The Chemical Basis of Life

The Chemical Basis of Life
Chapter 2 Lecture by Richard L. Myers

ELEMENTS, ATOMS, AND MOLECULES
Copyright © 2009 Pearson Education, Inc.

2.1 Living organisms are composed of about 25 chemical elements
Chemicals are at the base level of biological hierarchy They are arranged into higher and higher levels of structural organization Arrangement eventually leads to formation of living organisms Life results from the ordering of atoms into molecules and the interactions of these molecules within cells. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. Copyright © 2009 Pearson Education, Inc.

2.1 Living organisms are composed of about 25 chemical elements
Living organisms are composed of matter, which is anything that occupies space and has mass (weight) Matter is composed of chemical elements Element—a substance that cannot be broken down to other substances There are 92 elements in nature—only a few exist in a pure state Life requires 25 essential elements; some are called trace elements Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. Copyright © 2009 Pearson Education, Inc.

Table 2.1 Elements in the Human Body.

2.2 CONNECTION: Trace elements are common additives to food and water
Some trace elements are required to prevent disease Without iron, your body cannot transport oxygen An iodine deficiency prevents production of thyroid hormones, resulting in goiter Although nitrogen is not a trace element, a common deficiency in plants, especially those we grow for food, is nitrogen deficiency. Without fertilization in a field that is used year after year to produce food crops, the plants are less productive. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. 2. Many breakfast cereals are fortified with iron. As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect an abstract concept to a concrete example. Copyright © 2009 Pearson Education, Inc.

2.2 CONNECTION: Trace elements are common additives to food and water
Several chemicals are added to food for a variety of reasons Help preserve it Make it more nutritious Make it look better Check out the “Nutrition Facts” label on foods and drinks you purchase Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. 2. Many breakfast cereals are fortified with iron. As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect an abstract concept to a concrete example. Copyright © 2009 Pearson Education, Inc.

Figure 2.2B Nutrition facts from a fortified cereal.
The information in the main or top section of a Nutrition Facts label varies with each food product; it contains product-specific information (serving size, calories, and nutrient information). The bottom part contains a footnote with Daily Values (DVs) for 2,000 and 2,500 calorie diets. This footnote provides recommended dietary information for important nutrients, including fats, sodium, and fiber.

2.3 Elements can combine to form compounds
Compound—a substance consisting of two or more different elements combined in a fixed ratio There are many compounds that consist of only two elements Table salt (sodium chloride or NaCl) is an example Sodium is a metal, and chloride is a poisonous gas However, when chemically combined, an edible compound emerges Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed (for example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen). Copyright © 2009 Pearson Education, Inc.

+ Sodium Chlorine Sodium Chloride
Figure 2.3 The emergent properties of the edible compound sodium chloride. Sodium Chlorine Sodium Chloride

2.3 Elements can combine to form compounds
Many of the compounds in living organisms contain carbon, hydrogen, oxygen, and nitrogen DNA, for example, contains all four of these elements Interestingly, different arrangements of elements provide unique properties for each compound Students will soon learn about organic compounds, especially those that are based on the properties of carbon. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed (for example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen). Copyright © 2009 Pearson Education, Inc.

2.4 Atoms consist of protons, neutrons, and electrons
An atom is the smallest unit of matter that still retains the properties of a element Atoms are made of over a hundred subatomic particles, but only three are important for biological compounds Proton—has a single positive electrical charge Electron—has a single negative electrical charge Neutron—is electrically neutral Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. 2. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. 3. The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. Teaching Tips 1. Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.4 that an electron is about 1/2,000 the mass of a proton.) 2. The text in Module 2.4 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. 3. After sharing teaching tips 1 and 2 above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? 4. The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. Copyright © 2009 Pearson Education, Inc.

Elements differ in their number of protons, neutrons, and electrons Helium has two protons, two neutrons, and two electrons Carbon has six protons, six neutrons, and six electrons Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. 2. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. 3. The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. Teaching Tips 1. Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.4 that an electron is about 1/2,000 the mass of a proton.) 2. The text in Module 2.4 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. 3. After sharing teaching tips 1 and 2 above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? 4. The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. Copyright © 2009 Pearson Education, Inc.

