L5 Elodea-Onion-Cheek-Cell_Size Page 1 of 7 STUDENT LABORATORY PACKET Student s Full Name Lab #5: Elodea, Onion, Cheek Cells-Cell Size Lab Instructor Date Points Microscope # OBJECTIVES: a. to examine and compare plant and animal cells b. to determine the size of a cell STATE STANDARDS: Standard 4, Performance Indicator 1.2f, COMMON CORE STANDARDS: Reading Standard 3, 4 BACKGROUND: Dragonfly p. 168 chapter 7 ----------------------------------------------------PRE-LAB (Do Before Coming To Lab)------------------------------------------------ 1. The wet mount preparation of a thread viewed in the low-power field (100x) of a compound light microscope is shown in the diagram below. Diagram B shows the field of view as it appeared when the objective was switched to high power. Explain why the thread is not visible in diagram B. One millimeter (mm) contains 1000 microns (μ). Microns are also known as micrometers. The symbol for micron is the Greek letter μ. Convert the following and show your work. 2. A carrot cell is 100 microns. Express this as millimeters. 3. A small ant measures 5 mm in length. How long is the ant in microns? 4. Convert your height into microns. (hint: you must convert or measure your height in meters first!) 5. Express the length of the biological specimen in millimeters and microns.
L5 Elodea-Onion-Cheek-Cell_Size Page 2 of 7 ------------------------------------------------------Lab Exercise------------------------------------------------------ MATERIALS: Microscope, lens paper, slides, cover slips, water dropper bottle, scissors, tweezers, Elodea (Egeria naja may be substituted for Elodea), onion, prepared cheek epithelial slides, Lugol s iodine, clear metric ruler slide PROCEDURES AND OBSERVATIONS: Determining the Field Diameter: 1. Determine the diameter of the field of vision using the metric ruler slide provided under low power (yellow, 10x objective). Move the metric ruler slide until it is centered, forming a diameter in the circle 2. Estimate the field diameter by noting the number of millimeter lines in view. The space between a pair of black dashes is one millimeter. What is the diameter of the field of vision in millimeters? Convert to microns and record both below: Diameter in millimeters (mm) Diameter in micrometers (µm) Cheek Cells: 1. Obtain a slide of cheek cell epidermis. 2. Focus the cheek cells under low power. Then switch to high power. 3. Observe the cheek cells under high power. Draw 2-3 of them and label the cell membrane, cytoplasm and nucleus. Cheek Cell Size: 1. Focus the cheek cells back to low power. 2. Count the number of cheek cells that fit across the field of view, as if they were lined up next to each other side by side. Note that the cheek cells are Cheek (400x) more round than rectangular, so you only need to count them in one dimension. Because it is difficult to find a slide with cells across the entire field of view, you may have to estimate how many cells will fit across by looking at a few different fields of view. 3. Record your data below. 4. Calculate the average size of a cheek cell using the field diameter that you measured above. Number of cells that fit across field of view Average size of a cheek cell in mm Average size of a cheek cell in microns Elodea: 1. Clean a glass slide and cover slip. 2. Remove a small green leaf from near the tip of an Elodea plant. 3. Prepare a wet mount of the elodea leaf with distilled water by placing the flat leaf on a glass slide (You may be able to make the slide without adding a drop of water because the Elodea is already wet). 4. Place one side of a coverslip onto the drop of water at a 45 degree angle and gently lower the coverslip onto the drop. If water runs out from the edges of the coverslip, you may have added too much water. If there is an air space under the coverslip, you may have not added enough water, or you may have Elodea (400x)
L5 Elodea-Onion-Cheek-Cell_Size Page 3 of 7 placed the coverslip on the slide improperly. You may want to use a piece of paper towel to soak up extra water, or add a drop of water to the edge of the coverslip. You can also gently tap the coverslip with a pencil tip to drive some air bubbles out. 5. Observe the leaf under low power. Then switch to high power. 6. Locate the chloroplasts (oval green bodies) and try to find a cell where you can observe cyclosis, streaming of the cytoplasm. You should be able to see the chloroplasts move in a circular pattern. This shows that the cytoplasm is fluid and the organelles move within it. 7. Draw 2-3 adjacent cells (on previous page) and label the parts/organelles you observe. Be accurate. ---------------------------------------Suggested Break Point--------------------------------------- Onion Wet Mount: 1. Get a clean glass slide and cover slip. 2. Obtain a piece of onion. 3. Use your fingers (nails work well) or forceps, to carefully peel off a small piece of skin from the inner or concave side of the onion chunk. This piece should be thin and translucent; looking much like a piece of scotch tape. 4. To prepare a wet mount of the onion with distilled water, lay the onion skin flat on a glass slide. Make sure the skin doesn t fold over on itself (this can be tricky). 