Cells and Life

 

 

2 NOTES

Key Concepts

Vocabulary

Key Concepts

After this lesson, students should understand the Key Concepts and be able to answer these questions. Have students write each question in their Science Journals. Revisit each question as you cover its relevant content.

Vocabulary

Build a Class Definition

1. Write the word cell on chart paper or the board.

2. Ask: What are some common nonbiological uses of this word?

cell phone, prison cell, storm cell, spy cell. Explain to students that a cell is an individual unit within a large collection of identical, or similar, units.

3. Ask students how this general definition of a cell relates to the cells of organisms. Develop a class definition for cell. Students should record the agreed-on definition in their Science Journals.

Cells and Life

 What do you think?

Before you read, decide if you agree or disagree with each of these statements. As you read this lesson, see if you change your mind about any of the statements.

     1. Nonliving things have cells.

Disagree. Cells are found only in living things.

     2. Cells are made mostly of water.

Agree. Water makes up more than 70 percent of a cell’s volume.

 

Student answers will vary. Possible answer: All the cells in the plant came from the original cell in the seed.

 

ESSENTIAL QUESTIONS

• How did scientists’ understanding of cells develop?
• What basic substances make up a cell?

8 NOTES

Understanding Cells

The Cell Theory

Visual Literacy : Table 1

Teacher Demo

Differentiated Instruction

Differentiated Instruction

Fun Fact

Math Activity

Understanding Cells

After students read the first two paragraphs, use the scaffolded questions below to start a discussion to connect their experience in the Launch Lab to the lesson.

Guiding Questions

Why was it necessary to use a magnifying lens to look at the bean seed?

because the seeds are tiny

Why were early scientists unaware that cells existed?

The structures inside the seed are tiny and cannot be seen in great detail without magnification. Students should understand that the seeds they cut open, although small, are made up of many hundreds of cells. So it would have been hard for early scientists to see them without magnifying glasses.

What do you think you’d find if you cut open the bean seedling?

Students’ answers should relate to Hooke’s discovery of the many “small rooms” or cells in tree bark.

The Cell Theory

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Students may be aware that all living things are made of cells. But they may not realize that an organism can be a cell. Ask students to think about the following questions.

Guiding Questions

Where do new cells come from?

All cells come from preexisting cells.

Key Concept Check How did scientists’ understanding of cells develop?

by using better microscopes and looking for cells in many different places

Could the cell theory have been developed before the invention of the microscope?

Until the microscope was invented, it was impossible to see cells, so there would have been no way to demonstrate the validity of a cell theory.

A friend claims that plant cells and animal cells carry on similar functions. Do you agree or disagree with this claim? Explain your answer.

Accept all reasonable responses. Based on their understanding of the cell theory, students should know that Schleiden and Schwann realized that plant and animal cells have similar features and carry on similar functions, such as extracting energy from food and eliminating wastes.

Visual Literacy: Table 1

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Students may not understand how individual cells form a tissue. The top-right photograph in Table 1 shows how the cells in the leaves of this flower are packed closely together.

Ask: About how many cells are shown in this small portion of the leaf?

about 28

Ask: Would tissues hold together if there were gaps between them?

No. Tightly bound cell tissue gives the cell strength and shape.

Teacher Demo

A Close Look at Cells Using a microprojector, show prepared slides of plant, animal, and bacterial cells to the class. Tell students they are observing the smallest units of life. Ask students to compare and contrast the cells’ characteristics.

Differentiated Instruction

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Cells and Cell Theory Have students partner and use research resources available to them in class to learn more about the founders of the cell theory and their work that contributed to the concepts of the cell theory. Ask them to share what they discover with the class.

Differentiated Instruction

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Focusing on the Founders Have students select one of the pioneers of the cell theory, learn more about him or her, and write a one-page report about his or her work and its significance. Combine the reports in a folder and encourage students to take turns reading it.