Electron cloud Nucleus 2e–
Figure 2.4A Two models of a helium atom 2 Protons Mass number = 4 2 Neutrons 2 Electrons

Neutrons and protons are packed in the atom’s nucleus The negative charge of electrons and the positive charge of protons keep electrons near the nucleus The number of protons is the atom’s atomic number Carbon with 6 protons has an atomic number of 6 The mass number is the sum of the protons and neutrons in the nucleus (carbon-12 is written 12C) Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. 2. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. 3. The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. Teaching Tips 1. Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.4 that an electron is about 1/2,000 the mass of a proton.) 2. The text in Module 2.4 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. 3. After sharing teaching tips 1 and 2 above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? 4. The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. Copyright © 2009 Pearson Education, Inc.

Electron cloud 6e– Nucleus
Figure 2.4B Model of a carbon atom. 6 Protons Mass number = 12 6 Neutrons 6 Electrons

Although all atoms of an element have the same atomic number, some differ in mass number The variations are isotopes, which have the same numbers of protons and electrons but different numbers of neutrons One isotope of carbon has 8 neutrons instead of 6 (written 14C) Unlike 12C, 14C is an unstable (radioactive) isotope that gives off energy The isotope 12C is stable—its nucleus does not have a tendency to lose particles. The isotope 14C is radioactive—its nucleus decays spontaneously giving off particles and energy. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. 2. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. 3. The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. Teaching Tips 1. Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.4 that an electron is about 1/2,000 the mass of a proton.) 2. The text in Module 2.4 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. 3. After sharing teaching tips 1 and 2 above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? 4. The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. Copyright © 2009 Pearson Education, Inc.

Table 2.4 Isotopes of Carbon.

2.5 CONNECTION: Radioactive isotopes can help or harm us
Living cells cannot distinguish between isotopes of the same element Therefore, when radioactive compounds are used in metabolic processes, they act as tracers Radioactivity can be detected by instruments With instruments, the fate of radioactive tracers can be monitored in living organisms Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

Biologists use radioactive tracers in research Radioactive 14C was used to show the route of 14CO2 in formation of sugar during plant photosynthesis Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

Radioactive tracers are frequently used in medical diagnosis Sophisticated imaging instruments are used to detect them An imaging instrument that uses positron-emission tomography (PET) detects the location of injected radioactive materials PET is useful for diagnosing heart disorders and cancer and in brain research Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

Figure 2.5A Technician monitoring the output of a PET scanner

Healthy brain Alzheimer’s patient
Figure 2.5B PET images of brains of a healthy person (left) and a person with Alzheimer’s disease (right). Red and yellow colors indicate high levels of PIB bound to beta-amyloid plaques. Healthy brain Alzheimer’s patient

In addition to benefits, there are also dangers associated with using radioactive substances Uncontrolled exposure can cause damage to some molecules in a living cell, especially DNA Chemical bonds are broken by the emitted energy, which causes abnormal bonds to form One of the most serious environmental threats of radioactivity is radioactive fallout from nuclear accidents. A 1979 accident in the United States at the Three Mile Island nuclear power plant led to strict regulations about safety. In the Soviet Union on Saturday, April 26, 1986, the Number 4 reactor at the Chernobyl nuclear power plant blew apart and blasted through the surrounding concrete containment structure. Two workers died immediately in the blast, but over the following months 30 others died, most of them firefighters who battled to prevent the fire from spreading to the reactor in the next building. All died from radiation burns and radiation sickness following exposure to strong gamma and beta radiation. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

2.6 Electron arrangement determines the chemical properties of an atom
Only electrons are involved in chemical activity Electrons occur in energy levels called electron shells Information about the distribution of electrons is found in the periodic table of the elements Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

Hydrogen Helium First shell Lithium Beryllium Boron Carbon Nitrogen
Oxygen Fluorine Neon Second shell Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell Figure 2.6 The electron shell diagrams of the first 18 elements in the periodic table.

An atom may have one, two, or three electron shells The number of electrons in the outermost shell determines the chemical properties of the atom The first shell is full with two electrons, whereas the second and third will hold up to eight electrons Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

Atoms want to fill their outer electron shells To accomplish this, the atom can share, donate, or receive electrons This results in attractions between atoms called chemical bonds Chemical bonds cause atoms to stay close together. Recall that a compound is a substance consisting of two or more different elements combined in a fixed ratio. Chemical bonds are responsible for holding atoms together. Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Copyright © 2009 Pearson Education, Inc.

