economic english 9

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  1. – PHYSICAL SCIENCE – C ONTACT FORCE Maximum Maximum Contact forces are forces that exist as a result of an inter- Potential Potential action between objects that are physically in contact with Energy Energy one another. They include frictional forces, tensional forces, and normal forces. The friction force opposes the motion of an object across a surface. For example, if a glass moves across the surface of the dinner table, there exists a friction force in Maximum Kinetic Energy the direction opposite to the motion of the glass. Friction The change of potential energy into kinetic energy, and is the result of attractive intermolecular forces between kinetic energy into potential energy, in a pendulum the molecules of the surface of the glass and the surface of the table. Friction depends on the nature of the two Examples of potential energy include nuclear energy surfaces. For example, there would be less friction and chemical energy—energy is stored in the bonds that between the table and the glass if the table was moistened hold atoms and molecules together. Heat, hydrodynamic or lubricated with water. The glass would glide across the energy, and electromagnetic waves are examples of table more easily. Friction also depends on the degree to kinetic energy—energy associated with the movement of which the glass and the table are pressed together. Air molecules, water, and electrons or photons (increments resistance is a type of frictional force. of light). Tension is the force that is transmitted through a rope or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the rope or I nteractions of Energy wire and pulls on the objects on either end of the wire. and Matter The normal force is exerted on an object in contact with another stable object. For example, the dinner table Energy in all its forms can interact with matter. For exerts an upward force on a glass at rest on the surface of example, when heat energy interacts with molecules of the table. water, it makes them move faster and boil. Waves— including sound and seismic waves, waves on water, and N UCLEAR FORCE light waves—have energy and can transfer that energy Nuclear forces are very strong forces that hold the when they interact with matter. Consider what happens nucleus of an atom together. If nuclei of different atoms if you are standing by the ocean and a big wave rolls in. come close enough together, they can interact with one Sometimes, the energy carried by the wave is large another and reactions between the nuclei can occur. enough to knock you down. Forms of Energy Waves Energy is defined as the ability to do work. We have Energy is also carried by electromagnetic waves or light already stated that energy can’t be created or destroyed; waves. The energy of electromagnetic waves is related to it can only change form. Forms of energy include poten- their wavelengths. Electromagnetic waves include radio tial energy and kinetic energy. waves (the longest wavelength), microwaves, infrared Potential energy is stored energy. Kinetic energy is the radiation (radiant heat), visible light, ultraviolet radia- energy associated with motion. Look at the following tion, X-rays, and gamma rays. The wavelength depends illustration. As the pendulum swings, the energy is con- on the amount of energy the wave is carrying. Shorter verted from potential to kinetic, and back to potential. wavelengths carry more energy. When the hanging weight is at one of the high points, the When a wave hits a smooth surface, such as a mirror, gravitational potential energy is at the maximum, and it is reflected. Sound waves are reflected as echoes. Mat- kinetic energy is at the minimum. At the low point, the ter can also refract or bend waves. This is what happens kinetic energy is maximized, and gravitational potential when a ray of light traveling through air hits a water sur- energy is minimized. face. A part of the wave is reflected, and a part is refracted into the water. 230
  2. – PHYSICAL SCIENCE – Each kind of atom or molecule can gain or lose energy Key Concepts only in particular discrete amounts. When an atom gains energy, light at the wavelength associated with that This chapter gave you a crash course in the energy is absorbed. When an atom loses energy, light at basics of physical science. Here are the most the wavelength associated with that energy is emitted. important concepts to remember: These wavelengths can be used to identify elements. ➧ All matter is composed of tiny particles Nuclear Reactions called atoms. In a nuclear reaction, energy can be converted to matter ➧ Atoms combine with other atoms to form and matter can be converted to energy. In such processes, molecules. energy and matter are conserved, according to Einstein’s ➧ In a chemical reaction, atoms in molecules formula E = mc2, where E is the energy, m is the mass rearrange to form other molecules. (matter), and c is the speed of light. A nuclear reaction is ➧ The three common states of matter are different from a chemical reaction because in a nuclear solid, liquid, and gas. reaction, the particles in nuclei (protons and neutrons) ➧ The disorder in the universe is always interact, whereas in a chemical reaction, electrons are lost increasing. or gained by an atom. Two types of nuclear reactions are ➧ Mass and energy can’t be created or fusion and fission. destroyed. Fusion is a nuclear process in which two light nuclei ➧ Energy can change form and can be trans- combine to form one heavier nucleus. A fusion reaction ferred in interactions with matter. releases an amount of energy more than a million times greater than the energy released in a typical chemical reaction. This gain in energy is accompanied by a loss of mass. The sum of the masses of the two light nuclei is lower than the mass of the heavier nucleus produced. Nuclear fusion reactions are responsible for the energy output of the sun. Fission is a nuclear process in which a heavy nucleus splits into two lighter nuclei. Fission reaction was used in the first atomic bomb and is still used in nuclear power plants. Fission, like fusion, liberates a great amount of energy. The price for this energy is a loss in mass. A heavy nucleus that splits is heavier than the sum of the masses of the lighter nuclei that result. 231
  3. CHAPTER 24 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. L the nature of living things, from the smallest building blocks of life to the I F E S C I E N C E E X P LO R E S 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. 233