Fun Fact

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From cell theory to germ theory The discovery of unicellular organisms by Robert Hooke and other scientists led to the germ theory in the early 1800s. Some scientists proposed that many diseases are caused by germs—infectious organisms visible only with a microscope. Those scientists were widely ridiculed. It wasn’t until the late 1800s that the germ theory of disease was accepted. Before that, people did not know exactly where disease came from. Some thought it was from evil spirits or dirty air.

Math Activity

How small is small? A typical size for a bacterium is about 1 micron—one-millionth of a meter. A meter contains 1,000 millimeters. A pinhead is about 1 mm². How many 1-micron bacteria could fit on the head of a pin? Solution: Because there are 1,000 mm in a meter, there are 1,000 microns in a millimeter—1,000 bacteria lengths. If a pinhead has an area of 1 square millimeter, it will hold 1 million bacteria (1,000 × 1,000).

LinksCells-Lesson 1    Cells Alive Interactive Games   cell theory rap  Cell Theory Animation

The wacky history of cell theory  Cell Theory Timeline   BrainPop Cells  Short Video Hooke


Understanding Cells

Have you ever looked up at the night sky and tried to find other planets in our solar system? It is hard to see them without using a telescope. This is because the other planets are millions of kilometers away. Just like we can use telescopes to see other planets, we can use microscopes to see the basic units of all living things—cells. But people didn’t always know about cells. Because cells are so small, early scientists had no tools to study them. It took hundreds of years for scientists to learn about cells.
 

More than 300 years ago, an English scientist named Robert Hooke built a microscope. He used the microscope to look at cork, which is part of a cork oak tree’s bark. What he saw looked like the openings in a honeycomb, as shown in Figure 1. The openings reminded him of the small rooms, called cells, where monks lived. He called the structures cells, from the Latin word cellula (SEL yuh luh), which means “small rooms.”
 

(t) Bon Appetit/Alamy, (b) Omikron/Photo Researchers

Figure 1 To Robert Hooke, the cells of cork looked like the openings in a honeycomb.

 

The Cell Theory

After Hooke’s discovery, other scientists began making better microscopes and looking for cells in many other places, such as pond water and blood. The newer microscopes enabled scientists to see different structures inside cells. Matthias Schleiden (SHLI dun), a German scientist, used one of the new microscopes to look at plant cells. Around the same time, another German scientist, Theodor Schwann, used a microscope to study animal cells. Schleiden and Schwann realized that plant and animal cells have similar features and carry on similar functions, such as extracting energy from food and eliminating wastes. From this evidence, Schleiden and Schwann concluded that cells are the basic unit of life.

Almost two decades later, Rudolf Virchow (VUR koh), a German doctor, proposed that all cells come from preexisting cells, or cells that already exist. The observations made by Schleiden, Schwann, and Virchow were combined into one theory.

As illustrated in Table 1, the cell theory states that all living things are made of one or more cells, the cell is the smallest unit of life, and all new cells come from preexisting cells. After the development of the cell theory, scientists raised more questions about cells. If all living things are made of cells, what are cells made of?
 

1. Key Concept Check How did scientists’ understanding of cells develop?

by using better microscopes and looking for cells in many different places

(tl) Tim Fitzharris/Minden Pictures/Getty Images, (tr) James M. Bell/Photo Researchers, (c) Biophoto Associates/Photo Researchers, (bl, bc, br) Dr. Richard Kessel & Dr. Gene Shih/Visuals Unlimited/Getty Images

Table 1 Scientists developed the cell theory after studying cells with microscopes.

 

1 NOTE

Cell Theory Time Line

To complete the entries on the time line, students will need to do research. Provide them with access to the Internet or other research materials to look up key dates and facts. Divide the class into groups; assign one scientist to each group. Time lines should include the discoveries of Robert Hooke, Matthias Schleiden, Theodor Schwann, and Rudolph Virchow. Time lines also might include the discoveries of Hans and Zacharias Janssen and Anton van Leeuwenhoek.

