SECTION SIX

The Cell

All living things show similar characteristics, such as the ability to reproduce and the need for energy. In this section students are introduced to some of the basic characteristics of living things. The use of energy is examined closely, beginning with the production of ATP within mitochondria, the transfer of this energy through the organism, and the use of energy to maintain life. Activities in this section specifically focus on the characteristics of life, conversion of ATP to ADP, the steps of the cell cycle, cell organelles and their roles, roles of enzymes, differences between plant and animal cells, photosynthesis, and respiration.

6.1. CHARACTERISTICS OF LIFE

Is It Alive?

Biology is the study of living things. Scientists have a set of criteria that an organism must possess in order to be considered alive or once living. For example, viruses are not classified as living things because they cannot reproduce without the help of a host. In this activity you will determine some of the characteristics of living things.

Materials

Small animal (such as an ant) in a covered container (handle carefully; release after the activity is done)

Small plant (such as a potted fern or clump of grass)
Rock
Magnifying glass

Activity

1. Think of several different living things. Make a list of all the characteristics you think a living thing must possess to be considered “alive.”

2. Once your list is complete, look at the ant and the fern and see whether they possess all these attributes. You may want to add to or delete characteristics from your list at this time.

3. Now examine the rock and see which attributes it lacks.

Follow-Up Questions

1. What characteristics did you decide are common to living things?

2. What characteristics did the rock lack that ruled it out as a living thing? Did it have any characteristics that living things also possess?

Extension

Visualize a mushroom and an amoeba. You may need to look these up in a textbook or on the Internet. Do these two organisms meet the criteria for living things that you established? Do you need to add additional characteristics or delete some you established about the animal and plant you observed earlier?

6.2. ENERGY MOLECULES

ATP and ADP

Cells require energy for chemical reactions such as respiration and photosynthesis. Adenosine triphosphate (ATP) is the primary energy-supplying molecule in cells. An ATP molecule is made up of three phosphate groups, each with the formula PO₄. When ATP loses a phosphate group, it becomes adenosine diphosphate (ADP) and releases large amounts of energy to the cell. Eventually ADP reclaims a phosphate group and becomes high-energy ATP again. In this activity you will create a model of ATP and ADP.

Materials

Masking tape; Pen or marker; String (about two inches long); Small plastic container with lid

Activity

1. Place the small plastic container with lid on a table.

2. Tear off four one-inch pieces of masking tape. Write “PO₄” on three pieces of tape. Write “E” on the fourth piece.

3. Tape one PO₄ on the lid of the container. Place the second PO₄ near the bottom of the container (on the exterior) and the third near the middle of the container (on the exterior).

4. Remove the lid of the container. Tape the piece of string on the underside of the lid so it dangles downward. At the free end of the string attach the E.

5. Place the lid back on the container (with the E on the inside).The closed container represents a molecule of ATP. The E inside the container represents the energy of this molecule.

6. Remove the lid. You have removed a PO₄ and with it energy has been released. Now the molecule is called ADP. To make it ATP again, you must replace the lid.

Follow-Up Questions

1. How many phosphates does ATP contain? What does an ATP molecule possess that is important to cell function?

2. How many phosphates does ADP contain? What happens to it when it gains a phosphate?

Extension

Look up molecular diagrams of ATP and ADP. What elements are found in each of these energy molecules? What are some other energy molecules found in cells during cellular respiration?

6.3. ATP AND LACTIC ACID

Muscle Fatigue

The cells in your body get energy from the breakdown of carbon-rich compounds such as glucose. In the presence of oxygen, cells can completely convert glucose into large amounts of ATP, an energy molecule. During times of exercise, adequate supplies of oxygen may not be available to every cell. When oxygen levels are low, cells convert glucose to a small amount of energy and a waste product, lactic acid. When lactic acid builds up, it makes your muscles feel tired and sore. In this activity you will carry out an exercise to produce lactic acid in some of your muscles.

