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Thank you for purchasing this activity! We are confident you will find this activity both relevant and engaging. Please direct any questions or comments regarding this activity to: [email protected] 1-888-733-2467 This activity is copyrighted by the AIMS Education Foundation. All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means—graphic, electronic, or mechanical, including photocopying, taping, or information storage/retrieval systems—without written permission of the publisher unless such copying is expressly permitted by federal copyright law. The following is an exception to the foregoing statements: • A person or school purchasing this AIMS publication is hereby granted permission to make up to 200 copies of the student pages, provided these copies will be used for educational purposes. control center of the cell and regulates all the processes that occur within the cell. Some of the important structures of the plant cell are the cell wall which provides support and protection, the cell membrane which allows dissolved material to enter and leave the cell, and cytoplasm, the fluid that fills each cell in which other important cell parts can be found. What you will be able to see in either a wet mount or a stained, wet mount onion skin slide will depend upon the magnifying power of your microscope and the quality of the wet mount slides. The basic cell shape will be visible in good wet mount slides viewed at even 20X. Topic Onion cells Key Question What structures can be observed in onion cells? Focus Students will make a wet mount slide of onion cells and observe the cell walls. Students will stain a wet mount slide of onion cells and observe an onion cell’s nucleus, cytoplasm, cell wall, and cell membrane. Guiding Documents NCTM Standards • Use mathematics in other curriculum areas Management 1. This activity will take about two 45-minute periods. 2. It is suggested that the teacher review the procedure for preparing a wet mount slide with students. 3. Cut each onion in half and soak in half-filled cups of water over night. This makes it easy to separate the onion’s membrane. 4. Have tweezers, eyedroppers, water, and methylene blue stain ready. 5. When the students are ready to stain the onion slide, have them do this at special stations located around the room. Put dropper bottles of stain in tip proof boxes and set these boxes on newspaper. Project 2061 Benchmarks • Know why it is important in science to keep honest, clear, and accurate records. • Microscopes make it possible to see that living things are made mostly of cells. • All living things are composed of cells, from just one to many millions, whose details usually are visible only through a microscope. Math Ratio and proportion Geometry and spatial sense Procedure Preparing the wet mount slide of onion skin 1. Have students clean the slide and cover slip. 2. Direct them to break an onion slice in two. 3. Tell them to carefully pull the slice apart. 4. Have them use tweezers to pull off a very thin piece of onion skin. 5. Direct the students to place the skin in the center of their slide. Urge them to try to keep it from folding and to flatten it as much as possible. 6. Have them add a drop of water to the onion skin and cover with a cover slip. 7. Tell them to press the cover slip down carefully to remove any air bubbles. 8. Have students place the slide on the stage of the microscope, set it to low power, adjust the focus so the onion slice is clear, and draw four or five cells as they see them. Have them label the cell walls. 9. Direct them to switch to high power and try to identify the cell membrane, nucleus, and cytoplasm. Science Life science cells Integrated Processes Observing Recording data Comparing and contrasting Applying Materials Whole white onion Microscope Microscope slide Plastic cover slip Methylene blue (1% solution, see Resource Section) Dropper bottle with eyedropper Tweezers Background Information Although cells vary in size and shape, most have a similar cellular organization. The nucleus is the most prominent feature of a plant cell. The nucleus is the MAGNIFICENT MICROWORLD ADVENTURES Staining the onion cells 1. Have students lift up the cover slip and add one or two drops of methylene blue to the slide. 48 © 1995 AIMS Education Foundation 2. Direct them to lower the cover slip and examine the cells on high power. 3. Methylene blue stains different parts of the cells so that they can see structures they could not see before. Have the students draw lines from the labels (cell wall, cell membrane, cytoplasm, and nucleus) to the appropriate structures in their drawing. 11. Why are onion cells more like squares and rectangles than ovals? [The cells “fit together” without gaps at the corners. This makes the cells stronger.] Extensions 1. Have the students make wet mount slides using red and green onions. Compare the cell shapes. 2. Prepare wet mount slides of other plant cells (i.e., carrots or geraniums). Discussion 1. What is the general shape of the onion cells? [rectangular] 2. Describe what you saw without the stain. 3. Why do you think there are many cells close together? [strength and protection] 4. Is the onion skin composed of one cell or many cells? [many cells] 5. Why is it easier to see the onion cells after they are stained with methylene blue? [The stain creates contrast between light and dark structures.] 6. All plant cells have cell walls. What is the function of the cell wall? [to provide strength and protection] 7. Can you see a line on the inside of the cell walls? (probably not) What is this structure’s name? [the cell membrane] What is its function? [allows dissolved materials to enter and leave the cell] 8. Each cell has a control center. What is the control center called? [the nucleus] 9. Count the number of cells that are seen in the field of view under low power magnification and also high power magnification. Compare the number of cells observed in each field of view. Express these numbers as a ratio. For example, if you can see six cells under low power and two cells under high power, then the ratio is 6 : 2 or 3 : 1. 10. How many sides does an onion cell usually have? [four] What geometric shapes may be similar to the onion cell’s shapes? [four-sided polygons] MAGNIFICENT MICROWORLD ADVENTURES What You Should See Onion Rings Bausch & Lomb Elementary School Microscope Brock Optical Magiscope ® 20 X 25 X Learning Things, Inc. Radio Shack ® Super Microscope 50 X 49 Micronta ® 100 X © 1995 AIMS Education Foundation