CELLS ARE A BAG OF GOO

Transcription

1 Fifth Grade, 2013 CELLS ARE A BAG OF GOO By Michael E. Knotts, Curt M. Peterson, and Diana Anderson 2013 by Michael E. Knotts, Curt M. Peterson, and Diana Anderson, All Rights Reserved

2 Objective and Overview: This classroom experiment will examine the structure of a living chicken egg, in which the class will observe the outer protective shell, cell membrane, cytoplasm and nucleus. Students also will conduct experiments to demonstrate the that the cell membrane is semipermeable, i.e., it permits the passage of water, but not many other fluids, by the process of osmosis. The experiments described should be conducted over the course of three weeks to fully demonstrate the role of the cell membrane and the process of osmosis on egg cell viability. RELEVANT GEORGIA CURRICULUM STANDARDS S5L3. STUDENTS WILL DIAGRAM AND LABEL PARTS OF VARIOUS CELLS (PANT, ANIMAL, SINGLE-CELLED, MULTI-CELLED). a. Use magnifiers such as microscopes or hand lenses to observe cells and their structure. b. Identify parts of a plant cell (membrane, wall, cytoplasm, nucleus, chloroplasts) and of an animal cell (membrane, cytoplasm, and nucleus) and determine the function of the parts. c. Explain how cells in multicellular organisms are similar and different in structure and function to single-celled organisms. Georgia Fifth Grade Science Curriculum p. 7 (co-requisite content), dated 29 August ESSENTIAL QUESTION Does the cell membrane regulate the flow of substances into and out of the cytoplasm of a cell? FRAMING CONTEXT FOR EXPERIMENTS: Cytoplasm is the stuff of life and it is enclosed by a thin envelope called a cell membrane. The two major internal components of all living cells are the cytoplasm, sometimes referred to as the protoplasm, and the nucleus. As part of our research on this topic, we ll do some crazy stuff like dissolve away an egg s shell with a common acid to reveal the giant cell inside by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved Overview: Page 2

3 VOCABULARY Cytoplasm Cell Membrane Solution Dissolve Osmosis Permeable Semi-Permeable A clear, jelly like substance inside a cell s membrane. It contains all of the cell s internal structures. A thin, durable layer that surrounds and contains the cytoplasm of a cell. It is semi-permeable, letting certain molecules pass through while containing the cytoplasm inside and preventing most large molecules from passing into the cell. A uniform mixture of two liquids or gases. In the context of this experiment, sugar completely dissolved in water is a solution and food coloring completely mixed in water is a solution. To mix or blend. When solid sugar is mixed with water, the sugar molecules are said to dissolve into the water. The solid sugar disappears and the sugar molecules mix with the water and spread throughout. The movement of small molecules through a semi-permeable membrane that prevents large molecules from passing. Allowing a liquid or gas to pass through. A membrane is permeable if it allows some substance to pass through it. A window screen is permeable to air. Allowing some substances to pass but not others. A cell membrane is semi-permeable because it allows small molecules like water and dye to pass through but it prevents large molecules such as proteins from passing. Semi-permeability gives rise to osmosis. FORMAT OF THE ACTIVITY The class is divided into six groups as equal in size as practical (nominally four students). The exercise involves a few simple procedures and observations that occur within approximately two to three weeks. The exact timing isn t critical as long as each step is separated by about three days or more. It may be convenient to devote one session to the activity each week. Each session will likely be completed in about 30 minutes.

4 MATERIALS CHECKLIST Chicken eggs one per group of approximately four students Pint size Mason jars with lids one per group of approximately four students Distilled white vinegar (4% acetic acid) one pint per group of approximately four students (one gallon is sufficient for eight groups) Concentrated sugar solution (64% sucrose, 36% water by weight) or corn syrup one cup per group of approximately four students (1/2 gallon is sufficient for eight groups) Food coloring Add to indicate supplies have been procured 2013 by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved Overview: Page 4

