LAB: Part 1- Microscope Measurement of Cells

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LAB: Part 1- Microscope Measurement of Cells

Background:

Objects viewed through the microscope are too small to be measured using units such as the inch or the centimeter. Therefore, biologists use smaller units of measurement called the millimeter (mm) or the micrometer (µm). One centimeter (cm) equals 10 millimeters (mm). One millimeter (m) equals 1000 micrometers (µm).

Magnification causes us to lose the idea of actual size. You cannot hold a ruler up to a microorganism or plant cell while it is under the microscope. Therefore, the size must be measured indirectly- that is, it must be compared with the size of something you already know.

In this lab, you will measure your field of vision, while looking through your microscope, using an ordinary metric ruler. Armed with this information, you will then be able to estimate the sizes of microscopic objects without using the ruler. As an example, supposed you were asked the height of a person. You might say 5 feet 10 inches to 6 feet. This is an estimate. In a similar way you will be estimating the sizes of various microscopic objects. Most human cells, for example, are between 10 µm and 20µm in diameter, although some are larger. A bacterium that lives in your gut, E. coli, is about 1-3µm in diameter.

Objectives:

In this activity you will:

1.  Convert from millimeters to micrometers

2.  Determine the size of your microscope’s field of vision, in millimeters and micrometers, for all powers, and;

3.  Estimate the sizes of objects under the microscope

Pre-lab Questions

1.  The length of this paper is 11. What is wrong with this statement?

2.  How many millimeters (mm) are in 2 centimeters (cm)?

3.  How many micrometers (µm) are in 2 millimeters (mm)?

4.  a. If the diameter of the field of view on a particular microscope is 3 millimeters (mm) and a specimen took up ½ of the diameter, then how large would the specimen be in millimeters (mm)? (show your work)

b. How large would the specimen be in micrometers (µm)?

Materials

Compound microscope Cork Cells Prepared Slide of Blood Cells

Ruler Prepared slide of monocot Onion

Slides Iodine Droppers

Cover Slips

Part 1: Measuring and Conversions

Procedure:

1.  Look at the ruler without using the microscope. Notice that the smaller lines are spaced 1 millimeter (mm) apart.

2.  Place the ruler on the stage and focus on a small line using the 4x objective lens.

3.  Adjust your microscope to view under the 10x objective lens. Your objective and ocular lenses should be on 10x each. This gives you a total magnification of 100x (10x X 10x).

4.  Focus on a small line. Then, while looking through the microscope, move the ruler until one of the vertical lines is at the left edge of the field of view and one of the horizontal lines goes across the middle of the field of view, as illustrated below.

5.  Remembering that the lines are spaced 1 mm apart, estimate the diameter of the field of view when the magnification is 10x (total 100x). Express the diameter to the nearest tenth of a millimeter.

Convert the diameter above to µm below.

6.  Focus the slide on high power (40X objective lens). Can you use this slide to estimate the diameter when the magnification is 400X? Why?

7.  When you switch from low power to high power do you see more or less area? This is known as an inverse relationship- as one factor increases the other decreases.

8.  As you’ve noticed, the high power field is less than one millimeter in diameter, so it cannot be measured with the ruler. The diameter can be calculated mathematically. Check with the teacher to be sure the answers to 4 and 5 above are correct before proceeding.

9.  Since the magnification of the objectives is inversely proportional to the field size, you can use this formula to determine the high power field diameter:

High power field diameter = low power magnification

Low power field diameter high power magnification

Substitute the values you know into this formula to calculate the high power field diameter. SHOW YOUR SUBSTITUTIONS AND CALCULATIONS.

______= ______

10. Under low power, focus on a prepared slide of a monocot stem. The center of a monocot stem is filled with large pith cells. How many pith cells can fit across the diameter of the 100x field?

11. To calculate the diameter of a cell, divide the diameter of the low power field diameter (refer to #5 above) by the number of cells. Estimate the size of a monocot's pith in micrometers and record. Show your work. Size of cell=

12. Switch to high power (40x objective) and focus using the fine adjustment. How many pith cells fit across the diameter of the high power field of view?

13. To calculate the diameter of a cell, divide the diameter of the high power field diameter (refer to #9 above) by the number of cells. Estimate the size of a pith cell in micrometers and record. Size of cell=

14. Compare the answers in number 11 and 13. These answers should be similar, although they probably won’t be the same. (Remember, these measurements are estimations). If the answers are not similar, ask for help before proceeding. Why should the measurement of one cell be similar under both low and high power?

15. Observe a Monera prepared slide. To estimate the length of one Monera, position it in the fields so that that one end is touching the edge of the field (see diagram below). Next, estimate how many Monera, placed end-to-end, would extend from one edge to the other directly through the middle of the field. Divide the diameter by this number. Express your answers in micrometers. Show your work.

Low power # monera:

Low power field diameter

Length of monera at low power (show your work):

16. Switch to high power. Estimate the length of the Monera and record.

High power # monera:

High power field diameter:

Length of monera at high power (show work below):

17. Make a wet mount of a hair of about 1 cm long. Position the slide so that the hair runs from the top to the bottom of the field. Estimate the width of the hair under low and high power using the same method used for the Monera.

# hair widths at low power: # hair widths at high power:

Low power field diameter: High power field diameter:

Width of hair at 100X (show work) = Width of hair at 400x (show work):

Which of these answers do you think is more accurate? Why?

Analysis questions: Please answer in complete sentences.

1.  What happened to the field of view as the magnification increases?

2.  Your hair should have the same width at all powers. Explain.