- Cells are the basic functional units of all living organisms. They may exist singly or in aggregates. When cells join together to take to take on a specialized function within a larger organism, they form a tissue.
- There are two major divisions into which all cells fall: prokaryotic (organized nucleus absent) and eukaryotic (organized nucleus present). Bacteria make up the former division while the cells of plants, animals, fungi, protozoa, and algae compose the latter.
- Animal and plant cells share characteristics, which you will observe in this lab. They also differ in several important ways. Both animal and plant cells may occur unicellularly or within multicellular organisms. Because they often take on special functions within tissues, animal cells are frequently more specialized than plant cells. Epithelial (EP-uh-THEE-lee-ul) cells and blood cells are examples of different tissues.
- In this lab, you will look at epithelial cells in both plants and animals. Epithelial cells form the skin of the body surfaces and the linings of the inner surfaces. These cells are specialized for transportation of substances and protection. The individual cells of these layers may be shaped like cubes, columns, or be flat, depending on their location and function.
MATERIALS:
Compound microscope
Microscope slides
Cover slips
Forceps (tweezers)
Single-edged razor blade
Flat-edged toothpicks
Paper towel
Iodine solution
*Methyl-green stain
Onion
Sprigs of Elodea
Pictures of typical plant and animal cells from a textbook for reference*You can get a good substitute stain at the pet store. Either the green or the blue tropical fish medicine works great as a stain.
PRE-LAB PREPARATION:
- Place a beaker of water with the Elodea in it, under a strong light source about 30 minutes before the lab.
PROCEDURE:
Part 1: Plant Cells
- Onion bulbs are organized tissue that, under the appropriate conditions, will give rise to an entire plant. The curved pieces that flake away from a slice of onion are called scales. On the underside of each scale is a thin membrane called the epidermis.
1. Obtain a piece of onion and remove one of the scales from it. Use forceps to pull away the epidermis from the inner surface. Be careful not to wrinkle the membrane. Place a drop of water on the center of a microscope slide, cut a piece of membrane about 0.5 cm square with a single-edged razor blade. CAUTION: Handle the razor blade with care. Using a toothpick to straighten out any wrinkles, place the membrane sample in the drop of water. Take a cover slip, and carefully place it over the sample, lowering it at an angle to the slide. This helps keep air from being trapped under the cover slip. You have just made a wet mount.
2. Examine the epidermis first with the medium power objective of your microscope. Unstained specimens are often seen better with less light. Try reducing the illumination by adjusting the diaphragm of the microscope. Then examine it under high power.
- Question 1. How many layers thick is the epidermis?
- Question 2. What is the general shape of a typical cell?
3. To stain your specimen, remove your slide from the microscope stage. Place a drop of iodine on the side of the cover slip, touching its edge. CAUTION: iodine is toxic. Draw the water from underneath the cover slip with a scrap of paper towel placed edge to the opposite side of the cover slip from the iodine drop. The stain will be drawn under the cover slip to replace the water that the paper towel scrap absorbs.
4. Place the slide back on the microscope stage and observe as before. The iodine will stain the nucleus so it can be seen more clearly.
- Question 3. What does the nucleus look like under medium and high power?
- Question 4. Within an individual cell, where are the cytoplasm and the nucleus found? What general characteristic of plant cells can be inferred from observations of the cytoplasm and nucleus?
- Question 5. Make a diagram of several cells as observed under high power. Label the following structures in one cell: nucleus, cell wall, central vacuole, cytoplasm.
5. Obtain a single leaf of Elodea (from the young leaves at the tip) and prepare a wet mount as you did before. You may want to use only a small portion of the leaf tip, so it will lay flat on the slide.
- Question 6. What does Elodea look like under middle power?
6. Examine the chloroplasts under high power.
- Question 7. What does a single chloroplast look like?
- Question 8. Are the chloroplasts moving or stationary? Make an inference to explain this.
- Question 9. In what ways are the cells of onion epidermas and Elodea similar? Different?
- Question 10. What observable characteristics can be used as evidence for classifying a specimen as a plant? Use information from your textbook to help you with this question.
Part 2: Animal Cells
7. Prepare a slide of epithelial cells from your oral cavity, by the following procedure. Take a flat toothpick (a NEW one) and using the large end, scrape the inside of your cheek 3 or 4 times. Gently make a smear in the center of a clean slide, about the size of a dime. Carefully place 1 drop of methyl-green stain on the center of the smear. Place a cover slip over the drop of stain.
8. Examine the cells, first under middle power, then under high power. At first, the field of view will be light blue and the cells will be a slightly darker blue. After a few minutes, the field will lighten and the cells will become slightly purple.
- Question 11. Inside the mouth, these cells are joined together in a sheet. Why are they scattered here?
- Question 12. How are these animal cells different from the plant cells you observed?
- Question 13. Draw a few cells and label the cell membrane, nucleus, and cytoplasm.
Back to Biology Table of Contents
Back to Lab Dad's Laboratory
- There are two major divisions into which all cells fall: prokaryotic (organized nucleus absent) and eukaryotic (organized nucleus present). Bacteria make up the former division while the cells of plants, animals, fungi, protozoa, and algae compose the latter.
COMPARING PLANT AND ANIMAL CELLS
INTRODUCTION: