Focus on Life Sciences

Grade Seven

Now is an exciting time for the study of life sciences. Knowledge of biological systems is expanding rapidly, and the development of new technologies has led to major advances in medicine, agriculture, and envi­ronmental management. A foundation in modern biological sciences, with an emphasis on molecular biology, is essential for students who will become public school science teachers, college and university science professors and researchers, and specialists in technological fields.

Another definitive reason for a focus on life science in grade seven is the stu­dents’ own biological and behavioral transition into early adolescence. Young ado­lescents make decisions that may have an enormous influence on their lives. The study of life science provides a knowledge base on which adolescents can make well-informed and wise decisions about their health and behavior. The relevance of the curriculum to students’ lives helps students to maintain an interest in science and to expand their knowledge of the natural sciences.

The Health Framework for California Public Schools is a valuable resource for science teachers. It contains grade-level expectations for health education that pro-vide important connections to the life science curriculum. Specific statutes require parental notification regarding the teaching of topics related to human growth and development.

port of materials in plants and animals. They were also first introduced to cellular functions when they studied cellular respiration in animals and plants and photo-synthesis in plants. These studies are complemented in grade seven by new material on the cellular organelles responsible for those functions.

The standards in grade six covered ecology, and students in that grade learned how energy in the form of sunlight is transformed by producers into chemical en­ergy through the process of photosynthesis. The study of energy transfer through food webs provided a foundation for a more detailed exploration at the cellular level of how plant chloroplasts capture sunlight energy for photosynthesis and how mitochondria liberate energy for the work that cells do.

1.             All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:

1.a.          Students know cells function similarly in all living organisms.

There are fundamental aspects of cell function that are similar regardless of the organism in which the cell resides. For example, cells contain a DNA (deoxyribo­nucleic acid) genome (i.e., all genetic material in a cell) and express the genome by using a universal genetic code. The biochemical pathways in cells, such as those for cell division and energy production, are strikingly similar even though the cells serve different functions in and between organisms. Many proteins synthesized by cells have similar functions, such as serving as enzymes that promote chemical reac­tions in the cell. There are significant functional differences between cells in an organism as they become differentiated, or specialized (e.g., a liver or a brain cell). There are also significant differences between cells in different environments, such as the Escherichia bacterium living in an intestine or a Thermophilus bacterium living in a superheated geyser. Biological science has been greatly advanced by the uncov­ering of both similarities and differences among cells.

1.b.          Students know the characteristics that distinguish plant cells from ani­mal cells, including chloroplasts and cell walls.

Plant cells are surrounded by a cell wall (made primarily of cellulose) that is rigid and limits the shape of the cell membrane. Animal cells, however, are not sur­rounded by a cell wall, and their shape is defined by their underlying cytoskeleton. Many plant cells contain chloroplasts and a central vacuole, neither of which is found in animal cells. Those differences between plant and animal cells may be made apparent by microscopy as sections of plant and animal tissue are appropri­ately stained to highlight the structures. Images of cells are also available on the Internet and in textbooks. Labeled diagrams will help students learn about struc­tures that are too small to be seen with the use of classroom microscopes.

1.c.          Students know the nucleus is the repository for genetic information in plant and animal cells.

Chromosomes containing genes reside in the nucleus. When an interphase cell is observed by using a light microscope, the inside of the nucleus may appear to be homogeneous because the chromosomal DNA is not condensed. In an appropri­ately fixed and stained section of onion root (obtainable from commercial sources), the DNA will be visible as a disk-shaped area, apparently constrained within a nucleus. This is the best stage in which to visualize DNA in learning the content of the standard. If the root tissue had a high rate of growth at the time it was sectioned and fixed, a fraction of the cells may be in one of the stages of mitosis. In that case the chromosomes will be visibly condensed but will not be limited by a nuclear membrane. This phenomenon must be explained carefully so that students do not

develop a misconception about the distribution of DNA in a cell on the sole basis of their observation of mitotic chromosomes.

1.d.          Students know that mitochondria liberate energy for the work that cells do and that chloroplasts capture sunlight energy for photosyn­thesis.

Students may already understand that the food they eat provides them with energy in an informal sense. At the cellular level the mitochondrion is responsible for efficiently extracting the chemical energy from molecules that have been broken down mostly from ingested food. The energy liberated by mitochondria is still stored in the form of chemical energy but in molecules that are readily accessible for energy release. Chloroplasts use pigments to absorb the energy in sunlight. This captured energy is used to drive a chemical reaction within the chloroplast in which carbon dioxide from the air is used as a source of carbon to form sugar molecules from which mitochondria extract energy used in the cell.

1.e.          Students know cells divide to increase their numbers through a pro­cess of mitosis, which results in two daughter cells with identical sets of chromosomes.

Just as living organisms are said to have a life cycle that relates to their periods of growth and reproduction, cells are said to have a “cell cycle.” Cells reproduce themselves by a process called mitosis. The process takes place after a period of growth during which the DNA in the nucleus is replicated and cytoplasmic or­ganelles, such as mitochondria and chloroplasts, are doubled in number. During mitosis the replicated DNA chromosomes are segregated so that each daughter cell receives exactly the same number of chromosomes of each type (e.g., two of each type in a diploid organism). Students may observe mitotic chromosomes by light microscopy in a stained section of rapidly growing tissue. Time-lapse videos and movies of cell division will also help to illuminate the process of chromosome segre­gation.

1.f.          Students know that as multicellular organisms develop, their cells differentiate.

