CHAPTER of Life

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INTERACTIVITY

The Chemistry of Life2 C

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2.1 The Nature of Matter

2.2Properties of Water

2.3Carbon Compounds

2.4 Chemical Reactions and Enzymes

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HS-LS1-6, HS-ETS1-1, HS-ESS2-5

40 Chapter 2 The Chemistry of Life

CASE STUDY

Sadly, these were not the faces of children. The faces belonged to adults with cretinism, a condition of severely stunted growth—both physically and mentally.

Cretinism becomes apparent in infancy. Generally, the damage to the brain and body is permanent. People with cretinism grow up stunted, both physically and intellectually. In China, the disorder has been common in inland farming villages like Daxin. In some years, as many as 25 percent of the school-children in the Henan province were affected. However, cretinism seems confined to the inland provinces. It is effectively absent in the coastal regions of China.

The disorder occurs worldwide, and was once known as Alpine cretinism, named after inland regions in the Alps mountain range in Switzerland. The word “cretin” comes from crestin, a word in an Alpine French dialect that means “a fellow human being.” The term was a reminder to treat people afflicted with mental or physical disorders with care and compassion. Just as in China, the disorder is confined to inland regions, and is rarely found among people living along the seacoast.

Goiter is another condition that is com-mon in places where cretinism occurs. Goiter is a condition in which the neck and throat swell up. Goiter is produced when a gland known as the thyroid, increases in size so much that it causes a large bulge to appear at the base of the neck, where it is located. Unlike cretinism, goiter, which occurs in adult-hood, can be treated and reversed.

Some possible explanations for these dis-orders have proved false. Neither is inherited. Children born to families that had moved out of the affected regions develop normally, without any signs of cretinism. This is true even if a parent has the disorder. Neither cretinism nor goiter are caused by pollution or toxic chemicals, and they do not spread from person to person, like a communicable disease. Rather, the cause of both disorders is not something present in places like Daxin, but rather something that is missing. Can you figure out what that might be?

Throughout this chapter, look for con-nections to the CASE STUDY to help you answer these questions.

Life goes on slowly in the rural village of Daxin, much as it has for centuries. Daxin is

located in the landlocked province of Henan in central China. Chinese health workers once

spoke of Daxin with a sense of dread, knowing that many of its people suffered from seri-

ous intellectual disabilities. They remember how small people with curious, childlike faces

would peer out from the doorways of houses and the shadows of buildings.

Something is missing. But what?

Surface tension allows this double-crested basilisk to “run” across the water.

Unit 1 The Nature of Life 41

The Nature of Matter2.1LESS

ON

What are you made of? Just as buildings are made from bricks, steel, and wood, all forms of life are made from chemical compounds. When you breathe, eat, or drink, your body uses the substances in air, food, and water to carry out chemical reactions that keep you alive.

AtomsThe study of chemistry begins with the basic unit of matter, the atom. The concept of the atom came first from the Greek philoso-pher Democritus, nearly 2500 years ago. Democritus called the smallest fragment of any substance an atom, from the Greek word atomos, which means “unable to be cut.”

Atoms are incredibly small. Placed side by side, 100 million sulfur atoms would make a row only about 1 centimeter long—about the width of your little finger! Despite its extremely small size, an atom contains subatomic particles that are even smaller. Figure 2-1 shows the subatomic particles in a carbon atom. The subatomic par-ticles that make up atoms are protons, neutrons, and electrons.

Protons and Neutrons Protons and neutrons have about the same mass. However, protons are positively charged particles (+), and neutrons carry no charge at all. Strong forces bind protons and neutrons together to form the nucleus, at the center of the atom.

Electrons The electron is a negatively charged particle (−) with only 1/1840 the mass of a proton. Electrons are in constant motion in the space surrounding the nucleus. They are arranged in a series of shells or orbitals. The first shell can contain no more than two electrons, and the second shell, no more than eight. Atoms have equal numbers of electrons and protons, and are electrically neutral because the opposite charges cancel out.

KEY QUESTIONS• What three subatomic

particles make up atoms?

• How are all of the isotopes of an element similar?

• In what ways does a compound differ from its component elements?

• What are the main types of chemical bonds?

VOCABULARYatomnucleuselectronelementisotopecompoundionic bondioncovalent bondmoleculevan der Waals forces

READING TOOL

As you read your textbook, outline the headings and sub-headings throughout this lesson. Fill in the table in your Biology Foundations Workbook.


6

6

6

6

6

6

6

7

8

Isotopes of Carbon

Number of Protons

Number of Electrons

Carbon–12 (nonradioactive)

Carbon–13 (nonradioactive)

Carbon–14(radioactive)

Number of NeutronsIsotope

+ Proton

–

+ ProtonNeutronElectron–

Elements and IsotopesA chemical element is a pure substance that consists entirely of one type of atom. Elements are represented by one- or two-letter symbols. C, for example, stands for carbon, H for hydrogen, Na for sodium, and I for iodine. The number of protons in the nucleus of an element is called its atomic number. Carbon’s atomic number is 6, meaning that each atom of carbon has six protons and, consequently, six electrons.

Although there are nearly a hundred naturally occurring chemi-cal elements on Earth (see the Periodic Table in Appendix D), fewer than 20 of them are commonly found in living organisms. About 99 percent of the mass of living things is composed of just six elements: calcium, carbon, hydrogen, oxygen, nitrogen, and phosphorus. However, the remaining 1 percent, or trace elements, are also essen-tial. In fact, a lack of trace elements can stunt plant growth or damage the developing organs in unborn animals.

If all atoms are made of the same three elementary particles, then why do different elements have such different properties? Part of the answer is found in their electron shells. When two atoms interact, their shells overlap and may even swap electrons with each other. This affects how they can interact with other atoms and even how they may participate in chemical reactions. You might say that the number of electrons in that outer shell is the “face” that a particular atom shows to its neighbors.

Isotopes Atoms of an element may have different numbers of neutrons. For example, look at Figure 2-2. Although all atoms of carbon have six protons, they may have different numbers of neu-trons. Carbon-14, for example, has 8 neutrons. Atoms of the same element that differ in the number of neutrons they contain are known as isotopes.

The total number of protons and neutrons in the nucleus of an atom is called its mass number. Isotopes are identified by their mass numbers. The weighted average of the masses of an element’s isotopes is called its atomic mass. “Weighted” means that the abundance of each isotope in nature is considered when the average mass is calculated.

Although neutrons affect the atomic mass of an isotope, they do not affect its chemical properties. Isotopes have different masses, but their chemical properties are the same.

Figure 2-1

A Carbon Atom

All atoms have a nucleus of protons and neutrons. Electrons move around the nucleus.

Figure 2-2

Carbon Isotopes

Isotopes of carbon all have 6 protons but different numbers of neutrons—6, 7, or 8. Isotopes are identified by the total number of protons and neutrons in the nucleus: carbon-12, carbon-13, and carbon-14. Interpret Tables Which isotope of carbon is radioactive?

INTERACTIVITY

Explore an interactive periodic table to discover the properties of the elements.

2.1 The Nature of Matter 43

CASE STUDY

Figure 2-3

Radioactive Imaging of the Thyroid

A radioactive scan reveals the location of the thyroid gland at the base of the neck. Iodine, including the radioactive isotope 131I, is concentrated in the thyroid.

Figure 2-4

Sodium Chloride

Sodium (left) is a silvery metal. Chlorine (middle) is a poisonous yellow-green gas. Yet, the compound made from sodium and chlorine is sodium chloride—common table salt (right).

