Structure of Living Cells

Unit 3 Lab A Structure of Living Cells

Introduction For approximately 140 years biologists have recognized a cell as the basic unit of life. A cell is composed of many smaller parts called organelles which contribute to its total operation. Although a variety of cells exist, they frequently share many common organelles. A knowledge of the basic structure of cells is necessary in order to understand life processes of both simple and complex forms of life.

Objectives  Prepare a wet mount slide  State the advantage of staining cells.  Identify some of the basic cell structures found in plant cells  Describe the similarities and differences that occur in plant cells

Materials  Compound Microscope  Forceps  Tomato  Microscope slides  Iodine stain  Razor blade  Cover slips  Onion cut in quarters in water  Toothpicks  Dropper  Potato  Methylene Blue stain

Procedure

Part A: Onion Cells

1. Obtain an onion scale and remove a paper thin layer of tissue from the concave surface as shown. Place the tissue in a drop of water on the slide. Care should be taken to spread it flat without tearing the tissue. Add a cover slip, being careful to avoid trapping air bubbles. The cover slip must lie flat. Observe the preparation under low power.

Q1. What is the shape of the onion cell? Q2. What cell structure(s) can you identify?

2. With the cells under observation at low power, add a drop of iodine stain to the edge of the cover slip. Stains can cause permanent damage to clothing. Handle them carefully. A piece of paper toweling placed at the opposite edge of the cover slip will draw the iodine under the cover slip and into contact with the onion cells as shown in the picture.

3. Center several cells in the field of view. Switch to high power and observe the cells carefully. Remember that as magnification increases, field size decreases.

4. Sketch several onion cells showing as much detail as possible in one of the cells. The cells should be approximately 3 cm long on paper. Drawings are best made in pencil, since details can be shaded in. Label: cell wall, cytoplasm, nucleus, nucleoli. Labels should be printed horizontally on the paper. Part B: Potato Cells

1. Cut a small piece of potato about 5mm square. Using a razor blade, shave off a piece that is as thin a possible. Remember that light must pass through a sample before it can be viewed.

2. Prepare a wet mount by placing the potato sample in a drop of water, adding a drop of iodine, and placing a coverslip over the sample.

3. Examine the slide under low power. Q3. How do the potato cells differ from the onion cells? Q4. The black areas are starch grains being stored in cell organelles called leucoplasts. Why does a potato plant store starch in these cells?

4. Sketch several potato cells as viewed under low power. Again, add detail to one cell. Label: cell wall, leucoplasts.

Part C: Tomato Skin Cells

1. Remove a piece of skin from a tomato. Scrape all of the pulp from the skin and prepare a wet mount of the skin with the inner side down.

2. Observe the slide under low and high power. Red pigment is contained within cell organelles called chromoplasts.

Q5. How do the tomato cells compare to the onion and potato cells in size and shape?

3. Sketch the tomato skin cells. Label: cell wall, chromoplasts, and nucleus if seen.

Analysis

Q6. How can you tell that a cell has depth or thickness?

Q7. What is the reason for staining the cells before viewing them?

Q8. All the cells viewed are considered plant cells. What structure(s) did they all have?

Q9. The structure of a cell is related to its function. How do the laboratory observations in Part B support this statement?

Q10. If you were viewing animal cells, how would they differ from the cells observed in this activity?

Q11. How would you improve this lab?

Q12. How does this lab relate to everyday life?

(OPTIONAL) Part D: Cheek Cells

1. Using a toothpick, gently scrape the side of your cheek to obtain some cells. 2. Prepare a wet mount of the cells by wiping the toothpick over the center of the slide and placing a single drop of water over the cells. 3. Place your cover slip on the slide and add a drop of methylene blue stain to the edge of the cover slip. A piece of paper toweling placed at the opposite edge of the cover slip will draw the iodine under the cover slip and into contact with the cells. 4. Observe the slide under low and high power. 5. Sketch the cheek cells. Label: cell membrane, nucleus, cytoplasm

Q13. Compare the cheek cells you viewed in this section with the onion cells you viewed in Part A. DO NOT FORGET TO COMPLETE SKETCHES WITH LABELS (HIGH POWER) FOR EACH SECTION!!!