Cells

Grade Level

Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.

Time

Exercises 1, 2, and 3 take approximately 2 hours.

To Ponder

1.

Are cells alive? Why do you think this?

Yes! Cells have the characteristics that all living things have in common: order, metabolism, motility, responsiveness, reproduction, development, heredity, evolution, and adaptations. A cell is the smallest unit of life.

2.

Where are cells located in your body?

Your entire body is made of cells. Skin, blood, and your organs are all made of millions and millions of cells. All body parts are either made of cells or are products of cells.

3.

Where do the cells in your body get energy?

Our cells get energy from organic food molecules, such as sugars and starches, taken in from the environment. In cells of higher organisms, special organelles called mitochondria break these food molecules down. This process allows cells to capture chemical energy for other means.

4.

How do your cells know what to do? What directs their functioning?

Genes direct most cell functions. Genes are units of information encoded in the DNA of cells.

Supplies

  • cookies (about 5 for every group of 3 to 4 students)
  • Hershey kisses (10 kisses per group)
  • toothpicks (5 per group)
  • colored pipe cleaners (10 per group: 4 green, 3 white, 1 red, 2 yellow)
  • small gumdrops (10 per group, 4 green, 3 white, 1 red, 2 yellow)
  • pencils
  • scissors (1 pair per group)

Objectives

Once you have completed these exercises you should be able to:
1. Explain and apply cell theory.
2. Describe the appearance and function of the major components of a cell, including: cell membrane, cytoplasm, and the following membrane-bound organelles: nucleus, rough and smooth endoplasmic reticulum, mitochondria, chloroplast, and vacuole.
3. Describe the appearance and function of some subcellular structures, including ribosomes.
    4. Describe how respiration supports protein synthesis which includes copying of DNA into RNA (transcription) and translation of RNA into protein (see Figure 1).
    5.

Understand the roles of some important enzymes and macromolecules in protein synthesis, including RNA polymerase, transfer RNA, messenger RNA, and ribosomes.

Figure 1. Transfer of information in the cell.
 

Background
Information

Cells

According to the cell theory, proposed over 150 years ago:

  • All living things are composed of cells.
  • All cells come from pre-existing cells.
  • Cells are the smallest units of life.

Most cells are very, very small, so tiny that they can only be seen with the aid of a microscope. Your body is composed of billions of cells! Within your body, cells have different functions. We have blood cells, skin cells, brain cells...the list goes on. Despite their differences, cells in living organisms for the most part have similar structures and functions.

Question    1.

Have you ever seen a cell? When? What do you remember about it?

Chances are, many students will have seen a cell under a microscope. They may remember seeing the dark-staining nucleus or other subcellular structures, observing cell motility, or watching cells divide. They have all seen chicken eggs, which are specialized cells.

Powerful Idea

Cells found in the animal and plant kingdoms (with just a few exceptions) have these features in common:

  • cell membrane which serves as a boundary between the cell and the outside environment
  • cytoplasm containing organelles
  • nucleus containing hereditary material (DNA)
  • mitochondrion (plural, mitochondria), where cellular respiration takes place (the breakdown of sugars to produce energy for the cell, a process that uses oxygen and produces carbon dioxide and water)
  • smooth endoplasmic reticulum where lipids are made
  • rough endoplasmic reticulum where proteins are made with the help of ribosomes

Plant cells have, in addition to those components listed above, the following organelles

  • chloroplast which uses light energy to convert six carbon dioxide molecules into one organic six-carbon sugar
  • cell wall outside the cell membrane which provides additional strength
  • vacuole, a large organelle containing water, often with dissolved pigments, waste materials, or other substances

Cells come in many sizes and shapes, as illustrated in Figure 2a and b.

Figure 2. Images of Cells and a model of DNA

a. Postlethwait & Hobson, Figure 25.3, Blood Cells Transport Oxygen and Defend the Body [SEM*] b. P & H, Box 28.1 Figure 1, Corpulent Fat Cells with Red Blood Cells in the Background [SEM*] c. Model of a portion of the DNA double helix

Exercise 1

What are cells made of?

