The GED Science Exam - Life Science

L IFE SCIENCE EXPLORES the nature of living things, from the smallest building blocks of life to the larger principles that unify all living beings. Fundamental questions of life science include: ■ What constitutes life? ■ What are its building blocks and requirements? ■ How are the characteristics of life passed on from generation to generation? ■ How did life and different forms of life evolve? ■ How do organisms depend on their environment and on one another? ■ What kinds of behavior are common to living organisms? Before Anthony van Leeuwenhoek looked through his homemade microscope more than 300 years ago, people didn’t know that there were cells in our bodies or that there were microorganisms. Another common miscon- ception was that fleas, ants, and other pests came from dust or wheat. Leeuwenhoek saw blood cells in blood, found microorganisms in ponds, and showed that pests come from larvae that hatch from eggs laid by adult pests. However, it took more than 200 years for Leeuwenhoek’s observations to gain wide acceptance and find appli- cation in medicine. CHAPTER Life Science LIFE SCIENCE questions on the GED cover the topics studied in high school biology classes. In this chapter, you will review the basics of biology and learn the answers to some of the key questions scien- tists ask about the nature of life and living beings. 24 233  The Cell Today, we know that a cell is the building block of life. Every living organism is composed of one or more cells. All cells come from other cells. Cells are alive. If blood cells, for example, are removed from the body, given the right conditions, they can continue to live independently of the body. They are made up of organized parts, per- form chemical reactions, obtain energy from their sur- roundings, respond to their environments, change over time, reproduce, and share an evolutionary history. All cells contain a membrane, cytoplasm, and genetic material. More complex cells also contain cell organelles. Here is a description of cell components and the func- tions they serve. Also, refer to the figures on the next page. ■ The cell wall is made of cellulose, which sur- rounds, protects, and supports plant cells. Animal cells do not have a cell wall. ■ The plasma membrane is the outer membrane of the cell. It carefully regulates the transport of materials in and out of the cell and defines the cell’s boundaries. Membranes have selective per- meability—meaning that they allow the passage of certain molecules, but not others. A membrane is like a border crossing. Molecules need the molecular equivalent of a valid passport and a visa to get through. ■ The nucleus is a spherical structure, often found near the center of a cell. It is surrounded by a nuclear membrane and it contains genetic infor- mation inscribed along one or more molecules of DNA. The DNA acts as a library of information and a set of instructions for making new cells and cell components. To reproduce, every cell must be able to copy its genes to future generations. This is done by exact duplication of the DNA. ■ Cytoplasm is a fluid found within the cell mem- brane, but outside the nucleus. ■ Ribosomes are the sites of protein synthesis essen- tial in cell maintenance and cell reproduction. ■ Mitochondria are the powerhouses of the cell. They are the site of cellular respiration (break- down of chemical bonds to obtain energy) and production of ATP, a molecule that provides energy for many essential processes in all organ- isms. Cells that use a lot of energy, such as the cells of a human heart, have a large number of mitochondria. Mitochondria are unusual because unlike other cell organelles, they contain their own DNA and make some of their own proteins. ■ The endoplastic reticulum is a series of intercon- necting membranes associated with the storage, synthesis, and transport of proteins and other materials within the cell. ■ The Golgi complex is a series of small sacs that synthesizes, packages, and secretes cellular prod- ucts to the plasma membrane. Its function is directing the transport of material within the cell and exporting material out of the cell. ■ Lysosomes contain enzymes that help with intra- cellular digestion. Lysosomes have a large pres- ence in cells that actively engage in phagocytosis—the process by which cells con- sume large particles of food. White blood cells that often engulf and digest bacteria and cellular debris are abundant in lysosomes. ■ Vacuoles are found mainly in plants. They partic- ipate in digestion and the maintenance of water balance in the cell. ■ Centrioles are cylindrical structures found in the cytoplasm of animal cells. They participate in cell division. ■ Chloroplasts exist in the cells of plant leaves and in algae. They contain the green pigment chloro- phyll and are the site of photosynthesis—the process of using sunlight to make high energy sugar molecules. Ultimately, the food supply of most organisms depends on photosynthesis car- ried out by plants in the chloroplasts. ■ The nucleolus is located inside the nucleus. It is involved in the synthesis of ribosomes, which manufacture proteins. In a multicellular organism, individual cells specialize in different tasks. For example, red blood cells carry oxygen, white blood cells fight pathogens, and cells in plant leaves collect the energy from sunlight. This cellular organization enables an organism to lose and replace individual cells, and outlive the cells that it is composed of. For example, you can lose dead skin cells and give blood and still go on living. This differentiation or division of labor in multicellular organisms is accomplished by expression of different genes. – LIFE SCIENCE – 234  Molecular Basis of Heredity What an organism looks like and how it functions is determined largely by its genetic material. The basic principles of heredity were developed by Gregor Mendel, who experimented with pea plants in the 19th century. He mathematically analyzed the inherited traits (such as color and size) of a large number of plants over many generations. The units of heredity are genes carried on chromosomes. Genetics can explain why children look like their parents, and why they are, at the same time, not identical to the parents. Phenotype and Genotype The collection of physical and behavioral characteristics of an organism is called a phenotype. For example, your eye color, foot size, and ear shape are components of your phenotype. The genetic makeup of a cell or organ- ism is called the genotype. The genotype is like a cook- book for protein synthesis and use. Phenotype (what an organism looks like or how it acts) is determined by the genotype (its genes) and its environment. By environ- ment, we don’t mean the Earth, but the environment surrounding the cell or organism. For example, hor- mones in the mother’s body can influence the gene expression. Reproduction Asexual reproduction on the cellular level is called mito- sis. It requires only one parent cell, which, after exactly multiplying its genetic material, splits in two. The result- ing cells are genetically identical to each other and are clones of the original cell before it split. Sexual reproduction requires two parents. Most cells in an organism that reproduces sexually have two copies of each chromosome, called homologous pairs—one from each parent. These cells reproduce through mitosis. Gamete cells (sperm and egg cells) are exceptions. They carry only one copy of each chromosome, so that there are only half as many chromosomes as in the other cells. For example, human cells normally contain 46 chromo- somes, but human sperm and egg cells have 23 chro- mosomes. At fertilization, male and female gametes (sperm and egg) come together to form a zygote, and the number of chromosomes is restored by this union. The genetic information of a zygote is a mixture of genetic information from both parents. Gamete cells are manu- factured through a process called meiosis, whereby a cell multiples its genetic material once, but divides twice, producing four new cells, each contains half the number of chromosomes present in the original cell before divi- sion. In humans, gametes are produced in testes and ovaries. Meiosis causes genetic diversity within a species by generating combinations of genes different from those present in the parents. – LIFE SCIENCE – 235 2 2 2 2 2 2 2 2 2 2 2 Cytoplasm Endoplasmic reticulum Plasma membrane Nucleolus Nucleus Vacuole Cell wall Ribosomes Mitochondria Centriole Chloroplast Lysosome Animal Cell Plant Cell Golgi complex Alleles Alleles are alternative versions of the same gene. An organism with two copies of the same allele is homozy- gous, and one with two different alleles is heterozygous. For example, a human with one gene for blue eyes and one gene for brown eyes is heterozygous, while a human with two genes for blue eyes or two genes for brown eyes is homozygous. Which of the two genes is expressed is determined by the dominance of the gene. An allele is dominant if it alone determines the phe- notype of a heterozygote. In other words, if a plant has a gene for making yellow flowers and a gene for making red flowers, the color of the flower will be determined by the dominant gene. So if the gene for red flowers is dom- inant, a plant that has both the gene for red and the gene for yellow will look red. The gene for yellow flowers in this case is called