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Biology-Mr.Willets
Lab-Cell Study #1
The study of cells is called cytology. Since all living things are made of cells, this branch of biology is of obvious importance. Today and next time, you will examine several types of eukaryotic cells using the microscope. In each case, you hope to see the three main parts of all eukaryotic cells: the cell membrane, the cytoplasm and the nucleus along with some other specialized cell parts.
Part One- Onion Epidermal Cells
The part of the onion with which you are familiar is called a bulb. The cells of the onion bulb are used mostly for storage. Because of this, these cells do not have many specialized structures. You will be shown in class which part is the epidermis. We use this part because we can get a very thin piece.
a) Place 2-3 drops of water on a clean slide.
b) Remove a small section of onion epidermis and place carefully into the water. Be careful not to wrinkle the tissue. This can be tricky. If you mess up, start again. It will make the rest of this part easier.
c) Add a cover glass
d) Observe under scanning power. Switch to low power. Adjust the light for the best image. Notice the shape and color of the cells. You may see a nucleus in some cells.
e) Answer questions 1-2
f) Remove the slide from the microscope stage. Carefully raise the cover glass and add a drop of iodine. It is OK if the onion epidermis sticks to the cover glass. Replace the cover glass and observe under scanning power and then low power. If you cannot do this without messing up the slide, make a new slide and add the iodine before placing the cover glass on.
g) The iodine will darken the basic living material inside the cell (cytoplasm) and also darken the nucleus. The cytoplasm will appear grainy (dotted). Areas within the cell that appear less grainy are storage areas called vacuoles. They contain mostly water. Focus back and forth with the fine adjustment to see where the cytoplasm is (grainy areas) and where the vacuoles are (clearer areas).
h) Each cell has a nucleus. Some may not be as visible as others depending on how much iodine they absorbed. Inside the nucleus are smaller structures called nucleoli (sing-nucleolus). These appear as small circles. Look for them. The nucleoli make ribosomes.
i) Move the slide so that a nucleus in the center of the field and switch to high power. The nucleoli should be more visible now.
j) Still under high power, move the slide to get a better look at the cytoplasm, the vacuoles, and the outer edge of the cell. This outer edge is made of two parts- the cell wall (found only in plant cells) and the cell membrane (found in all cells). They are right next to each other and appear as one unit. The cell wall is the outer part. Notice its thickness. Later in today's lab, when you examine animal cells (your own), you will notice the outer edge is not as thick. That is because animals cells only have the cell membrane.
k) Answer questions 3-7
Part Two- Cells of Elodea (This is the scientific name for the plant we will use and so it is in italics)
Elodea is a fresh-water plant with very thin leaves. This will allow us to observe the cells in the leaf without having to slice the leaf. Elodea is often used in fish tanks to help keep the water clean and to provide oxygen for the fish.
a) Bring a slide and cover glass to the front desk. Add 2-3 drops of water from the dropper bottle. Place a leaf of Elodea on the slide, outside of the curve facing up. Add a cover glass.
b) Observe under scanning power and then switch to low power. Look for a section where the cells are easily seen. There are two layers of cells in this leaf so you may see one layer by looking through another. Be careful that this does not confuse you. When you have found a good section, switch to high power.
c) The green structures inside the cell are chloroplasts. This is where food is made. Although you will not see it, the cytoplasm is around the chloroplasts. The chloroplasts and cytoplasm are located along the inside edge of the cells because the center of the cell is filled with water and appears clear. This is called the central vacuole and serves as a storage area for the cell. The central vacuole pushes the rest of the cell to the edge. The outer edge of the cell is the cell wall and cell membrane. Notice its thickness.
d) Search to find a cell where the chloroplasts are moving. It is actually the cytoplasm that is moving and carrying the chloroplasts along with it. This is called cytoplasmic streaming. The nucleus is mixed in with the chloroplasts. It is unlikely you will see it but look anyway.
e) Answer questions 8-11
Part Three- Human Cheek Epithelial Cells
The inner lining of your cheek and the lining of other body parts are made of cells called epithelial cells. These can be easily removed without doing any harm to your precious body.
a) Gently scrape the inside of your cheek with a clean toothpick as shown in class.
b) Smear the material on a clean slide. Add one drop of iodine and one drop of water.
c) Observe under scanning power to find some cells then switch to low power. Some of the cells will have folded and wrinkled. Find some cells that are flat and smooth. The iodine will make the cytoplasm more visible and the nucleus should be easily seen. Normally these cells would be colorless. Notice that there is no central vacuole but that the cytoplasm fills the cell completely. Also, notice the outer edge is thinner than what you saw in the plant cells in parts one and two. There is no cell wall, just a cell membrane. Notice that the cells are not a regular geometric shape like the plant cells mostly were. These differences are because you are an animal (grrrr!!).
g) Center a cell and switch to high power to observe more closely.
h) Answer question 12-13
Biology-Mr.Willets
Cell Lab #1- Data Sheet
When estimating sizes of cells, use the chart that shows the length of each part of the pointer. With our scopes: low power is 100X, high power is 400X
Part One- Onion Epidermal Cells
1) In the circle below, draw a group of 10 onion epidermal cells as they appear under low power. Estimate the length and width of an average cell.
Specimen __________________________ / _______X
Length ____________microns
Width ____________microns
2) Are all the cells about the same size? ___________ What is the color of the inside of the cells? _________________________________
3) When stained, the grainy looking areas are what part of the cell? ________________________
4) The clearer looking areas inside the cells are ______________________. These are used for ______________________ and contain mostly __________________
5) Does every cell have a nucleus? __________ Is the nucleus located in the same position in every
cell? ____________ What are the smaller round structures sometimes seen inside the nucleus? __________________________
6) The diagram below is of an onion epidermal cell. Label the following parts: cell wall, cell membrane, cytoplasm, vacuole, nucleus,
7) Draw a single onion epidermal cell about the same size as the diagram above.. Draw it as it actually looks using the scope. Do a nice job- Use a pencil. Where the cytoplasm appears grainy, use little dots. No labels needed.
Part Two- Elodea
8) The chloroplasts in Elodea move so that each can get to the upper part of the cell and be exposed to __________________ needed to make food.
9) What pushes the cytoplasm to the edge of the cell? __________________________
10) In the circle below draw several Elodea leaf cells as seen under high power. This diagram should just show the basic shape and size. Estimate the length and width of the cell.
Specimen _________________________/ _______X
Length ________________microns
Width _________________microns
11) Draw a diagram of one Elodea leaf cell. Make it as large as the diagram in question #6. Include and label the following structures: Cell wall, cell membrane, chloroplasts, cytoplasm, central vacuole.
Part Three- Human Cheek Epithelial Cells
12)In the circle below, draw several human cheek cells as seen under high power. Estimate the size of a cell. Below your circle draw one cheek cell greatly enlarged. Label- cytoplasm, nucleus, cell membrane
Specimen____________________/________X
Size_________________
13) In general, plants cells have a more ___________________ shape than animal cells. Also, the outer edge of plant cells is much __________________ because of the _____________________ they have.