 Question   1.

What are the four classes of large organic molecules found in living things?

  • lipids
  • proteins
  • nucleic acids
  • carbohydrates

Organic molecules are both derived from living things and they have a carbon backbone. They tend to be relatively large and complex, and consist of subunits which are combined to form larger molecules.

    2.

Name three inorganic molecules that commonly occur in living things.

Water, oxygen (O2), carbon dioxide, and salt are four possibilities. Inorganic molecules commonly occur in both the living and non-living worlds. The inorganic molecules found in living things are small and contain some of the lighter atoms in the periodic table.

 To Do   3.

Refer to a textbook to draw a simple diagram of an animal cell in the template provided in Drawing 1. Draw to scale and label the following structures:

nucleus mitochondrion
cell membrane ribosome
rough endoplasmic reticulum (rough ER)

 
Drawing 1. Animal Cell

 

    4.

Also, refer to a textbook to draw a simple diagram of a plant cell in Drawing 2. Draw to scale and label the following structures:

cell membrane cell wall
chloroplast cytoplasm
mitochondrion nucleus
ribosome vacuole
rough endoplasmic reticulum (rough ER)

 
Drawing 2. Plant Cell

 

  To Notice   5. Note that drawings can be misleading. For example, a drawing typically shows one or a few mitochondria in a cell, but cells actually contain many mitochondria, sometimes 10,000 or more. Also, drawings are two-dimensional whereas cells are three-dimensional.
  To Do   6. In your drawings, briefly note of the functions of each organelle.
  Question   7.

How are plant cells different from animal cells? In what ways are they similar?

In contrast to animal cells, plant cells have chloroplasts, organelles which take in energy from the sun to create sugar molecules during the process of photosynthesis. Plant cells also have cell walls which provide physical strength and counter-balance the pressure produced by the vacuole (creating a rigid cell).

Both plant and animal cells are eukaryotic cells. They have many organelles in common such as nucleus, mitochondria, rough and smooth endoplasmic reticulum, cytoplasm and cell membrane. Both plant and animal cells respire (take in oxygen to break down sugars and generate cellular energy) 24 hours per day.

    8.

Does cellular respiration occur in plant cells? Explain.

Yes! Although plant cells can create their own molecules of sugar using solar energy, they still need a way to derive chemical energy from these sugars. Cellular respiration allows plants to obtain usable energy. This cellular respiration process takes place in the mitochondria of plant cells.

  Question   9.

What are the membranes in cells made of?

Membranes are fluid arrangements of phospholipid molecules. The phospholipids form a bilayer with the fatty tails on the interior and the polar phosphate heads on the outer edges. Phospholipids are amphipathic (part hydrophobic and part hydrophilic). The phospholipid bilayer contains many embedded proteins, protein channels, and glycoproteins (proteins with sugars attached).

    10.

Figure 3 shows a portion of a cell membrane. Label the parts of the molecules shown.

Figure 3. Cell Membrane

 

    11.

The cell membrane is selectively permeable, allowing only certain substances from the outside environment to enter the cell. What sorts of molecules pass through a phospholipid membrane easily? What sorts of substances do not pass through a membrane readily?

Small, uncharged molecules like water and non-polar molecules like oxygen and carbon dioxide pass through cell membranes easily. Large, polar molecules such as sugars do not pass through semipermeable membranes without help. Also, charged ions such as the chloride ion and the sodium ion do not passively move through membranes. In order to get these substances through membranes, cells use transport proteins or channels.

Exercise 2

Dynamic Cell Simulation Introduction

 Background 1. Cells are constantly making and breaking down molecules of all types. Proteins make up a diverse and important category of large biological molecules. Proteins manage biochemical reactions, provide physical strength in cells, aid in cell-to-cell communication, and many other tasks.
2. Your group will go through a simplified simulation of how proteins are made in the cell. The goal is to see the living cell as a site of constant metabolic activity, not a motionless structure. Another goal is to appreciate how the functions of different organelles are interconnected.
3. Members of each lab group will perform different functions within the simulated cell. The end result will be the production of a small protein.