In-Class Activity: Cell Theory Time Line

 

As a class, create a time line of the important discoveries that led to the development of the cell theory. Draw a time line on the board or chart paper. Add entries at the appropriate time points. For each entry on the time line, note the scientist and any relevant facts about the discovery. Once the time line is complete, copy the diagram in your Science Journal to help you understand the key principles of the cell theory.

Analyze and Conclude

1. What invention enabled scientists to study cells and develop the cell theory?

the microscope

2. Which three scientists discovered evidence that led directly to the development of the cell theory?

Matthias Schleiden, Theodor Schwann, and Rudolf Virchow

3. What are the three principles of the cell theory?

All living things are made of one or more cells. The cell is the smallest unit of life. All new cells come from preexisting cells.

 

1 NOTE

REVIEW VOCABULARY

Explain that, in science, a theory is not just a guess.

Ask: What is a hypothesis and how does it differ from a theory?

Explain that a hypothesis is an idea about how something works. Scientists propose a hypothesis and then make observations and/or conduct experiments that they hope will enable them to develop their hypothesis into a solid theory.

REVIEW VOCABULARY

theory
explanation of things or events based on scientific knowledge resulting from many observations and experiments

 

12 NOTES

Basic Cell Substances

The Main Ingredient — Water

Visual Literacy : Figure 2

Macromolecules

Differentiated Instruction

Teacher Demo

Differentiated Instruction

Differentiated Instruction

Differentiated Instruction

Teacher Demo

Real - World Science

Fun Fact

Basic Cell Substances

Cells contain many macromolecules, or large molecules composed of a large number of smaller molecules joined together. Use the scaffolded questions below to informally assess students’ comprehension of this concept.

Guiding Questions

In what ways are the macromolecules in cells similar to long trains or a brick wall?

Just as a long train consists of many connected cars, a macromolecule consists of many connected smaller molecules. Macromolecules are also like a brick wall, because they are made up of smaller pieces joined together.

Why do cells need water to function?

All cells and the molecules that cells are made of need water. Water is essential for all living things on Earth.

The Main Ingredient—Water

Water makes up most of a cell’s volume, and it is an ideal liquid for dissolving many substances. Use the scaffolded questions below to informally assess students’ comprehension of the role of water in cells.

Guiding Questions

What functions does water play in the body?

It fills and surrounds cells and it helps maintain the body’s homeostasis, or stable internal environment.

Visual Check Which part of the salt crystal is attracted to the oxygen in the water molecule?

sodium

Why do scientists think that Mars and some of the moons of the solar system may contain some form of life?

Because they have discovered evidence of water there. Only where there is water present is life as we know it possible.

Visual Literacy: Figure 2

Explain that sodium chloride (salt) is held together by ionic bonds. When dissolved in water, sodium chloride separates into positive sodium ions and negative chloride ions. Ions are atoms that have gained or lost one or more electrons and acquired an electric charge.

Macromolecules

This is a quick reiteration of the information about macromolecules, naming the four macromolecules found in cells: nucleic acids, proteins, lipids, and carbohydrates. Be sure students know these four macromolecules before discussing each in more detail.

Students should learn the primary functions of the four macromolecules as explained in the callouts on the photograph of the cell in Figure 3. They will learn more later in the lesson.

Guiding Questions

What are the four kinds of macromolecules?

nucleic acids, lipids, proteins, and carbohydrates

Which of the four kinds of macromolecules has a precise function and location inside of the cell in Figure 3?

Nucleic acids are found in the cell’s nucleus, which is indicated in the photograph of the cell. DNA never leaves the nucleus because of its large size. RNA is smaller and single-stranded. It has the ability to enter and exit the nucleus, so RNA is found both inside and outside the nucleus. The other three macromolecules are found at various places in the cell.

Nucleic Acids

Students need to understand that there are two kinds of nucleic acids and that they work together to make proteins.

Guiding Questions

What does DNA make, and what does RNA make?

DNA makes RNA, and RNA makes proteins.

If the order of nucleotides in a section of DNA gets mixed-up when new DNA is made, what is the end result?