recessive, as it doesn’t contribute to the phenotype (appearance) of a heterozygote (a plant con- taining two different alleles). The only way this plant would make yellow flowers is if it had two recessive genes—two genes both coding for yellow flowers. For some genes, dominance is only partial and two different alleles can be expressed. In the case of partial dominance, a plant that has a gene that codes for red flowers and a gene that codes for white flowers would produce pink flowers. A Punnett square can be used to represent the possi- ble phenotypes that offspring of parents with known genotypes could have. Take the example with the yellow and red flower. Let’s label the gene for the dominant red gene as R and the gene for yellow flowers as r. Cross a plant with yellow flowers (genotype must be rr) with a plant with red flowers and genotype Rr. What possible genotypes and phenotypes can the offspring have? In a Punnett square, the genes of one parent are listed on one side of the square and the genes of the other parent on the other side of the square. They are then combined in the offspring as illustrated here: The possible genotypes of the offspring are listed inside the square. Their genotype will be either Rr or rr, causing them to be either red or yellow, respectively. Sex Determination In many organisms, one of the sexes can have a pair of unmatched chromosomes. In humans, the male has an X chromosome and a much smaller Y chromosome, while the female has two X chromosomes. The combination XX (female) or XY (male) determines the sex of humans. In birds, the males have a matched pair of sex chromosomes (WW), while females have an unmatched pair (WZ). In humans, the sex chromosome supplied by the male determines the sex of the offspring. In birds, the female sex chromosome determines the sex. Plants, as well as many animals, lack sex chromo- somes. The sex in these organisms is determined by other factors, such as plant hormones or temperature. Identical twins result when a fertilized egg splits in two. Identical twins have identical chromosomes and can be either two girls or two boys. Two children of different sex born at the same time can’t possibly be identical twins. Such twins are fraternal. Fraternal twins can also be of the same sex. They are genetically not any more alike than siblings born at different times. Fraternal twins result when two different eggs are fertilized by two dif- ferent sperm cells. When meiosis goes wrong, the usual number of chro- mosomes can be altered. An example of this is Down’s syndrome, a genetic disease caused by the presence of an extra chromosome. Changes in DNA (mutations) occur randomly and spontaneously at low rates. Mutations occur more fre- quently when DNA is exposed to mutagens, including ultraviolet light, X-rays, and certain chemicals. Most mutations are either harmful to or don’t affect the organ- ism. In rare cases, however, a mutation can be beneficial to an organism and can help it survive or reproduce. Ultimately, genetic diversity depends on mutations, as mutations are the only source of completely new genetic material. Only mutations in germ cells can create the variation that changes an organism’s offspring. Plant rr RRr Rr rrr rr Plant – LIFE SCIENCE – 236  Biological Evolution Mutations cause change over time. The result of a series of such changes is evolution, or as Darwin put it, “descent with modification.” The great diversity on our planet is the result of more than 3.5 billion years of evo- lution. The theory of evolution argues that all species on Earth originated from common ancestors. Evidence for Evolution Several factors have led scientists to accept the theory of evolution. The main factors are described here. ■ Fossil record. One of the most convincing forms of evidence is the fossil record. Fossils are the remains of past life. Fossils are often located in sedimentary rocks, which form during compres- sion of settling mud, debris, and sand. The order of layers of sedimentary rock is consistent with the proposed sequence in which life on Earth evolved. The simplest organisms are located at the bottom layer, while top layers contain increas- ingly complex and modern organisms, a pattern that suggests evolution. ■ Biogeography. Another form of evidence comes from the fact that species tend to resemble neigh- boring species in different habitats more than they resemble species in similar, but far away, habitats. ■ Comparative anatomy. Comparative anatomy provides us with another line of evidence. It refers to the fact that the limb bones of different species, for example, are similar. Species that closely resemble one another are considered more closely related than species that do