Exercise 3

Simulation Set Up

 To Do 1.

Collect the materials for the lab. Table 1 summarizes what each item represents in the simulation. Refer back to this table as needed during the simulation.

Table 1. Materials and cell structures they represent.
Material: Represents:
paper models organelles
lab table or desktop cell cytoplasm
5 cookies glucose molecules
10 Hershey's kisses ATP molecules
10 colored gumdrops amino acids
5 toothpicks peptide bonds
10 colored pipe cleaners transfer RNAs

2. Set up your cell. First, make models of organelles. One group member cuts out the organelle models shown in Figure 4 from their handout. You only need one set of organelles for each group. This simulation will only involve two organelles: the mitochondrion and the nucleus; and a ribosome (which is a structure, not an organelle).
 

Figure 4. Organelle Models for Dynamic Cell Simulation (not drawn to scale)
 
Ribosome

 

 

3. Place the organelles on your desk or lab table. Pretend that your desk surface is the cytoplasm of the cell and that the edges represent the cell membrane.
4. Next, divide up the tasks. There will be four players. Table 2 describes the different roles group members will play.
 
Table 2. Different roles of players in the simulation.
Player 1 "Sugar Splitting" Enzymes In the cytoplasm and mitochondria, sugar-splitting enzymes break bonds in sugar molecules to release stored energy. In the process, energy is captured in the bonds of another molecule, ATP, which carries chemical energy to other parts of the cell.
Player 2 RNA Polymerase In the nucleus, the RNA polymerase copies small segments of DNA to make a complementary RNA molecule. The copy, a "messenger" RNA molecule, leaves the nucleus and heads to a ribosome.
Player 3 Transfer RNA(tRNA) Manager In the cytoplasm, transfer RNA is linked to an amino acid. This requires energy, usually provided by ATP. The transfer RNA then carries its attached amino acid to the ribosome.
Player 4 Ribosome Attached to the endoplasmic reticulum, the ribosome "reads" the messenger RNA (mRNA) in three-letter "words," or codons. A transfer RNA brings in an amino acid corresponding to each codon. The amino acids are joined together into a chain with help from the ribosome.
 Question   5.

What is an enzyme, anyway?

 

 

Exercise 4

Harvesting Energy from Glucose in the Mitochondrion
To Do 1. To start the simulation of the process of cellular respiration, Player 1 places the glucose molecules (cookies) in the mitochondrion and puts the ATP molecules (Hershey's kisses) off to the side.
2. Player 1 (mitochondrial "sugar splitting" enzymes) breaks a cookie in half. This represents breaking the bonds which hold a glucose molecule together. This process uses oxygen gas and releases carbon dioxide and water.
Question 3.

Does the breaking of chemical bonds in glucose release energy or consume energy? Explain?

Chemical bonds contain chemical energy. When they are broken, this energy is released, which is what you see when wood burns. Wood is made of cellulose, that is, long chains of sugars. During burning, the cellulose sugars are rapidly combining with oxygen, a process that releases heat energy. The same process occurs in the mitochondrion, but at a controlled rate, and some of the energy is transferred to other molecules such as ATP.

Background 4. The cell stores some of the energy released from the bonds in glucose in special bonds of ATP molecules. ATP is called the "universal energy carrier of cells" because it can travel to other places in the cell and provide energy for other chemical reactions (such as protein synthesis).
To Do 5.

Each time a cookie is broken in half, two ATP molecules (Hershey's kisses) are produced. Player 1 breaks a cookie in half to produce two ATP molecules.

Then, he/she breaks the two halves in half again. This will break two more bonds and produce four more Hershey's kisses, or six total ATP for each glucose molecule.

6. Player 1 now transports the six ATP molecules to the cytoplasm where they will be used by other members of the group.
 

Exercise 5

Reading the Genetic Code in the Nucleus

Background 1. Now that some cellular energy has been stored in ATP molecules, the next stop is the nucleus, where a message will be created that can be sent out to direct the creation of a protein.
2.