The protein that gets made from the instructions in that segment of DNA will not be made correctly and will probably not function properly. This is called a mutation.

Since DNA contains all the instructions for cells to grow, divide, and respond to the environment, why does an organism need proteins?

Proteins are involved in all the growing, responding, and dividing. They are an important component of the body’s many tissues, and they carry out many vital functions.

Proteins, Lipids, and Carbohydrates

These are the main constituents of cells, along with nucleic acids. Students should learn that each substance has important functions.

Guiding Questions

How are proteins manufactured in a cell?

RNA makes them by linking together building blocks called amino acids.

Key Concept Check What basic substances make up a cell?

nucleic acids, proteins, lipids, and carbohydrates

Reading Check Why are lipids important to cells?

Because lipids don’t mix with water, they act as protective barriers in cells. They also make up cholesterols and hormones that play important roles in our bodies.

Differentiated Instruction

Macromolecules and Other Macros Have students use a dictionary to find other words beginning with the prefix macro–. Have them write down the words and their meanings.

Teacher Demo

Macromolecules Illustrate the nature of a macromolecule. Bring to class a collection of a dozen or more units that can be linked together, such as buttons of various colors. Tell students that these units represent small molecules in a cell. Then show students a group of identical or similar units that have been joined with thread or string. Explain that this represents a macromolecule. Ask students to compare the individual buttons to the buttons that are joined with threads or string. Students should note that the joined buttons contain the individual buttons.

Differentiated Instruction

Cell Substances Have students join in a discussion about the four main substances in cells and what they do.

Differentiated Instruction

Zeroing In Have students, using reference resources available in class, find the names of some particular proteins and learn what they do. Have them share their findings with the class.

Differentiated Instruction

Learning About Nuclei Explain that nucleic acids are so called because they are found in the nucleus of the cell. Explain that a cell isn’t the only thing with a nucleus. Although different from the nucleus in a living cell, atoms and comets have nuclei, and the word also applies to the central and most important part of an organization or group. Have students learn more about the word nucleus.

Teacher Demo

Proteins, Carbohydrates, and Lipids Bring in some everyday examples of these substances. For example, crack an egg and explain that the white is almost pure protein (about 40 kinds of proteins) mixed with water. The yolk of the egg contains lipids, including cholesterol. You can find many common examples of the three substances on the Internet. The point of the exercise is to show that the substances are part of our lives, not just the components of cells.

Real-World Science

Close Encounters Have students investigate proteins, carbohydrates, and lipids on their own and find examples of these substances that they encounter on a daily basis. For example, a hamburger is full of protein but also contains lipids in the form of saturated fats.

Fun Fact

Tasty Lipids It is the fats in foods that give them much of their flavor. Many foods that are considered “junk food,” such as ice cream and corn chips, are high in fat. Controlling one’s weight often requires eating less of these foods and more of foods that provide good nutrition with less or no fat, such as whole-grain bread and fruit. Fats aren’t all bad, though. You need a certain amount of fat in your diet for your cells to function properly. And some fats, such as those in olive oil and certain kinds of fish, are thought to protect people against heart disease.

Basic Cell Substances

Have you ever watched a train travel down a railroad track? The locomotive pulls train cars that are hooked together. Like a train, many of the substances in cells are made of smaller parts that are joined together. These substances, called macromolecules, form by joining many small molecules together. As you will read later in this lesson, macromolecules have many important roles in cells. But macromolecules cannot function without one of the most important substances in cells—water.

The Main Ingredient—Water
 The main ingredient in any cell is water. It makes up more than 70 percent of a cell’s volume and is essential for life. Why is water such an important molecule? In addition to making up a large part of the inside of cells, water also surrounds cells. The water surrounding your cells helps to insulate your body, which maintains homeostasis, or a stable internal environment.

The structure of a water molecule makes it ideal for dissolving many other substances. Substances must be in a liquid to move into and out of cells. A water molecule has two areas:

• An area that is more negative (−), called the negative end; this end can attract the positive part of
   another substance.

