Systems

Students should understand that systems are artificial constructions created by people for certain purposes -- to gain a better understanding of the natural world or to design an effective technology.

Understanding the construct of a system entails identifying and defining its boundaries, identifying its component parts, identifying the interrelations and interconnections among the component parts, and identifying the inputs and outputs of the system.

Systems should be embedded in life science learning at the three grade levels in the following ways:

Grade 4

Systems should be approached at the level of organisms. Students should have broad and rich acquaintance with structure/function relationships as a precursor to a more thorough knowledge of organ systems by grade 8. Understanding examples of food chains and interdependencies among organisms, say, within an aquarium, are precursors to understanding complex systems.

Grade 8

Students should understand that an organism is made up of organ systems that have structure/function adaptations and interconnections among other organ systems.

Interdependence of plants and animals in communities should be understood by grade 8: plants consumers decomposers. Students should be able to explain specific examples such as purple loose-strife replacing cattails and the effects of the introduction of rabbits into Australia.

Disease and health should be understood in systems terms. If a part of a system is put out of kilter by disease, for example, the whole system is affected. Taking drugs or smoking by an individual may have an impact on another system (organism); for example, secondary smoking effects on children of smoking parents or fetal damage from drugs. A measles vaccine taken by an individual, or not taken, affects the whole population of a region or further, depending on migration patterns. If a specific animal or plant population becomes unhealthy (for example, fish poisoned, raccoons diseased, or species of grass infected by virus), the food chain and, therefore, the rest of the community are affected.

Grade 12

Ecosystems should be understood in their full complexity, including interrelationships of plants and animals with one another as well as with the physical components of a system. Students also need to recognize the effects of human activity on ecosystems and the limitations on human activity imposed by natural systems.

At this level, the cell should be understood both as a system in itself and as a component of a system.

Patterns of Change

Patterns of Change is a particularly valuable theme in the life sciences because a conceptual understanding of patterns of change can be developed in the context of several different levels in the hierarchy of biological organization. At the cellular/organismal level, the primary patterns of change are the growth and development that occur throughout the life of organisms. At the population level, the primary patterns of change are the changes in population growth over relatively short periods of time and the evolutionary changes that occur over longer periods of time. At the community/ecosystem level, the primary patterns of change are those that involve the nonliving and living components of ecosystems during the process of succession. Patterns of change may be linear, or they may be cyclical; for example, many of the patterns of change that occur within cells are related to homeostasis, in which a change leads to feedback reactions that result in a return to conditions that existed before the change. An understanding of cyclical patterns of change can also be developed in the context of ecosystems (nutrient cycles) and in the context of organisms (life cycles).

Regardless of the context in which an understanding of the patterns of change theme is developed, students should be able to recognize patterns of similarity and difference; to recognize how these patterns change over time; and to transfer their understanding of a familiar pattern of change to a new, unfamiliar situation.

To understand the conceptual basis for the patterns of change theme, students must begin to develop an understanding of major ideas by the 4th grade, continue to develop their understanding through the 8th grade, and integrate their understanding with their knowledge in the 12th grade.

Grade 4

Understanding patterns of change at the organismal level

• Life cycles (including growth and metamorphosis)

• Concept of biotic potential, birth rates, and survival rates

• Diversity of many types of plants and animals (an important preconcept for the understanding of evolution)

• Variation within species (focus on humans, dogs, and cats)

• Food chains (also important for the systems theme)

Grade 8

Understanding patterns of change at the organismal level

• Growth, development, and reproduction of the human organism

• Homeostasis of body systems

• Adaptation and natural selection, including learned and instinctive behavior

• Variation and similarity among many different organisms, including humans

• Food webs (also part of the systems theme)

• Environmental effects of human activity (also part of the systems theme)

Grade 12

Students should have acquired an understanding of the following concepts and developed the ability to integrate them into the patterns of change theme.