5 STUDENT HANDOUT AND INSTRUCTIONS Each group should follow the procedures outlined below. Allow 2 or more days between each major step. DISSOLVE THE SHELL OF AN EGG USING VINEGAR 1. Carefully place an egg in the bottom of a one pint Mason jar. 2. Pour white distilled vinegar over the egg, filling the jar about ¾ full. 3. Wash hands to eliminate salmonella bacteria that may have transferred from the egg.. 4. Attach the lid but make sure it remains loose. Do not tighten! 5. Mark the jar so that you know which one belongs to your group. 6. Set aside in a safe location in the classroom, not in direct sunlight. Over the course of two or three day, the vinegar will undergo a chemical reaction with the components of the egg shell, primarily limestone or calcium carbonate, to dissolve away the shell revealing the internal large cell that is enclosed by a cell membrane. A large amount of carbon dioxide, CO 2, is given off during the dissolution of the egg shell, and it will be released into the vinegar solution and also will accumulate as small bubbles on the surface of the egg shell initially, and then the egg cell membrane. EXPAND THE EGG WITH WATER 1. Observe the egg. With the shell gone, it will have expanded significantly. Compare to a fresh egg. 2. Pour out most of the vinegar, being very careful not to pour out the egg. Don t touch the egg since it is very fragile. Also, uncooked eggs sometimes contain harmful salmonella bacteria. 3. Fill the Mason jar with clean water, screw on the lid, and set aside. It is safe to tighten the lid at this point since the shell and vinegar are gone. After the egg shell was dissolved, the single large cell inside was revealed. It absorbed water from the vinegar solution and swelled in size, so now the cell is larger than when it was surrounded by the egg shell. This process will continue in plain water. The cell expands as the cell membrane is stretched by the uptake of water Teacher Guide: Page by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved

6 across the cell membrane into the cytoplasm. Essentially, the cell membrane is permeable to water. In other words, water molecules can readily move through the membrane either into or out of the cytoplasm. However, the much larger protein molecules in the egg s cytoplasm can t move through the cell membrane. Think of the membrane like a coffee filter or a screen with tiny holes. Small molecules can move through, but big molecules cant. Since there is a natural tendency for water molecules to move from a region with higher concentration (more water molecules per unit of volume) to a region of lesser concentration (fewer water molecules per unit of volume), water slowly flows into the egg. This process is called osmosis. As water enters the egg s cytoplasm, pressure builds up inside and causes the membrane to swell like a balloon. CONTRACT THE EGG IN CORN SYRUP 1. After observing the swollen egg cell without its shell, students will perform the opposite experiment from experiment II. 2. Carefully pour out the water, being sure not to damage the egg or pour it out. Don t touch the egg since it is very fragile. Also, uncooked eggs sometimes contain harmful salmonella bacteria. 3. Fill the Mason jar half way with concentrated 64% sugar solution. This is a thick, slightly yellow syrup made by dissolving 6.4 pounds of sugar in 3.4 pounds of water (0.43 gallons). If sugar solution isn t available, corn syrup (for example Karo brand) is a more expensive substitute. 4. Watch the egg over the course of a few days. It will slowly shrink and the egg cell membrane will begin to have a wrinkled appearance. Look carefully to see a visible layer surrounding the outside of the cell membrane. The layer mentioned above is formed by water moving out of the egg and into the surrounding sugar solution. The concentration of water molecules in the sugar solution is significantly lower than in the egg and water flows into the sugar solution until the concentration inside the egg is equal to that outside the egg. To speed up the process, it helps to gently swirl the egg around in the jar to more thoroughly mix the water molecules with the sugar solution. EXPAND THE EGG AGAIN WITH WATER AND DYE MOLECURES 1. Again, gently pour out the contents of the jar, being careful not to damage or touch the egg by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved Student Guide: Page 2

7 2. Refill the jar with clean tap water, screw on the lid, and gently swirl the jar to mix. 3. Empty the jar, and refill with water. Do this one or two more times to wash off the sugar solution, ending up with a jar full of water (and an egg). 4. Add a few drops of food coloring to the water. 5. Observe the shrunken egg cell periodically for about an hour. Notice that the egg cell again takes up water and swells back to its swollen state before being placed in the sugar solution. 6. Leave the egg cell in the colored tap water solution overnight. 7. Empty the water and replace with clean tap water. Notice that some of the dye has diffused into the cytoplasm of the egg cell across the cell membrane. Water has quickly moved into the cell by crossing through the egg cell membrane since the concentration of water molecules inside the egg (in the cytoplasm) is much lower than the surrounding volume of pure or dyed water outside. The food color dye molecules also move across the membrane for the same reason. Some of the dye molecules get stuck in the membrane, making it slightly darker than the egg s interior. These experiments demonstrate that the egg cell s membrane is semi-permeable. It allows small molecules such as water and dye to pass through, but it prevents large molecules such as sugar and protein (in the cytoplasm) from passing. CONCLUSION The cell membrane protects the interior contents of the cell from the external environment The cell membrane is semipermeable permitting the passage of water and other small molecules such as the blue dye, but limiting the passage of other molecules such as corn syrup The physical movement of water and other molecules across a semi-permeable membrane is defined as osmosis Over time, osmosis equalizes water concentration inside and outside of a cell. Teacher Guide: Page by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved

8 This page intentionally left blank by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved Student Guide: Page 4

9 NOTES FOR TEACHERS In this exercise, students are focusing their attention on experiments to study the flow or movement of water across a semi-permeable membrane. Before they can observe this phenomenon, they first must dissolve the egg shell way from the underlying plasma or cell membrane. This is done by placing an egg in a vinegar solution. Vinegar is approximately a 4 % acetic acid solution. It reacts with calcium carbonate, the principal component of the egg shell. As the acetic acid reacts with the calcium carbonate, carbon dioxide (CO 2 ) is released from this reaction and accumulates on the surface of the dissolving egg shell as tiny, but visible, bubbles. As the egg shell is completely dissolved, large numbers of CO 2 bubbles will appear on the surface of the egg cell membrane, and a frothy or white scummy substance will appear on the surface of the egg cell or on the surface of the vinegar solution, representing the dissolved egg shell. Dissolution of the egg shell will generally occur within about two days, so if the Teacher and Volunteers wished to move to the next sections, i.e., II. and III. of this experiment, they probably could begin section II. by day 3 or 4. Of course the egg shell residue and CO 2 bubbles should be rinsed away by gently pouring off the vinegar and rinsing the naked egg cell with a couple of rinses of tap water. At this point, the students should observe that the naked egg has swollen and is now larger than the original egg that contained the egg shell. You can keep some intact eggs in the room so the students can see the difference in sizes of the egg with a shell vs the naked egg without a shell. Encourage the students to read through their Lab Topic so they can record their observations in the Student Guide and also answer the question in the Student Guide. In section II. of the lab topic an explanation is provided for why the naked egg cell enlarges. Essentially, the protoplasm of the egg cell consists of approximately 90 % water, whereas the acetic acid solution consists of about 96 % water. Water always moves from a region of higher water concentration (the vinegar in this case) to a region of lower water concentration (the naked egg cell), and thus as water moves across the semi-permeable egg cell membrane, the naked egg swells in size. If the vinegar solution is replaced with pure tap water, then the egg cell will continue to swell, because tap water contains 100 % water. In section III.a., the vinegar or pure water solution is now replaced with corn syrup, which has very little water content, and water will immediately begin to diffuse out of the naked egg across its semi-permeable cell membrane into the syrup, and the egg cell will shrink. This process of shrinkage will occur within a matter of hours, so by the second or third day in the syrup solution, students will be able to observe that the naked egg has shrunken in size and appears somewhat wrinkled in appearance like a prune. Teacher Guide: Page by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved

10 When students now perform the final section, III. c., the students should observe the blue colored water inside the egg cell indicating it has diffused across the semi-permeable cell membrane into the cytoplasm of the cell, and the naked egg cell has swollen again to a size larger than the size of the original egg that contained an intact shell. The process of osmosis could be repeated again and again using the naked egg until the cell began to decompose, but steps II and III have sufficiently illustrated that the cell membrane is semi-permeable and that water and other small molecules such as the dye molecules can move across this membrane. This process is osmosis. FOLLOW-UP QUESTIONS FOR THE CHILDREN TO CONFIRM THEIR UNDERSTANDING 1. Why did the bubbles form on the outside of the egg s shell? Because the acetic acid in the vinegar solution reacted with the calcium carbonate in the egg shell causing it to dissolve and release carbon dioxide (CO 2 ) in a gaseous form and liquid water (H 2 0) into the vinegar solution. The water molecules aren t visible but the CO 2 is because it exists in a gaseous form (bubbles). 2. Was the egg cell larger in size after the egg shell is dissolved? If so, how can you explain this change in size? Yes, it was larger. The answers to these questions are embedded in the Student Handout above, but if students were not using/reading their lab handout, then they may not know the answer. If they can t answer this question initially, then they could be encouraged to read or re-read their handout in order to find the answer. 3. Did the egg cell shrink in size in the corn syrup? What was the purpose of using the food color dye? What physical process has been demonstrated by the use of the blue dye? Is a semi-permeable membrane required for this process? Explain. Yes, it shrunk in size. The food color demonstrates that water can move across the egg cell membrane, because the membrane is semi-permeable meaning that it will permit the passage of water and/or other small molecules like the dye molecules, but it will not permit the passage of larger molecules like sugar molecules or proteins from the inside of the egg. Osmosis is the physical process. The answers to these questions are explained in more detail in the student handout. Important!!!! Please note: When the students have completed this experiment the naked egg should be discarded in a container of bleach. We do not advise the students to puncture or otherwise play with the naked egg cells. They also should be discouraged from handling the naked egg during the experiment or when the experiment is completed by Michael Knotts, Curt Peterson, and Diana Anderson, All Rights Reserved Teacher Guide: Page 2