In most multicellular organisms there is a division of labor among cells. Some cells in humans are brain cells; others are stomach, skin, or muscle cells. Although those cells are clearly different, their ancestry can be traced back to a single fertilized egg. During the development of an embryo, some cells become fixed in their devel­opmental program and are said to be differentiated. For example, cells that will eventually divide to give rise to the stomach and intestines are distinguished at a very early stage from cells that will divide to give rise to the central nervous system and eyes. At later stages of development, a more fine-grained differentiation takes place. For example, some cells in the retina of the eye become rod cells (for vision in dim light) and others become cone cells (for color vision). After differentiation, most cells in humans lose the ability to become other types of cells.

In plants the cells often retain the ability to differentiate into other tissues. For example, a leaf of an African violet can set roots in soil and develop into a new plant. Although the leaf is clearly differentiated, it is not fixed in its developmental potential in the way that animal cells typically are (an exception being the animal’s germ cells that lead to eggs and sperm).

ence. Gregor Mendel’s studies of pea plants revealed the concept of genes and the rules for the inheritance of traits. Today it is understood that those rules are based on the chemical composition and structure of DNA. Students in grade seven will learn some of those rules, which will serve as a foundation for high school biologi­cal sciences.

2.  A typical cell of any organism contains genetic instructions that specify its traits. Those traits may be modified by environmental influences. As a basis for understanding this concept:

2.a.          Students know the differences between the life cycles and reproduction methods of sexual and asexual organisms.

Sexual reproduction entails fertilization, an event in animals that requires the fusion of an egg cell with a sperm cell. The fertilized egg (the zygote) goes through a series of cell divisions (mitosis) and developmental steps to generate a new organ-ism genetically related to its parents. Pollination of flowering plants and growth of a new genetically related plant from seed should also be presented as examples of a sexual life cycle.

Some organisms exclusively reproduce without a fertilization event. This method is called asexual reproduction. Protists (single-celled eukaryotic organisms) often have no known sexual cycle and reproduce solely by mitotic division. Fungi and plants often have both sexual and asexual methods of reproduction. For ex-ample, plants may be propagated from a seed (sexual method) or a cutting (asexual method). Although a seed is related to two parental plants, a cutting is genetically identical to the plant from which it was taken. Some primitive animals, such as the flatworm Planaria, can divide themselves asexually into two genetically identical organisms. Asexual reproduction should not be confused with reproduction in primitive animals such as nematodes. Asexual reproduction should also not be con-fused with hermaphroditic sexual reproduction that entails fusion of eggs and sperm generated by a single organism.

2. b.         Students know sexual reproduction produces offspring that inherit half their genes from each parent.

Sexual reproduction combines the genetic material from two different cells. In most animal species, including humans, the genetic information is contributed from two different parents, nearly half from the biological mother and half from the biological father. Mitochondria DNA is derived solely from the mother, mak­ing possible the tracing of heritage from grandmothers to grandchildren with great certainty. During fertilization the egg and sperm cells combine their single sets of chromosomes to form a zygote containing two sets, or the diploid number, of chro­mosomes for a species (half from each parent).

2. c.         Students know an inherited trait can be determined by one or more genes.

In the preceding standard the idea of genes was introduced to students as some-thing inherited from each parent in roughly equal quantities. This standard draws a correlation between genes and the inherited traits or features of an organism. For example, attached or unattached earlobes is an inherited trait typically determined by a single gene (inherited from each parent). Having attached or unattached ear-lobes is very likely just one visible manifestation of that particular gene, which may have many other important roles during development that have not been cataloged. A single gene may affect more than one trait or feature in an organism. Many traits, such as hair and eye color, are determined by multiple genes and do not have simple patterns of inheritance. Although an organism’s genes define every inherited trait, there is not always a one-to-one correspondence between trait and gene.

2. d.         Students know plant and animal cells contain many thousands of differ­ent genes and typically have two copies of every gene. The two copies (or alleles) of the gene may or may not be identical, and one may be dominant in determining the phenotype while the other is recessive.

This standard introduces some principles of Mendelian genetics. The most sig­nificant concept is that genes exist in multiple versions, called alleles, and these units of heredity are not typically changed during mating. Prior to acceptance of Mendel’s laws, people believed that the mixing of genetic information was similar to mixing paint; the information (like red or white paint) could be blended to form a combined version (like pink paint) that could be blended still further (making it more white or more red). Using true-breeding strains of peas with variation of a single gene (such as flower color), Mendel showed that this model of blending was incorrect.

In grade seven students will learn that every person has tens of thousands of genes and that there are slight variations, or alleles, of these genes in every indi­vidual. Using the correct vocabulary is important: A person with a genetic disorder does not have the gene for that trait, but it might be said that the person has the genetic allele for that trait. Every person has every gene (and usually in two copies), but some people have an abnormal or different version (or versions) that can lead to a disorder or different trait. The genetic traits of an individual are determined by which alleles of genes are inherited from each parent and how those alleles work together. Some alleles are dominant, meaning that they overcome the influence of the other (recessive) alleles. In grade seven students learn to interpret the genotype-phenotype relationship in offspring (for example, on a premade Punnett Square diagram). In high school biology students will learn many of the details of genetics. Therefore, because this standard provides a foundation for transmission genetics in high school biology, the details of genetics (including the construction of the Punnett Square model) may be deferred.

2. e.         Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell.