• An area that is more positive (+), called the positive end; this end can attract the negative part of
   another substance.

Examine Figure 2 to see how the positive and negative ends of water molecules dissolve salt crystals.

FoodCollection/SuperStock

Figure 2 The positive and negative ends of a water molecule attract the positive and negative parts of another substance, similar to the way magnets are attracted to each other.

Visual Check Which part of the salt crystal is attracted to the oxygen in the water molecule?

sodium

 

Macromolecules

Although water is essential for life, all cells contain other substances that enable them to function. Recall that macro-molecules are large molecules that form when smaller molecules join together. As shown in Figure 3, there are four types of macromolecules in cells: nucleic acids, proteins, lipids, and carbohydrates. Each type of macromolecule has unique functions in a cell. These functions range from growth and communication to movement and storage.

Nucleic Acids Both deoxyribonucleic (dee AHK sih ri boh noo klee ihk) acid (DNA) and ribonucleic (ri boh noo KLEE ihk) acid (RNA) are nucleic acids. Nucleic acids are macromolecules that form when long chains of molecules called nucleotides (NEW klee uh tidz) join together. The order of nucleotides in DNA and RNA is important. If you change the order of words in a sentence, you can change the meaning of the sentence. In a similar way, changing the order of nucleotides in DNA and

RNA can change the genetic information in a cell.

Nucleic acids are important in cells because they contain genetic information. This information can pass from parents to offspring. DNA includes instructions for cell growth, cell reproduction, and cell processes that enable a cell to respond to its environment. DNA is used to make RNA. RNA is used to make proteins.

Proteins The macromolecules necessary for nearly everything cells do are proteins. Proteins are long chains of amino acid molecules. You just read that RNA is used to make proteins. RNA contains instructions for joining amino acids together.

Cells contain hundreds of proteins. Each protein has a unique function. Some proteins help cells communicate with each other. Other proteins transport substances around inside cells. Some proteins, such as amylase (AM uh lays) in saliva, help break down nutrients in food. Other proteins, such as keratin (KER uh tun)—a protein found in hair, horns, and feathers—provide structural support.

Lipids Another group of macromolecules found in cells is lipids. A lipid is a large macromolecule that does not dissolve in water. Because lipids do not mix with water, they play an important role as protective barriers in cells. They are also the major part of cell membranes. Lipids play roles in energy storage and in cell communication. Examples of lipids are cholesterol (kuh LES tuh rawl), phospholipids (fahs foh LIH pids), and vitamin A.

1. Reading Check Why are lipids important to cells?
 

Because lipids don’t mix with water, they act as protective barriers in cells. They also make up cholesterols and hormones that play important roles in our bodies.

Carbohydrates One sugar molecule, two sugar molecules, or a long chain of sugar molecules make up carbohydrates (kar boh HI drayts). Carbohydrates store energy, provide structural support, and are needed for communication between cells. Sugars and starches are carbohydrates that store energy. Fruits contain sugars. Breads and pastas are mostly starch. The energy in sugars and starches can be released quickly through chemical reactions in cells. Cellulose is a carbohydrate in the cell walls in plants that provides structural support.

2. Key Concept Check What basic substances make up a cell?

nucleic acids, proteins, lipids, and carbohydrates

Dr. Gopal Murti/Photo Researchers

Figure 3 Each type of macromolecule has a special function in a cell.

 

1 NOTE

WORD ORIGIN

Ask: Why are these molecules called macromolecules?

They have this name because they are large molecules. The prefix macro– comes from the Greek word makro–, meaning “long and large.”

WORD ORIGIN

macromolecule
from Greek makro–, means “long”; and Latin molecula, means “mass”

 

1 NOTE

How can you observe DNA?

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Prep: 10 min Class: 25 min

Purpose
To introduce students to DNA, an important macromolecule found in cells, as seen at various magnifications.