Radioactive Isotopes Some isotopes are radioac-tive, meaning that their nuclei are unstable and break down at a constant rate over time. The radiation these isotopes give off can be dangerous, but radioactive iso-topes have a number of important scientific and practical uses. Geologists can determine the ages of rocks and fossils by analyzing the isotopes found in them. Radiation from certain isotopes can be used to detect cancer and to kill bacteria that cause food to spoil. Radioactive isotopes can also be used as labels or “tracers” to fol-low the movements of substances within organisms. For example, if the radioactive isotope of iodine, 131I, is injected into the body, in just a few minutes nearly all of the radioactivity is found in just one place—a gland in the front of the neck called the thyroid (See Figure 2-3).

READING CHECK Define What is an isotope?

Chemical CompoundsIn nature, most elements are found combined with other elements in compounds. A chemical compound is a substance formed by the chemical combination of two or more elements in definite proportions. Scientists show the composition of compounds by a kind of short-hand known as a chemical formula. Water, which contains two atoms of hydrogen for each atom of oxygen, has the chemical formula H2O. The formula for table salt, NaCl, indicates that the elements that make up table salt—sodium and chlorine—combine in a 1:1 ratio.

The physical and chemical properties of a compound are usually very different from those of the elements from which it is formed. For example, hydrogen and oxygen, which are gases at room temperature, can combine explosively and form liquid water. Sodium is a silver-colored metal that is soft enough to cut with a knife. It reacts explosively with water. Chlorine is very reactive, too. It is a poisonous, yellow-green gas that was used as a weapon in World War I. But the compound sodium chloride, more commonly known as table salt, shown in Figure 2-4, is a white solid that dissolves easily in water. As you know, sodium chloride is not poisonous. In fact, it is essential for the survival of most living things.


Protons +11Electrons –10

Charge +1


Charge 0


Charge 0


Charge –1

Transferof electron

Sodium atom (Na) Chlorine atom (Cl)+ +Sodium ion (Na+) Chloride ion (Cl–)

– –

– –

– –

– –

–

–

H H

O

Water molecule (H2O)

Chemical BondsThe atoms in compounds are held together by chemical bonds. Much of chemistry is devoted to understanding how and when chemical bonds form. Bond formation involves the electrons that surround each atomic nucleus. The electrons in an atom’s outer shell that are available to form bonds are called valence electrons. The main types of chemical bonds are ionic bonds and covalent bonds.

Ionic Bonds An ionic bond is formed when one or more elec-trons are transferred from one atom to another. Recall that atoms are electrically neutral when they have equal numbers of protons and electrons. A neutral atom that loses electrons becomes positively charged. A neutral atom that gains electrons has a negative charge. These positively and negatively charged atoms are known as ions.

Figure 2-5

Ionic Bonding

The compound sodium chlo-ride forms when sodium loses its valence electron to chlorine.

Figure 2-5 shows how ionic bonds form between sodium and chlorine in table salt. A sodium atom loses its one electron from its outer shell to become a sodium ion (Na+). A chlorine atom gains an electron and becomes a chloride ion (Cl−). In a salt crystal, there are trillions of sodium and chloride ions. These oppositely charged ions have a strong attraction for each other, forming an ionic bond.

Covalent Bonds Sometimes electrons are shared by atoms instead of being transferred. What does it mean to share electrons? It means that the moving electrons actually travel about the nuclei of both atoms, forming a covalent bond. When the atoms share one electron from each atom, a single covalent bond is formed. Sometimes the atoms share four electrons to form a double bond. In other cases, atoms can share six electrons, forming a triple bond. The structure that results when atoms are joined together by cova-lent bonds is called a molecule, as shown by the diagram of a water molecule in Figure 2-6. The molecule is the smallest unit of most compounds.

READING CHECK Compare How are ionic and covalent bonds alike? How are they different?

Figure 2-6

Covalent Bonding

In a water molecule, each hydrogen atom shares two electrons with the oxygen atom.

Discover how electrons are involved in chemical bonds.

INTERACTIVITY

INTERACTIVITY

2.1 The Nature of Matter 45

Weak Interactions While the strongest chemical bonds are ionic or covalent, one of the most interesting things about the chemistry of living things is the importance of weak interactions between atoms and molecules. Within a living cell, many molecules interact only briefly to send signals, carry out chemical reactions, or copy information from one molecule to another. One example of these weak interactions is the attraction between molecules known as van der Waals forces. These forces produce a slight attraction between the molecules when they are very close together. If two molecules have shapes that match so they can fit against each other with very little space between them, these forces may be strong enough to hold the molecules together. The combined van der Waals forces along the feet of a gecko are strong enough to hold the gecko to a wall, as shown in Figure 2-7.

Hydrogen bonds are another form of weak interaction. These bonds form between a hydrogen atom of one molecule and an oxy-gen or nitrogen atom of a neighboring molecule. Because hydrogen bonds are essential to understanding the special properties of water, they will be considered in detail in the next lesson.

Figure 2-7

Van der Waals Forces

The underside of each foot of this gecko is covered by millions of tiny hairlike projections. The projections themselves are made of even finer fibers, creating more surface area for “sticking” to surfaces at the molecular level. This allows geckos to scurry up walls and across ceilings.

KEY QUESTIONS

1. Describe the major subatomic particles that make up an atom.

2. Explain why isotopes of an element have the same chemical properties.

3. Compare the physical and chemical properties of a compound to those of the elements of which it is composed.

4. What are the differences between ionic bonds and covalent bonds?

CRITICAL THINKING

5. Use Models How could you use red, blue, and yellow marbles to model a carbon atom?

6. Evaluate Models Evaluate the model you described in question 5. Describe its usefulness and its limitations.

7. Synthesize Information A calcium atom tends to lose two electrons to become a calcium ion, while a chlorine atom tends to gain one electron to become a chloride ion. Given this information, write out the chemical formula for the compound calcium chloride.

LESSON 2.1 Review

BUILD VOCABULARY

Academic Words The noun interaction means “a shared action or influence.” The inter-actions between molecules due to van der Waals forces can hold them together.

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H

HO

(–)

(+)

HydrogenBond

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Looking back at our planet, an astronaut in space once said that if other beings have seen the Earth, they must surely call it “the blue planet.” He referred, of course, to the oceans of water that cover nearly three fourths of Earth’s surface. The very presence of liquid water tells a scientist that life may be present on such a planet. Why should life itself be connected so strongly to something so ordinary that we often take it for granted? The answer is that there is some-thing very special about water and the role it plays in living things.

The Water MoleculeWater is one of the few compounds found in a liquid state over most of Earth’s surface. Water (H2O) looks like an ordinary molecule. However, there is more to the story.

Polarity With 8 protons, water’s oxygen nucleus attracts electrons more strongly than the single protons of water’s two hydrogen nuclei. As a result, water’s shared electrons are more likely to be found near

2.2Properties of Water

KEY QUESTIONS• How does the

structure of water contribute to its unique properties?

• How does water’s polarity influence its properties as a solvent?

• Why is it important for cells to buffer solutions against rapid changes in pH?

the oxygen nucleus. So, water has a partial negative charge on one end and a partial positive charge on the other. A molecule in which the charges are unevenly distributed is said to be polar, because the molecule is a bit like a magnet with two poles.

Because of their partial charges, polar molecules such as water can attract each other. The attraction between a hydro-gen atom with a partial positive charge and another atom with a partial negative charge is known as a hydrogen bond, which is shown in Figure 2-8.

Figure 2-8

Hydrogen Bonding

Each molecule of water can form multiple hydrogen bonds with other water molecules.

INTERACTIVITY

VOCABULARYhydrogen bond • cohesionadhesion • mixturesolution • solute • solvent suspension • pH scaleacid • base • buffer

READING TOOL

As you read the section of the lesson under The Water Molecule, use the table in your Biology Foundations Workbook to list the causes and effects of the properties of water.

HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.

2.2 Properties of Water 47

INTERACTIVITY

Explore the properties of water that make it so impor-tant to life on Earth.

