Idea Transcript
The Bronx High School of Science Valerie Reidy, Principal Name
Department of Biology J. Donahue, IA Asst. Principal Date
ACTIVITY 14: Water Balance in Cells: Investigating osmosis in purple onion tissue (full lab write-up) Introduction: In today's laboratory exercise, you will investigate the relationship between the concentration of solutions in the external environment of the cell and water movement. When you have completed this investigation, you will write the laboratory report in the form of a research paper. The format to use is described at the end of this Activity. In most plant cells there is a balance between water inside and water outside the cells. If this balance is upset, water may flow into or out of the cell through the cell membrane and cell wall. Such movement is called osmosis. In this investigation, we will vary the osmotic environment and observe the changes that occur in the purple onion epidermal cells. Materials: microscope, lens paper, slide, coverslip, dissecting needle, piece of Purple onion, absorbent paper towel, biological waste pail, soap water, dropper bottles of: a) 0.9% salt solution (99.1% H20 + 0.9% NaCl) b) 10.0% salt solution (90.0% H2O+ 10.0% NaCl) c) distilled water (100% H20 + 0% NaCl). Procedures & Observations: 1) Obtain a piece of onion from the stock supply. 2) Separate the epidermal cell layer containing the purple pigment (outer layer) from the piece of onion by cracking it toward the convex surface. 3) Move the two pieces apart by pulling the thin pigment-containing layer from the fleshy portion. 4) Detach the purple epidermal layer and place it on a clean slide, stretching the skin so that it is flat and not overlapping. Dispose of the remnant of the onion in the biological waste pail. 5) Put one or two drops of 0.9% salt solution (solution a) on top of the purple onion and add the coverslip. 6) Under low power, locate an area that is deeply pigmented and one cell-layer thick. This salt solution (0.9% salt) approximates the normal internal environment of these living cells. 7) Find an area where the cells may be clearly seen and center them. Switch to high power. 8) Draw a few adjoining well-pigmented cells. Label all parts. Include the cell wall, cell membrane, pigmented cytoplasm and the nucleus.
Figure 1. PURPLE ONION EPIDERMAL CELLS IN 0.9% SALT SOLUTION Magnification 430X 9) Change the osmotic environment of the extracellular fluid surrounding these same cells, using the 10.0% salt solution (solution b). In order to do this, you will use a replacement technique. Put a few drops of the
Activity 14: Revised September 2003
Page 1 of 3
10.0% salt solution on the left edge of the coverslip, and then draw this solution through under the coverslip by drawing out the existing 0.9% salt solution with a small piece of absorbent paper placed at the right edge of the coverslip. (See the diagram below).
Figure 2.
absorbent paper towel
10a) Observe for a few minutes and then draw the cells as they now appear.
Figure 3. PURPLE ONION EPIDERMAL CELLS IN 10.0% SALT SOLUTION 430X 10b) Describe any changes you have observed. (answer on a separate sheet of paper) 11) Again change the osmotic environment surrounding these cells using the replacement technique with the distilled water (solution c). 12a) Resume your observation and again draw the cells after a few minutes.
Figure 4. PURPLE ONION EPIDERMAL CELLS IN 0.0% SALT SOLUTION (Magnification, 430X)
12b) Describe any changes you have observed. (answer on a separate sheet of paper) 13) CLEAN-UP: Remove the coverslip and discard in the appropriate container. Swirl the slide in the containers provided and dry it with clean paper towel. Summary Questions: 1) Explain the differences observed in the onion epidermal cell when the solution was changed from a concentration of 0.9% salt to a concentration of 10.0% salt. 2) Explain the differences observed in the onion epidermal cell when the solution was changed from a concentration of 10.0% salt to distilled water 3) The 0.9% salt solution is an example of an isotonic solution because its concentration of dissolved materials (solute) and its concentration of water (solvent) is identical to that of living protoplasm. In comparison, the 10.0% salt solution is said to be hypertonic to the cells, because there is a greater amount of solute, and therefore, less solvent than is present in living cells. The distilled water is hypotonic to the cells because it contains no solute, which is therefore less than the solute in the living cell; it is pure solvent. Make a general statement describing the factors that determine whether water will move in or out of the cell using the terminology introduced in the paragraph above.