Materials

Unopened large can of food or hand weight
Watch with a second hand

Activity

1. Pick up the can of food or a hand weight with your nonwriting hand.

2. Place your elbow on the desk and do one bicep curl by lifting and lowering the can.

3. Once you have the hang of it, do this exercise as quickly and as many times as possible for one minute. After a minute, note how your arm feels. Are you tired in any part of your arm? If not, repeat the bicep curl for another sixty seconds.

Follow-Up Questions

1. The act of doing the bicep curls was a biological process, and all biological processes require energy. After the exercise, did your arm feel tired? If so, what part of your arm?

2. What do you think happened on a cellular level to account for your tiredness?

3. Do you think some people in the class felt more fatigued than others? Explain your answer.

Extension

While standing, rise up and down on your toes repeatedly for one minute. How do you feel? What area of your leg is the sorest? Find out which students in your class run or exercise a lot. How did their responses to this exercise differ from those of students who do not exercise often?

6.4. THE CELL CYCLE, PART ONE

Getting Started

The life cycle of a cell includes a period of growth followed by mitosis, or cell division. During the growth period, called interphase, cells increase in size. Just before cell division, chromosomes, structures that hold the cell’s genetic material, copy or replicate themselves so that each new cell will have a copy. This activity will summarize the first steps of the cell cycle.

Materials

Paper plate; Small ball of red modeling clay; Small ball of blue modeling clay; Two thumbtacks

Activity

1. Use the red clay to make two rod-shaped chromosomes about the size of your little finger. Make two similar chromosomes from blue clay.

2. The cell cycle begins with interphase. Place one red chromosome and one blue chromosome on the paper plate. These represent the cell’s genetic material during interphase.

3. When it is almost time for a cell to divide, it makes a copy of its chromosomes. To show the replication of the red chromosome, place the second red rod next to the original. The original red rod and its copy are now called sister chromatids.

4. Connect these two sister chromatids with a thumbtack to represent the centromere, the place where sister chromatids come in contact.

5. To show the replication of the blue chromosome, place the second blue rod next to the original. Both blue rods are now sister chromatids.

6. Connect these two sister chromatids with a thumbtack.

Follow-Up Questions

1. How many chromosomes did the cell have in interphase?

2. What happened to these chromosomes?

3. What structure holds sister chromatids together?

Extension

Explain how this activity would differ if your paper-plate cell had forty-six chromosomes instead of two.

6.5. THE CELL CYCLE, PART TWO

The Process

After copying its chromosomes, a cell is ready to divide into two new or daughter cells. Cell division has several steps: prophase, metaphase, anaphase, and telophase. During these steps, the original chromosomes and their copies split up. Next, the cytoplasm separates in a process called cytokinesis. The two new cells formed by cell division have the same number of chromosomes. They also have the same number of chromosomes as the original cell.

Materials

Paper plate with sister chromatids from Activity 6.4; Two paper plates; Scissors

Activity

1. In prophase, the first step, the red and blue chromosomes are ready for cell division.

2. Move the red and blue chromosomes to the center of the plate to simulate metaphase.

3. Remove the centromere holding the two red sister chromatids together and separate them slightly. Do the same for the blue sister chromatids. This step demonstrates what happens in anaphase. Each sister chromatid is now called a chromosome.

4. During telophase, chromosomes reach opposite ends of the cell and the cell begins to divide. Further separate the chromosomes. Use scissors to partially cut the top and bottom sections of the plate.

5. Continue to drag the chromosomes off the old plate. Place each on a daughter cell plate to simulate cytokinesis, division of the cytoplasm.

Follow-Up Questions

1. The cell in this demonstration began mitosis with two chromosomes. How many chromosomes did it have at the end of mitosis?

2. Why must a cell make a copy of its chromosomes before cell division?

Extension

Look up a description of the process of meiosis. Model this process using clay and paper plates.

6.6. CELL TRANSPORT

When It Comes to Cells, Small Is Good

Why are single-celled organisms so small? Why can’t an amoeba grow to the size of an elephant? Cells rely on diffusion to get nutrients. Diffusion is the movement of particles from areas of high concentration to areas of low concentration. Nutrients must be delivered to all parts of the cell. This activity will show why large size can be a problem.

