Biology Microscope Questions

Biology Microscope Questions

Description

Exercise 6

THE MICROSCOPE AND THE CELL

Student Learning Outcomes:

At the completion of this exercise you should:

(1) Be able to label a compound microscope diagram indicating its 12 basic parts.

(2) Be able to describe how to adjust a microscope in order to observe a cell under scanning, low, and high power magnifications.

(3) Be able to estimate the size of a cell or other object, in millimeters and micrometers, under high and low power magnifications.

(4) Be able to identify the cell membrane, nucleus, and cytoplasm of an animal cell. Be able to identify the cell wall, nucleus, and cytoplasm of a plant cell.

1. The Microscope

Introduction

When many people hear the word biology, they picture a person looking through a microscope, and students entering a beginning biology course often expect to use the microscope a great deal. In practice, whether a professional biologist regularly uses a microscope depends on his/her area of special interest. Nevertheless, it is a valuable tool with which all biologists are familiar. When using a microscope, we should remember this: it is only an extension of our eyes which allows us to examine individual cells and other structures which are otherwise too small to be seen. Today we will develop the basic microscope skills necessary to successfully study cells in this and some of the subsequent laboratory exercises this semester.

1. The Compound Microscope

The compound light microscope, one of the two types of microscopes you will use in this class, must be cared for and operated correctly. You should spend considerable time during this period learning how to use it, and you should pay particular attention to how it differs in form and function from the dissecting microscope. See Figure 1. One of the major differences between the two microscopes is that the compound microscope has higher powers of magnification than does the dissecting microscope. All specimens to be viewed and studied with the compound microscope must be mounted on a glass slide and covered by a coverslip. (Since objects viewed through the dissecting microscope are usually larger, coverslips are not often used.) The correct technique of mounting a specimen on a slide will be demonstrated.

Procedure:

The proper way to carry the compound light microscope will be demonstrated. Always use two hands. Make sure that the cord is not dangling to prevent a tripping hazard. One hand should be holding the base of the microscope, while the other should hold the arm of the microscope.

Figure 1. General diagram of a compound light microscope. You this image and the one provide in my introductory PowerPoint slides–they both provide nice views of the microscope parts.

Always remember that the microscope is a delicate instrument. Please care for it and use it correctly.

Practice the Parts of the Compound Microscope.

In order to practice for your upcoming quiz, practice locating the following parts of the compound light microscope on the diagram below. You should learn these terms before proceeding. Refer to my definitions provided in the introductory presentation as well. If you feel comfortable with the microscope parts and functions from watching the video, proceed to the next page

1. OCULAR or Eye Piece
   • Contains lenses to increase magnification
   • May be replaced with another of lower or higher magnification
2. BODY TUBE
   • Holds lenses of ocular and objectives at the proper working distance from each other
3. NOSEPIECE
   • Permits interchange of scanning, low, and high power objectives
4. OBJECTIVES
   • Contains lenses of different magnifications
5. SLIDE (SPECIMEN) HOLDER
   • Mechanical stage with adjustment knobs
6. STAGE
   • Supports slide over opening that admits light from mirror or lamp

 

1. CONDENSOR & IRIS DIAPHRAGM
   • Concentrates & focuses light
   • Regulates amount of light passing through the specimen
2. LIGHT SOURCE
   • Directs light upward through diaphragm and hole in stage
3. COARSE ADJUSTMENT
   • Moves stage up and down approximately to the correct distance
4. FINE ADJUSTMENT
   • Permits exact focusing by moving stage up or down very slightly
5. ARM
   • Supports body tube and coarse adjustment
6. BASE
   • Firm support that bears weight of microsco

1. Checking the Microscope’s Condition Before Use

Prior to using the microscope, check the condition of the microscope from the prior class. You don’t have to answer a-f since you don’t have a microscope in front of you.

For steps g, h, i, and j, refer to the Virtual Lab Images and state which choice (A or B) was left correctly. The images for i and j are out of order.