Chromosomes in eukaryotes are complexes of DNA and protein. Chromo­somes organize the genetic make-up of a cell into discrete units. Humans, for ex-ample, have 23 pairs of chromosomes that vary in size. When looking through a microscope at an appropriately stained section of onion root tip, students may see cells that are engaged in mitosis and that have visible, condensed chromosome structures. The proteins in a chromosome help to support its structure and func­tion, but the genetic information of a cell is uniquely stored in the DNA compo­nent of the chromosome.

who traveled widely; students in grade three are retracing his steps when they de­velop their knowledge of organisms in a wide variety of earth biomes. Students in grade four learn about the survivability (or fitness) of plants and animals in an envi­ronment, and students in grade six are provided with a background in earth sci­ence. The standards in this set provide a foundation for learning about natural selection in grade seven and understanding the fossil record to be a line of evidence for the evolution of plants and animals.

3.             Biological evolution accounts for the diversity of species developed through gradual processes over many generations. As a basis for understanding this concept:

3 .a.         Students know both genetic variation and environmental factors are causes of evolution and diversity of organisms.

In grade two students learned that some characteristics of an organism are in­herited from the parents and that some are caused or influenced by the environ­ment. They also learned that there is variation among individuals in a population. This standard takes these simple ideas to much greater depth by explicitly referring to environmental factors and genetic variation. Environmental factors are a cause of natural selection, but as the term selection implies, there must also be favorable and unfavorable traits uncovered in the population. Genetic variability must precede natural selection, or there is some risk that no individuals in the population will survive a crisis. This principle is evident in the worldwide cheetah population and in other endangered species with much genetic homogeneity. Having little genetic variation to spread the risk makes a population more susceptible to extinction, for example, by succumbing to an infectious disease for which there is no natural resis­tance.

3. b.         Students know the reasoning used by Charles Darwin in reaching his conclusion that natural selection is the mechanism of evolution.

In his book On the Origin of Species by Means of Natural Selection, Charles Dar­win explained his line of reasoning for natural selection as the primary mechanism for evolution.5 Darwin proposed that differences between offspring would occur randomly. Some of those differences would be hereditary and affect an individual offspring’s ability to survive and reproduce within a particular environment and ecological setting. With the passage of succeeding generations, those individuals best suited to particular environments would tend to have more progeny and those less well suited would have fewer progeny or even become extinct. Darwin called this process natural selection because environmental and ecological conditions es­sentially “select” certain characteristics of plants and animals for survival and repro­duction. Darwin proposed that over very long periods of time, natural selection acting on different individuals within a population of organisms might account for all the great varieties of species seen today and for the great number of extinct and nonextinct species found in the fossil record. Darwin’s proposal that natural selec­tion is the mechanism for evolution was drawn in part from the ideas of Thomas Malthus’ Essay on the Principle of Population.6 Malthus presented his argument that human populations have a tendency to grow faster than their food supply, causing shortages and a “struggle for existence.” Darwin’s observations in the Galapagos Islands led him to think that this “struggle for existence” might be generalized to animals and plants.

 3. c.        Students know how independent lines of evidence from geology, fossils, and comparative anatomy provide the bases for the theory of evolution.

Independent lines of evidence from geology, the fossil record, molecular biol­ogy, and studies of comparative anatomy support the theory of evolution. Many decades before Darwin proposed his theory, geologists knew that sedimentary rocks formed an important history of life on Earth. Geologically younger rock layers are usually near the top, and older layers are successively closer to the bottom of sedi­mentary formations. Sometimes the normal sequence of sedimentary layers has been overturned by tectonically caused folding and faulting, resulting in older rock units resting on top of younger units.

Some of the organisms that lived in or were buried by the original sediment were preserved as fossils while the sediment hardened into rock. The process of fossilization preserves evidence of ancient life forms, and geologic interpretation of the enclosing sedimentary rock yields valuable information about the environments in which those ancient organisms lived. Paleontologists find more recently evolved organisms in the geologically younger layers of sedimentary rocks and more ancient life forms in the older layers of rocks. Original material (e.g., shell and/or bone) may be preserved as found, but chemical means may sometimes be used to alter or preserve it.

Radioactive dating provides another highly accurate method of confirming the age of rocks and fossils. Comparative anatomists study similarities and differences among organisms. Anatomists have been able to discover significant similarities in the skeletal architecture and musculature of all vertebrates from fish to humans. The most plausible explanation for this finding is that all vertebrates descended from a common ancestor.

3. d.         Students know how to construct a simple branching diagram to clas­sify living groups of organisms by shared derived characteristics and how to expand the diagram to include fossil organisms.

Evolutionary relationships among living organisms and their ancestors can be displayed in a diagram that resembles a branching tree. Groups of similar living species belong to a genus, similar genera belong to a family, similar families belong to an order, similar orders belong to a class, and similar classes belong to a phylum. Working back in time from the shared derived characteristics of each living species contained in the diagram will show the evolutionary relationships leading to a com­mon ancestor. The classification of organisms according to their characteristics is called systematics. It is based on a system developed in 1758 by the Swedish botanist and explorer Carolus Linnaeus.

3. e.         Students know that extinction of a species occurs when the environ­ment changes and the adaptive characteristics of a species are insufficient for its survival.