  4. – LIFE SCIENCE – T he Cell cells of a human heart, have a large number of mitochondria. Mitochondria are unusual because unlike other cell organelles, they contain their Today, we know that a cell is the building block of life. own DNA and make some of their own proteins. Every living organism is composed of one or more cells. The endoplastic reticulum is a series of intercon- ■ All cells come from other cells. Cells are alive. If blood necting membranes associated with the storage, cells, for example, are removed from the body, given the synthesis, and transport of proteins and other right conditions, they can continue to live independently materials within the cell. of the body. They are made up of organized parts, per- The Golgi complex is a series of small sacs that ■ form chemical reactions, obtain energy from their sur- synthesizes, packages, and secretes cellular prod- roundings, respond to their environments, change over ucts to the plasma membrane. Its function is time, reproduce, and share an evolutionary history. directing the transport of material within the cell All cells contain a membrane, cytoplasm, and genetic and exporting material out of the cell. material. More complex cells also contain cell organelles. Lysosomes contain enzymes that help with intra- ■ Here is a description of cell components and the func- cellular digestion. Lysosomes have a large pres- tions they serve. Also, refer to the figures on the next page. ence in cells that actively engage in phagocytosis—the process by which cells con- The cell wall is made of cellulose, which sur- ■ sume large particles of food. White blood cells rounds, protects, and supports plant cells. Animal that often engulf and digest bacteria and cellular cells do not have a cell wall. debris are abundant in lysosomes. The plasma membrane is the outer membrane of ■ Vacuoles are found mainly in plants. They partic- ■ the cell. It carefully regulates the transport of ipate in digestion and the maintenance of water materials in and out of the cell and defines the balance in the cell. cell’s boundaries. Membranes have selective per- Centrioles are cylindrical structures found in the ■ meability—meaning that they allow the passage cytoplasm of animal cells. They participate in cell of certain molecules, but not others. A membrane division. is like a border crossing. Molecules need the Chloroplasts exist in the cells of plant leaves and ■ molecular equivalent of a valid passport and a in algae. They contain the green pigment chloro- visa to get through. phyll and are the site of photosynthesis—the The nucleus is a spherical structure, often found ■ process of using sunlight to make high energy near the center of a cell. It is surrounded by a sugar molecules. Ultimately, the food supply of nuclear membrane and it contains genetic infor- most organisms depends on photosynthesis car- mation inscribed along one or more molecules of ried out by plants in the chloroplasts. DNA. The DNA acts as a library of information The nucleolus is located inside the nucleus. It is ■ and a set of instructions for making new cells and involved in the synthesis of ribosomes, which cell components. To reproduce, every cell must be manufacture proteins. able to copy its genes to future generations. This In a multicellular organism, individual cells is done by exact duplication of the DNA. specialize in different tasks. For example, red Cytoplasm is a fluid found within the cell mem- ■ blood cells carry oxygen, white blood cells fight brane, but outside the nucleus. pathogens, and cells in plant leaves collect the Ribosomes are the sites of protein synthesis essen- ■ energy from sunlight. This cellular organization tial in cell maintenance and cell reproduction. enables an organism to lose and replace individual Mitochondria are the powerhouses of the cell. ■ cells, and outlive the cells that it is composed of. They are the site of cellular respiration (break- For example, you can lose dead skin cells and give down of chemical bonds to obtain energy) and blood and still go on living. This differentiation or production of ATP, a molecule that provides division of labor in multicellular organisms is energy for many essential processes in all organ- accomplished by expression of different genes. isms. Cells that use a lot of energy, such as the 234
  5. – LIFE SCIENCE – Lysosome Chloroplast 2 Cytoplasm 2 Endoplasmic 2 2 reticulum Golgi complex Plasma membrane Nucleolus Nucleus 2 Vacuole 2 2 2 Cell wall 2 2 Ribosomes Centriole 2 Mitochondria Plant Cell Animal Cell M olecular Basis of Heredity Reproduction Asexual reproduction on the cellular level is called mito- sis. It requires only one parent cell, which, after exactly What an organism looks like and how it functions is multiplying its genetic material, splits in two. The result- determined largely by its genetic material. The basic ing cells are genetically identical to each other and are principles of heredity were developed by Gregor Mendel, clones of the original cell before it split. who experimented with pea plants in the 19th century. Sexual reproduction requires two parents. Most cells He mathematically analyzed the inherited traits (such as in an organism that reproduces sexually have two copies color and size) of a large number of plants over many of each chromosome, called homologous pairs—one generations. The units of heredity are genes carried on from each parent. These cells reproduce through mitosis. chromosomes. Genetics can explain why children look Gamete cells (sperm and egg cells) are exceptions. They like their parents, and why they are, at the same time, not carry only one copy of each chromosome, so that there identical to the parents. are only half as many chromosomes as in the other cells. For example, human cells normally contain 46 chromo- Phenotype and Genotype somes, but human sperm and egg cells have 23 chro- The collection of physical and behavioral characteristics mosomes. At fertilization, male and female gametes of an organism is called a phenotype. For example, your (sperm and egg) come together to form a zygote, and the eye color, foot size, and ear shape are components of number of chromosomes is restored by this union. The your phenotype. The genetic makeup of a cell or organ- genetic information of a zygote is a mixture of genetic ism is called the genotype. The genotype is like a cook- information from both parents. Gamete cells are manu- book for protein synthesis and use. Phenotype (what an factured through a process called meiosis, whereby a cell organism looks like or how it acts) is determined by the multiples its genetic material once, but divides twice, genotype (its genes) and its environment. By environ- producing four new cells, each contains half the number ment, we don’t mean the Earth, but the environment of chromosomes present in the original cell before divi- surrounding the cell or organism. For example, hor- sion. In humans, gametes are produced in testes and mones in the mother’s body can influence the gene ovaries. Meiosis causes genetic diversity within a species expression. by generating combinations of genes different from those present in the parents. 235



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