The DNA always remains in the nucleus, protected by the nuclear membrane (except during cell division). The genetic "blueprints" for any protein are found encoded in genes, which are sections of the DNA. DNA is a nucleic acid and is composed of four types of molecules called nucleotides. We often use letters to represent the four different types of nucleotide molecules.

  • A=adenine
  • T=thymine
  • C=cytosine
  • G=guanine
3. RNA is also a nucleic acid and are made of a long chain of nucleotides. However, RNA does not contain thymine (T); it contains uracil (U), a similar nucleotide, instead.
4. One type of RNA is called messenger RNA (mRNA). It is a copy of a DNA gene and it is made in the nucleus. The process of copying a DNA gene into mRNA is called transcription. Unlike DNA, mRNA moves out of the nucleus into the cytoplasm, where it directs the creation of a specific protein.
 
Figure 5: DNA template and space for generating the corresponding mRNA molecule according to the rules that follow.
 
Background 5. A section of a DNA gene, 30 nucleotides long, is shown in Figure 5. It has been split in half to fit on the page. Player 2 (RNA polymerase enzyme) reads the DNA gene shown in Figure 5 and transcribes it into a mRNA message. The mRNA is not identical to the DNA; it is complementary. This is almost like translating the DNA into a code which is written in the mRNA. Figure 6 gives the rules for converting the language of DNA into the language of mRNA.
 
Figure 6. Rules for converting DNA to RNA. The U stands for uracil, a nucleotide which is only found in RNA. RNA contains uracil in place of thymine, which occurs in DNA.
DNA   mRNA
T--- pairs with --->A
A--- pairs with --->U*
C--- pairs with --->G
G--- pairs with --->C
To Do 6. Player 2 starts reading the DNA in Figure 5 and then writes the complementary letter in the mRNA row below the DNA row. The first three nucleotides in Figure 5 have been transcribed for you.
7. When transcription is finished, Player 2 cuts out the newly formed strip of mRNA from the page (leaving the DNA strip behind), and tapes the two pieces together at the center to form one continuous strip. The strip is moved out of the nucleus over to the ribosome, where it will direct the synthesis of a protein. Your group only needs one copy of mRNA for the simulation.
 Background   8. Look at the mRNA model you made with your group. Notice how the strip is separated into sequences of three nucleotides; these are called codons. Codons are like words; they call for one amino acid to be linked into a growing protein chain. We will use only four different types of codons for our simulation. These codons specify one of four kinds of amino acids (gumdrops) to be placed in the protein model. Table 4 shows which gumdrop each of these four codons specifies. (In real cells the mRNA contains 64 different codons.)
 

Table 4. Codons and their corresponding tRNAs and "amino acids"
mRNA Codons tRNA "Amino Acid"
UUG green pipe cleaner green gumdrop
CAG yellow pipe cleaner yellow gumdrop
GAA white pipe cleaner white gumdrop
GCA red pipe cleaner red gumdrop

 To Do   9. The mRNA contains ten codons of four different types. On the mRNA model, directly below each codon (in the bottom row labeled "AA" in Figure 5), Player 4 indicates which amino acid (gumdrop) is to be incorporated into the protein chain using Table 4. The first amino acid has been indicated in the mRNA model for you.

Exercise 6

Preparing tRNAs for Protein Synthesis
Background 1.

The function of tRNA is to pick up amino acids in the cytoplasm and to align them on the ribosome in the order specified by the mRNA. Each type of tRNA molecule carries only one specific kind of amino acid. At one end of the tRNA molecule is a site where the amino acid is attached. On the other end is a complementary site called an anticodon which can recognize a specific mRNA codon.

