atoms may join together to form molecules [4D(6-8)/1]. This is usually done briefly in the definition of molecule and found amid sophisticated and unnecessary details about the structure of the atom and types of chemical bonds. Even though all textbooks include this prerequisite idea, only one book relates it to the work of the cell (part of Idea c).

You can think of chemical reactions in cells as either gluing atoms or molecules together or tearing molecules apart. Extracting energy from sugar, for example, involves ripping apart the carbon backbones of sugar molecules. Making a protein is a matter of sticking amino acid molecules together to form long chains.

Biology: Principles & Explorations, p. 80s

This passage includes two examples of processes carried out in cells that are explained in terms of attaching or breaking apart molecules. Unfortunately, this nice connection is found two chapters after the initial presentation of prerequisite and the key idea and thus it is a questionable link between the two.

Other prerequisite ideas are found infrequently, usually in only two textbooks. The exception is the prerequisite that shapes are particularly important in how large molecules interact (part of prerequisite 4D(9-12)/8). This prerequisite is found in four textbooks, but the statements refer specifically to enzymes or proteins, not large molecules. One textbook uses the example of cystic fibrosis to explain how the shape of a protein in the cell membrane is important to cell’s functioning, thus relating the shape of a protein to completing a task for the cell.

Any changes in a protein’s structure can alter or impair its function and, in turn, threaten the cell’s normal activities. The effects can be devastating. Consider, for example, the disorder known as cystic fibrosis….In cystic fibrosis patients, the gene encodes for a misshapen protein channel that cannot function properly.

Biology: Visualizing Life, p. 71s

Connections between key ideas and related ideas.
Overall important related ideas are presented poorly in these textbooks and only infrequently are the related ideas connected to the key ideas. For the most part, the two related ideas about proteins are presented but only sometimes are there specific statements connecting them to the key ideas. In two textbooks, the related idea about the structure of the protein is sometimes connected to the work of the cell in a statement such as:

Proteins form into complex three-dimensional shapes to become key cell structures and regulators of cell functions.

Biology: The Dynamics of Life, p. 294s

This passage continues by providing examples of the structures and functions of proteins within cells. However, like other connective statements found in these high school biology textbooks, this statement comes several chapters after the sections that specifically address the structure of proteins and the work of the cell. Connective statements and statements that explain the relationship between key ideas must be in the same section as the key ideas they connect or the key ideas must be reestablished when the connection is made for it to be effective.

Likewise, the related idea that some proteins assist the cell in replicating genetic information, repairing cell structures, controlling what can enter and leave the cell, and generally catalyzing and regulating molecular interactions (AAAS, 1989, p. 63) is presented in most textbooks. However, the presentation is distributed throughout several chapters and is linked infrequently to the appropriate key ideas, namely, that cells assemble molecules (mostly proteins) to do work (Idea c), that cells function best within a narrow range or temperature and acidity (Idea d), and that cell functions are regulated (Idea e).

Lastly, some textbooks mention part of a related idea, that some cells have specialized tasks (AAAS, 1989, p. 63). However, textbooks do not explain the other part of this idea, that the specialized tasks of cells are in addition to the basic cellular functions common to all cells. Furthermore, only once is this related idea linked to the appropriate key idea, that cells have specialized parts for specialized functions (Idea b).

The related ideas about systems (including that systems often contain subsystems [11A(6-8)/3] and that parts of a system are related to others parts—specifically that the outputs of one part can be the inputs of another [11A(6-8)/2]) are not presented in these textbooks.

Outside of a few connective statements found in these textbooks, related ideas are not presented in ways that further explain or strengthen the understanding of the key ideas.

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