Project 2061

AAAS Project 2061 Biology Textbooks Evaluation

Key Ideas Used for the Evaluation

Topic: Cell Structure and Function

Ideas that served as the basis of the analysis were drawn from Science for all Americans, Benchmarks for Science Literacy, and the National Science Education Standards.

• Every cell is covered by a membrane that controls what can enter and leave the cell. In all but quite primitive cells, a complex network of proteins provides organization and shape and, for animal cells, movement.
• Within the cell are specialized parts for the transport of materials, energy capture and release, protein building, waste disposal, passing information, and movement.
• The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins.
• 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.
• 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.

• Matter is transformed in living systems.
  • Plants make sugar molecules from carbon dioxide (in the air) and water.
  • Plants break down the sugar molecules they have synthesized into carbon dioxide and water, use them as building materials for body structures, or store them for later use.
  • Other organisms break down the stored sugars or the body structures of the plants they eat (or in the animals they eat) into simpler substances, use them as building materials, or store them for later use.
  • 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.
• Energy is transformed in living systems.
  • Plants use the energy from light to make ``energy rich'' sugar molecules.
  • Plants get energy to grow and function by breaking down the sugar molecules. Some of the energy is released as heat.
  • 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 into the environment as heat.
  • There is an irreversible flow of energy from captured sunlight into dissipated heat:
    At each link in a food web, some energy is stored in newly-made structures but much is dissipated into the environment as heat (by energy releasing processes in cells).
    Each successive stage in a food web captures only a small fraction of the energy content of organisms it consumes. Continual input of energy from sunlight keeps the process going.
• 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.

• The information [for specifying characteristics of an organism] passed from parents to offspring is coded in DNA molecules.
• DNA molecules are long chains linking just four kinds of smaller molecules, whose precise sequence encodes genetic information.
• Genes are segments of DNA molecules. Each DNA molecule contains thousands of discrete genes.
• The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires.
• A change in even a single atom in the DNA molecule can change the protein that is produced.
• Inserting, deleting, or substituting DNA segments can alter genes.
• Mutation of a DNA segment may not make much difference, may fatally disrupt the operation of the cell, or may change the successful operation of the cell in a significant way.
• An altered gene may be passed on to every cell that develops from it.
  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.
• Heritable characteristics can be observed in molecular and whole-organism levels--in structure, chemistry, or behavior.

• The basic idea of biological evolution is that the earth's present-day species developed (over many generations) from earlier, distinctly different species.
• 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.
• 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.
• 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.
• New heritable characteristics can result from new combinations of parents' genes or from mutations of genes in reproductive cells.
• When an environment changes (in this sense, other organisms are also part of the environment), the advantage or disadvantage of characteristics can change.
• Natural selection leads to organisms that are well suited for survival in particular environments.

Continued: Criteria for Evaluating the Quality of Instructional Support