Understanding patterns of change at the cellular/organismal level

• Growth and development of cells, including an understanding of the importance of mitosis and meiosis

• Patterns of evolution, mechanisms for evolution, the consequences of evolution (such as speciation and diversity through time), and evidence for evolution

• Nutrient cycles and the impact of human activity on those cycles

• Succession, both natural and as a result of human disturbance

Models

Models of objects and events in nature can be used as approaches to understanding. As such, they have limits and involve simplifying assumptions but also possess generalizability and, sometimes, predictive power. Models are composed of groups of interrelated concepts selected to represent the interrelations of objects or events in nature or in the laboratory. Models need not be deemed correct to be useful but may represent attempts to help identify relevant variables to build evermore useful representations.

Models may be conceptual and consist of word descriptions or drawings. Models can also be mathematical, consisting of equations or other formal representations. Finally, physical models consist of physical objects that possess or represent some characteristics of the real thing.

The solar system is often modeled conceptually in the classroom by describing the planets as huge balls moving about an even larger Sun. A mathematical model of the solar system should include quantitative descriptions of the gravitational forces between the planets and the Sun as determined by their masses and distances from one another, and might include the shape of a planet's orbit as being elliptical. And finally, a physical model of the solar system might consist of a series of scale-size balls placed at appropriate distances throughout a room or hallway.

Other examples from the Earth and physical sciences include models of shorelines and continental plates as well as stick-and-ball models of molecules. Physical models, such as those of the eye, leaf, and human torso, have been used in the life sciences for decades. Experiments with animals serving as models of human beings have been used to understand the effects of medical treatments that might be useful in preventing human diseases, and bacteria have been used to model population growth and decay.

Similarly, conceptual models are common in both the biological and physical sciences. The simplified treatment of photosynthesis; the stages of meiosis and mitosis, accounting for an electrical current in a "water flow" analogy; and the characterization of gas molecules as bouncing balls are examples of commonly used conceptual models.

Mathematical models such as the gas laws and Newton's laws of motion are major components of the physical sciences. In addition, some mathematical models, such as Mendel's laws, have been part of the biological sciences for most of this century, whereas the Hardy-Weinberg formulation for describing ecosystems mathematically has become part of introductory biological knowledge more recently.

Models often serve as prototypes in technology, and in that case may be full-sized representations of the final product. However, models can be used to test the workings of technology without costly investments in full-scale objects. Small boats and airplanes are tested in tanks and wind tunnels before their full-size counterparts are built. In this way, many experiments can be tested inexpensively to optimize the design.

Models can be easily developed as a theme and can be linked to the immediate experiences of children because they have grown up with a variety of toys. Children readily understand that most toys are models that look like the real objects -- such as cars, airplanes, babies, and animals -- but do not possess all the attributes of those objects. Many of these toys are models, sometimes scale models, of objects from the natural world. For example, models of dinosaurs enable children to develop ideas about what these creatures were like.

The models theme has been selected because of the importance of enabling students to distinguish the idealizations of models from the phenomena themselves. Students need to understand that a model of the human eye does not represent all aspects of human eyes as they occur in human organisms. The model is a simplification, leaving unrepresented the many important variations in human eye structure, yet the simplification has utility in illuminating some features of the eye and enables new questions about the eye to be generated.

Students need to understand the limitations and simplifying assumptions that underlie the varied models used in the natural sciences. For example, beliefs that models are replicas of "real" objects or events can negate the critical concept of variation that many models do not take into account. Although generalized models, such as a generalized graph of growth in populations, are useful, they are not to be confused with a graph of the growth of a particular organism or population or with a graph of data from a single experiment.

Grade 4

At this level, models should be identified by students as representations of objects or events. Students can examine both conceptual and physical models in terms of how they are like and not like the object or event being represented. Examples can be models of insects, seeds, leaves, and other physical objects. These models and others in the sciences can be linked to children's experiences with scale models of cars, dinosaurs, doll furniture, and so forth.

Grade 8

Students should have knowledge of both conceptual and physical models and their uses and limitations. For example, when asked to illustrate their understanding of vertebrate structure and function with models of skeletons of different vertebrates, students need to be aware of variations in real skeletons and the generalized nature of the replicas.

Grade 12

Mathematical, physical, and conceptual models should be familiar to students beyond grade 8. It is appropriate to assess students' ability to formalize the concept of models and their uses and limitations in the natural sciences and in technology.

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