1. Was the microscope placed back in its assigned compartment with the arm facing out toward you?
2. Was the cord wrapped between the stage and objectives with the plug tucked inside the cord?
3. Was the cord relatively untangled?
4. Was the ocular lens clean? (If not, clean the dirty lens with the appropriate lens cleaner and lens paper (not a paper towel).)
5. Was the light turned off?
6. Check that the condenser is at its highest point, directly below the stage. There is a knob connected to the condenser that allows it to be moved up and down.
7. Which image has the stage centered so that the stage clips don’t hang over the edge of the stage?

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1. Which image has the scanning objective lens (4x) left over the stage?

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1. Which image has the stage lowered to the lowest setting possible position? If the stage wasn’t properly lowered, you’d use the coarse focus knob to do so.

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1. Which image doesn’t have any slides remaining on the stage?

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1. The Lenses of the Compound Microscope

1. The two lenses of the compound microscope are the ocular and the objective lenses. These lenses should be cleaned only if needed. Use only lens paper to clean the lenses, not paper towels or Kimwipes. If the lens is very dirty, consult your instructor.
2. Lenses for Magnification: The oculars and objectives contribute to the total magnification of the image of a specimen.

The ocular (or eyepiece) magnifies objects ten times their diameter–it is said to have a magnifying power of 10x.

Note that there are three objectives that are mounted in a nosepiece which revolves so that any objective can be brought into the functional position in line with the body tube. Varying objectives can be installed depending on the needs of the laboratory or class, so you need to check the magnification on your particular microscope. Observe the objective lenses, and record the color and magnification power for each below:

Use this picture of the three different objective lenses for reference:

• The very short objective is called the scanning objective. The scanning objective typically has a magnifying power of ___x, depending what has been installed on your microscope. (color: ___________),
• a medium-length one called the low power objective. The low power objective also has a magnifying power of ___x. (color: ___________),
• and a long one called the high power (also called “high-dry”, see reference to “wet” or oil lens below) objective. The high power objective has a magnifying power of ___x or more. (color: ___________).

 

1. How to Use the Compound Microscope

Read Powerpoint slides #7-9 in my introductory presentation and watch the following video: How to use the Microscope. This video is essentially the lead instructor carrying out the steps of “how to focus on a specimen” that I described in the introductory presentation. This is the same youtube link provided on page 7 of the virtual images. I don’t have access to these pdfs and the lead instructor saved them as pdfs, where most of the links are not functioning. It’s messy :(( I’ll always provide the link here within the lab exercise itself.

1. Calculating Total Magnification: The ocular lenses and the lenses in the three objectives have different powers of magnification. When you view through a microscope, the magnification is provided by both ocular and objective lenses.

To determine the total magnification of any object viewed with the microscope, multiply the magnifying power of the ocular by the magnifying power of the objective in use. For example, if the low power objective is in use, the total magnification of the object would be 10 x 10 or 100X. (Meaning that the image formed by the lenses is 100 times larger than it would appear with the naked eye.

Question 1. Calculate the total magnification that would be obtained when using the microscope with the scanning power objective in place.

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Question 2. Calculate the total magnification that would be obtained when using the microscope with the low power objective in place.

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Question 3. Calculate the total magnification that would be obtained when using the microscope with the high power objective in place.

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Question 4. Now, write the generalized “equation” for the calculation of the total magnification. Your equation should use words, not numbers.

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Use the virtual images (starting on page 8) for instructions on using a virtual microscope for completing questions #5-12. The link provided on page 8 of the virtual images does not work, so here is another copy of the link to: Microscope Basics. Complete steps #3-9 on pages 8 and 9 of the virtual images. You can skips steps 1 and 2, because the link that I’ve provided will take you directly to “Microscope Basics”. This link will allow you to view: Microscope Overview, Microscope Lens, and Microscope Care.