Extinction of a species occurs when the adaptive characteristics of the species are no longer sufficient to allow the species to survive under changing environmen­tal conditions. Evidence from the fossil record indicates that most of the species that once lived on Earth are now extinct. Biological adaptations are produced through the evolutionary process. Random mutations in the DNA of different in­dividuals (plants or animals) produce variations of particular traits in a population of organisms. These mutations result in some individuals acquiring characteristics that give them and their offspring an advantage in surviving and reproducing in their present environments or in a different environment. The offspring of indi­viduals in which these advantageous characteristics are not present may decline in numbers and eventually become extinct, or they may continue to coexist with the offspring of individuals that have the mutational advantage. Natural selection will then lead to the existence of populations better able to survive and reproduce under any one particular environmental condition. However, when particular adaptive characteristics of a species are no longer sufficient for the survival of that species under changing environmental conditions (such as increased competition for re-sources, newly introduced predators, loss of habitat), that species may become ex-tinct. There are many different environmental causes of the extinction of species.

focus in this standard set is on using the geologic evidence to better understand life on Earth, past and present.

This standard set presents two great ideas from the geologic sciences to make clear the relationship between life and geology: (1) the concept of uniformitarian-ism; and (2) the principle of superposition. Uniformitarianism refers to the use of features, phenomena, and processes that are observable today to interpret the an­cient geologic record. The idea is that small, slow changes can yield large cumula­tive results over long periods of time. Standard 4.c states a simplified version of the principle of superposition when it indicates that the oldest rock layers are generally found at the bottom of a sequence of rock layers. The principle of superposition is the basis for establishing relative time sequences (i.e., determining what is older and what is younger). Geologic records indicate that both local and global catastrophic events have occurred, including asteroid/comet impacts, that have significantly affected life on Earth. Both the evidence and the impact on life should be addressed in this standard set.

4.             Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:

4.a.          Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time.

This standard approaches two different but related ideas in the geologic sci­ences. The first (uniformitarianism) uses the present as the key to the past. For ex-ample, ripples preserved in ancient sedimentary rock are identical to ripples made by running water in mud and sand today. This idea is only one example of how geologists use the present to interpret features and processes in the geologic past. The second idea (superposition) states that the vastness of geologic time allows even very slow processes, if they continue long enough, to produce enormous effects. Perhaps the most important example of this idea is the dramatic change in the ar­rangement of the continents (continental drift) caused by the slow movement of lithospheric plates (approximately 5 centimeters per year) during the course of many millions of years. One piece of evidence for plate tectonics, including Pangaea, is the fossil record. The coherence of species in the fossil record is seen when geologic history is properly understood.

4. b.         Students know the history of life on Earth has been disrupted by major catastrophic events, such as major volcanic eruptions or the impacts of asteroids.

The subject of major catastrophic events is important because such events, al­though rare in the history of Earth, have had a significant effect on the shaping of Earth’s surface and on the evolutionary development of life. Most of the time geo­logic processes proceed almost imperceptibly, only to be interrupted periodically by the impact of a large meteor or by a major volcanic eruption. The immediate effect of both types of catastrophic events is much the same: injection of large amounts of fine-grained particulate matter into the atmosphere, an event that may have imme­diate regional or even global consequences for the climate by causing both short-and long-term changes in habitats.

4. c.         Students know that the rock cycle includes the formation of new sedi­ment and rocks and that rocks are often found in layers, with the oldest generally on the bottom.

Whenever rocks are uplifted and exposed to the atmosphere, they are subject to processes that can break them down. Purely physical processes, such as abrasion and freezing/thawing cycles, break rocks into smaller pieces. At the same time reac­tions with constituents of the atmosphere, principally acidic rain and oxygen, may cause chemical changes in the minerals that constitute the rocks and result in the

formation of new types of minerals. The net result is called sediment. It consists of rock and mineral fragments, various dissolved ions, and whatever biological debris happens to be lying around. The sediment is removed by erosion from the sites where it formed and is transported by water, wind, or ice to other sites; there the sediment is deposited and eventually lithified to form new sedimentary rock. The biological portion of accumulated sediment may be fossilized and preserved, pro­viding a partial record of existing life in the source area of the sediment.

Superposition, the fossil record, and related principles, such as crosscutting and inclusions, together form the basis for dating the relative ages of rocks. Students should realize that relative dating establishes only the order of events, not quantita­tive estimates of when those events actually occurred.

4. d.         Students know that evidence from geologic layers and radioactive dating indicates Earth is approximately 4.6 billion years old and that life on this planet has existed for more than 3 billion years.

Relative age-dating (see Standard 4.c) provides information about the relative sequence of events in the history of Earth. Absolute dating (putting a numerical estimate on the age of a particular rock sample) requires the use of a reliable “clock” in the form of the radioactive decay of certain naturally occurring elements. Those elements are disaggregated into the various minerals at the time those minerals are formed, generally during the crystallization of igneous rocks. Thus the newly formed minerals in the igneous rock contain only the original radioactive form of the element (parent) and none of the products of radioactive decay (daughter prod­ucts), which are different from the parent. The rate of transformation by radioac­tive decay from parent to daughter elements can be measured experimentally. This rate is usually expressed as a half-life, which is defined as the amount of time it takes to change one-half of the atoms of the parent element to daughter products.

Earth’s surface is always being reworked because of plate tectonics and erosion; therefore, very little of the planet’s original material is available for dating. How-ever, moon rocks and meteorites, thought to be the same age as Earth, can also be dated. All the available evidence points to Earth and the solar system being ap­proximately 4.6 billion years old. The earliest rocks containing evidence of life are slightly more than 3 billion years old.

4. e.         Students know fossils provide evidence of how life and environmental conditions have changed.