Figure 6. Pipe Cleaner tRNA model
 

To Do 2. Player 3 makes four different-colored tRNA models which can carry the four types of amino acids to the ribosome. Player 3 bends four colored pipe cleaners into the "cloverleaf" shape shown in Figure 6. This is approximately the shape of all tRNA molecules in cells, (although each type has a different sequence of nucleotides). An amino acid (gumdrop) matching the color of the tRNA is attached by sticking the sharp end of the pipe cleaner into the gumdrop. (Note: only tRNAs and amino acids of the same color are attached together!).
 

Exercise 7

Protein Synthesis on the Ribosome
To Do 1. The final step of protein synthesis requires the cooperation of both Player 3 (transfer RNA manager) and Player 4 (ribosome). Player 3 should have the pipe cleaners, toothpicks, and gumdrops nearby.
Question 2.

A ribosome directs the creation of protein molecules. What are the subunits of proteins?

Proteins are polymers (long chains) composed of amino acids. One protein differs from the rest in the length of the protein chain and the particular order and type of amino acids included in it.

Background 3.

Examine the ribosome model, noting its two distinct regions. One holds the strip of mRNA, moving along it three nucleotides at a time. The other region has two sites which can hold tRNA molecules.

Figure 7. Ribosome model.
 

To Do 4. Player 4 cuts along the dotted lines in the two mRNA binding regions.
5.

Player 4 (ribosome) begins at the left-most codon on the mRNA strip. Slide the ribosome onto the mRNA strip. Notice that the mRNA binding region of the ribosome shows two codons through the cut-outs. Player 3 determines which tRNA + amino acid should be shuttled to the mRNA to begin the protein chain, and hands it to Player 4. Player 4 lines the appropriate tRNA up with the first mRNA codon in the first site on the ribosome. Player 3 then reads the second codon and identifies the second tRNA + amino acid and hands it to Player 4. Player 4 lines it up with the second mRNA codon in the second site on the ribosome (see Fig. 8).

Figure 8. Beginning protein synthesis.
 

6.

A peptide bond is formed between the two amino acids which are held side by side on the ribosome. Player 4 breaks a toothpick in half and uses it to link the two gumdrops together. At the same time the peptide bond is formed, Player 3 breaks the bond between the first amino acid and its tRNA molecule by removing the end of the green pipe cleaner from the green gumdrop.

Figure 9. Peptide bond formation.
 

Question 7.

The formation of a peptide bond requires an input of chemical energy. From where might chemical energy which has been stored in the cell be obtained?

The ATP molecules your cell have been created with the energy released from the breaking of the bonds of glucose molecules (cookies). The bonds in ATP contain chemical energy which can be used to drive the attachment reaction. ATP molecules are found in the cytoplasm.

 To Do   8.

The chocolate kiss represents the stored energy in the ATP molecule and it is eaten or set aside as the energy is consumed to create a peptide bond.

*In reality, in your cells, the stored energy from a total of four ATP molecules is required to form each peptide bond.

    9. Player 4 moves the ribosome down the strip of mRNA so that the second codon is now in the first ribosome site. The tRNA attached to the small amino acid chain moves from the second binding site to the first binding site when the ribosome moves along the mRNA strip. Player 3 moves the "empty" tRNA back to the cytoplasm and "recharges it" (by attaching another amino acid of the same color). This recharged tRNA will later carry another amino acid to the ribosome.
Figure 10. tRNA leaves, ribosome moves ahead one codon.
 
    10.

Player 3 determines which tRNA with attached amino acid corresponds to the third mRNA codon and brings it into position.

Figure 11. New tRNA enters ribosome.
 

    11.

Player 4 then links the third amino acid to the second with a peptide bond (toothpick) and simultaneously releases the white tRNA attached to the second amino acid.

Figure 12. The second peptide bonds.
 

    12.

All three amino acids are now attached to the third tRNA.

Figure 13. The "empty" tRNA leaves the ribosome.
 