2.7 Ionic bonds are attractions between ions of opposite charge
An ion is an atom or molecule with an electrical charge resulting from gain or loss of electrons When an electron is lost, a positive charge results; when one is gained, a negative charge results Two ions with opposite charges attract each other When the attraction holds the ions together, it is called an ionic bond Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (ionic, covalent, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example: Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) Animation: Ionic Bonds Copyright © 2009 Pearson Education, Inc.

Na Sodium atom Cl Chlorine atom
Transfer of electron Na Sodium atom Cl Chlorine atom Figure 2.7A Formation of an ionic bond, producing sodium chloride.

Sodium chloride (NaCl)
Transfer of electron + – Na Sodium atom Cl Chlorine atom Na+ Sodium ion Cl– Chloride ion Figure 2.7A Formation of an ionic bond, producing sodium chloride. Sodium chloride (NaCl)

2.8 Covalent bonds join atoms into molecules through electron sharing
A covalent bond results when atoms share outer-shell electrons A molecule is formed when atoms are held together by covalent bonds Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. 2. Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.8 and the differences and limitations of representations of atomic structure. The contrast in Figure 2.8 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.4. Teaching Tips 1. Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (ionic, covalent, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example: Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) 2. Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) Animation: Covalent Bonds Copyright © 2009 Pearson Education, Inc.

Table 2.8 Alternative Ways to Represent Four Common Molecules.

2.9 Unequal electron sharing creates polar molecules
Atoms in a covalently bonded molecule continually compete for shared electrons The attraction (pull) for shared electrons is called electronegativity More electronegative atoms pull harder Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Modules 2.9 and 2.10 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Copyright © 2009 Pearson Education, Inc.

In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity The bonds formed are called nonpolar covalent bonds Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Modules 2.9 and 2.10 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Copyright © 2009 Pearson Education, Inc.

Water has atoms with different electronegativities Oxygen attracts the shared electrons more strongly than hydrogen So, the shared electrons spend more time near oxygen The result is a polar covalent bond Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Modules 2.9 and 2.10 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Copyright © 2009 Pearson Education, Inc.

In H2O the oxygen atom has a slight negative charge and the hydrogens have a slight positive charge Molecules with this unequal distribution of charges are called polar molecules Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Modules 2.9 and 2.10 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Copyright © 2009 Pearson Education, Inc.

(–) (–) O H H Figure 2.9 A water molecule. (+) (+)

2.10 Hydrogen bonds are weak bonds important in the chemistry of life
Some chemical bonds are weaker than covalent bonds Hydrogen, as part of a polar covalent bond, will share attractions with other electronegative atoms Examples are oxygen and nitrogen Water molecules are electrically attracted to oppositely charged regions on neighboring molecules Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond Student Misconceptions 1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Modules 2.9 and 2.10 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Animation: Water Structure Copyright © 2009 Pearson Education, Inc.

Hydrogen bond Figure 2.10 Hydrogen bonds between water molecules.

WATER’S LIFE-SUPPORTING PROPERTIES
Copyright © 2009 Pearson Education, Inc.

2.11 Hydrogen bonds make liquid water cohesive
Hydrogen bonding causes molecules to stick together, a property called cohesion Cohesion is much stronger for water than other liquids This is useful in plants that depend upon cohesion to help transport water and nutrients up the plant Water displays emergent properties resulting from the hydrogen bond that orders water molecules. Cohesion is just one of these properties. Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water both stick to each other. 2. Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. Copyright © 2009 Pearson Education, Inc.

2.11 Hydrogen bonds make liquid water cohesive
Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid Hydrogen bonds are responsible for surface tension Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water both stick to each other. 2. Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. Animation: Water Transport Copyright © 2009 Pearson Education, Inc.

Figure 2.11 Surface tension allows a water strider to walk on water.