Fossils provide evidence of the environments and types of life that existed in the past. As an ancient environment changed, so did the organisms it supported. Thus environmental changes are reflected by the classes of organisms that evolved during the period of environmental change. Uniformitarianism is the foundation on which these interpretations are based. For example, ancient animals exhibiting approximately the same shell shape and thickness as that of the modern clam prob­ably lived in the same environment as clams do today. By examining fossil evidence and noting changes in life types over time, geologists can reconstruct the environ­mental changes that accompanied (perhaps caused) the changes in life types.

4. f.         Students know how movements of Earth’s continental and oceanic plates through time, with associated changes in climate and geo­graphic connections, have affected the past and present distribution of organisms.

Darwin’s work on finches in the Galapagos Island demonstrated clearly the effect of isolation on the distribution of organisms. Geographic separation of indi­viduals in a species prevented the populations from interbreeding. This separation may have led to the accumulation of genetic changes in the two populations, changes that eventually defined them as different species. Plate tectonic movements of lithospheric plates and the uplift of mountain ranges divided (albeit slowly) populations of plant and animal species and isolated the divisions from one an-other. This principle was illustrated in the fossil record of dinosaur species. Some dinosaurs, as well as other species that were restricted to specific continents after geologic separation, were uniformly distributed prior to continental separation.

4. g.        Students know how to explain significant developments and extinc­tions of plant and animal life on the geologic time scale.

Many changes that life has undergone during the history of Earth have been gradual, occurring as organisms adapt to slowly changing environments, evolve into new species, or become extinct. This principle is a fundamental tenet of uni­formitarianism. But even uniformitarianism is not consistently true. For example, very early Earth on which the first life appeared was considerably different from the planet of today. Little oxygen was in the atmosphere, and no ozone layer was in the stratosphere to protect against harmful solar radiation. The earliest life was there-fore anaerobic and had to be protected from solar radiation. Evidence for this single-celled life can be found in rocks that are slightly more than 3 billion years old.

Photosynthetic cyanobacteria, once referred to as blue-green algae, were an early addition to the prehistoric ecosystem. These early organisms are seen in the fossil record and were very successful, so much so that they still exist worldwide and are essentially unchanged in form after billions of years.

The slow change in Earth’s life has been punctuated by sudden events—cata­strophic ones—when viewed on the vast geologic time scale. One such remarkable event occurred about 600 million years ago. It is known as the Cambrian Explo­sion because of the sudden appearance of many different kinds of life, including many new multicellular animals that, for the first time, had preservable hard parts, such as shells and exoskeletons.

At various times life on Earth has also suffered from catastrophic mass extinc­tions in which the vast majority of species quickly died out. The greatest such event happened about 250 million years ago toward the end of the Paleozoic Era. It is

known as the Permian extinction, and as much as 90 percent of marine species may have died out. Another famous mass extinction occurred at the end of the Meso­zoic Era and is known as the Cretaceous-Tertiary (K-T) extinction. At the time all species of dinosaurs died out, as did about half of all the plant and animal groups. Evidence is mounting to indicate that this catastrophic event was caused by the impact of an asteroid.

grade three studied the external physical characteristics of organisms and consid­ered their functions as a matter of adaptation. Students in grade seven will deepen their understanding of internal structures, a topic that was introduced in grade five.

Anatomists and physiologists consider at different levels the internal structures of living organisms. Mammals have discrete organs, many of which work together as systems. For example, the adrenal and pituitary glands are parts of the endocrine system, and the kidneys and bladder are parts of the excretory system. Flowering plants have tissues, such as xylem and phloem, that are part of a vascular system. Organs themselves may have specific tissues; for example, the white and gray mat­ter of the brain can serve multiple functions. The pancreas produces both digestive enzymes and blood hormones.

Students in grade seven learn about the musculoskeletal system, the basic func­tions of the reproductive organs of humans, and the structures that help to sustain a developing fetus. Students also study the intricate structures of the eye and ear, which have well-understood functions in sight and hearing. Although many topics are covered in this section, they are all grouped in the fields of anatomy and physi­ology.

5.             The anatomy and physiology of plants and animals illustrate the complementary nature of structure and function. As a basis for understanding this concept:

5.a.          Students know plants and animals have levels of organization for structure and function, including cells, tissues, organs, organ systems, and the whole organism.

Protists, such as amoebae, consist of only one cell. All the functions necessary to sustain the life of these organisms must be carried out within that one cell. Multicellular organisms, such as plants and animals, tend to have cellular special­ization (differentiation), which means individual cells or tissues may take on spe­cific functions within the organism. For example, the musculoskeletal system of animals comprises individual muscle groups (e.g., biceps) that are bundles of muscle fibers, which are themselves groups of muscle cells, working together to make pos­sible movements of the organism. Within individual muscle cells are organelles, such as the mitochondria, that help provide the energy for muscle contraction.

5.b.          Students know organ systems function because of the contributions of individual organs, tissues, and cells. The failure of any part can affect the entire system.

Students learned in grade five how blood circulates through the body and how oxygen, O2, and carbon dioxide, CO2, are exchanged in the lungs and tissues. The pulmonary–circulatory system functions as a whole because of the functions of its individual components. A person may die from a heart attack (from failure of the heart), suffocation or pneumonia (from insufficient gas exchange in the lungs), shock (from loss of blood volume), or a stroke (sometimes caused by an insufficient gas exchange with brain tissues due to the blockage of blood vessels).

5.c.          Students know how bones and muscles work together to provide a structural framework for movement.