    13. Once a tRNA has released its amino acid to the growing protein chain, Player 3 moves it back to the cytoplasm where another amino acid is attached.
    14. The process of protein synthesis continues until the end of the mRNA is reached. Your group will need more ATP to finish the protein. Player 1 "breaks up" more glucose (cookies) as needed.
    15. Proteins fold up in unique ways. The shape they take dictates their chemical properties and their function in the cell. Some proteins fold up in a long spiral, kind of like a telephone cord. Others have a more irregular, globular shape. You can try "folding" your protein as it is synthesized.
    16. When synthesis is complete, the protein is imported out of the cell and can be consumed by hungry team members along with the other candy remnants.
 
Exercise 8 Connections and Review
Question    1.

Protein synthesis is an exquisitely complex process that occurs very fast. The sequence of amino acids in a protein is determined by information contained in what molecules?

The nucleic acids, DNA and RNA, are the information-containing molecules.

    2.

Explain the flow of information involved in protein synthesis.

Information is permanently stored in DNA. It is copied from DNA into mRNA so it can be moved out of the nucleus. Transfer RNAs "read" the mRNA codons by matching up with complementary anticodons. Sixty-one of the 64 codons in mRNA code for amino acids, and they require 61 different tRNAs. The information can be summarized as in Figure 1:

    3.

How is it possible to store all the information needed to construct a living thing in molecules having just four nucleotide bases?

The four bases are like letters of an alphabet. We can write and infinite number of sentences with our 26-letter English alphabet. A computer can generate the same infinite number of sentences with a two-letter alphabet composed of zero and one. Living things can accomplish the same feat with a four-letter alphabet.

    4.

Where do all the enzymes involved in protein synthesis come from?

They are proteins and therefore are made by the process of protein synthesis described in this lesson, using information in the genes. You may wonder how an egg can produce the proteins it needs to grow and to produce proteins. Where do they come from if there are no enzymes around to assist in protein synthesis? The fertilized egg actually inherits these critical enzymes directly from the mother; they are in the cytoplasm of the egg cell. So there is really more we inherit from our mothers other than just DNA!

Supplementary
Resources

Sayre, Anne. Rosalind Franklin and DNA. New York, NY : Norton. 1978.

Cell Biology, The Biology Project, University of Arizona <http://www.biology.arizona.edu/cell_bio/tutorials/pev/main.html>

The Access Excellence Collection: Biology Lesson Ideas Sponsored by Genentech <http://www.accessexcellence.org>

Postletwait, J.H. & Hopson, J.L. (1995). The Nature of Life. McGraw Hill, Inc.

Storey, Richard D. "Textbook Errors and Misconceptions in Biology: Cell Energetics." The American Biology Teacher. Vol. 54, No. 3. pp. 161-166. March 1992.

Storey, Richard D. "Textbook Errors and Misconceptions in Biology: Cell Physiology." The American Biology Teacher. Vol. 54, No. 4. pp. 200-203. April 1992.

Related
AAAS
Benchmarks

Chapter 5: THE LIVING ENVIRONMENT
Section C: Cells

Grade 3-5 Benchmark 1 of 2
Some living things consist of a single cell. Like familiar organisms, they need food, water, and air; a way to dispose of waste; and an environment they can live in.

Section C: Cells

Grade 6-8 Benchmark 1 of 4
All living things are composed of cells, from just one to many millions, whose details usually are visible only through a microscope . Different body tissues and organs are made up of different kinds of cells. The cells in similar tissues and organs in other animals are similar to those in human beings but differ somewhat from cells found in plants.

Section C: Cells

Grade 6-8 Benchmark 3 of 4
Within cells, many of the basic functions of organisms--such as extracting energy from food and getting rid of waste--are carried out. The way in which cells function is similar in all living organisms.

Section C: Cells

Grade 9-12 Benchmark 3 of 8
The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. Protein molecules are long, usually folded chains made from 20 different kinds of amino acid molecules. The function of each protein molecule depends on its specific sequence of amino acids and the shape the chain takes is a consequence of attractions between the chain's parts.

Section C: Cells

Grade 9-12 Benchmark 4 of 8
The genetic information in DNA molecules provides instructions for assembling protein molecules. The code used is virtually the same for all life forms.