Special Properties of Water Hydrogen bonds are not as strong as covalent or ionic bonds, but they give one of life’s most important molecules many of its unique characteristics. Because water is a polar molecule, it is able to form multiple hydrogen bonds, which account for many of water’s special properties. These include the fact that water expands slightly upon freezing, making ice less dense than liquid water. Hydrogen bonding also explains water’s ability to dissolve so many other substances, a property essential in living cells.

Cohesion The attraction between molecules of the same sub-stance is called cohesion. Because a single water molecule may be involved in as many as four hydrogen bonds at the same time, water is extremely cohesive. Cohesion causes water molecules to be drawn together, which is why drops of water form beads on a smooth sur-face. Cohesion also produces surface tension, explaining why some insects and spiders can walk on a pond’s surface.

Adhesion The attraction between molecules of different sub-stances is called adhesion. Have you ever been asked to read the volume in a graduated cylinder at eye level? If so, you may have noticed that the surface of the water in the graduated cylinder dips slightly in the center. This is because the adhesion between water molecules and glass molecules is stronger than the cohesion between water molecules. Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draws water out of the roots of a plant and up into its stems and leaves. Cohesion holds the column of water together as it rises.

Heat Capacity Because of hydrogen bonding, water’s heat capacity is relatively high. A substance’s heat capacity is the amount of energy needed to raise its temperature by making its molecules move faster. This allows bodies of water, such as oceans and lakes, to absorb large amounts of heat with only small changes in tem-perature. The organisms living in the water are thus protected from drastic changes in temperature.

Water in Living Things Living things are composed mostly of water. Water accounts for approximately 60 percent of the mass of the human body. As a result, the chemical reactions that take place within living things do so in a water environment. Nearly everything that cells do—from growth and development to movement—takes place by means of chemical reactions in a water environment. That’s why all living things, even those found in the driest places on Earth, depend upon a source of water.

READING CHECK Compare and Contrast How are cohesion and adhesion similar? How are they different?


WaterWater

Na+Na+

Cl– Cl–

Solutions and SuspensionsWater is often found as part of a mixture. A mixture is a material composed of two or more elements or compounds that are physi-cally mixed together but not chemically combined. Earth’s atmo-sphere is a mixture of nitrogen, oxygen, carbon dioxide, and other gases. Living things are in part composed of mixtures involving water. Two types of mixtures that can be made with water are solu-tions and suspensions.

Solutions If a crystal of table salt is placed in a glass of warm water, sodium and chloride ions on the surface of the crystal are attracted to the polar water molecules. Ions break away from the crystal and are surrounded by water molecules, as illustrated in Figure 2-9. The ions gradually become dispersed in the water, form-ing a type of mixture called a solution. All the components of a solution are evenly distributed throughout the solution. In a saltwater solution, table salt is the solute—the substance that is dissolved. Water is the solvent—the substance in which the solute dissolves.

Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Water easily dissolves salts, sugars, minerals, gases, and even other solvents such as alcohol. Without exaggeration, water is life’s most important solvent. But even water has limits. When a given amount of water has dissolved all of the solute it can, the solution is said to be saturated.

Suspensions Some materials do not dissolve when placed in water but separate into pieces so small that they do not settle out. The movement of water molecules keeps the small particles suspended. Such mixtures of water and nondissolved material are known as suspensions. Some of the most important biological fluids are both solutions and suspensions. The blood that circulates through your body is mostly water. The water in the blood contains many dissolved compounds. However, blood also contains cells and other undissolved particles that remain in suspension as the blood moves through the body.

READING CHECK Classify In a cup of tea, what is the solvent? What is the solute?

Figure 2-9

A Salt Solution

When an ionic compound such as sodium chloride is placed in water, water mol-ecules surround and separate the positive and negative ions.

Interpret Diagrams What happens to the sodium ions and chloride ions in the solution?

READING TOOL

Complete a T-chart to compare and contrast solutions and suspensions. Be sure to include examples of each type of mixture.


Acids, Bases, and pHWater molecules sometimes split apart to form ions. This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction.

H2O ⥮ H+ + OH−

water hydrogen ion hydroxide ion

How often does this happen? In pure water, about 1 water molecule in 550 million splits to form ions in this way. Because the number of positive hydrogen ions produced is equal to the number of negative hydroxide ions pro-duced, pure water is electrically neutral.

The pH Scale Chemists have devised a measurement system called the pH scale to indicate the concentration of H+ ions in solution. As Figure 2-10 shows, the pH scale ranges from 0 to 14. At a pH of 7, the concentration of H+ ions and OH− ions is equal. Pure water has a pH of 7. Solutions with a pH below 7 are called acidic because they have more H+ ions than OH− ions. The lower the pH, the greater the acidity. Solutions with a pH above 7 are called basic because they have more OH− ions than H+ ions. The higher the pH, the more basic the solution. Each step on the pH scale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10 times as many H+ ions as a liter of a solution with a pH of 5.

Figure 2-10

The pH Scale

The concentration of H+ ions determines whether solutions are acidic or basic. The most basic material on this scale is oven cleaner. The most acidic material on this pH scale is stomach acid.

Guided InquiryQuick Lab

Acidic and Basic Foods1. Construct a data table. Include spaces for

food samples to be tested, predicted pH, and actual pH.

2. Predict whether the food samples provided are acidic or basic.

3. Tear off a 2-inch piece of pH paper for each sample you will test.

4. Test each food sample and record your obser-vations in your data table. Touch the cut sur-face of each sample to a square of pH paper. Use a dropper pipette to place a drop of any liquid sample on a square of pH paper. Record the pH of each sample in your data table.

Oven cleaner

Incr

easi

ngly

Bas

ic

Neutral

Incr

easi

ngly

Aci

dic

Bleach

Ammonia solution

Soap

SeawaterToothpaste

Human bloodPure waterMilkNormal rainfall

Acid rainTomato juice

Lemon juice

Stomach acid

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

ANALYZE AND CONCLUDE

1. Analyze Data Use the pH measurements to classify the foods as acidic and basic. Was your prediction correct?

2. Construct an Explanation Based on your observations, are you able to classify the foods according to pH? For example, what pH group would you generalize most fruits to to be in? Explain your response using the data you collected.


Acids Where do all those extra H+ ions in a low-pH solution come from? They come from acids. An acid is any compound that releases H+ ions into solution. Acidic solutions contain higher concentrations of H+ ions than pure water and have pH values below 7. The hydrochloric acid (HCl) produced by the stomach to help digest food is a strong acid with a pH of 1.5 to 3.0.

Bases A base is a compound that produces hydroxide (OH−) ions in solution. Basic, or alkaline, solutions contain lower concentra-tions of H+ ions than pure water and have pH values above 7. Strong bases, such as the lye (commonly NaOH) used in soapmaking, tend to have pH values ranging from 11 to 14.

Buffers The internal pH of most cells in the human body must generally be kept between 6.5 and 7.5. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells. Thus, controlling pH is important for maintaining homeostasis. One of the ways that organisms control pH is through dissolved com-pounds called buffers. Buffers are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH. You can see one example of how buffers work in Figure 2-11. Blood, for example, has a normal pH of 7.4. Sudden changes in blood pH are usually prevented by a number of chemical buffers, such as bicar-bonate and phosphate ions. Buffers dissolved in life’s fluids play an important role in maintaining homeostasis.

Figure 2-11

Buffers

Buffers help prevent drastic changes in pH. Adding acid to an unbuffered solution causes the pH of the unbuffered solu-tion to decrease. If the solution contains a buffer, however, adding the acid will cause only a slight change in pH.

KEY QUESTIONS

1. What does it mean when a molecule is said to be “polar”?

2. Why is water such a good solvent?

3. What is a buffer? Why is it useful to cells?

CRITICAL THINKING

4. Classify Identify some beverages that are mix-tures. Classify them as solutions, suspensions, or neither. Explain your classifications.

5. Cite Evidence Describe two observations of water that provide evidence that water molecules are attracted to one another.