The skeletal system in animals provides support and protection. Muscles are attached to bones by tendons and work in coordination with the bones and the ner­vous system to cause movement through coordinated contraction and relaxation of different muscle groups. For example, a muscle in the arm called the biceps causes bending of the arm at the elbow so that the angle between the bones (humerus and ulna) decreases. The triceps on the back of the arm causes bending so that the same angle increases. This flexion and extension of the arm is a good example of muscle groups that are coordinated. Even in a lifting motion in which one of those two muscle groups is ostensibly the prime mover of the bone (e.g., “curling” a weight with the biceps), the opposing muscle group is involved in producing a smooth, controlled motion of the arm and protecting the joint from strong contraction.

5.d.          Students know how the reproductive organs of the human female and male generate eggs and sperm and how sexual activity may lead to fertilization and pregnancy.

In males the testes in the external scrotum are the reproductive structures that produce sperm. Immature sperm cells in the walls of the seminiferous tubules of each testis mature into flagellated cells that are transported and stored in the epi­didymis. During sexual arousal millions of sperm may be transported to the urethra and ejaculated through the penis. Some sperm may exit through the penis before ejaculation (i.e., without the man’s knowledge), and sexual activity that does not result in ejaculation may nonetheless lead to the release of sperm, fertilization, and pregnancy.

In females the ovaries are the reproductive structures that produce and store eggs, also called oocytes (pronounced “oh-oh-sights”). An egg develops within an ovarian structure called a follicle. A mature follicle can rupture through the wall of the ovary, releasing the egg during the process of ovulation. The egg is then trans-ported by one of the Fallopian tubes to the uterus. If the female, at or around this time, engages in sexual activity that results in sperm being deposited in or near the vagina, a sperm cell can travel through the vagina to the uterus or Fallopian tubes and fertilize the egg. A fertilized egg may implant in the uterus and develop, mean­ing that the female is pregnant and may deliver a baby approximately nine months later. If the fertilized egg fails to implant and begin development, or if the egg is not fertilized, it will be sloughed off along with several layers of cells lining the uterus and leave the female’s vagina during menstruation. One of the first signs of pregnancy is that a woman’s regular monthly menstrual cycle stops.

5.e.          Students know the function of the umbilicus and placenta during pregnancy.

The placenta is an organ that develops from fetal tissue in the uterus during pregnancy. It is responsible for providing oxygen to the developing fetus. The um­bilical cord (which enters the body at the umbilicus, or navel) is a cord containing arteries and veins that connect the fetus to the placenta. Although the blood of the mother and of her fetus do not mix together, oxygen and nutrients pass from the mother’s blood to the fetus. Wastes, such as carbon dioxide from the fetus, are re-moved. The placenta helps to nourish and protect the fetus; however, most drugs and alcohol can easily pass from the mother’s blood into the blood of the fetus, as can many infectious viruses, such as the human immunodeficiency virus (the source of AIDS).

5.f.          Students know the structures and processes by which flowering plants generate pollen, ovules, seeds, and fruit.

Flowering plants, or angiosperms, reproduce sexually by generating gametes in the form of sperms and ova. The reproductive structure of the angiosperms is the flower, which may contain male or female parts or both. Stamens are the male re-productive structures within the flower. Each stamen is composed of an anther, the structure that produces pollen granules, and a filament, the long thin stalk that con­nects the anther to the base of the flower (receptacle). The pistil is the female repro­ductive structure located in the center of the flower. The pistil consists of the stigma, which receives the pollen grains, and the style, a long thin stalk that acts as a guide for the pollen tube. The pollen tube, in turn, provides a migration path for the sperm of the pollen grain down to the ovary at the base of the pistil. The ovary contains one or more ovules, inside of which develop the ova. After fertilization the ovule develops into a seed with the developing embryo inside surrounded by a food source (the endosperm) for the plant embryo. The surrounding ovary may then enlarge and mature into a fruit that can contain one or more seeds.

 5.g.        Students know how to relate the structures of the eye and ear to their functions.

The eye works much like a camera. The eye is equipped with a lens that brings an image into focus on a sheet of light-sensitive cells called the retina, which is equivalent in a camera to a sheet of film or a video chip. The amount of light enter­ing the eye is controlled by the iris, which is an adjustable circular aperture. In bright lighting the iris contracts and the pupil (the open area that appears black) be-comes smaller in diameter to admit less light. In dim lighting the iris relaxes and the pupil becomes larger to admit more light. The lens of the eye refracts (or bends) the light, much as a magnifying glass does, and focuses an image on the retina. The lens is flexible, and its shape changes when focusing on nearby or distant objects. The retina contains cells that are sensitive to bright colors (cone cells) and others that are sensitive to dim lighting (rod cells). The cells in the retina generate an electrical sig­nal that travels to the brain, which can interpret the visual pattern. Investigative ac­tivities with lenses may be practiced both in this standard set and in Standard Set 6, “Physical Principles in Living Systems,” which describes the optics of sight.

The external ear (i.e., the part that can be seen) helps to collect sound waves and direct them to the middle ear. Many mammals (e.g., cats and many breeds of dogs) can redirect their external ears to detect faint sounds and determine the direction from which a sound is coming. The middle ear consists of a vibrating eardrum, or tympanic membrane, and three small bones (the malleus, or hammer; incus, or anvil; and stapes, or stirrup) that form a series of levers connecting the eardrum to the in­ner ear. Two small muscles control the tension on the eardrum and middle ear bones to reduce or increase the loudness of sound being transmitted. The inner ear, or labyrinth, contains the sensory cells that turn the waves of sound or pressure into electrical signals that are sent to the brain.

