THE EXPERIMENT
This lab is the first of several for investigating
biodiversity. Biodiversity is basically the variety of kinds
of critters-- high biodiversity means a sample has many different
types of critters.
Today's lab is about the first few steps of the
scientific method: asking questions and making preliminary
observations about biodiversity. You will be observing and counting microscopic critters
from various ponds or creeks or the rocks or sticks in them.
Using a microscope, you will try to find
and identify as many types of critters in a sample as possible. As you find the critters on your slides, use a table
to "virtually sort" them into different species, describing or drawing or naming
the different types.
As you record your observations for one sample, compare your
observations with other students' observations, especially those comparable to your sample only from a different sized pond or rock or whatever.
Do not tabulate critters smaller than 50 µm (=0.05 mm) long; in other words, ignore
critters which look like nothing more than a dot at the lowest
(weakest) power of your microscope. (However, you should record a note describing their presence and abundance; it could be
important for your work in Lab 4 or later.)
Identifying the species you find: The
goal is to figure out how many species you have in each sample. It's not necessary to
identify or name each species, but you do need to try our critter
ID key to figure out at least general categories, like "insect" or
"protozoa" or "algae." Then if you have two different types of
protozoa, you could just record them as "protozoa a" and "protozoa b."
If you have time and skill, however, you could use the reference sources on the
internet (linked to the critter ID key) to get more
specific.
| This week your job is to find how many
different species (=different kinds) of microscopic critters
you can identify an a sample from various creeks and
ponds. List the names (or
rough sketches or descriptions) of each species you find. You may want to group the
critters by category, like those on the example, or maybe by size or some other
characteristic. Next week (Lab 4) we will do more work
with critter ID. |
Example
of the pages you could finish today. You might add extra notes in another
column or in footnotes or whatever. Neatness does not count. Your
paper does not have to be yellow. |
| Sample #2: Critters
found in the smaller lily pond |
| category |
name
or
description or sketch |
size |
Number Found |
notes (optional) |
| algae |
diatom a:
diatom b:
Volvox?
eukaryotic filament
prokary. filament |
10-4
m
10-4 m but 1mm long
|
1
23
1
14
6 |
seems like
more diversity in algae than in animals in this sample |
| protozoa |
ciliated thing |
10-5m |
7 |
0.01mm |
| and you might want to make more notes
just in case you forget important observations before next week |
larger animals |
mosquito larva
roundworm
copepod |
1
mm?
10-4 m
|
1
1
5 |
Reading the following details before lab can make your job
easier:
Your grade for this lab will be based on
the hand-written observations (and ideas or insights) you hand in,
like the yellow charts above. A real biologist interested in biodiversity would keep
careful, detailed records of her observations. She would
also write down any ideas or insights (even half-baked ideas)
which could be relevant for writing a hypothesis about patterns
and causes of biodiversity.
How to use the
Bio103 microscopes
Thanks to several generous donors, all the microscopes in this lab are
almost brand new. You must be very careful to use both hands while carrying them, and you
must never touch the glass lenses with anything except lens paper, found in little
booklets in little drawers in all biology labs.
You will not be tested on the names of the parts of a microscope, but
you do need to know how to use the parts. We'll show you in lab, but
here's a (p)review:
First, you put a drop of the sample on a glass slide and cover the drop
with a small plastic cover slide. Figure out how to clip the slide into the mechanism
which moves the slide around the stage.
Turn on the light at the base of the scope, and note the disk under the stage
which controls how much light goes through the slide. Use the stage knobs to move the best
part of the slide over the light hole.
Rotate the lowest-power (red) lens (called "objective") until it is aimed down toward the slide and the light hole below it. Use the big
knob on the side of the microscope until the lens is almost touching the slide. Look
through the ocular lens (at the top) while reversing the big knob slowly from its closest point until
you see something. Note that you can improve the view three ways:
- The big knob
- The smaller "fine-tuning" knob
- Changing the light disk under the stage holding the slide.
These microscopes have three sets of lenses (red, yellow, and blue
"objectives) which can be rotated to increase the magnification with only minor
change of focus. When you have trouble, always start with the lowest power (red) to find
your critter. If you switch to the yellow and then the blue lens, you should need only the
smaller "fine-tuning" knob and maybe the light-adjustment disk.
In the lab we have more pictures and printed information, and actual
human help if you want it. For the true science nerd, there are some good web sites that
tell anybody much more than they need to know about microscopes:
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How to
estimate sizes of microscopic critters
 |
These photographs are various views of the
letter "e" and a transparent ruler. We'll go
over them at the beginning of the lab period. There's
space below for you to take notes.
|
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|
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The basic idea here is to judge the length of anything you see by
comparing it to something which can be measured. Your cover slide is about 20 mm wide; so
something that spans about a fourth of the cover slide is about 5 mm long. Notice that biologists almost always use metric system
measurements. If you're clueless, see table 1.1, textbook p.
2.
The smallest thing you can really identify with the naked eye is about 1 mm, although you
could probably see something five times smaller (0.2 mm or 20 mm or 20 x 10-6m).
When you look through the microscope, the diameter of the field of view (the circle you
see) can be calculated or even measured by focusing on a transparent ruler or grid of
known size. Check it out yourself.
| LENS |
ROUGH RULER: diameter of entire field |
smallest thing you can see well (1/100 of the field) |
| none |
|
0.2 mm (human hair) |
| red (4x) |
less than 5 mm |
0.05 mm |
| yellow (10x) |
less than 2 mm |
0.02 mm = 2x10-5 m |
| blue (40x) |
less than 0.5 mm |
0.005 mm =5x10-6 m = 5000 nm |
| (best possible light lens) |
|
(200 nm) |
| (electron microscopes) |
|
(<0.2nm) |
With the red objective, a critter which spans a fourth of your cover slide now
stretches all the way across your field of view. Something ten times smaller than the
diameter should be about 0.5 mm. You could check to see if the same object spans your
entire field with the blue objective.
0.005 mm =5 mm
(micrometers um) = 5000 nm = 5
x 10-6 m.
If the line above is only Greek to you, you're advised to check out
one of these helps for the metric system:
table 1.1, textbook p. 2.
http://www.ex.ac.uk/cimt/dictunit/dictunit.htm
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What you need to know from this lab for future tests
- Once you've finished this lab, you're supposed to have good understanding
of the sizes of cells (mostly 0.1 mm) and how few of their
parts you can see with a light microscope. The relative
size of things is an important aspect of understanding biology.
- You also should be able to compare light
microscopes with the electron microscopes described in the
textbook. (See the cell
tables).
- Be able to define these terms: biodiversity, protozoa, algae, metric system, µm and mm (both
measures of length; 1000 µm = 1 mm; 50 µm (=0.05 mm) is the size of a dot at the lowest
power of your microscope).
- You could be asked to identify or discuss the possible sources of
experimental error or bias in the procedures for collecting and counting critters.
- In the lab we also have some flash cards on biomolecules and
atoms for your reviewing pleasure.
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