6. Integrate Information Discuss how the proper-ties of water help Earth support life.

LESSON 2.2 Review

Unbuffered base + acid = acidic pH

Base Neutral Acid

Buffered base + acid = basic pH

INTERACTIVITY

Explore how buffers help stabilize the pH of the blood during exercise.

HS-ESS2-5


H

H

H C H

H

H C C H

H

H

H

H

H

CC

CC

C

CHHH

H

C C C C

H

H

Methane ButadieneAcetylene Benzene

LESS

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2.3

Chemists once called the compounds in living things “organic,” believing they were different from nonliving compounds. Today we understand that living things obey the same chemical principles as nonliving. But the term “organic chemistry” is still around, referring today to the chemistry of carbon.

The Chemistry of CarbonWhat’s the big deal about carbon? Why is it so interesting that it has its own branch of chemistry? There are two reasons for this. First, car-bon atoms have four valence electrons, allowing them to form strong covalent bonds with many other elements. Carbon can bond with many elements—including hydrogen, oxygen, phosphorus, sulfur, and nitrogen—to form compounds with many different chemical properties. Living organisms depend upon these compounds.

Even more important, one carbon atom can bond to another, which gives carbon the ability to form chains that are almost unlim-ited in length. As shown in Figure 2-12, these carbon-carbon bonds can be single, double, or triple covalent bonds. Chains of carbon atoms can even close up on themselves to form rings, as shown by the structure of benzene. No other element matches carbon’s versa-tility or the size of molecules that carbon can build.

Carbon Compounds

KEY QUESTIONS• What elements does

carbon bond with to make up life’s molecules?

• What are the functions of each of the four groups of macromolecules?

Figure 2-12

Carbon Structure

Carbon can form single, double, or triple bonds with other atoms. Each line between atoms in a molecular drawing represents one covalent bond.

A fern fossil made of carbon

HS-LS1-6: Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.

VOCABULARYmonomer • polymercarbohydratelipid • nucleotidenucleic acid • proteinamino acid

READING TOOL

As you read, identify the similarities and differences between the different groups of macromolecules. Take notes in your Biology Foundations Workbook.


BUILD VOCABULARY

Prefixes The prefix mono- means one, the prefix di- means “two,” and the prefix poly- means “many.”

MacromoleculesThe large organic molecules found in living things are known as macromolecules, literally—“giant molecules”—because of their size. Most macromolecules are produced by a process known as polymer-ization (pah lih mur ih ZAY shun), in which larger compounds are built by joining smaller ones together. The smaller units, or monomers, are joined together to form polymers. The monomers in a polymer may be identical, like the links on a metal watchband, or different, like the beads in a multicolored necklace. Figure 2-13 illustrates the process of polymerization.

Four major groups of macromolecules are found in living things: carbohydrates, lipids, nucleic acids, and proteins. As you read about these molecules, compare their structures and functions.

Carbohydrates Examples of carbohydrates include sugar, starch, and cellulose. Carbohydrates are made up of carbon, hydrogen, and oxygen atoms, usually in a ratio of 1 : 2 : 1. Organisms use car-bohydrates to store and release energy, as well as for structural support and protection. The breakdown of sugars, such as glucose, supplies immediate energy for cell activities. Many organisms store extra sugar as complex carbohydrates known as starches. The mono-mers in starch polymers are sugar molecules.

Simple Sugars Single sugar molecules are also known as monosaccharides (mahn oh SAK uh rydz). Besides glucose, which is shown in Figure 2-14, monosaccharides include galactose, which is a component of milk, and fructose, which is found in many fruits. Ordinary table sugar, sucrose, is a disaccharide, a compound made by joining together two simple sugars, fructose and glucose.

Complex Carbohydrates The macromolecules formed by joining many monosaccharides together are known as polysaccharides. Many animals store excess sugar in a polysaccharide called glycogen. When the level of glucose in your blood runs low, glycogen is broken down into glucose, which is then released into the blood. The glycogen stored in your muscles supplies the energy for muscle contraction and, thus, for movement.

Figure 2-13

Polymerization

When monomers join together, they form polymers.

Figure 2-14

Carbohydrates

Starches form when sugar molecules join together in a long chain. Potatoes are made up mostly of starches, as are foods like bread and pasta.

Polymerization

Monomers

Polymer

Starch

H

CH2OH

H OH

HO

H

C C

OH H

C O

C

OH

H

C

Glucose

Discover the stinky chemicals in the durian fruit.

Video

2.3 Carbon Compounds 53

Starches and Cellulose Plants use a slightly different polysaccha-ride, called starch, to store excess sugar. Plants also make an impor-tant polysaccharide called cellulose. Tough, flexible cellulose fibers give plants much of their strength and rigidity. Cellulose is the major component of both wood and paper, so you may actually be looking at cellulose if you are reading these words on a printed page.

Lipids Lipids are a large and varied group of macromolecules that are generally not soluble in water. Lipids are made mostly from carbon and hydrogen atoms, as shown in Figure 2-15. Lipids include the compounds we call fats, oils, and waxes. Lipids can be used to store energy, and they form important parts of biological membranes and waterproof coverings. Many lipids, such as steroid hormones, function as chemical messengers.

Trace ElementsJust four elements—oxygen, carbon, hydrogen, and nitrogen—make up 96 percent of living things. The table shows the percentages of some other elements.

1. Construct an Explanation Is the importance of an element in the body related to its per-centage of body weight? Cite the evidence in the table to support your explanation.

2. Evaluate Claims A student claims that the four types of macromolecules make up all of the important compounds of the human body. Provide evidence and reasoning to support or refute this claim.

CASE STUDY Analyzing Data

Other trace elements include fluorine, copper, zinc, and iodine.

ElementPercentage of Body Weight Uses

Phosphorus 1.0 Formation of bones and teeth

Potassium 0.25 Regulation of nerve function

Sulfur 0.25 Present in two amino acids

Sodium 0.15 Regulation of nerve function, blood levels

Chlorine 0.15 Fluid balance

Magnesium 0.05 Bone and muscle function

Iron 0.006 Carrying oxygen in the blood

Figure 2-15

Lipids

Lipid molecules, like this triglycer-ide, are built from fatty acids and glycerol. Olive oil, which contains mainly unsaturated fatty acids, is liquid at room temperature.

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GlycerolFatty acids

INTERACTIVITY

Explore how dietary fat affects blood cholesterol levels.


Many lipids are formed when a glycerol molecule combines with compounds called fatty acids. If each carbon atom in these fatty acid chains is joined to another carbon atom by a single bond, the lipid is said to be saturated because the fatty acids contain the maximum pos-sible number of hydrogen atoms. If there is at least one carbon-carbon double bond in a fatty acid, the fatty acid is said to be unsaturated. Lipids whose fatty acids contain more than one double bond are said to be polyunsaturated. If the terms saturated and polyunsaturated seem familiar, you have probably seen them on food package labels. Lipids that contain unsaturated fatty acids, such as olive oil, tend to be liquids at room temperature. Other cooking oils, such as corn oil, sesame oil, canola oil, and peanut oil, contain polyunsaturated lipids.

READING CHECK Compare How are saturated fats different from unsaturated fats?

Nucleic Acids As shown in Figure 2-16, nucleotides are mono-mers that consist of three components: a 5-carbon sugar, a phos-phate group (–PO4), and a nitrogenous base. Nucleic acids are polymers assembled from nucleotides. Some nucleotides, includ-ing the compound known as adenosine triphosphate (ATP), have important functions in capturing and transferring chemical energy. Individual nucleotides can be joined by covalent bonds to form a polynucleotide, or nucleic acid.

There are two kinds of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). As their names indicate, RNA contains the sugar ribose while DNA contains the sugar deoxyribose. The sequence of bases in both DNA and RNA contains information used by the cell to build other molecules such as proteins. Nucleic acids store and transmit hereditary, or genetic, information.