not resemble one another. For example, a horse and a donkey are considered more closely related than a horse and a frog. Biological classifications (kingdom, phylum, class, order, family, genus, and species) are based on how organisms are related. Organ- isms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. ■ Embryology. Embryology provides another form of evidence for evolution. Embryos go through the developmental stages of their ancestors to some degree. The early embryos of fish, amphib- ians, reptiles, birds, and mammals all have com- mon features, such as tails. ■ Comparative molecular biology. Comparative molecular biology confirms the lines of descent suggested by comparative anatomy and fossil record. Darwin also proposed that evolution occurs gradually, through mutations and natural selection. He argued that some genes or combinations of genes give an individual a survival or reproductive advantage, increasing the chance that these useful combinations of genes will make it to future generations. Whether a given trait is advantageous depends on the environment of the organism. Natural selection is only one of several mechanisms by which gene frequency in a population changes. Other factors include mating patterns and breeding between popula- tions.  Interdependence of Organisms The species in communities interact in many ways. They compete for space and resources, and they can be related as predator and prey, or as host and parasite. Plants and other photosynthetic organisms harness and convert solar energy and supply the rest of the food chain. Herbivores (plant eaters) obtain energy directly from plants. Carnivores are meat eaters and obtain energy by eating other animals. Decomposers feed on dead organisms. The flow of energy can then be repre- sented as follows: Sun → Photosynthetic organisms → Herbivores → Carnivores → Decomposers The food chain is not the only example of the inter- dependence of organisms. Species often have to compete for food and space, so that the increase in population of one can cause the decrease in population of the other. Organisms also may have a symbiotic relationship (live in close association), which could be classified as parasitism, mutualism, or commensalism. In a parasitic relationship, one organism benefits at the expense of the other. Commensalism is symbiosis in which one organ- ism benefits and the other is neither harmed nor rewarded. In mutualism, both organisms benefit. Under ideal conditions, with ample food and space and no predators, all living organisms have the capacity to reproduce to infinite number. However, resources are limited, limiting the population of a species. – LIFE SCIENCE – 237 Humans probably come closest to being a species with seemingly infinite reproductive capacity. Our population keeps increasing. Our only danger seems to come from viruses and bacteria, which at this point, we more or less have under control. When we need more food, we grow more, and when we need more space, we clear some by killing off other biomes. By doing this, humans modify ecosystems and destroy habitats through direct harvest- ing, pollution, atmospheric changes, and other factors. This attitude is threatening current global stability and has the potential to cause irreparable damage.  Behavior of Organisms Even the most primitive unicellular organisms can act to maintain homeostasis. More complex organisms have nervous systems. The simplest organism found to have learning capability is a worm, suggesting a more complex nervous system. The function of the nervous system is collection and interpretation of sensory signals as trans- mission of messages from the center of the nervous sys- tem (brain in humans) to other parts of the body. The nervous system is made of nerve cells, or neurons, which conduct signals in the form of electrical impulses. Nerve cells communicate by secreting excitatory or inhibitory molecules called neurotransmitters. Many legal and ille- gal drugs act on the brain by disrupting the secretion or absorption of neurotransmitters. Many animals have sense organs that enable them to detect light, sound, and specific chemicals. These organs provide the animals with information about the outside world. Animals engage in innate and learned social behavior. These behaviors include hunting or searching for food, nesting, migrating, playing, caring for their young, fighting for mates, and fighting for territory. Plants also respond to stimuli. They turn toward the sun and let their roots run deeper when they need water. – LIFE SCIENCE – 238 E ARTH AND SPACE science are concerned with the formation of the Earth, the solar system and the universe, the history of Earth (its mountains, continents and ocean floors), the weather and seasons on Earth, the energy in the Earth system, and the chemical cycles on Earth.  