When you get to step 6 on page 9 of the virtual images, ignore “Click on Launch and Learn and Complete” and instead go to the following link: Virtual Microscope and press launch. This link will allow you to view the microscope slide of the printed letter “e” so that you can answer questions #5-7 and #9. Then proceed to step 7 of the virtual images. The letter “e” slide is in the “sample slides” section of the slide box. The slide box is grey and has a question mark on it. Skip step 9 and skip pages 10-13, 17, and 20-23 of the virtual images

At this point, you should be seeing a blurry image of the letter “e” slide within the Virtual Microscope simulation. You will use this virtual microscope to complete questions #5-7 and #9 below.

Question 5. Click on the blurry letter e and drag it down toward the bottom of the circle (known as the Field of View or FOV), which direction does the stage move? (Towards or away from you?) You’ll have to keep an eye on the microscope to the left of the letter “e”. This represents the movement of the same microscope that you’re looking through.

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Question 6. What happens to the stage when you click and drag the “e” to the top of the FOV? (Does it move towards or away from you?)

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Question 7. The virtual person who placed this “e” on the stage, placed it so that the slide label is facing upward (right side up) and able to be read. However, how does the blurry “e” look through the FOV? (Right side up or upside down?)

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Slide the Coarse Focus Adjustment knob to the right until the letter “e” is clear. Then move the Fine Focus slider until it is even clearer.

 

Question 9.Switch to 10X and then to 40X. Pay attention to whether the FOV gets lighter or darker. Remember that opening the iris diaphragm allows more light to pass through the slide–”brightening” the image. On a real microscope, would you open or close the iris diaphragm as you increased from 4X to 40X?

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Question 10. What is the low to high rule? (What order should the objective lenses be used in?) Look for the answer in the video: Microscope Basics. This video is also great for learning how to use the microscope! You can stop watching at 3:15.

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Question 11. Why could you possibly only be seeing part of an image? Look for the answers to these in the video: Common Microscope Mistakes

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Question 12. Why can’t I find the same part of the specimen once I move to a higher magnification? Look for the answers to these in the video: Common Microscope Mistakes You can stop watching at 3:10.

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7. Measuring Microscopic Objects

Frequently, the biologist needs to know the dimensions of the specimen he or she is examining under the microscope. The following method will enable you to obtain an estimate of the size of an object by comparing it with Field of View. The Field of View is the diameter of the illuminated circle observed through the microscope lenses. Watch my introductory video for a tutorial on this section.

To do this it will first be necessary to measure the size of the field. For this measurement, as well as others you will be making in subsequent laboratory work, the metric system will be used.

Procedure:

1. Place a short, clear plastic ruler over the opening in the center of the stage so that the lines are visible through the microscope. Place it on top of the stage clips, not under them, or you may bend the stage clips.
2. Line up one of the vertical lines so that it is just visible at the left side of the circular field of view, using your scanning-power (4x) objective.
3. The distance from the center of one line to the center of the next line is 1 millimeter (mm). Count the number of millimeters included from one side of the field to the opposite side. If the right side of the field does not coincide with one of the lines, you will have to estimate the portion of a millimeter. For example, the diagram above shows 1.7 mm.

Obviously you are not able to complete steps 1-3 from home, but if you did use a metric ruler to measure the field of view under scanning power, you would have found out that the diameter of the FOV under scanning power is 5 mm.

 

Question 16.What is the FOV (in millimeters) under scanning-power? Write the units (mm) after the number. I just gave you the answer above and you can also find it in my introductory video.

FOVscanning = _______ mm

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4. For most microscopic measurements we need a much smaller unit than the millimeter. Scientists use the micrometer (µm), more commonly called the micron, which is one-thousandth of a millimeter (0.001 mm). (Micro- is symbolized by the Greek letter µ or mu.)

Question 17. How many microns are there in one millimeter? (Hint: which is smaller, micron or millimeter?