Students may explore the structure of the mammalian eye by performing a dis­section. They should be able to identify and explain the function of the different parts of the eye. Students may learn the structure and function of the human ear by building a model from simple materials. Students should be able to identify the dif­ferent parts of the ear and explain how those parts work together to transmit sensory information through sound waves. The sensory cells lining the cochlea are stimu­lated by the sound waves, causing nerve impulses to be transmitted through the au­ditory nerve to the brain.

Suggestions are made to relate the study of these topics to the eye, muscles, bones, tendons, and heart.

The human eye contains receptors that detect incoming visible light emitted by a luminous object or reflected from an illuminated object. Until the early 1900s physicists believed that the properties of light could be completely understood by viewing light as a wave of electromagnetic energy that was supported by an elusive medium—the so-called ether—that was imagined to pervade even a vacuum. The nature of light still seems mysterious to most people because light manifests the properties of both a wave and a particle. In most experiences geometric optics, which treats light as rays traveling in straight lines, adequately accounts for reflec­tion and refraction, mirrors, and lenses. Before starting these topics, students should be able to measure angles, do ratio and proportion problems, and use gram mass weights and metersticks.

Students in grade seven can and should learn how levers confer a mechanical advantage. Given a lever, students should be able to identify the fulcrum and four important quantities: effort distance, effort force, resistance distance, and resistance force. If three of those quantities are known, students should be able to calculate the fourth quantity. Students can make simple levers and hinges (and other simple machines, if time permits) to show how levers can be used to increase forces at the expense of distances or distances at the expense of forces. Metersticks, weight hold­ers, hooked weights, and pivoted supports are commercially available for students to make a straightforward investigation of the operation of levers. A key element of this standard set is to relate the physical principles to the function of muscle and bone in the body. Pressure, a subject that was introduced to students in the context of atmospheric pressure in earth science, is now discussed in the context of blood pressure and heart function.

6.             Physical principles underlie biological structures and functions. As a basis for understanding this concept:

 6.a.         Students know visible light is a small band within a very broad electromagnetic spectrum.

Visible light is a part of a continuum known as the electromagnetic spectrum that extends on both sides of the visible region. This continuum includes the very long wavelengths, such as those of AM and FM radio and TV; the slightly shorter wavelengths, such as radar, microwave, and infrared radiation; and visible light that has wavelengths just less than one-millionth of a meter long. The wavelengths of electromagnetic radiation that the human eye can see vary from about 800 nanom­eters (0.0000008 m, or red light) to 400 nanometers (0.0000004 m, or blue/violet light). The colors of the visible spectrum are traditionally described as red, orange, yellow, green, blue, indigo, and violet but are actually a continuous spectrum.

6.b.          Students know that for an object to be seen, light emitted by or scattered from it must be detected by the eye.

This standard deals with the physical principles of the interaction of light with matter. After the initial interaction light rays from an object must pass from the object to the eye. The interactions with those parts of the eye that focus the light, creating an image on the retina, and transfer the light into electrical impulses, which are interpreted by the brain, all depend on the information in the light that enters the eye. This information arises from the initial interaction of the light with the object or the nature of the light emitted by the light source(s) or both. The color and brightness of the light that is emitted or reflected from an object depend on the color, brightness, and angle of incidence from the source illuminating the object. The object then absorbs, reflects, or refracts the illuminating light and im­parts a color and brightness. That color is attributed to the object, but color really depends on the source of light and the way the object interacts with it.

This process scatters light in all directions. The eye detects only the light that enters it. This light first encounters the front, rounded, transparent surface of the eye (the cornea), where most of the focusing occurs. Next, it enters the interior of the eyeball through the pupil and passes through the lens, which acts to further focus the light to accommodate both near and far objects. The focused light then falls on the receptors (the rod cells and the cone cells) in the retina, is converted into electrical impulses, and is transferred by the optic nerve to the visual cortex of the brain.

6.c.          Students know light travels in straight lines if the medium it travels through does not change.

In a vacuum or in a uniformly transparent material, light travels in straight lines. At the interface between two media or between a vacuum and a medium, light rays will bend if they enter at an angle other than perpendicular to the inter-face. The light-bending properties of objects should be explored. However, trans-parent materials, such as air, may have differing densities and cause light to bend as it passes through the material. For example, the air heated by a campfire can cause objects to appear to shimmer because the path of the light is not a straight line. The variations in the density of the atmosphere are what cause the stars to twinkle. When light travels from one transparent medium (such as air) into another trans-parent medium with different optical properties (such as water), the path of the light may bend (or be refracted) depending on the angle of the ray of light in rela­tion to the surface between the two media.

A pencil placed in a glass half full of water will appear bent. By analyzing the path of the light from various points on the pencil to the eye of the observer, stu­dents will be able to confirm that the path of the light did change direction as it passed from one medium into another.

6.d.          Students know how simple lenses are used in a magnifying glass, the eye, a camera, a telescope, and a microscope.