Energy in the Earth Systems Energy and matter can’t be created or destroyed. But energy can change form and travel great distances. Solar Energy The sun’s energy reaches our planet in the form of light radiation. Plants use this light to synthesize sugar mol- ecules, which we consume when we eat the plants. We obtain energy from the sugar molecules and our bodies use it. Ultimately, our energy comes from the sun. The sun also drives the Earth’s geochemical cycles, which will be discussed in the next section. The sun heats the Earth’s surface and drives convection within the atmosphere and oceans, producing winds and ocean currents. The winds cause waves on the surface of oceans and lakes. The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Strong winds cause large CHAPTER Earth and Space Science HUMANS HAVE always wondered about the origin of the Earth and the universe that surrounds it. What kinds of matter and energy are in the universe? How did the universe begin? How has the Earth evolved? This chapter will answer these fundamental questions and review the key concepts of Earth and space science. 25 239 waves. Tsunamis, or tidal waves, are different. They result from underwater earthquakes, volcanic eruptions, or landslides, not wind. Energy from the Core Another source of Earth’s energy comes from Earth’s core. We distinguish four main layers of Earth: the inner core, the outer core, the rocky mantle, and the crust. The inner core is a solid mass of iron with a temperature of about 7,000° F. Most likely, the high temperature is caused by radioactive decay of uranium and other radioactive elements. The inner core is approximately 1,500 miles in diameter. The outer core is a mass of molten iron that surrounds the solid inner core. Electri- cal currents generated from this area produce the earth’s magnetic field. The rocky mantle is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium and is about 1,750 miles thick. This mantle accounts for most of the Earth’s mass. When parts of this layer become hot enough, they turn to slow moving molten rock, or magma. The Earth’s crust is a layer from four to 25 miles thick, consisting of sand and rock. The upper mantle is rigid and is part of the litho- sphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The crust is divided into plates that drift slowly (only a few cen- timeters each year) on the less rigid mantle. Oceanic crust is thinner than continental crust. This motion of the plates is caused by convection (heat) currents, which carry heat from the hot inner mantle to the cooler outer mantle. The motion results in earthquakes and volcanic eruptions. This process is called plate tectonics. Tectonics Evidence suggests that about 200 million years ago, all continents were a part of one landmass, named Pangaea. Over the years, the continents slowly separated through the movement of plates in a process called continental drift. The movement of the plates is attributed to con- vection currents in the mantle. The theory of plate tec- tonics says that there are now twelve large plates that slowly move on the mantle. According to this theory, earthquakes and volcanic eruptions occur along the lines where plates collide. Dramatic changes on Earth’s land- scape and ocean floor are caused by collision of plates. These changes include the formation of mountains and valleys.  Geochemical Cycles Water, carbon, and nitrogen are recycled in the bios- phere. A water molecule in the cell of your eye could have been, at some point, in the ocean, in the atmosphere, in a leaf of a tree, or in the cell of a bear’s foot. The circula- tion of elements in the biosphere is called a geochemical cycle. Water Oceans cover 70% of the Earth’s surface and contain more than 97% of all water on Earth. Sunlight evapo- rates the water from the oceans, rivers, and lakes. Living beings need water for both the outside and the inside of their cells. In fact, vertebrates (you included) are about 70% water. Plants contain even more water. Most of the water passes through a plant unaltered. Plants draw on water from the soil and release it as vapor through pores in their leaves, through a process called transpiration. Our atmosphere can’t hold a lot of water. Evaporated water condenses to form clouds that produce rain or snow on to the Earth’s surface. Overall, water moves from the oceans to the land because more rainfall reaches the land than is evaporated from the land. (See the figure on the next page.) Carbon Carbon is found in the oceans in the form of bicarbon- ate ions (HCO 3 − ), in the atmosphere, in the form