Proteins Proteins are macromolecules containing nitrogen as well as carbon, hydrogen, and oxygen. Proteins are polymers of mole-cules called amino acids. Amino acids are compounds with an amino group (–NH2) on one end and a carboxyl group (–COOH) on the other end. In addition to serving as the building blocks of proteins, many amino acids serve other purposes. The amino acid tyrosine, shown in Figure 2-17, is used to produce a hormone, or chemical messenger, known as thyroxine. Thyroxine is produced in the thyroid gland from tyrosine and as many as four atoms of iodine.

Figure 2-16

A Nucleotide

The monomers that make up a nucleic acid are nucleotides. Each nucleotide has a 5-carbon sugar, a phosphate group, and a nitrogenous base.

Explore how the body breaks down sugar mole-cules in order to build other types of macromolecules needed by the body.

INTERACTIVITY

C

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General Structure of Amino AcidsH

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N

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O

OH

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Aminogroup

Phosphategroup

5-carbon sugar

Nitrogenousbase

Figure 2-17

Amino Acids

All amino acids have the same basic structure. Only the R group differs among them. Amino acids join together to form proteins or may help form compounds like thyroxine.

CASE STUDY


Peptide Bonding Covalent bonds called peptide bonds link amino acids together to form a polypeptide. As shown in Figure 2-18, the amino group (–NH2) of one amino acid links to the carboxyl group (–COOH) of another amino acid. When the two groups react, they release a water molecule (H2O) as the peptide bond forms.

Function A protein is a functional molecule built from one or more polypeptides. Proteins can have a variety of shapes and sizes, and they serve a variety of purposes as well. Some pro-teins function to control the rate of reactions and regulate cell processes. Others form important cellular structures, while still others transport substances into or out of cells or help to fight disease. Hair (shown in Figure 2-19) and nails are made of protein.

Proteins enable the cells of the body to communicate and interact. Many cells have proteins exposed on their surfaces that act as receptors to certain compounds. When a receptor encoun-ters such a compound, it transmits a chemical signal into the cell, setting off a response. The result may be an increase or decrease in cellular activity, the production of a new protein, or a change in the cell’s pattern of growth and development.

For example, thyroxine binds to cells with a specific receptor protein on their surfaces. That binding increases cell activity. In this way, the thyroid gland can send chemical signals throughout the body that control the activities of millions of cells. If thyroxine levels are too low, especially during childhood, development of the brain and nervous system can be affected.

Structure More than 20 different amino acids are found in nature. All amino acids are identical in the regions where they may be joined together by covalent bonds. This uniformity allows any amino acid to be joined to any other amino acid by linking an amino group to a carboxyl group.

Proteins are among the most diverse macromolecules. The rea-son is that amino acids differ from each other in a side chain called the R-group, which can have a range of different properties. Some R-groups are acidic and some are basic. Some are polar, some are non-polar, and some even contain large ring structures. Two of the amino acids—methionine and cysteine—contain sulfur in their R-groups.

Figure 2-18

Peptide Bonding

Peptide bonds are formed between the amino group of one amino acid and the carboxyl group of another amino acid. In this dia-gram, the amino groups are shaded in green, the carboxyl groups are shaded in blue, and the R-groups are shaded in purple.

Figure 2-19

Protein

Hair and nails are made of a tough protein called keratin.

Formation of Peptide Bond

SerineAlanine

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Levels of Organization Amino acids are assembled into poly-peptide chains according to instructions coded in DNA. However, proteins do not take on a linear shape. Instead, the polypeptides bend and twist into three-dimensional shapes.

To help understand these large molecules, scientists describe pro-teins as having four levels of structure. A protein’s primary structure is the sequence of its amino acids. Secondary structure is the folding or coiling of the polypeptide chain. Tertiary structure is the complete three-dimensional arrangement of a polypeptide chain. Proteins with more than one chain are said to have a fourth level of structure, describing the way in which the different polypeptides are arranged with respect to each other. Figure 2-20 shows the structure of hemo-globin, a protein found in red blood cells that transports oxygen in the bloodstream.

The shape of a protein is maintained by a variety of forces, includ-ing ionic and covalent bonds, as well as van der Waals forces and hydrogen bonds. In the next lesson, you will learn why a protein’s shape is so important.

KEY QUESTIONS

1. What properties of carbon explain carbon’s ability to form different large and complex structures?

2. What are the four major categories of macromol-ecules? Describe the basic structures and the primary functions of each.

CRITICAL THINKING

3. Compare and Contrast What three elements do all macromolecules share? Explain how the chemical properties of lipids, nucleic acids, and proteins differ from carbohydrates.

4. Construct an Explanation How does the struc-ture of an amino acid relate to its function in cellular processes? Use the role of amino acids in the structure of proteins as supportive evidence.

5. Integrate Information What elements differenti-ate the amino acids of a protein from the sugars of a carbohydrate?

LESSON 2.3 Review

Figure 2-20

Protein Structure

The protein hemoglobin con-sists of four subunits. The iron-containing heme group in the center of each subunit gives hemoglobin its red color. An oxygen molecule binds tightly to each heme group.

INTERACTIVITY

Hemegroup Amino

acids

HS-LS1-6


Living things are made up of chemical compounds—some simple and some complex. But chemistry isn’t just what life is made of—chemistry is also what life does. Everything that happens in an organism—its growth, its interaction with the environment, its repro-duction, and even its movement—is based on chemical reactions. Even the twinkle of a firefly’s body comes from a chemical reaction.

Chemical ReactionsA chemical reaction is a process that changes, or transforms, one set of compounds into another. An important scientific principle is that mass and energy are conserved during chemical transforma-tions. This is also true for chemical reactions that occur in living organisms. Some chemical reactions occur slowly, such as the com-bination of iron and oxygen to form an iron oxide called rust. Other reactions occur quickly. The elements or compounds that engage in a chemical reaction are known as reactants. The elements or com-pounds produced by a chemical reaction are known as products.

Chemical reactions involve changes in the chemical bonds that join atoms in compounds.

Energy in ReactionsEnergy is released or absorbed whenever chemical bonds are formed or broken. This means that chemical reactions also involve changes in energy. Some chemical reactions release energy, and other reactions absorb energy. Energy changes are one of the most important factors in determining whether a chemical reaction will occur. Chemical reactions that release energy often occur on their own, or spontaneously. Chemical reactions that absorb energy require a source of energy.

Chemical Reactions and Enzymes2.4 LE

SSO

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KEY QUESTIONS• What happens to

chemical bonds during chemical reactions?

• How do energy changes affect whether a chemical reaction will occur?

• What role do enzymes play in living things, and what affects their function?

VOCABULARYchemical reactionreactantproductactivation energycatalystenzymesubstrate

READING TOOL

As you read the lesson, complete the concept map in your Biology Foundations Workbook that shows the relationship between vocabulary terms.

HS-LS1-6: Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.


An example of an energy-releasing reaction is the burning of hydrogen gas, in which hydrogen reacts with oxygen to produce water vapor.

2H2 + O2 → 2H2O

The energy is released in the form of heat, and sometimes—when hydrogen gas explodes—as light and sound.

The reverse reaction, in which water is changed into hydrogen and oxygen gas, absorbs so much energy that it generally doesn’t occur by itself. In fact, the only practical way to reverse the reaction is to pass an electrical current through water. Thus, in one direction the reaction releases energy, and in the other direction the reaction requires energy.

Energy Sources In order to stay alive, organisms need to carry out reactions that require energy. Because matter and energy are conserved in chemical reactions, every organism must have a source of energy to carry out chemical reactions. Plants get that energy by trapping and storing the energy from sunlight in energy-rich com-pounds. Animals consume plants or other animals for food. Then chemical reactions break apart the food and capture its energy.

