Science Homework Help

Lab Exercise 4:

The Microscope: Gateway to the Secrets of the Cell

It would seem ironic that after thousands of years of civilization and the practice of science, the compound microscope and the telescope happened to be invented by different people less than 20 years apart. But, in fact, the basic principles behind both instruments are fundamentally the same: they both use a series of lenses to make things larger, whether it is a distant interstellar nebula or a microbe in a drop of water. When Galileo turned his new telescope to the night sky and Hooke his microscope to a small slice of cork, the universe became both a great deal bigger and smaller than anyone could have imagined.

In this first lab exercise, you will be becoming familiar with the function of a light compound microscope. This is an

One of the first light microscopes (ca 1660)

important first step in this course, as you will be asked to use the microscope to see and identify a great variety of very small objects, including microorganisms and cell types. It is important that in the upcoming laboratory exercises, you spend more time examining and documenting your specimen slides and less time trying to figure out how to work your microscope.

Of all the things we may view using a microscope, none is more awe-inspiring than the simplest: the cell. A cell is the smallest unit of life; anything smaller doesn’t qualify as a living thing. Proteins, nucleic acids, lipids and carbohydrates, each and of themselves are not alive. But assemble them in a very precise and orderly way, and the entire assemblage is alive. It would be a defensible claim to say that the cell is evolution’s

The oldest published image known to have been made using a microscope. Bees, by Francesco Stelluti, 1630.

greatest achievement, and the most mysterious.

Cells were first described by the British scientist Robert Hooke, who used a microscope to examine a thin slice of cork from the bark of an oak tree. Microscopy has come a long way – today’s electron microscopes use a beam of electrons to

resolve objects and are used for study the fine details of the cell surface or for exploring the internal structure of a cell.

Tasks: (1) Be able to identify the name and function of each component of your microscope; (2) Be able to effectively apply the functions of the microscope when examining a slide; (3) Draw representative examples of microscopic specimens, noting

distinctive characteristics of each; (4) Develop techniques in slide preparation, including staining; and 5) Understand the role of osmosis in cell function

Part A: Viewing Prepared Slides

Collect the following slides: the letter “e” (#1103), thread (#1102), and the tick and mite (#2314). View each of the slides on the microscope and draw what you observe. Refer to the instructions above and focus on getting the sharpest image you can and on honing your microscope-viewing skills. While viewing the selected prepared slides, determine whether the following aspects of the microscope image either increase or decrease when total magnification increases: size of the field of view? working distance? light intensity? resolution? working distance?

Examine the prepared slide of the letter “e” with the scanning lens (4X) and adjust the iris diaphragm for the correct light intensity. Without adjusting the iris diaphragm further, switch first to the 10X and then to high power (40X).

GOTO4-5 What happens to the light intensity as the magnification is increased?

GOTO4-6 Prepare a drawing of the ‘e’ at 40x total magnification.

GOTO4-7 The field of view is the circular lighted area you see when you look through the microscope. What happens to the actual amount of field visible (not the size of the object being viewed) when magnification is increased?

Examine the prepared slide of two well-known arthropods: the tick and the mite (look closely, both species are on the same slide).

GOTO4-8 Preparing two separate drawings: one of the tick and the other, the mite.

Focus on the “threads” slide and examine it under low power. Now adjust to high power, and slowly focus away from the slide with the fine adjustment until the fibers are out of focus. Then very slowly focus toward the slide until the first

set of fibers comes into focus. These are on top. Continue to focus until the middle set of fibers comes into focus and finally the bottom set.

GOTO4-9 Prepare a drawing of the threads (color pencils will be made available to you). What is the color of the top thread?

Part B: Preparing Specimen SlidesFollow these instructions to prepare the following slides, draw what you see in your field of view, and record observations:

1.Your own cheek cells (obtained from an epithelial scrape). This is done by using a clean toothpick which is vigorously scraped across the inside surface of your cheek. The resultant cells that were detached are rubbed onto a clean slide. Next, add a very small amount of methyl blue stain to the sample on the slide and swoosh it around with the tip of the toothpick. Finally, place a cover slip over the sample and examine it under the microscope. The methyl blue is a stain that binds to protein in the cells, making the nucleus visible (your own DNA is in each one!). If it doesn’t work the first time, you may need to make another attempt if time permits.

GOTO4-10 Prepare a drawing of your cheek cells.

2.Pond water. You instructor will provide you with a container fresh pond water. Use the droppers provided to take a small sample (usually best from the bottom) and place it on a clean slide. Do not use any stain! Use a cover slip, but do not press down too hard or you may crush your slide’s occupants. Examine your slide under the microscope. Prepare a second slide using the same techniques, but now add methyl cellulose to the pond water. Record your observations.

GOTO4-11 Do you observe any differences in the two preparations? Explain.