Activity 14: Revised October 2002
Page 2 of 3
4) Based on the concept of osmosis, hypothesize what will happen when: a) An onion epidermal cell is left in distilled water for a prolonged period of time. Answer: The cell will swell due to the movement of water inside, but will not burst because of the cell wall. b) A red blood cell is left in distilled water for a prolonged period of time. Answer: The red blood cell will swell because of the movement of water inside and will eventually burst because there is no strong support such as a cell wall. c) Define the term plasmolysis. Why it is a misnomer? Answer: Lysis implies the bursting of a cell but here the cell actually shrinks but remains intact. Application of Concepts: 1) Why should vegetables be stored in humid conditions in order to retain their crispness? Answer: Under humid conditions there is more water outside than inside. Therefore the water will stay inside the vegetables and not diffuse out into the hypotonic environment. This will keep the vegetables crisp. 2) Sugar or salt can be used as food preservatives. This action is due to the fact that sugar and salt in high concentrations can kill bacteria that cause food spoilage. Based on the concepts learned in this laboratory exercise, explain how the high concentrations of sugar or salt preserve the food. Answer: Sugar and salt make the external environment hypertonic. The water in the food and in the bacteria will tend to leave and diffuse to the outside to attain equilibrium. When bacteria dehydrate they cannot carry out life functions and so they cannot reproduce. 3) Some state roads are salted heavily in the winter, creating an environmental problem. Based on observations you made in this laboratory activity, explain how high levels of salt from roadways could harm organisms. Answer: The salt from the roads washes into the surrounding soil creating a hypertonic environment for the plant roots. Therefore water will not flow into the plant but out of it and the plant will wilt and eventually die. 4) When a person in the hospital is given fluid intravenously (an I.V.), the fluid is typically a saline (salt) solution with about the same water concentration as human body tissues. Explain how the use of distilled water in place of this saline solution would be expected to upset the patient’s homeostasis. Your answer should refer to the process of diffusion. Answer: Distilled water makes the blood hypotonic to the body tissues including the red blood cells. Therefore the water will enter the red blood cells and may cause them to lyse. Additionally, water will enter the tissue spaces and cause swelling. 5) Many fresh-water one-celled organisms have structures called contractile vacuoles. Explain why these organelles are necessary for organisms living in a hypotonic environment. Name the process that causes water to flow into these organisms. Explain why contractile vacuoles would be of little value to one-celled organisms living in the ocean (salt water). Answer: In a hypotonic environment water will enter the organisms because there is more solute inside than outside. The contractile vacuoles pump the water that enters by osmosis back outside. Without the contractile vacuole the water will continue to enter and eventually the organism will rupture. In ocean water, the solute concentration is much higher than inside the organism. Therefore the water will leave the organism not enter it and contractile vacuoles are not necessary. 6) Popcorn sold at most movie theaters is very salty, causing people to become thirsty and buy soft drinks. Describe in scientific terms why the salty popcorn causes this thirst. You should mention changes in specific body cells in your answer. Answer: The cheek cells and the cells of the pharynx will lose water to the external environment. Also the increased salt in the blood will cause water loss through the kidneys. This will cause dehydration and result in the feeling of thirst. 7) In many animals, the blood transports glucose rather than starch to all the cells of the body. Starches in many foods are digested to yield glucose. Based on what you learned in this laboratory activity, explain why the digestion of starch to glucose is necessary. Answer: Starch must be digested to glucose because the starch is too large to pass through the cell membrane. Glucose is a small molecule and can diffuse through the cell membrane in the small intestine to allow the glucose to enter the blood stream.
Activity 14: Revised October 2003 Writing the Research Report
Page 3 of 3
In order to prepare for using a research paper format, get your thoughts clear by answering the questions in the discussion section above. You will use this information in your report. The research paper typically has four parts: the introduction, the materials and methods, the results (observations and data), and the discussion. The information, which should be included in each part, is described below. THE INTRODUCTION: Includes background information on the problem being studied, and ends with a statement of that problem. For this report, you may use the Introduction at the beginning of the lab sheet. However this must be restated in your own words. Supplement the introduction with information from your textbooks and class notes. MATERIALS AND METHODS: This is not a numbered, step-by-step protocol. It is a past tense description of what you did; presented in sufficient detail so that the reader would be able to repeat your experiment. Summarize in your own words. RESULTS: This section includes all information gathered from your investigation procedures. It frequently contains not only a written description of what you have found, but also drawings, diagrams, data tables, photographs, and graphs. Diagrams, photographs and graphs are labeled as Figures while the data and tables are labeled as Tables. These are presented in the same order they appear in the methods section. In the purple onion epidermal cell report, you should include the information gathered from your drawings and observations. DISCUSSION: This is the section in which the results are analyzed, interpreted, and applied to new situations. Any discrepancies between what you expected and what you actually found is discussed here; sources of error are identified. The problem is again addressed and a hypothesis is stated. In the purple onion epidermal cell report, the answers to Summary Questions 1-5 should be integrated into the discussion section.