Activation Energy Chemical reactions that release energy do not always occur spontaneously. Otherwise, the pages of a book might burst into flames without warning. The cellulose in paper burns only if you light it with a flame, which supplies enough energy to get the reaction started. The energy that is needed to get a reac-tion started is called its activation energy. As Figure 2-22 shows, acti-vation energy is involved in chemical reactions regardless of whether the overall chemical reaction releases energy or absorbs energy.

READING CHECK Interpret Graphs Look at Figure 2-22. How does the energy of the reactants and products differ between an energy-absorbing reaction and an energy-releasing reaction?

Figure 2-22

Activation Energy

The peak of each graph represents the energy needed for the reaction to go forward. The differ-ence between this required energy and the energy of the reactants is the activation energy.

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Figure 2-21

Chemical Reactions

Burning is a type of chemical reac-tion that releases energy. When wood burns it releases energy in the form of heat and light.

2.4 Chemical Reactions and Enzymes 59

EnzymesSome chemical reactions that are essential to life would happen so slowly or require such high activa-tion energies that they could never take place on their own. These chemical reactions are made possible by a process that would make any chemist proud—by catalysts made by living cells. A catalyst is a substance that speeds up the rate of a chemical reaction without being consumed by the reaction. Catalysts work by lowering a reaction’s activation energy.

Nature’s Catalysts Enzymes are biological catalysts, and most enzymes are proteins. The role of enzymes is to speed up chemical reactions that take place in cells. Like other catalysts, enzymes act by lowering activation energies, as illustrated by the graph in Figure 2-23. Lowering the activation energy has a dramatic effect on how quickly the reaction is com-pleted. How big an effect does it have? Consider the reaction in which carbon dioxide combines with water to produce carbonic acid, as shown in Figure 2-24.

Left to itself, this reaction is so slow that carbon dioxide might build up in the body faster than the bloodstream could remove it. Fortunately, the blood-stream contains an enzyme called carbonic anhydrase that speeds up the reaction by a factor of 10 million. With carbonic anhydrase, the reaction takes place immediately and carbon dioxide is removed from the blood quickly.

Enzymes are very specific, generally catalyzing only one chemical reaction. For this reason, part of an enzyme’s name is usually derived from the reaction it catalyzes. Carbonic anhydrase gets its name because it also catalyzes the reverse reaction, which removes water from carbonic acid.

The Enzyme-Substrate Complex How do enzymes do their jobs? For a chemical reaction to occur, the reactants must collide with each other with sufficient energy that existing bonds will be broken and new bonds will be formed. If the reactants do not have enough energy, they will be unchanged after the collision.

Enzymes provide a site where reactants can be brought together, reducing the energy needed for the reaction. The reactants of enzyme-catalyzed reactions are known as substrates. In the reaction catalyzed by carbonic anhydrase, the substrates are water and carbon dioxide.

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anhydrase)

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Figure 2-23

Effect of Enzymes

Notice how the addition of an enzyme lowers the activation energy in this reaction. The enzyme speeds up the reaction.

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Figure 2-24

An Enzyme-Catalyzed Reaction

Carbonic anhydrase binds both substrates: carbon dioxide and water. The substrates react to form carbonic acid.

ANIMATION

INTERACTIVITY

Investigate the changes to matter and energy in an enzyme-catalyzed reaction.


The substrates bind to a site on the enzyme called the active site. The active site and the substrates have complementary shapes, and they may be held together by weak interactions such as hydrogen bonds and van der Waals forces. The fit is so precise that the active site and substrates are often compared to a lock and key. You can even think of the catalyst as being the key that turns on a chemical reaction machine, allowing products to be created much faster than they would be without the catalyst.

Regulation of Enzyme Activity Enzymes play essential roles in chemical pathways, making materials that cells need, releasing energy, and transferring information. Because the activity of an enzyme depends upon the structure of its active site, conditions that tend to change protein structure can affect enzyme activity. These conditions include high temperature and extreme pH, which may weaken hydrogen bonds, causing proteins to unfold and disrupting active site structure.

Not surprisingly, the enzymes produced by human cells generally work best at temperatures close to 37°C, the normal temperature of the human body. Similarly, the stomach enzyme pepsin, which acts on food in the stomach, works best under acidic conditions. In addition, the activities of most enzymes are regulated by molecules that carry chemi-cal signals within cells, switching enzymes “on” or “off” as needed.

KEY QUESTIONS

1. What changes occur to chemical bonds during a chemical reaction?

2. How does the change in energy of a chemical reaction predict whether or not the reaction will occur?

3. Explain the role of enzymes and how they affect the chemical reactions of living things.

CRITICAL THINKING

4. Use Models Explain why a key that fits into a lock is a useful model for the function of enzymes.

5. Construct an Explanation Explain how consum-ing an acid-neutralizing antacid might affect protein digestion. Apply the concept of activa-tion energy to support your explanation.

6. Plan an Investigation Predict which tempera-ture—20°C, 39°C, or 50°C—you would expect a human enzyme to function best. Plan a simple investigation to test your hypothesis.

LESSON 2.4 Review

Explore enzymes and the variables that affect them.

INTERACTIVITY

Temperature and EnzymesProblem How does temperature affect the rate of an enzyme-catalyzed reaction?

Cells must regulate their content of hydrogen peroxide (H2O2), a chemical that helps fight infec-tions but can be harmful in high concentrations. Catalase is the enzyme that catalyzes the breakup of hydrogen peroxide into water and oxygen. In this lab, you will investigate how temperature affects the function of catalase.

You can find this lab in your digital course.

Exploration Lab Open-Ended Inquiry

HS-LS1-6

HS-LS1-6

2.4 Chemical Reactions and Enzymes 61

Something

CASE STUDY WRAP-UP

The element iodine makes up a small fraction of the human body. Nevertheless, a steady supply of iodine is very important for good health.

Make Your CaseThe human diet requires certain trace elements. A key trace ele-ment was missing from the diets of children and adults in Daxin, China. Try to identify the missing element, and then to connect it with both cretinism and goiter.

Communicate Information1. Conduct Research Using library or Internet resources, identify the

missing element, and describe why it is important to human health. Explain why the lack of this element causes cretinism in children and goiter in adults. Explain why goiter is reversible, but cretinism is not.

2. Communicate Information Explain why these disorders are usually found in landlocked regions far from the coast. Are there ways in which the diets of people in places like Daxin could be supplemented to prevent both disorders?

is missing. But what?


Careers on the CaseWork Toward a SolutionDiet is the key to preventing many health prob-lems, including iodine deficiency. Food scien-tists work to make sure that the food people eat is healthy and safe.

Food ScientistFood scientists study ways to make food healthier, to keep food fresh, and to produce food more efficiently. They may work in laboratories or packaging plants.

Society on the CaseFukushima and Thyroid CancerIn March 2011, a tsunami overwhelmed a coastal nuclear power station in Fukushima, Japan. A cooling system failed, leading to a meltdown of its radioactive core, an explosion, and the release of radioactive material. Knowing that one radio-active isotope in particular may cause thyroid cancer, authorities debated whether to distribute tablets of non-radioactive potassium iodide (KI). Potassium iodide helps block radioactive iodine from being absorbed by the thyroid gland.

Fearing that some people might panic and take too much potassium iodide, which can be toxic, the authorities decided not to widely distribute the tablets, although employees of the power company and their families did take the tablets. A 2014 survey after the accident found 75 cases of thyroid cancer among evacuated chil-dren, a rate much higher than the pre-accident incidence of 1 case per million.

Some people think that authorities made the right decision in balancing the potential risks and benefits of widespread distribution of KI tablets. Others disagree. What would you recommend if a similar emergency occurred in your commu-nity today?

Issues involving science, technology, and society require that members of the community study the issues and engage in informed dis-cussion. People need to understand the goals, costs, constraints, and trade-offs to make wise decisions for themselves and their communities.Watch this video to learn about

other careers in biology.

