Introduction

The purpose of this lab was to use the microscope and identify cells such as animal cells and plant cells. This subject is important because in Biology, we will be using the microscope many times during different laboratory exercises. The microscope is used for looking at many specimens that cannot be seen with the naked eye. Humans only have a resolution, the ability to separate or distinguish two or more objects that are close together, of 0.1 millimeters. The average microscope has a resolving power up to 0.2 micrometers. In this lab, we adjusted the resolution on the microscope to have a better look at the specimens that were observed. In addition, we needed to look at contrasts of some specimens in this lab. Contrast is defined as being able to see different parts of the specimen at hand. In this lab, in order to increase the contrast of some specimens, we stained the samples using Methylene Blue and Water. The main hypothesis of this lab was, can we use the compound microscope to look at samples that we normally cannot see with our unaided eyes?

Materials, Methods, and Results

In this laboratory exercise, our main instrument was the compound microscope. In order to prepare the samples for observation certain materials were used. I used tools such as:

  • Compound Microscope
  • Toothpicks
  • Slides
  • Cover slips
  • Methylene Blue
  • Water
  • Onion bulb
  • Forceps
  • Razor blades
  • Onion bulb
  • Zebrina stem
  • Laboratory Exercise Manual
  • Pencil

There were three mini-lab procedures carried out during this lab. The first lab exercise was observing animal cells, in this case, my cheek cells. The second lab exercise was observing plant cells, in this case, onion epidermis. The third lab exercise was observing chloroplasts and biological crystals, in this case, a thin section from the Zebrina plant.

The first thing that was done in this lab exercise was gather materials. I worked with two other classmates that sat at my table.

Observation of animal cells (squamous epithelium of a cheek)

Using a toothpick, I carefully scarped the inside of my cheek to get the cells. I then spread it across the slide, added the Methylene Blue cheek-cell  solution and then covered the slide with a cover slip. I placed the slide in the center of the stage and made sure it was secured with the stage clip. The objective lens was already at 10X magnification, so I switched it to 40X magnification. I moved the stage closer up using the Coarse Adjustment. The specimen wasn’t exactly in the middle, so I had to move the slide around using the X-Y stage control so that I was looking directly at the sample. I also adjusted the lighting of the microscope using the diaphragm. I then switched the magnification to 40X. I adjusted the Fine Adjustment to get a sharper image of the cell. I was able to see the cheek cell correctly. I was able to see the Cytoplasm, Nucleus, and the Cell Membrane.

Observation of plant cells (onion epidermis)

For this observation, a plant cell was to be seen. An onion bulb was retrieved. Using the forceps, I removed a small slice of the onion and onion-cellcarefully and quickly put it on the slide. I also added water to ensure that the onion slice would not dry out. I adjusted the lighting again using the diaphragm, to contrast the compartments of the cell. I moved the stage closer up using the Coarse Adjustment and switched the magnification to 10X. I was able to see the Nucleus’s and Cell Walls between each cell.

Observation of chloroplast and biological crystals

For this observation, I looked at a small section from the Zebrina stem. The stem was gotten from the bucket in front of the classroom. The small section was obtained by slicing a tiny amount of the stem using the razor blade. It was placed on the slide, followed by the water. When first observed, nothing clear could be seen. It appeared to be that the Zebrina stem was cut too thick. The stem had to be cut once more, and this time much thinner, but not too thin. The same procedure was repeated again, zabrina-celladding water, placing the cover slip, putting the slide on the stage, adjusting the stage, and making the image sharper. The second sample proved to be much better. The magnification was already positioned at 10X magnification, which made the cell much clearer to see. The Cell Walls, Cytoplasm, Nucleus’s, Chloroplasts, and Crystals were able to be seen.

Discussion

  • Calculate the resolving power of the lenses on your microscope.

To find the resolving power for each of the lenses on the compound microscope, I used the Abbey equation. I plugged in the appropriate numbers into the variables, where d = resolution (nm), 0.612 was given, l = wavelength of light used (550 nm), and NA = numerical aperture. I repeated this equation for each magnification, getting the resolving power for each of the lens.

Magnification Working Distance Numerical Aperture Resolving Power
10X 5.1 mm .22 1530
40X 0.15 mm .65 517.8461538
100X (Immersion) 0.05 mm 1.25 269.28
  • Will this resolution be attained with each sample you look at? What will be some of the interfering factors?

Each resolution for each sample is different. Some samples will require you to choose a higher magnification or lower magnification. Some interfering factors can be using contaminated samples such as, dirty slides, using the wrong stains or dyes, using the incorrect sample, broken slides and many more. Other interfering factors could be for using incorrectly using the equipment or broken equipment such as, broken objective lens, broken illuminators, the light might be too high or too dim and many more.

  • What happens to resolving powers as the NA increases?

When the Numerical Aperture increases, the resolving power will decrease.

  • What differences can you observe between animal cells (cheek epithelium) and plant cells (onion epidermis)? Think of the size, shape and cellular components.

The onion epidermis cell is the only cell that has a cell wall. In addition, it is the only cell that has a chloroplast, where the photosynthesis can happen. The cheek epithelium cell is the only one that has centrioles, the barrel-shaped organelle that is responsible for helping organize chromosomes during cell division. Furthermore, the presentation of the onion cells was positioned right next to each other, on top of each other, below each other, like a checker board. The cheek cells were bunched up together at some areas, almost overlapping each other. The onion cells almost looked rectangular shaped, whereas the cheek cells look oval shaped.

  • How many chloroplasts would you estimate are in each cell of Zebrina?

From looking at the Zebrina slide, I would estimate that there was about 50 chloroplasts.

  • What function do you think the calcium oxalate crystals have?

The calcium oxalate is a calcium salt of oxalic acid. It forms crystals known as raphides, which appears to be what I saw when l looked at the Zebrina sample. Interestingly enough, while reading about calcium oxalate, I discovered that it is a major constituent of human kidney stones, founded in urine. From observing the calcium oxalate crystals, it looks like spikes/needles. From what I know about spikes, they serve as a weapon. So my assumption is that these crystals are used as a defense of some sort.

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