Materials

Potato cube with a 1 cm³ volume (prepared by the teacher)
Potato cube with a 3 cm³ volume (prepared by the teacher)
Small beaker of Lugol’s solution (purchased) or iodine
Plastic knife
Forceps
Paper towel

Activity

1. Place both cubes of potato in the beaker of Lugol’s solution. Potatoes contain starch, which takes on a blue-black color in Lugol’s solution.

2. Leave the potato cubes in the solution for four minutes.

3. Use forceps to remove each of the cubes and place them on a paper towel on a table.

4. Both cubes should have a blue-black coloration. To see how far the solution has traveled into the potato cubes, use the knife to cut the cubes open and look inside.

Follow-Up Questions

1. In which cube did the Lugol’s solution (or iodine) get closer to the center?

2. The cubes represent cells. Would nutrients be more likely to diffuse to all parts of a small cell or a large cell?

3. Oxygen diffuses into cells in a similar way to how Lugol’s solution diffuses into potato cubes. Would you expect a large cell to be able to supply oxygen to all of its parts?

Extension

Try this activity with potato cubes of different sizes. Allow more time for soaking in Lugol’s solution. Do you get similar results?

6.7. PROTEINS AS ENZYMES

Saltine Crackers and Amylase

A protein is a large molecule found in cells. The unique structure of a protein determines its function. Enzymes are proteins that speed up chemical reactions, helping transform one substance into another. The enzyme amylase, found in saliva, changes starch into sugar. In this activity you will experience how this transformation occurs.

Materials

One-half of an unsalted saltine cracker
Small paper cup of water

Activity

1. Remove any gum or candy you may have in your mouth and drink the cup of water.

2. Place the saltine in your mouth, but do not chew it. Just hold it in your mouth.

3. Note any changes in taste you experience over the next few minutes.

Follow-Up Questions

1. What did the saltine taste like when you first put it in your mouth?

2. Did you experience any change in taste over the time you held the saltine in your mouth? Explain.

3. Discuss what you think was happening in your mouth while you held the saltine.

Extension

Verify what actually occurred in this activity by placing a few drops of iodine in a test tube containing a sample of cornstarch mixed with a little water. Iodine turns purple when exposed to a starch. Collect some of your saliva and mix it with cornstarch in another test tube. Allow a few minutes for the reaction. Test this with iodine. What do think will happen?

6.8. PLANT CELL OR ANIMAL CELL

Shoestring Venn Diagram

Although plant and animal cells share some basic similarities, they also have some distinctions. For instance, both plant and animal cells have a nucleus, but only plant cells have a cell plate, a divider that forms during cell division. In this activity you will group terms in a Venn diagram depending on whether they represent characteristics of plant cells, animal cells, or both.

Materials

Two colored shoestrings; Notebook paper; Scissors

Activity

1. Write the following terms double-spaced on your notebook paper:

Animal cell
Plant cell
Both
centrioles
cell membrane
flagella
chloroplasts
cell wall
ribosomes
mitochondria
cytokinesis
large vacuoles

2. Cut out the terms so that each term is on a small rectangle of paper.

3. On a table, make a circle with each shoestring, overlapping the circles so you have three separate areas as in a Venn diagram.

4. Look over the terms you have cut out. Place “Plant cell” over the left circle, “Both” over the middle where the circles overlap, and “Animal cell” over the right circle. See Figure 6.1.

5. Place the remainder of the terms in their proper locations within the circles. Refer to Figure 6.2 if you need reminders about animal and plant cells.

Follow-Up Questions

1. Which terms did you place in the animal cell region?

2. Which terms did you place in the plant cell region?

3. Which terms were characteristic of both plant and animal cells?

FIGURE 6.1. Shoestring Venn Diagram

FIGURE 6.2. Animal and Plant Cells

Extension

Plants and animals are made up of eukaryotic cells, cells with nuclei and membrane bound organelles. Bacteria are prokaryotic cells, cells without nuclei or membrane-bound organelles. Use your shoestring Venn diagram to classify the following terms as prokaryotic, eukaryotic, or both:

DNA
mitochondria
cell membrane
undergoes mitosis
earliest cells on Earth
chromatin