Adhesion Water-conducting cells Cohesion 150 µm
Direction of water movement Cohesion Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 3.3 Water Transport in Plants. 150 µm

2.12 Water’s hydrogen bonds moderate temperature
Because of hydrogen bonding, water has a greater ability to resist temperature change than other liquids Heat is the energy associated with movement of atoms and molecules in matter Temperature measures the intensity of heat Heat must be absorbed to break hydrogen bonds; heat is released when hydrogen bonds form Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. 2. Students at all levels struggle with the distinction between heat and temperature. Students might also expect that all ice is about the same temperature, 0°C. Redefining and correcting misunderstandings often takes more class time and energy than introducing previously unknown concepts. Teaching Tips 1. Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. 2. The following analogies may help students to understand the relation between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students didn’t take the test. (b) The authors note that the performance of a track team would drop if the fastest sprinters did not compete. In both analogies, removing the top performers lowers the average, just like the evaporation of the most active water molecules cools the evaporative surface. 3. It’s not the heat, it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. Copyright © 2009 Pearson Education, Inc.

Figure 2.12 Evaporative cooling.
Evaporative cooling is a process in which water molecules with the greatest energy leave. Perspiration is an example of a benefit from this process. When water molecules leave, the water molecules left behind cool down and therefore cool the body surface.

2.13 Ice is less dense than liquid water
Water can exist as a gas, liquid, and solid Water is less dense as a solid, a property due to hydrogen bonding Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. Ask your students if the ocean levels would change if ice did not float. They can try this experiment to find out, or you can begin class with the demonstration and watch the progress throughout the class period. Place several large chunks of ice in a glass and fill the glass up completely with water to the top rim. Thus, the ice cubes should be sticking up above the top of the filled glass. Will the glass overflow when the ice melts? (No.) This phenomenon is important when we consider the potential consequences of global warming. If floating glaciers melt, ocean levels will not be affected. However, if the ice over land melts, we can expect higher ocean levels. 2. Module 2.13 notes the insulating effect of ice forming at the surface of a lake. This phenomenon would not occur if ice were denser than water. Challenge students to think of other consequences from the expansion of water when it forms ice. (These include the ability to widen cracks in rocks, roads, and sidewalks!) Copyright © 2009 Pearson Education, Inc.

2.13 Ice is less dense than liquid water
When water freezes, each molecule forms a stable hydrogen bond with four neighbors A three-dimensional crystal results There is space between the water molecules Ice is less dense than water, so it floats Floating ice provides a barrier for the water below. The water stays considerably warmer than the air above the ice and can serve as the ideal habitat for some marine life. Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. Ask your students if the ocean levels would change if ice did not float. They can try this experiment to find out, or you can begin class with the demonstration and watch the progress throughout the class period. Place several large chunks of ice in a glass and fill the glass up completely with water to the top rim. Thus, the ice cubes should be sticking up above the top of the filled glass. Will the glass overflow when the ice melts? (No.) This phenomenon is important when we consider the potential consequences of global warming. If floating glaciers melt, ocean levels will not be affected. However, if the ice over land melts, we can expect higher ocean levels. 2. Module 2.13 notes the insulating effect of ice forming at the surface of a lake. This phenomenon would not occur if ice were denser than water. Challenge students to think of other consequences from the expansion of water when it forms ice. (These include the ability to widen cracks in rocks, roads, and sidewalks!) Copyright © 2009 Pearson Education, Inc.

constantly break and re-form
Hydrogen bond Figure 2.13A Hydrogen bonds between water molecules in ice and water. Ice Hydrogen bonds are stable Liquid water Hydrogen bonds constantly break and re-form

2.14 Water is the solvent of life
A solution is a liquid consisting of a uniform mixture of two or more substances The dissolving agent is the solvent The substance that is dissolved is the solute Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. A simple demonstration of a solute dissolving in a solvent can focus students’ attention on the process when discussing solutions. Using colored water and white sugar or salt may make it easier to see and reference while you are discussing the process. Such simple visual aids add life to a lecture. (You might also add corn oil to the top of the solution to demonstrate the properties of hydrophobic substances, and challenge your class to explain why oil and water do not mix.) Copyright © 2009 Pearson Education, Inc.

2.14 Water is the solvent of life
Water is a versatile solvent that is fundamental to life processes Its versatility results from its polarity Table salt is an example of a solute that will go into solution in water Sodium and chloride ions and water are attracted to each other because of their charges Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties requires high-level evaluative analysis. Teaching Tips 1. A simple demonstration of a solute dissolving in a solvent can focus students’ attention on the process when discussing solutions. Using colored water and white sugar or salt may make it easier to see and reference while you are discussing the process. Such simple visual aids add life to a lecture. (You might also add corn oil to the top of the solution to demonstrate the properties of hydrophobic substances, and challenge your class to explain why oil and water do not mix.) Copyright © 2009 Pearson Education, Inc.