Combinations of lenses are used in telescopes and microscopes to magnify ob­jects. The cornea of the eye plays the major role of a lens in transforming the rays of light diverging from an object into rays of light converging to a focus on the

retina. To provide instruction in this standard, teachers may use magnifiers. Simple magnifiers of plastic (or glass) are inexpensive and easily obtained. A magnifier is a converging optic because it can convert rays of light diverging from an object to rays of light converging to form an image. Magnifiers are characterized by their focal lengths, which may be found by lifting a lens up from a table until the sharp­est image of a ceiling light is formed. The distance from the magnifier to the image on the tabletop is the focal length. If the magnifier is held at a distance shorter than the focal distance above a printed page, the print is seen magnified because the lens creates an enlarged, virtual image instead of a real image. If the magnifier is held at a distance greater than the focal length above the page, what is seen depends on where the observer’s eye is located. The light leaving the lens is now converging so that if the eye intercepts the converging rays, no sharp image will be seen. If the eye is located far enough above the page, the rays from the lens converge to form a real image and pass through it. The eye is now intercepting diverging rays and sees the print upside down.

6.e.          Students know that white light is a mixture of many wavelengths (colors) and that retinal cells react differently to different wave-lengths.

White (visible) light may be dispersed into a spectrum of colors: from red at the longest wavelength to violet at the shortest wavelength. A glass or plastic prism dis­perses white light into the colors of the spectrum because the angle of refraction is different for each of the different wavelengths (colors). A diffraction grating of closely spaced grooves can also be used to separate white light into various colors because different wavelengths (i.e., different colors) interfere constructively after reflection at different angles. Teachers should present both these effects to show the nature of white light.

The human perception of color is due to specialized color light receptor cells in the retina of the eye. These specialized cells (called cone cells) make color vision possible. Full-color printing is achieved by the use of just four ink colors (usually magenta, yellow, and cyan along with a very dark purple or black). The four colors are printed in combinations of dot patterns too small to perceive (resolve) with the human eye. Color images in magazines are commonly produced in this way.

6.f.          Students know light can be reflected, refracted, transmitted, and absorbed by matter.

The interaction of light with matter may be classified as reflection, refraction, transmission, or absorption. Light transmitted through air and transparent, uni­form materials continues to travel in a straight line. However, when rays of light encounter a surface between two materials or two media, such as air and water or air and glass, the light may be reflected or refracted at the surface. The angle at which the light is reflected or refracted from its original path follows principles that depend on the optical properties of the materials, such as the angle of incidence being equal to the angle of reflection. The principles of refraction are what make it possible for lenses to focus and magnify images.

Light travels (is transmitted) through a transparent medium by a process of absorption and reemission of the light energy by the atoms of the medium. Opaque and translucent objects absorb and scatter light from their original direction much more strongly than do transparent objects. Optically denser materials, such as glass, cause light to travel more slowly than do less optically dense materials, such as wa­ter and especially air. Light travels through air just slightly more slowly than through a vacuum. Rays of light may be observed to change direction, or refract (a consequence of light changing speed), in going from one medium to another. However, if light enters a new medium perpendicular to its surface, the light con­tinues in a straight line so that refraction is not observed (even though the light is traveling at a different speed in the second medium). Impurities or imperfections in transparent materials or media cause some of the light to be scattered out of a beam. Smoke, fog, and clouds decrease visibility because they scatter light.

6.g.         Students know the angle of reflection of a light beam is equal to the angle of incidence.

When a light beam encounters a shiny reflecting surface, the angle of reflection is the same as the angle of incidence. The angle is usually measured in relation to the surface normal.

6. h.        Students know how to compare joints in the body (wrist, shoulder, thigh) with structures used in machines and simple devices (hinge, ball-and-socket, and sliding joints).

Archimedes is credited with first understanding that a rigid rod (a lever) able to rotate about a fixed pivot point (a fulcrum) can be used to turn a small force into a large force. Joints in the body act as pivot points for bones acting as levers, and muscles provide the force. There are three classes of levers, which are defined by the relative positions of the applied force causing the action, the placement of the ful­crum, and the resistant object being moved. A lever provides one of two principal advantages: It can amplify the force being applied so that a small force applied over a long distance can create a large force over a short distance. This principle is useful to know in lifting heavy objects. The alternative is typical of levers in the human body: A large force applied over a short distance in a short time can be amplified into long, rapid motions, such as in running or in swinging a baseball bat.

6.i.          Students know how levers confer mechanical advantage and how the application of this principle applies to the musculoskeletal system.

A lever can be used to take advantage of force or speed (or motion). A bone is the lever; a joint is the pivot point (or fulcrum); muscles supply the force; and con­nective tissues transfer the force to locations that usually give an individual the leverage to increase his or her speed of motion of foot, arm, or hand. Students can

make simple levers and hinges (and other simple machines, if time permits) to show how levers may be used to increase force at the expense of distance or distance at the expense of force. Metersticks, weight holders, hooked weights, and pivoted sup-ports are commercially available for students to make straightforward investigations of the operation of levers. These or other hands-on laboratory activities using first-, second-, and third-class levers in simple equipment will make the “law of the lever” more real than will solving a set of mathematical proportion problems or merely identifying the parts of a lever from drawings or pictures.

6.j.          Students know that contractions of the heart generate blood pressure and that heart valves prevent backflow of blood in the circulatory system.

The heart is a pump in which blood enters a chamber through a blood vessel; a valve closes off the blood vessel to prevent the blood from flowing in the wrong direction; and the heart muscle contracts. This action “squeezes” the blood and increases the pressure to force the blood into another blood vessel. Pressure is de-fined as force per unit area and is measured in various units, such as millimeters of mercury (mmHg). Students may learn more about the physiology of the heart by reading science texts and studying models.

information from a variety of resources is an important part of scientific inquiry and experimental design. Many types of print and electronic resources are available in the school library to support teaching and learning science. The skills needed to search out and recognize accurate and useful resources are complex and generally require significant knowledge of the topic.