GOTO4-12 Select one object in the pond water and prepare a drawing of it.

3.Cork Cells. Robert Hooke was the first to coin the term “cells” to describe they “honeycomb” of “pores” that he observed while viewing cork under a microscope. He was indeed looking at plant cells and remarked that the boxlike cells of cork reminded him of the cells of a monastery. Cork cells make up the bulk of the outer bark of a woody plant. A fully formed cork cell will accumulate large amounts of waterproofing compounds in their cell wall, which eventually leads to the cell’s death. At maturity, the cells are dead and have no cytoplasm or internal structures, but aid in plant defense and water conservation.

Cut a thin section of cork and make a wet mount by placing a drop of water on a clean dry slide and placing the thin slice of cork on top of it.

Observe the cells under low power. Locate an area of the specimen where the cells are one or a few layers and can be easily be viewed in sharp focus. Then, view under high power.

Notice the uniform size and general appearance of these cells. These are dead cells with no internal contents, just a thick, waxy cell wall.

GOTO4-13 Prepare a drawing of your cork cell preparation.

Part C: Using a Dissecting Microscope

A dissecting microscope (see right) is used for viewing objects that are too thick for light to pass through. It allows for the examination of bulkier objects or opaque objects that don’t require as much magnification to view (4X-40X). The dissection scopes also have dual lens so allow for stereoscopic view. Because there is a much greater amount of space between the lens and the specimen, it is possible to manipulate the

specimen while viewing it. For example, the parts of a flower may be dissected under the lens – a procedure that is certainly not possible in the higher compound microscope!

A few specimens in the lab have been set up for you using dissecting scopes. View the specimens on display under the dissecting microscopes.

GOTO4-14 Draw and label your observations of the specimens under the dissecting microscopes (including total magnification).

At the end of this lab, you will be asked to summarize the advantages and disadvantages to working with dissecting versus compound microscopes.

Part D: Putting it all Together – A Demonstration of Osmosis in Plant Cells

Homeostasis is the ability of cells to maintain an internal cellular environment despite changes in the extra-cellular environment. Homeostatic mechanisms have evolved that address temperature, pH, ion concentrations, and in this activity, the movement of water across the cellular membrane. Without some form of cellular osmoregulatory ability, there are very few microenvironments on Earth that would support life, as we know it. In this exercise, you will be observing changes in the osmotic balance in plant cells. Osmotic balance refers to the relative movement of water and out of the cell. Osmosis can be a significant problem for all cells, as the cellular membrane is permeable to water. If too much water enters the cell, the cell will expand and perhaps eventual burst, an event called lysis. If, instead, water leaves the cell, the cell shrivels (crenation). The key is to maintain a state of osmotic balance between the water inside and outside the cell.

A cell may find itself in only one of three possible osmotic environments:

Hypertonic environment: The net movement of water is out of the cell, due to the higher concentration of solutes (dissolved materials) outside the cell. This is what occurs when freshwater cells are placed in saltwater.

Hypotonic environment: The net movement of water is into the cell due to the higher concentration of solutes inside the cell. This is what occurs when saltwater cells are placed in freshwater.

Isotonic environment: In this condition the concentration of solutes is the same inside and outside the cell, so there is no net movement of water either direction across the membrane.

GOTO4-15 Which osmotic state do you suspect is maintained in animal cells as these cells lack a supportive cell wall like that of plants?

Osmosis in Plant Cells Procedure:

1.Prepare a slide of a signal Elodea leaf. Add additional water from the water plant container and cover with a cover slip to keep it wet. Draw the cells that you see, noting the location of the chloroplasts within the cells.

2.Place a couple of drops of 5% saltwater at one edge of the cover slip.

3.While one member of your team is looking through the scope, place a piece of paper towel with a torn edge at the side of the cover slip opposite the saltwater. The paper towel will pull the saltwater under the cover slip. Note any changes that occur in the appearance of the cells.

4.Repeat the experiment with 10% saltwater, then distilled water, noting any changes you observed in the cells.

5.See if it is possible to change the saltwater treated cells by subsequently flushing the leaf with distilled water.

GOTO4-16 Prepare drawings of your plant cells under both osmotic conditions.

The capacity to take in water creates a fluid pressure in plant tissue called turgor pressure. You may have seen the effects of a loss of turgor pressure when you fail to water your houseplant and the leaves begin to droop.

End of Lab Exercise Questions/Tasks:

GOTO4-17 Microscopes have come a long way since their invention centuries ago. Nevertheless, ask your instructor why many colleges tend to hold onto their older scopes rather than purchase new instruments (beyond the issue of cost). Clue: It’s all about the glass!

GOTO4-18 Complete the summary table comparing the features and functions of the compound and dissecting microscopes.