1 mm = ______ µm

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Question 18. What is the FOV at scanning power in microns? Hint: This requires you to convert your answer from question 16 into micrometers.

FOVscanning = _____ µm

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5. Remove the ruler, and return it to the bin on the side lab bench.
6. Turn the high-power objective lens into place.If you did this in person, you would notice that the field of view is less than 1 mm (you wouldn’t be able to measure it with a metric ruler). Instead of measuring the field of view under high power with a ruler, it is more practical and accurate to obtain the diameter indirectly by using a simple calculation. Watch my introductory video to learn how to make this calculation.

Question 19. Calculate the FOV under high power in microns (µm). You answer from question 18 will be used in your calculation.

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Question 20. Convert the diameter of your field of view under high power that you just calculated in question 19 into millimeters.

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Question 21. How did the FOV diameter change when you moved from scanning power to high power? (Did it increase or decrease?)

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Question 22. Is the relationship between magnification and FOV direct (both values increase together) or inverse (as one value increases, the other decreases)?

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1. Structure and Function

Introduction

Two words you will come across many times in a biology course are structure and function. Our concern with these words goes beyond a simple dictionary definition According to Webster’s New World Dictionary, function is defined as; “the normal or characteristic action of anything; especially, any of the natural, specialized actions of an organ or part of an animal or plant.” Structure is defined as, “something composed of interrelated parts forming an organism or organization.” A third word we must consider is complementarity. When we say complementarity of structure and function, we expand the meaning of the words. Now we mean that the way something is built (structure) complements (adds to or benefits) its function.

Before we begin, let’s talk about a series of relationships found in the world of biology. The phrase ‘levels of organization’ refers to a series of relationships from small (very, very small) to large (very, very large) entities. We’ll start with the atom. See if you can figure how the terms below are related to each other.

atom

molecules

organelles

cell

tissue

organ

system

organism

population

community

ecosystem

If you figured out that each succeeding term is composed of the elements preceding it (meaning that the terms become more inclusive), then you were correct. As you might already know, atoms of various types make up molecules. Two atoms of hydrogen and one atom of oxygen make a molecule of water (H2O). One more example, six atoms of carbon, twelve atoms of hydrogen, and six atoms oxygen make one molecule of (C6H12O6) glucose

The next step is a big one. Putting together millions of molecules of many types in a particular way, we have a cell. Cells are special structures. They are the first and smallest forms of life. All living things are either a single cell or composed of many cells. This is the “cell theory,” a primary concept of biology. Another part of the cell theory is that all cells come from pre-existing cells.

Let us move on. If we put together a large group of similar cells, we have tissue. Millions of nerve cells make up nerve tissue. Millions of smooth muscle cells make up smooth muscle tissue and so on.

The correct combination of different tissues makes an organ. A leaf is an example of a plant organ. It is composed of epidermal cells, palisade cells, mesophyll cells (spongy layer), and vascular bundles. Other examples of organs which are composed of tissues are the heart, lungs, and the kidney. (The rest of the list on the previous page continues on with the type of relationships we have just described. You will be exposed to many of these relationships during the course.)

1. What is a Cell?

What is a cell really like? By looking at a slide of animal or plant cells do you get an idea of what a cell in a living organism is like? Hardly! First of all, you can’t see the thousands of things going on in an ordinary cell because with an ordinary light microscope they’re too small or so thin that they’re transparent.

Although there are hundreds of thousands of different kinds of cells in the living world, they all work in the same general way. Sure, some have special parts so they are able to perform special functions, but all cells have basic parts which make them similar.

How many different makes (kinds of motor vehicles) can you think of? How many kinds of chairs or hammers? We see here the concept of different types of models, but each has basically the same parts modified for different purposes.

What is it like inside of a cell? Is there a lot of space, is it dark, cold, dry? Well, with very few exceptions there are no “empty” spaces in cells. For the most part cells are filled with a liquid a little thicker than milk and not as thick as syrup. Of course, there are exceptions to this also.