of car- bon dioxide, in living organisms, and in fossil fuels (such as coal, oil, and natural gas). Plants remove carbon diox- ide from the atmosphere and convert it to sugars through photosynthesis. The sugar in plants enters the food chain, first reaching herbivores, then carnivores, and finally scavengers and decomposers. All these organ- isms release carbon dioxide back into the atmosphere when they breathe. The oceans contain 500 times more carbon than the atmosphere. Bicarbonate ions (HCO 3 – ) settle to the bottoms of oceans and form sedimentary rocks. Fossil fuels represent the largest reserve of carbon on Earth. Fossil fuels come from the carbon of organisms that had lived millions of years ago. Burning fossil fuels releases energy, which is why these fuels are used to power human contraptions. When fossil fuels burn, car- bon dioxide is released into the atmosphere. Since the Industrial Revolution, people have increased the concentration of carbon dioxide in the atmosphere – EARTH AND SPACE SCIENCE – 240 30% by burning fossil fuels and cutting down forests, which reduce the concentration of carbon dioxide. Car- bon dioxide in the atmosphere can trap solar energy—a process known as the greenhouse effect. By trapping solar energy, carbon dioxide and other greenhouse gases can cause global warming—an increase of temperatures on Earth. In the last 100 years, the temperatures have increased by 1° C. This doesn’t seem like much, but the temperature increase is already creating noticeable cli- mate changes and problems. Many species are migrating to colder areas, and regions that normally have ample rainfall have experienced droughts. Perhaps the most dangerous consequence of global warming is the melting of polar ice. Glaciers worldwide are already melting, and the polar ice caps have begun to break up at the edges. If enough of this ice melts, coastal cities could experience severe flooding. Reducing carbon dioxide concentrations in the atmosphere, either by finding new energy sources or by actively removing the carbon dioxide that forms, is a challenge to today’s scientists. (See the figure on the next page.) Nitrogen The main component of air in the atmosphere is nitro- gen gas (N 2 ). Nitrogen accounts for about 78% of the atmosphere. However, very few organisms can use the form of nitrogen obtained directly from the atmosphere. This is because the bond between two atoms in the nitro- gen gas molecule is tough to break, and only a few bac- teria have enzymes that can make it happen. These bacteria can convert the nitrogen gas into ammonium ions (NH 4 + ). Bacteria that do this are called nitrifying or nitrogen-fixing bacteria. – EARTH AND SPACE SCIENCE – 241 Run-off from glaciers, snow rivers, and lakes Precipitation Precipitation Evaporation and transpiration Ocean Groundwater flow Another source of nitrogen for the non-nitrogen-fixing organisms is lightning. Lightning carries tremendous energy, which is able to cause nitrogen gas to convert to ammonium ions (NH 4 + ) and nitrate ions (NO 3 − )—fixed nitrogen. Plants, animals, and most other organisms can only use fixed nitrogen. Plants obtain fixed nitrogen from soil and use it to synthesize amino acids and proteins. Ani- mals obtain fixed nitrogen by eating plants, or other animals. When they break up proteins, animals lose nitrogen in the form of ammonia (fish), urea (mam- mals), or uric acid (birds, reptiles, and insects). Decom- posers obtain energy from urea and uric acid by converting them back into ammonia, which can be used again by plants. The amount of fixed nitrogen in the soil is low, because bacteria break down most the ammonium ion into another set of molecules (nitrite and nitrate), through a process called nitrification. Other bacteria con- vert the nitrite and nitrate back into nitrogen gas, which is released into the atmosphere. This process is called denitrification. This limited amount of nitrogen has kept organisms in balance for millions of years. However, the growing human population presents a threat to this stability. In order to increase the growth rate of crops, humans man- ufacture and use huge amounts of fertilizer, increasing the amount of nitrogen in the soil. This has disrupted whole ecosystems, since, with extra nitrogen present, some organisms thrive and displace others. In the long run, too much nitrogen decreases the fertility of soil by depriving it of essential minerals, such as calcium. Burning fossil fuels and forests also releases nitrogen. All forms of fixed nitrogen are greenhouse gases that cause global warming. In addition, nitric oxide, a gas released when fossil fuels are burned, can convert into nitric acid, a main component of acid rain. Acid rain destroys habitats. People are already suffering the consequences of the pollution they have caused. Preventing further damage to the ecosystem and fixing the damage that has been done is another challenge for today’s scientists. – EARTH AND SPACE SCIENCE – 242 CO 2 in atmosphere Photosynthesis (land) Photosynthesis (water) Burning fossil fuels Burning forests Respiration (organisms on land and in water) [...]