Microscope Use and Cell Observation
Objectives: To teach students what a compound microscope is and how to use one.
To observe both animal and plant cells under a microscope and to identify
cell membrane, cell wall, and nucleus.
Grade Level: 4th Grade
Materials:
Compound Microscopes (1 per 2 students)
Glass Microscope Slides (1 per student)
Cover slips
Water with eye dropper
Methylene Blue stain
Toothpicks
Onion (red work well)
Bowl with weak Clorox solution (for disinfecting slides and toothpicks)
PowerPoint Presentation "Microscope Use"
Optional:
Live protozoa (Euglena, Paramecium, Amoeba)
Leaf lettuce
Prepared slides of blood
Other prepared slides
Procedure:
Introduction to the Microscope
This activity was done at The University of Mississippi in a biology laboratory
where compound microscopes and lab tables were available, but it can be done
anywhere the microscopes can be set up.
To begin this activity, the students should be introduced to microscopes. Explain
to them what a microscope is and a bit about how it works. A PowerPoint presentation
works well for this. You may want to discuss other types of microscopes and talk about
when each would be used (dissecting, SEM, TEM). Once the microscope has been
introduced as a useful piece of equipment, show the students how to use them. Start
out by introducing each part of the microscope, describe how it works and what it does
(for example, the oculars (eyepieces) are magnifying lenses that make the transition
between the specimen, objectives and the eye). At this point the students should be
able to look at and manipulate a microscope in front of them. It is easier for the
students to understand if they can see the parts and manipulate them rather than just
being told how to work it. This will save time and frustration when the students are
trying to look at real specimens. Parts that are most important for the student to know
include: oculars, objectives, focus knobs, stage, base, light and the slide clips. Other
parts can be added, but for simplicity these are the only ones included in the
presentation. As the parts are introduced, take the time to demonstrate proper focusing
procedures, use of different objectives to get more or less resolution and discuss
magnification.
Introduction to Cells
Cells are the fundamental units of life, because a cell is the simplest unit capable
of independent existence and all living things are made of cells. Some organisms are
made up of one cell (unicellular, amoeba) and some are made up of many cells
(multicellular, animals, plants). Cells are made up of 90% water and may contain
several different types of internal structure. Prokaryotic cells (bacteria) have a nuclear
region but no internal membrane system and are very tiny. Eukaryotic cells (protists,
fungi, plants, animals) are usually larger, contain a nucleus and have several internal
membrane bound structures called organelles (eg nucleus). For this activity the
students will look at different types of eukaryotic cells including plant and animal cells.
Animals are multicellular organisms that are heterotrophic (obtain food from
external sources). Animal cells are surrounded by a cell membrane that contains all of
the cell internally. The cell membrane is a phospholipid bilayer that is semi-permeable
(that is it lets some molecules in/out and not others). The cell membrane is not a rigid
structure and unless some force (other cells, internal or external matrix) contains it, it
cannot maintain its shape and will often expand to its greatest perimeter (circle). Plant
cells are surrounded by a cell membrane but are also surrounded by a cell wall made of
cellulose that gives the cells structure. These cellulose cell walls are the building blocks
for wood and fiber in plants. Both animal and plant cells have a nucleus. The nucleus
is a membrane bound structure in the cell that contains much of the cell's DNA. DNA is
a molecule that contains the code for building the proteins all organisms need to grow
and reproduce. The nucleus is usually visible in a cell at moderate magnification (40100x). Plant cells also have chloroplasts that may be visible. These are the structures
that contain chlorophyll (photosynthetic pigment) and are therefore green. Also visible
in plant cells may be vacuoles (open spaces). Other internal structure in plant and
animal cells are usually too small to be seen at the magnifications that a studentcompound microscope has.
Slide Preparation and Observation
Human cheek cells are a fairly large, easily obtained example of animals cells for
observation with a microscope. Because these are obtained from the students' mouths,
care should be taken to adequately disinfect materials that come into contact with the
cells. Each student should:
1. Scrape cells from the inside of their cheek. This can be done with the side of
a toothpick. This should be done by carefully wiping the toothpick against the
skin on the inside of the cheek (do not gouge out chunks of cheek!).
2. Smear the cells onto a slide by wiping the toothpick across the slide.
3. Air dry the cells onto the slide by gently blowing on the slide.
4. Place a small drop of methylene blue stain in the center of the slide (where
the cells were smeared).
5. Cover the stain with a cover slip.
6. Place a fairly large drop of water adjacent to one side of the cover slip and
using a paper towel as a wick, draw the water under the cover slip towards
the other side. This will remove excess stain, leaving behind the cheek cells
stained blue.
7. Place the slide (cover slip up) onto the stage of the microscope.
8. Observe cheek cells at different magnifications.
9. Have the students draw what they see under the microscope, labeling cell
membrane and nucleus. They should also note the magnification (remember
that the total magnification is the ocular x objective).
10. Place all toothpicks and used slides into a bowl with weak Clorox for
disinfection. Rinse and re-use.
Onions are an easily obtainable source of a single layer of plant cells. This can
be done as a demonstration or individually by the students.
1. Remove the outer papery skin of an onion and then carefully peel back the
first "meaty" layer. Under this layer should be a very thin, translucent
membranous layer that can be peeled off the second "meaty" layer (usually
white, not red). Pull approximately 1cm2 piece of this thin layer from the
onion.
2. Place a drop of water onto a clean slide.
3. Place the onion membrane piece on top of the water and add a small drop of
water on top of the membrane.
4. Cover with a cover slip.
5. Place the slide under the microscope and observe onion cells at different
magnification.
6. Have the students draw what they see under the microscope, labeling cell
walls and nucleus. Because the onion bulb is not the photosynthetic portion
of the onion plant, the cells will not have visible chloroplasts.
7. Have the students write a short paragraph comparing the onion and cheek
cells and hypothesize as to why the onion would need a cell wall and the
cheek cells don't.
Extensions:
1. Use a piece of leaf lettuce (peel a thin layer) to see other types of plant cells
(puzzle-piece shaped) and chloroplasts.
2. Show a prepared blood smear to demonstrate red and white blood cells.
3. Show living protists (Amoeba, Euglena or Paramecium) to demonstrate
unicellular organisms.