Ion in Salt solution crystal
Figure 2.14 A crystal of salt (NaCl) dissolving in water. Ion in solution Salt crystal

2.15 The chemistry of life is sensitive to acidic and basic conditions
A few water molecules can break apart into ions Some are hydrogen ions (H+) Some are hydroxide ions (OH–) Both are extremely reactive A balance between the two is critical for chemical processes to occur in a living organism Teaching Tips 1. Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in labs, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Copyright © 2009 Pearson Education, Inc.

Chemicals other than water can contribute H+ to a solution They are called acids An example is hydrochloric acid (HCl) This is the acid in your stomach that aids in digestion An acidic solution has a higher concentration of H+ than OH– Teaching Tips 1. Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in labs, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Copyright © 2009 Pearson Education, Inc.

Some chemicals accept hydrogen ions and remove them from solution These chemicals are called bases For example, sodium hydroxide (NaOH) provides OH– that combines with H+ to produce H2O (water) This reduces the H+ concentration Teaching Tips 1. Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in labs, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Copyright © 2009 Pearson Education, Inc.

A pH scale (pH = potential of hydrogen) is used to describe whether a solution is acidic or basic pH ranges from 0 (most acidic) to 14 (most basic) A solution that is neither acidic or basic is neutral (pH = 7) Teaching Tips 1. Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in labs, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Copyright © 2009 Pearson Education, Inc.

(Higher concentration of H+) (Lower concentration of H+)
pH scale 1 Battery acid 2 Lemon juice, gastric juice 3 Grapefruit juice, soft drink, vinegar, beer (Higher concentration of H+) Increasingly ACIDIC Acidic solution 4 Tomato juice 5 Rain water 6 Human urine Saliva NEUTRAL [H+]=OH–] 7 Pure water Human blood, tears 8 Seawater Neutral solution 9 Figure 2.15 The pH scale represents the relative concentration of H+ and OH–. 10 (Lower concentration of H+) Increasingly BASIC Milk of magnesia 11 Household ammonia 12 Household bleach 13 Oven cleaner Basic solution 14

Acidic solution Basic solution Neutral solution
Figure 2.15 The pH scale represents the relative concentration of H+ and OH–. Acidic solution Neutral solution Basic solution

2.17 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water
An important question is, has life evolved elsewhere? Water is necessary for life as we know it The National Aeronautics and Space Administration (NASA) has evidence that water was once abundant on Mars Scientists have proposed that reservoirs of water beneath the surface of Mars could harbor microbial life Teaching Tips 1. The SETI (Search For Extraterrestrial Intelligence) Institute’s Mission is “to explore, understand, and explain the origin, nature, and prevalence of life in the universe.” Your students might enjoy exploring this respected scientific organization’s website, Copyright © 2009 Pearson Education, Inc.

August 1999 September 2005 New deposit
Figure 2.17 Gullies on Mars that might indicate recent water flow. New deposit

2.18 Chemical reactions make and break bonds, changing the composition of matter
You learned that the structure of atoms and molecules determines the way they behave Remember that atoms combine to form molecules Hydrogen and oxygen can react to form water 2H2 + O H2O Student Misconceptions and Concerns 1. A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips 1. The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions (noted just above). Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) Copyright © 2009 Pearson Education, Inc.

2.18 Chemical reactions make and break bonds, changing the composition of matter
The formation of water from hydrogen and oxygen is an example of a chemical reaction The reactants (H2 and O2) are converted to H2O, the product Organisms do not make water, but they do carry out a large number of chemical reactions that rearrange matter Photosynthesis is an example where plants drive a sequence of chemical reactions that produce glucose Student Misconceptions and Concerns 1. A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips 1. The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions (noted just above). Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) Copyright © 2009 Pearson Education, Inc.

2 H2 O2 2 H2O Figure 2.18 Breaking and making bonds in a chemical reaction.

You should now be able to
Describe the importance of chemical elements to living organisms Explain the formation of compounds Describe the structure of an atom Distinguish between ionic, hydrogen, and covalent bonds List and define the life-supporting properties of water Explain the pH scale and the formation of acid and base solutions Define a chemical reaction and explain how it changes the composition of matter Copyright © 2009 Pearson Education, Inc.

The Chemical Basis of Life

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