... through the floor of the ocean and flows from fissures Origin of the Earth and the Solar System The sun, the Earth, and the rest of the solar system formed 4.6 billion years ago, according to the solar nebula theory This theory states that the solar system was initially a large cloud of gas and dust, which most likely originated from the explosions of nearby stars This cloud is named the solar nebula The. .. their moons orbit the sun as well These planets include Mercury and Venus, which are closer to the sun than the Earth is, and Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, which are further away from the sun It takes about one year for the Earth to complete its orbit around the sun The rotation of the Earth around its axis causes the change between day and night The tilt in the Earth’s axis gives... formed at the central, densest point of the nebula One argument that supports this hypothesis is that planets closer to the sun are composed of heavier elements, while light, gaseous planets are farthest from the sun The solar nebula theory also states that planets form in conjunction with stars This component of the theory is supported by the fact that other stars have planets and that the age of... SPACE SCIENCE – where it is cooled by the water, resulting in the formation of igneous rocks As the molten material flows from the fissure, it forms ridges adjacent to it Origin and Evolution of the Earth System Earth Basics Most people know that the Earth is round and revolves around its axis in about 24 hours It is a part of the solar system, with the sun in its center Eight other planets and their... age of moon rocks is comparable to the age of the Earth Origin and Evolution of the Universe Nobody knows for sure how the universe originated According to the Big Bang theory, the universe began in a hot, dense state under high pressure between ten and 20 billion years ago The Big Bang theory also postulates that the universe has been expanding since its origination The universe is still expanding and... pressure deep beneath the earth’s surface Rock cycle is the transformation of one rock type into another Molten rock material cools and solidifies either at or below the surface of the earth to form igneous rocks Weathering and erosion break the rocks down into smaller grains, producing soil The soil is carried by wind, water, and gravity and is eventually deposited as sediment The sediments are deposited... that the rate of expansion of the universe is increasing Whether the universe will continue to expand forever, eventually reach an equilibrium size, or shrink back into a small, dense, hot mass is unknown Stars are formed by the gravitational attraction of countless hydrogen and helium molecules The stars became gravitationally bound to other stars, forming galaxies The solar system is part of the Milky... result from cooling of molten rock If the cooling from molten rock occurred quickly on or near the earth’s surface, it is called volcanic igneous rock If the cooling took place slowly, deep beneath the surface, it is called plutonic igneous rock Sedimentary rocks are formed in layers in response to pressure on accumulated sediments Metamorphic rocks are formed when either igneous or sedimentary rocks are... bound to other stars, forming galaxies The solar system is part of the Milky Way galaxy, which, in addition to the sun, contains about 200 billion other stars The energy of stars stems from nuclear reactions, mainly the fusion of hydrogen atoms to form helium Nuclear processes in stars lead to the formation of elements 243 ... together and cemented or lithified, forming sedimentary rocks Variations in temperature and pressure can cause chemical and physical changes in igneous and sedimentary rocks to form metamorphic rocks When exposed to higher temperatures, metamorphic rocks may be partially melted, resulting in the creation once again of igneous rocks, starting the cycle all over again Molten material from inside the earth . environ- ment, we don’t mean the Earth, but the environment surrounding the cell or organism. For example, hor- mones in the mother’s body can influence the. when they need water. – LIFE SCIENCE – 238 E ARTH AND SPACE science are concerned with the formation of the Earth, the solar system and the universe, the