You know how a pinball machine lights up and bells ring and scores flash on the screen when the pinball hits a post. Well, this is similar to a living cell. At any one instant in a living cell there are hundreds of thousands of complex chemical reactions occurring. If each of these chemical reactions gave off light and sound like a pinball machine, it would probably be like the Fourth of July Light Show at San Diego Stadium.

1. Microscopic Examination of Cell

Remember that all the cells you are observing have three dimensions: length, width, and depth. Keep in mind that you are initially only seeing two dimensions. The third dimension (depth) may be appreciated by moving your fine adjustment as you watch the image through your ocular.

1. Animal Cell (Human Cheek Cell)

Procedure:

Simple animal cells are easily obtained from your own body. As described below, prepare a wet mount (a temporary preparation used when the observations are expected to be completed within a single laboratory period).

Watch the following video while reading steps a-d (below): Preparing a Cheek Cell Wet Mount Slide

1. Slide: Wash and dry a glass slide. Obtain a new toothpick from the front table. Using the flat end of the toothpick gently scrape the inside of your cheek. You will not see much on the toothpick but spread what is there onto your slide.
2. Stain: Cover the specimen on the slide with a small drop of methylene blue. (This is a stain that will make the cells more visible). Next to the stain, add a small drop of tap water from the dropper bottle provided. Stir the drops together with your toothpick.
3. Coverslip: Carefully cover the preparation with a clean plastic coverslip as follows:

Place one edge of the coverslip near to the drop. The stain and water with which you mixed the cells will flow along the junction of the edge of the coverslip and the slide. Carefully lower the coverslip over the specimen keeping the edge of the coverslip in contact with the slide. In this way, the water will flow slowly and uniformly about the specimen and force out air bubbles from beneath the coverslip. (A few air bubbles are not a serious problem for your first slide.)

1. Excess fluid: If liquid spills out from under the coverslip, gently blot the excess with a paper towel, so that it will not later drip onto the stage of the microscope

Read steps e and f (below) and refer to page 14 of the virtual images to see human cheek cells as they appear under scanning and low power magnification. Refer to page 15 of the virtual images to see a human cheek cell as it appears under high power.

1. Scan cells: Observe the cells through the microscope, beginning with the scanning lens. When you look at the cells they will be separated and spread out over the slide. Recognize that most of the white circles or ovals on your slide are likely air or water bubbles. Some small groups of cells will stick to one another in little blue aggregates. Move your slide (using the stage knobs) to where there are lighter blue areas.
2. Higher magnification: To see the cells well, move up to low power and then to high power. (Don’t forget to use only your fine focus knob for low and high power objectives. Adjust your iris diaphragm to change lighting). Notice the darker-stained oval nucleus in each cell.

Question 24. Cheek cells have been described as having a modified “fried egg” shape: flat except for the small lump at the nucleus. Can you tell that the cells are flat? Look for cells overlapping one another. Also, the edges of some cells may be “folded” back. Draw a cheek cell, and label the cell membrane, nucleus, and cytoplasm. This is covered in my introductory video as well. Also an image is shown on page 15 of the virtual images.

Make your drawing by hand, take a picture and paste it in place of this text. From the menu above, choose Insert > Image.

Question 25. Refer to page 16 of the virtual images. Recall the calculation for the FOV diameter under high power. Estimate the size of a single cheek cell (at its widest point). One way to look at this is to estimate how many of these cells (side-by-side) it would take to cross the entire diameter of the field of view. Then divide the FOV by the number of cells that it would take to cross. The equation below looks weird, but it is just the diameter of the FOV under high power divided by the number of cells that you estimate would fit across.

Width of single cheek cell = FOVhigh . = ______ µm

Number of cells across diameter

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2. Plant Cell (Onion storage leaf cells)

Watch the following video while reading steps a-f (below): Preparing an Onion Epidermal Tissue Wet Mount Slide

Procedure:

Make a wet mount slide of your onion cells in a fashion similar to the one with the human cheek cell.