Video

Case Study Wrap-Up 63

H

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Water molecule (H2O)

CHAPTER 2

STUDY GUIDE

2.1 The Nature of MatterAtoms are extremely small, and they are made of even smaller particles called protons, neutrons, and electrons.

A chemical element is a pure substance made of one type of atom. Of the more than 100 elements, only about two dozen are common in living things. However, other elements are important to life in small amounts.

Atoms of different elements join together to form compounds. Chemical bonds hold atoms together in a compound. The two main types of chemical bonds are covalent and ionic.

• atom• nucleus• electron• element• isotope• compound

• ionic bond• ion• covalent bond• molecule• van der Waals forces

2.2 Properties of WaterAll living things depend on water. A water mol-ecule (H2O) is polar, which means one of its ends has a slight positive charge and another end has a slight negative charge.

Hydrogen bonds form between the relatively positive and relatively negative ends of adjacent molecules. These bonds account for water’s special properties, including its ability to dissolve many other substances.

Water solutions may be acidic or basic, which is measured by pH. In blood and similar fluids, com-pounds called buffers help keep pH within toler-able limits.

• hydrogen bond• cohesion• adhesion• mixture• solution• solute

• solvent• suspension• pH scale• acid• base• buffer

Lesson ReviewsGo to your Biology Foundations Workbook for longer versions of these lesson summaries.

Classify What kind of bonds are shown in this diagram?

Use Models Add dotted lines to show the hydrogen bonds in this diagram of water molecules.


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CH2OH

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C C

OH H

C O

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2.4 Chemical Reactions and EnzymesChemical reactions always involve changes in the chemical bonds that join atoms in compounds.

Chemical reactions that release energy often occur on their own, or spontaneously. Other reactions depend on an input of energy.

Living things make and use enzymes, which act as catalysts to speed up a reaction. Enzymes depend on many conditions, including temperature and pH, to function properly.

• chemical reaction• reactant• product• activation energy

• catalyst• enzyme• substrate

Organize InformationComplete the main idea table to review the chapter.

Interpret Graphs Does the reaction described by this graph absorb or release energy? Explain your reasoning.

Main Idea Details

All living things are made up of compounds. 1.

All living things depend on liquid water. 2.

All living things depend on compounds of carbon. 3.

The chemical compounds of living things are constantly changing.

4.

2.3 Carbon CompoundsCarbon atoms are able to form long chains that may include many other elements. As a result, the molecules of carbon compounds can be long, varied, and stable. Living things contain four types of macromolecules, based on carbon. These are carbohydrates, lipids, nucleic acids, and proteins.

Carbohydrates include sugars and starches. They store and release energy. Lipids include fats, oils, and waxes. Some lipids function as chemical messengers. Nucleic acids carry genetic informa-tion. Proteins have many roles: they may control chemical reactions, form cell structures, or carry substances in and out of cells.

• monomer• polymer• carbohydrate• lipid• nucleotide• nucleic acid• protein• amino acid

Classify Identify the monomers and polymer in this example of a macromolecule.

Study Guide 65

Design a Solution

If you’ve ever been caught in a rain-storm, you know how quickly your

clothes can get drenched with water. Most of your everyday clothes are made of materials that absorb water. But some of your clothes—think of your raincoat or your windbreaker—are made of materials that are waterproof.

The leaves of plants are naturally waterproof. The top and bottom surfaces of leaves are covered with a thin, waxy layer that acts as a protective barrier. Some leaves are more waterproof than others. In fact, some are so waterproof that scien-tists call them hydrophobic, or “water fearing.”

The leaves of the lotus flower are a well-known example. Water on a lotus leaf immediately beads up and rolls off the leaf surface, almost as if the leaf itself were pushing the water away. Try doing a video search for “lotus effect” and “hydrophobic effect,” and you can see for yourself.

What makes a hydrophobic substance “fear” water? Now that you know about some of the special properties of water, investigate the phe-nomenon of hydrophobicity on your own or with a partner. Find out what causes the lotus effect, and discuss the technological innovations that have been inspired by it.

STEM

PERFORMANCE-BASED ASSESSMENT

Harnessing theFear of Water

HS-ETS1-1, CCSS.ELA-LITERACY.RST.9-10.3, CCSS.ELA-LITERACY.WHST.9-10.2


1. Obtain Information Search for and watch an online video clip that shows the lotus effect. What do you think causes this phenomenon? Discuss possible answers with your classmates. Then do your own research to deepen your understanding of what you observed.

2. Construct an Explanation Write a scientific explanation of the lotus effect. Your explanation should include descriptions of the microscopic structure of the leaf surface and the physical properties of the materials involved. You should also identify the function that the lotus effect provides to the plant. What problem or need does it solve?

3. Develop Models Draw a diagram or build a model that illustrates the main points of your explanation. Ask your classmates to identify any gaps or weaknesses in your diagram or model. Then refine your work based on the feedback.

ENGINEERING PROJECT

4. Construct Explanations Hydrophobic phenom-ena in the natural world have inspired inventions in the fields of materials science and nanotech-nology. Identify a product that uses hydrophobic technology, and specify the problem or need that it solves. What claims have been made about the product’s effectiveness? Explain how you could scientifically test the validity of such claims and how the data generated from your procedure would be used.

5. Design a Solution Think of a problem or need in your own world that could be solved by hydro-phobic technology. What are the criteria for a successful solution to this problem? Discuss ideas for possible solutions, and then write a proposal outlining how you would design a solution to the problem.

Performance-Based Assessment 67

CHAPTER 2

ASSESSMENT

KEY QUESTIONS AND TERMS

2.1 The Nature of Matter1. In the atom, which particles are in constant motion

around the nucleus?a. protonsb. protons and neutronsc. neutronsd. electrons

2. How do the properties of sodium chloride (NaCl) compare with the properties of its component ele-ments, sodium (Na) and chlorine (Cl)?

3. How do the isotopes of an element differ?

4. Describe the electric charges of the three main subatomic particles.

5. When might van der Waals forces be strong enough to hold two molecules together?

6. How does the behavior of an electron change when it forms a covalent bond?

2.2 Properties of Water

7. What are produced when a base is mixed with water?a. hydrogen ionsb. hydroxide ionsc. oxygen atomsd. oxygen ions

8. Which type of bonds hold two or more water molecules together, as shown?a. covalent bondsb. van der Waals forcesc. hydrogen bondsd. ionic bonds

H

HO

(–)

(+)

9. What is the function of a buffer, such as bicarbon-ate ions in the blood?

10. What does the pH scale measure?

11. What property of water molecules allows water to dissolve many substances?

12. What is the difference between the solute and solvent? Give an example.

13. Why is water essential to all living things?

2.3 Carbon Compounds

14. Carbohydrates may form larger carbon-based macromolecules by combining with which elements?a. sodium, potassium, nitrogenb. nitrogen, sulfur, phosphorusc. potassium, sodium, sulfurd. silicon, phosphorus, sodium

15. Which type of macromolecule stores genetic information?a. carbohydratesb. proteinsc. lipidsd. nucleic acids

16. Which type of macromolecule regulates cell processes or transports material into and out of cells?a. carbohydratesb. proteinsc. lipidsd. nucleic acids

17. What is the relationship between monomers and a polymer?

18. What are three major roles of proteins?

19. What is the general structure of an amino acid?

20. Describe the parts of a nucleotide.

HS-LS1-6

HS-ESS2-5


Water

Na+

Cl–

2.4 Chemical Reactions and Enzymes

21. Which are the catalysts of reactions in living things?a. enzymesb. lipidsc. carbohydratesd. substrates

22. What changes during a chemical reaction between two compounds?a. number of atomsb. chemical bonds c. total massd. total energy

23. Which type of chemical reaction tends to occur on its own, or spontaneously?

24. What is the name for the amount of energy that a reaction needs to get started?

25. How do enzymes act as catalysts in a chemical reaction?

26. What changes to the environment can affect the activity of enzymes?

CRITICAL THINKING

27. Integrate Information An oxygen atom has eight protons. From this information, can you determine numbers of neutrons and electrons in an oxygen atom? Explain.

