Using a Microscope to Investigate Cell Structure

Name______________________ Date __________________ Period _______

Purpose: You will become familiar with the parts and use of a compound microscope. You will also be able to explain the similarities and differences between plant and animal cells. You will be able to identify each cell part and state its function. Introduction: In the 17th century Robert Hooke built a microscope powerful enough to see objects at greater magnification than had previously been possible. Hooke used his new microscope to examine many things—minerals, cloth, small plants, and small animals. After examining a thin piece of cork, he observed many individual units making up the cork. He published a report in 1665 in which he called these units “cells” because they reminded him of the small cubicles that monks lived in. Over the next 150 years, scientists realized that all living things are composed of cells. Cells are the basic functional units of all living organisms. They may exist singly or in aggregates (more than one). When cells join together to take on a specialized function within a larger organism, they form a tissue. There are 2 major division into which all cells fall—prokaryotic (organized nucleus is absent) and eukaryotes (organized nucleus is present). Bacteria make up the prokaryotic division while the cells of plants, animals, fungi, protozoa, and algae make up the eukaryotic division. Animal and plant cells share many characteristics, which you will observe in this lab. They also differ in several important ways. During this lab, try to see if you can tell what the similarities and differences are between animal and plant cells.

Procedure: Part I: Learning the Compound Microscope 1. Locate each microscope part listed in the data table and shown in the diagram. Complete the following table as you go. Microscope Part Eyepiece (Ocular lens) Objective lens Stage Diaphragm Base Fine Adjustment knob Coarse Adjustment knob Arm Lamp

Function

2. Observe the magnification power (a number followed by X) of the eyepiece and the low- and high-power objectives. What are the magnification powers of the objectives? a. low-power objective __________ b. high-power objective__________ 3. To determine the overall magnification of the image you are observing you must multiply the magnification power of the ocular lens and the objective lens together. a. What is the magnification of the ocular lens? ____________ b. If the eyepiece on a microscope has a magnification of 10x, what is the total magnification with a 10x objective? ____________ 4. Raise the objectives (or lower the stage) as far as possible by turning the coarseadjustment knob toward or away from you. Turn the lower power objective into position over the stage. While observing the stage from eye level, use the coarseadjustment knob to position the objective as close to the slide as it will go without touching the slide. 5. Look through the eyepiece. Always keep both eyes open as you look into the eyepiece. If the lens is dirty, ask your teacher for special lens paper to clean it. Focus with the coarse-adjustment knob by turning it away from you. Never focus objectives downward. You risk smashing the lens into the stage. 6. Complete focusing by slowly turning the fine-adjustment knob back and forth. When the object you are viewing is in focus and exactly in the middle or your field of vision, switch to a higher power objective. You should be able to focus on the object again by only adjusting the fine-adjustment knob. Pre-lab Questions: 1. What is the basic structural feature that distinguishes plant and animal cells from bacteria? _______________________________________________________________________ 2. When cells join together to take on a specialized function, what do they form? _______________________________________________________________________ Part II: Plant Cells 1. Take you slide to your teacher and obtain a small sample of onion epidermis (thin outer membrane). 2. Prepare a wet mount slide of the epidermis. •

Place one drop of water in the middle of a clean glass microscope slide.

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Carefully place the onion epidermis in the drop of water. Make sure it lies flat on the microscope slide.

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Hold a coverslip at a 45° angle to the slide at the edge of the water drop. Lower the coverslip slowly to avoid forming air bubbles. Under the microscope, air bubbles look round and have dark edges.

3. Examine the epidermis first on low, then on high power. Draw what you see under high power in the space below.

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a. What is the general shape of a typical plant cell? _______________________ b. Stain your sample of onion cells by lifting the coverslip and placing a drop of iodine (Lugol’s solution) at the center of the onion sample. Iodine stains the starch and cellulose in the cells. 4. Examine your stained cells. Begin on low and then move to high power. Draw your cells under high power in the space below. Label: cell wall, cell membrane, nucleus, cytoplasm

_________X a. Within an individual cell, where are the cytoplasm and the nucleus found? _______________________________________________________________ 5. Clean off your slide and make a wet mount slide of Elodea using the young leaves found at the tip of the plant. Use the prepared slide of Elodea if available. If using a prepared slide—do not stain. 6. Observe these cells under low and then high power. Draw them under high power in the space below. Label: cell wall, cell membrane, nucleus, cytoplasm, and chloroplast

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a. What does Elodea look like under low power? _______________________________________________________________ b. What does a single chloroplast look like? _______________________________________________________________ c. In what ways are the cells of onion epidermis and Elodea similar? _________________________________________________________________ _________________________________________________________________ ___________________________________________________________ Different?_________________________________________________________ _________________________________________________________________ ___________________________________________________________ d. What observable characteristics can be used as evidence for classifying a specimen as a plant? _________________________________________________________________ _____________________________________________________________ Part III: Animal Cells 1. Using the toothpick, gently scrape the inside cheek of your mouth removing a small amount of cells. Be careful not to dig into your skin. A gentle scraping will remove more than enough cells. 2. Place a drop of methylene blue stain on a clean microscope slide and smear the cheek cell sample into the stain. Methylene blue stains animal cells to make the nucleus more visible. 3. Place a coverslip on the methylene blue/cheek cell mixture. Observe under low, and then high power. 4. Draw under high power the cheek cells in the space below. Label: cell membrane, nucleus, cytoplasm.

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a. Inside your mouth, these cells are joined together in a sheet. Why are they scattered here? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ b. Describe the general shape of these cells. _________________________________________________________________ _________________________________________________________________ 5. Clean off your slides and coverslips (Be careful). Make sure to dry them and then return them to their proper place. Post-lab Questions 1. When looking in the microscope, what cellular structures did you observe in plant cells but not in animal cells? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 2. In what observable ways are plant and animal cells structurally similar? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 3. How are they structurally different? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 4. Onions are classified as green plants. Where in the onion plant are the cells that contain the green chloroplasts located? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________