5. Add one or two drops of water from the dropper bottle. 6. Place one side of the coverslip onto the drop at a 45 degree angle and gently lower the coverslip onto the drop. Using this procedure helps prevent air bubbles from being trapped under the coverslip. 7. If water runs out from the edges of the coverslip, you may have added too much water. If there is an air space under the coverslip, you may have not added enough water, or you may have placed the coverslip on the slide improperly. You may want to use a piece of paper towel to soak up extra water, or add a drop of water to the edge of the coverslip. You can also gently tap the coverslip with a pencil tip to drive some air bubbles out. 8. Observe the onion under low power. Then switch to high power. Do not yet draw the onion. Stain the onion cells using the Draw-Through method: 1. Swing the low power objective into place, but keep the cells in focus. 2. Carefully place a drop of Lugol s iodine at one edge of the coverslip. Hold a small piece of paper toweling at the opposite edge with a forceps to draw off the water and allow the iodine to seep through. Look at the picture on the right for help. 3. Try to observe the stain advancing across the field of view. 4. Allow 1-3min for the stain to seep across. 5. Observe the onion cells under low power. Draw 2-3 adjacent cells in the space below. Locate and label the nucleus, cell wall, cell membrane. 6. Switch to the high power objective. Use only the fine adjustment when focusing. Draw one onion cell. Locate and label the nucleus, cell wall, cell membrane and nucleolus. 7. Save this slide. Onion (100x) Onion (400x)
L5 Elodea-Onion-Cheek-Cell_Size Page 4 of 7 Measuring Onion Cells: 1. Examine your onion slide under low power. Align the slide such that the cells run lengthwise in a straight line. Count the number of cell lengthwise along where you think the diameter would be (the widest part of the circle.) Record below. 2. Count the number of cells that fit widthwise across the field. Record below. 3. Estimate the average length and width of an onion cell in mm and then in microns. Record below. (Hint: In your calculations, use the diameter of the field of view determined above.) Number of cells that fit across field of view Lengthwise Widthwise Average size of an onion cell in mm Length Width Average size of an onion cell in microns Length Width ANALYSIS AND CONCLUSION: 1. For each, provide the location and description as seen under the microscope. If a structure was not visible under the microscope (or is not present in a particular type of cell), place an X in the table: Organelles Onion Elodea Cheek Cells 2. Complete the following chart Comparison between elodea, onion cell and cheek cells based on your knowledge of animal and plant cells. Present in Onion (O) Organelles Present in Elodea (E) Present in Cheek (C) Function
L5 Elodea-Onion-Cheek-Cell_Size Page 5 of 7 Student Answer Packet Student s Full Name Lab #5: Elodea, Onion, Cheek Cells-Cell Size Lab Instructor Date Points FINAL GRADE Pre-Lab Completed as homework: Max. value 1pt. pt. Procedure/Observations: Max. value 4 pts. pts. Field Diameter: What is the diameter of the field of vision in millimeters? Convert to microns and record both below: Diameter in millimeters (mm) Diameter in micrometers (µm) Cheek Cells and Elodea: Draw 2-3 adjacent cells for each. Label the cell membrane, cytoplasm, and nucleus of the cheek cells. Label the parts/organelles of the elodea that you observe. Cheek (400x) Cheek Cell Size: What is the size of an average cheek cell? Fill in the table. Number of cells that fit across field of view Average size of an cheek cell in mm Average size of an cheek cell in microns
L5 Elodea-Onion-Cheek-Cell_Size Page 6 of 7 Onion: Draw 2-3 adjacent cells under low power and label the nucleus, cell wall and cell membrane. Draw one onion cell under high power and label the nucleus, cell wall, cell membrane and nucleolus. Onion (100x) Onion (400x) Onion Cell Size: How big is an average onion cell? Fill out the table. Number of cells that fit across field of view Lengthwise Widthwise Average size of an onion cell in mm Length Width Average size of an onion cell in microns Length Width ANALYSIS AND CONCLUSION: Max. value 5 pts. pts. 1. For each, provide the location and description as seen under the microscope. If a structure was not visible under the microscope (or is not present in a particular type of cell), place an X in the table: Organelles Onion Elodea Cheek Cells
L5 Elodea-Onion-Cheek-Cell_Size Page 7 of 7 2. Complete the following chart Comparison between elodea, onion cell and cheek cells based on your knowledge of animal and plant cells. Organelles Present in Onion (O) Present in Elodea (E) Present in Cheek (C) Function EXTRA CREDIT: Using the data from the lab, a) Calculate the average area of an onion skin cell, b) Draw a 1in. x 1 in square box below, then c) Calculate how many onion cells would fit into it. (Hint: you can determine the area of an onion cell, and calculate the area of a 1 inch x 1 inch square box in microns) Show All Work!