Materials
Student: microscope, onion root-tip slides (or other slides in which DNA can clearly be seen and cells can easily be counted) Optional: board and chalk (or overhead projector, transparency, and marker) if you choose to fill in class data on the board

Before You Begin
Familiarize students with the major macromolecules that are found in cells, specifically DNA.

Guide the Investigation
• If there are not enough slides or microscopes for all students, have them work in pairs. Assign half the class to observe the cells in 100× and the other half to observe the cells in 400×.

• Be sure students handle the slides with caution.

• Troubleshooting The class may need to review how to properly handle and use a microscope. Visit each student or student pair to ensure they are seeing what they are supposed to see in their field of view. Help students discover how to figure out the percentage of cells with visible DNA (number of cells with visible DNA/total number of cells).

 

Higher magnifications limit the field of view, so it will affect percentages.

Lesson Review

Visual Summary

The cell theory summarizes the main principles for understanding that the cell is the basic unit of life

James M. Bell/Photo Researchers
 

Water is the main ingredient in every cell.

A nucleic acid, such as DNA, contains the genetic information for a cell.

Dr. Gopal Murti/Photo Researchers

 

What do you think NOW?

You first read the statements below at the beginning of the lesson.

1. Nonliving things have cells.

2. Cells are made mostly of water.

    Did you change your mind about whether you agree or disagree with the statements? Rewrite any
   false statements to make them true.

 

Lesson Assessment

Use Vocabulary

1. The __________ __________ states that the cell is the basic unit of all living things.

cell theory

2. Distinguish between a carbohydrate and a lipid.

Carbohydrates are sugars that store energy, provide structural support, and are needed for communication between cells. Lipids are fats which are not soluble in water and act as protective barriers in cells.

3. Use the term nucleic acid in a sentence.

Sample answers might include: DNA and RNA are nucleic acids. Nucleic acids are made up of nucleotides. Nucleic acids contain genetic information.

Understand Key Concepts

4. Which macromolecule is made from amino acids?

     A. lipid

     B. protein

     C. carbohydrate

     D. nucleic acid

5. Describe how the invention of the microscope helped scientists understand cells.

The invention of the microscope enabled scientists to see the structure and function of cells.

6. Compare the functions of DNA and proteins in a cell.

DNA stores genetic information, whereas proteins are used for many functions, such as communication between cells, transport of materials, structural support, and the breaking down nutrients.

7. Cholesterol is which type of macromolecule?

     A. carbohydrate

     B. lipid

     C. nucleic acid

     D. protein

8. Genetic information is stored in which macromolecule?

     A. DNA

     B. glucose

     C. lipid

     D. starch
 

Interpret Graphics

9. Summarize Copy and fill in the graphic organizer below to summarize the main principles of the
     cell theory.

 

10. Analyze How does the structure of the water molecule shown below enable it to interact with    
     other water molecules?

Each water molecule has an area that is more negative and an area that is more positive. The negative oxygen areas can attract positive hydrogens on other nearby water molecules. The hydrogens in the positive area of the water molecule can attract oxygens in negative areas of other water molecules.

Critical Thinking

11. Summarize the functions of lipids in cells.

Lipids provide protective barriers around cells and play roles in energy storage and cell communication.

12. Hypothesize why carbohydrates are found in plant cell walls.

Carbohydrates are found in plant-cell walls because they provide structural support.

13. Evaluate the importance of the microscope to biology.

The microscope was important to biology because it enabled scientists to discover and study the structure of the cell, which is the foundation of all living things.

14. Summarize the role of water in cells.

Water is the most abundant component of a cell, making up 70 percent of a cell’s volume. Water provides a stable environment inside and outside a cell. Many substances can be dissolved in water.

15. Hypothesize how new cells form from existing cells.

Existing cells produce new cells by dividing.

16. Assess According to the cell theory, the cell is the smallest unit of life. How does this concept 
     relate to the function of cells?
 

All cells carry on similar functions to sustain life, such as extracting energy from food and eliminating wastes.

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