28. Interpret Visuals Describe the process shown in this diagram. What will the outcome of the pro-cess be?

29. Apply Scientific Reasoning By identifying the properties of water (H2O), can you predict or infer the properties of its component elements, hydrogen and oxygen? Explain why or why not.

30. Communicate Information Describe the basic molecular structures and primary func-tions of the four major categories of biological macromolecules.

31. Design an Experiment Suggest one or two simple experiments to determine whether a solid white substance is a lipid or a carbohydrate. What evidence would you need to support each hypothesis?

32. Synthesize Information In a multistep process, cells can combine the reactants glucose (C6H12O6) and oxygen (O2) to form the products carbon dioxide (CO2) and water (H2O). Some cells can also perform the reverse of this process. Must one, both, or neither of the processes release energy? Explain.

33. Construct an Explanation The temperature of the interior of the human body is about 37°C (98.6°F), regardless of the air temperature. Explain the importance of a constant body temperature given the role of enzymes as catalysts in the body.

34. Integrate Information In a series of chemical reactions, sugar molecules are combined with other reactants to form a protein. What elements must be included in the reactants? Explain your reasoning.

35. Plan an Investigation Like other enzymes, carbonic anhydrase is affected by the pH of the solution surrounding it. Describe the steps of an investigation to identify the ideal pH for the enzyme.

36. Construct an Explanation Changing the tem-perature or pH can change an enzyme's shape. Explain how changing the temperature or pH might affect the function of an enzyme.

37. Construct an Explanation How does the high heat capacity of water contribute to the ability of a river, lake, or ocean to support life?

38. Integrate Information How does water‘s versatil-ity as a solvent help living things survive?

HS-LS1-6

HS-LS1-6

Chapter Assessment 69

ASSESSMENTCHAPTER 2

CROSSCUTTING CONCEPTS

39. Energy and Matter A burning log releases energy in the form of heat and light. Describe the changes to matter at the level of atoms and molecules that cause this energy to be released.

40. Cause and Effect As part of the digestive pro-cess, the human stomach produces hydrochloric acid, HCl. Sometimes excess acid causes discom-fort. In such a case, a person might take an antacid such as magnesium hydroxide, Mg(OH)2. Explain how this substance can reduce the amount of acid in the stomach.

MATH CONNECTIONS

Analyze and Interpret Data

The half-life is a unit of time that measures the stabil-ity of an isotope. During the time span of one half-life, half of a supply of the isotope has decayed into other isotopes or elements. The least stable isotopes have half-lives that are fractions of a second.

The table below shows half-lives and other properties of six isotopes of carbon. Use this table to answer questions 41–43.

Isotope

Number of

Protons

Number of

Neutrons

Abundance (percentage of all carbon

atoms) Half-lifeCarbon-10 6 4 0 19.29

seconds

Carbon-11 6 5 0 20.33 minutes

Carbon-12 6 6 98.9 (stable)

Carbon-13 6 7 1.1 (stable)

Carbon-14 6 8 Less than 0.1 5730 years

Carbon-15 6 9 0 2.45 seconds

41. Identify Patterns Describe the pattern in half-lives and abundance shown by the data.

42. Apply Scientific Reasoning Among the six iso-topes shown in the table, which would be most useful for labeling, or tracing, a carbon compound through an organism? Explain.

43. Reason Quantitatively A molecule of a com-plex carbohydrate is made of 12 carbon atoms. Assuming that carbon and other atoms combine in the expected ratio, calculate the numbers of hydrogen and oxygen atoms in the molecule.

44. Model With Mathematics A hydrogen atom has 1 proton and 1 electron, while an oxygen atom has 8 protons and 8 electrons. Construct a table to show the changes in protons and electrons when a water molecule (H2O) splits apart to form a hydro-gen ion (H+) and a hydroxide ion (OH–).

LANGUAGE ARTS CONNECTIONS

Write About Science

45. Write Explanatory Texts The four main types of macromolecules are carbohydrates, lipids, nucleic acids, and proteins. Write a method for classifying a macromolecule based on its chemi-cal composition.

46. Write Informative Texts Write a paragraph that explains the role of enzymes in biological systems. Focus on a specific reaction, such as the combin-ing of carbon dioxide and water to form carbonic acid. Note the importance of the shapes of the enzyme active site and substrate in the reaction.

Read About Science

47. Summarize Text Describe three properties of water that help explain why it is essential to all living things.

48. Integrate With Visuals Review Figure 2-13, which shows how monomers combine to form polymers. How does the diagram help explain the formation of each type of macromolecule?

CCSS.MATH.CONTENT.MP2, CCSS.MATH.CONTENT.MP4, CCSS.MATH.CONTENT.HSN.Q.A.1

CCSS.ELA-LITERACY.RST.9-10.2, CCSS.ELA-LITERACY.RST.9-10.7

CCSS.ELA-LITERACY.WHST.9-10.2, CCSS.ELA-LITERACY.WHST.9-10.4


End-of-Course Test Practice 71

END-OF-COURSE TEST PRACTICECHAPTER 2

Questions 1–2

The diagram shows the structure of glucose, a mol-ecule that organisms make and use.

Starch

H

CH2OH

H OH

HO

H

C C

OH H

C O

C

OH

H

C

Glucose

1. Which of these statements describes how organ-isms could use molecules of glucose? I. Several glucose molecules could be assembled

into a larger molecule. II. Glucose molecules could be broken apart to

form smaller molecules. III. The atoms of glucose molecules could be com-

bined with other elements to form a different molecule.

A. I onlyB. III onlyC. I and II onlyD. I and III onlyE. I, II, and III

2. What structural feature does glucose share with larger organic molecules, including DNA and proteins?A. A basic structure, or backbone, formed by

chains of carbon atomsB. A ratio of 1 carbon atom to 2 hydrogen atoms

to 1 oxygen atomC. The presence of double covalent bonds

between carbon atomsD. Strong bonds between carbon atoms that

cannot be brokenE. Ionic bonds between carbon atoms and

oxygen atoms

3. Roger is comparing a model of a sugar molecule to a model of an amino acid. Which of the follow-ing evidence statements would be supported by the two models?A. Sugars and amino acids are made of exactly

the same elements.B. Sugars and amino acids have the same structure.C. Sugars and animo acids are made of the same

elements but amino acids also contain nitrogen.D. Sugars and amino acids are made of the same

elements but sugars also contain phosphorus.E. Sugars and amino acids are both polymers.

4. Lisa is investigating a chemical reaction involving carbon compounds. Which of these results could NOT occur, according to scientific laws about chemical reactions, matter, and energy?A. The reaction releases energy to the environment.B. The reaction absorbs energy from the environment.C. The mass of carbon in the products is greater

than the mass of carbon in the reactants.D. The mass of carbon in the products is equal to

the mass of carbon in the reactants.E. The reaction occurs faster in the presence of a

certain protein.

5. Two students are developing a computer simula-tion of a chemical reaction that forms amino acids. Their simulation uses colored spheres to represent atoms of different elements. It uses lines con-necting the spheres to represent chemical bonds. For the simulation to be accurate, which of these features should be included?A. Lines that break and reform between the spheresB. Spheres that break apart into small piecesC. Spheres that disappear during the simulation of

the reactionD. Spheres that appear during the simulation of

the reactionE. Lines that never break once they are placed

If You Have Trouble With…Question 1 2 3 4 5

See Lesson 2.3 2.3 2.3 2.4 2.4

Performance Expectation HS-LS1-6 HS-LS1-6 HS-LS1-6 HS-LS1-6 HS-LS1-6

ASSESSMENT

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