Vehicles for understanding and doing are the themes common to all science. Those selected for inclusion in this document come from Benchmarks for Science Literacy [6] and include the unifying concepts and processes for grades K-12 described in the National Science Education Standards. [7] Four major themes are identified - systems, models, constancy and change, and measurement and scale. It should be understood that the learning important for scientifically literate individuals might well be organized using a different set of themes. The model presented here is but one way of doing so.
Systems
A system is a group of objects, ideas, processes, and/or organisms that influence one another or function together. Systems are human creations that help people better understand their world. For example, it is easier to understand the digestive system, even in all its complexity, than it is to comprehend the entire system of the human body. For full understanding of the human body, an individual must know about all of the systems' parts, how they function and interact with other parts, and even how the entire system interacts with other systems. Sometimes, it is also useful to comprehend both the inputs to a system (energy into an ecosystem, for example) as well as its outputs (e.g., sound from a set of speakers).
Systems are functional and exhibit patterns of organization and hierarchy that are not random. For example, in the life sciences, cells make up tissues, which make up organs, which, in turn, fashion body systems. Thus, in order to understand how the digestive system processes a hamburger, french fries, and a soft drink into molecules that can be distributed to and used by individual cells, a student should know about the structure and function of each organ in the system, how these organs work together to accomplish the task, and how the various types of cells and tissues contribute to the process. Even each cell itself is a system made of many parts which must work together cooperatively to perform well the functions of the cell.
The purpose for which a system is created dictates the scope and size of that system. If a mechanic wishes to fix a car's air conditioning, then he or she might define the system as one which includes all of the mechanical and electrical components, including blowers, conduits, condensers, and electric controls. On the other hand, if the mechanic believes that the problem lies only in the electrical controls, then a much smaller (although still very complex) system can be defined. Thus, many systems are composed of smaller subsystems, with each subsystem having its own internal parts and interactions. Humanity itself can be considered a subsystem in some cases because the person is part of a complex culture and economic system, for example, while at the same time being a system unto himself/herself. Furthermore, a person is composed of various subsystems.
Models
Models are simplified representations of objects/ processes, or systems that help scientists understand and describe how things work. Models need not be accurate representations of phenomena: rather, they serve their purpose best when they facilitate understanding and learning. For example, the Bohr model of the atom is no longer considered to be an accurate representation of a real atom, yet it is helpful to younger students in understanding atomic theory.
Models can be physical, conceptual, or mathematical. Some physical models are devices that behave like the real thing, for example, a model car or airplane. Other physical models, referred to as "manipulatives," are used to simulate situations-for example, gumdrops and toothpicks to simulate the atoms and bonds in a molecule. Conceptual models explain the unfamiliar by comparing it to something familiar and are understood through imagery, metaphor, or analogy. Describing the flow of electricity through a conductor by comparing it to the plumbing system of a house is a conceptual model. Many times conceptual models do not fit all the attributes of the system that one is trying to understand. Sometimes a model is too simple; at other times only certain attributes of mathematical models describe how components of a system interrelate. Equations used in the physical sciences are such models because they describe phenomena such as force, current, and energy mathematically.
Well into the middle grades, students need to work primarily with physical models first and then with conceptual models. Because of limitations in their abstract thinking abilities they need concrete models to understand both simple and complex ideas. For example, the motion and relative position of the earth, moon, and sun produce eclipses that are better understood through the manipulation of a physical model. Likewise the action of the heart is better understood when compared to a pump, and the work of the digestive system is better understood when compared to a waste treatment plant. Computer simulations are useful conceptual models for furthering an understanding of complex scientific concepts.
As each student matures and becomes a more abstract thinker, increased emphasis can be placed upon mathematical models. Before this time, understanding the mathematics that describes a falling body is not usually possible. Yet once abstract reasoning is feasible, then the richness of mathematics added to conceptual cognizance makes many aspects of science more comprehensible .
Constancy and Change
Science and mathematics are often concerned with understanding, creating, and/or controlling change. Of course, change can vary widely from no change at all (we call this constancy), to almost infinite change. Other terms related to change and constancy are stability and equilibrium (terms that define a physical system when energy for action dissipates), conservation (when a quantity is reduced in one place, it is increased equivalently in another), and symmetry (which describes constancy of form). While these broad notions show up in all the sciences, stability, equilibrium, and conservation are most important to the physical sciences; conservation and symmetry are critical to the life sciences.
Some changes take place so slowly that they go almost unperceived by humans. Changes in the earth's crust, such as mountain building, fall into this category. Other changes, such as the relationship between the numbers of predators and prey, the recycling of matter in the ecosystem, or seasonal changes, are cyclical and need to be observed for extended periods of time in order to perceive the patterns of constancy and change. Still other changes take place so fast or occur in systems so small that people have come to use technology to make sense of them. Molecular biology and atomic physics include many examples of systems that cannot be studied directly.
The concepts of diversity, variation, adaptation, and natural selection relate directly to constancy and change. As the environment changes, natural selection chooses the expression of a variable trait which best enables the individual to adapt, survive, and reproduce. Gradually, over a long period of time, the species changes or evolves. When, a particular trait is observed for only a short time, however, little or no change in the population may be observed. Thus, the population trait being observed appears to be stable or constant. A very different conclusion would be reached if the same trait were studied over a long time interval.
Measurement and Scale
Measurement quantifies observations of objects and phenomena, making comparisons more accurate. Size, volume, area, weight, mass, temperature, distance, velocity, and many other attributes can be directly measured in science labs, in classrooms, and beyond the school walls; other attributes, such as evidence of the energy related to atomic particles, can be indirectly measured. Measurement is a critical theme and a crucial skill in science and operates throughout all the sciences. As each student's sophistication increases with age and experience, so too does the ability to make more precise measurements and to use more refined and complex instruments for measurement.
Scale within our world varies so widely that it can easily boggle the mind. Estimating the number of stars in the universe or comprehending the size of a subatomic particle is difficult even for the most experienced scientist. To do so, we use mathematical representations that in themselves are difficult to understand. Scale includes not only the idea of ratios and of the upper and lower limits of variables, but also the notion that some "laws" of science operate only within a certain range. Students' understanding of measurement and scale increases as they have a variety of experiences with magnitudes and the effects of altering them.
Table 5 presents the understandings that all of Georgia's students should master relative to the vehicles for understanding and doing discussed in this section. For a more complete discussion of these themes, see Chapter 11 of Benchmarks for Scientific Literacy (AAAS, 1993), Science For All Americans (AAAS, 1990), and the National Science Education Standards (NRC, 1996). Also refer to measurement and scale in Table 2 in the mathematics section of this framework and the measurement standards in Curriculum and Evaluation Standards for School Mathematics (NCTM, 1989).