1. Slide: Wash and dry a glass slide and coverslip.
2. Obtain your peel.Cut a piece of red onion away from the remainder. If the onion is in a baggy, return the unused portion of onion into the baggy. Bend the onion until you see a layer of membrane separate from the other parts of the onion. (This is the membrane in between the layers. It’s thin and transparent.) Place the layer on your slide and use a razor to cut it from the rest of the onion.
3. Stain: Optional: If you are using a white onion, add a small drop of methylene blue to the onion. If you are using the purple part of a red onion, then you don’t need the methylene blue, just add a small drop of water.
4. Coverslip: Carefully, cover the preparation with a clean plastic coverslip as directed above with the human cheek cell.
5. Excess fluid: If liquid spills out from under the coverslip, gently blot the excess with a paper towel, so that it will not later drip onto the stage of the microscope.

Read steps g and h (below) and refer to page 18 of the virtual images to see onion cells as they appear under scanning and low power magnification. Refer to page 19 of the virtual images to see onion cells as they appear under high power.

1. Scan cells: Observe the cells through the microscope, beginning with the scanning lens.
2. Higher magnification: To see the cells well, move up to low power and then to high power. (Don’t forget, you may use the coarse and fine adjustment knobs to focus on the scanning lens, but use only your fine focus knob for low and high power objectives. Adjust your iris diaphragm to change lighting). Notice the darker-stained oval nucleus in each cell (note this is not easily seen in the virtual images).

Question 26. Onion cells (and plant cells, in general) look a bit more like a brick wall. The cells may overlap one another, depending on whether you accidentally folded the membrane peel. Draw a few onion cells, and label the cell wall, nucleus, and cytoplasm for one of them. Refer to the image on the right side of page 18 of the virtual images (under low power) or the images on slide #12 of the introductory presentation for your drawing.

Make your drawing by hand, take a picture and paste it in place of this text. From the menu above, choose Insert > Image.

Question 27. Why do the onion cells appears “blocky”, rather than rounded like the animal cell? Think back to Module 5 “Cell Structure”.

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Question 29. Refer to page 19 of the virtual images. Recall the calculation for the FOV diameter under high power. Estimate the size of a single onion cell (along its longest edge).

Length of single onion = FOVhigh . = ___________µm

Number of cells across diameter

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Additional Cells for Study

Protist Cell (Paramecium)

Paramecium is a single-celled organism belonging to the Protist Kingdom commonly found in freshwater environments. Even though it is composed of only one cell, paramecium has a highly complex structure. One may observe such activities as ingestion of food, locomotion, excretion of waste, as well as certain interesting structural features such as the hair-like cilia which cover the surface of the cell.

Watch the following videos: Paramecium and Paramecium Eating to answer questions 30-32.

Question 30.Draw a Paramecium. Label the cell membrane. You may or may not see evidence of the cilia on the cell membrane. Label where the cilia would be.

Make your drawing by hand, take a picture and paste it in place of this text. From the menu above, choose Insert > Image.

Question 31. Can you see movement inside the Paramecium? Does there appear to be a single location where the yeasts enter the cell? This is called the oral groove. What is the oral groove used for?

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Observations of a Mixed Live Culture

Use the following videos: Protists-Amoeba, Euglena, Paramecium, Amoeba, and Euglena to answer question 33 and to see what some common single-celled protists from a wet mount of pond water look like.

Procedure:

1. Prepare a wet mount of the live culture (“pond water” or “hay infusion”) at the front table. Use one drop of “Proto-Slo” or equivalent with one drop of the culture.

Question 33:Draw the shapes of two different organisms. Identify the organisms if you can.

Make your drawing by hand, take a picture and paste it in place of this text. From the menu above, choose Insert > Image.