Biology 115 Spring Semester: Lab 2 (Cells: Structure and Function)
This lab is designed to give you an opportunity to see that organisms are, in fact, composed of
cells. We will look at both of the major cell types – prokaryotes and eukaryotes. You will examine
organisms from the Kingdoms Bacteria (also known as Eubacteria), Protista, Animalia, and Plantae. As
you proceed with your investigation, consider the fact that you are a living organism, composed of the very
things you are observing, i.e. you are literally ‘cells looking at cells’.
Prokaryotic Cells
Cells (the smallest individual units of life) are divided into two basic categories: prokaryotic
cells, and eukaryotic cells. The cells of every organism apart from the bacteria are eukaryotic. Only
bacteria have prokaryotic cells. There are two kingdoms of prokaryotic cells, the Bacteria (Eubacteria) and
the Archaebacteria. These two prokaryotic kingdoms are so different from each other that they are actually
classified into different domains, the Domain Bacteria (contains only Kingdom Bacteria) and the Domain
Archaea (contains only Kingdom Archaebacteria). We will only examine the Bacteria today.
Prokaryotic cells differ from eukaryotic cells in that they lack a membrane-bound nucleus and
membrane-bound organelles. Prokaryotic cells are simpler in structure and are thought to have evolved
before eukaryotic cells. However, both prokaryotic and eukaryotic cells share many similarities. Both have
a plasma membrane, cytoplasm, DNA, ribosomes, and similar enzyme systems. And, like plants and fungi,
many prokaryotes also have a cell wall. Prokaryotic cells are generally much smaller than eukaryotic cells,
and thus require higher magnification to see (about 1000X). However, some of the prokaryotes you will
examine today are larger and all can easily be seen with a total magnification of 400X.
Bacteria can be classified by shape into several groups, three of which are: coccus (spherical
bacteria); bacillus (rod-shaped bacteria); and spirillum (corkscrew-shaped bacteria). Additionally, cocci
and bacilli can be classified by the structure of their cell wall. Today, you will examine Lactobacillus.
Cyanobacteria (blue-green algae) are also prokaryotic cells, often living in aggregated colonies,
that perform photosynthesis to obtain energy in fundamentally the same way that chloroplasts (the
photosynthetic organelles in plants) do. The photosynthetic pigment, chlorophyll a, is contained in
organized membranous systems called thylakoids. Today, we will examine 2 genera of cyanobacteria,
Oscillatoria and Gleocapsa.
Eukaryotic Cells
Eukaryotic cells have a membrane-bound nucleus and organelles. Today, we will look at three
kingdoms of eukaryotic cells with the microscope: Protista, Animalia, and Plantae. However, first you
should use the diagrams provided and the descriptions below to become familiar with cell structure prior to
looking at cells with the microscope.
Plasma Membrane
The plasma membrane surrounds each cell and regulates which materials enter and leave the cell. It
consists of a double layer of phospholipid and protein. In addition to the plasma membrane, some
eukaryotic cells, such as plant cells, also have a rigid cell wall surrounding the cell membrane. This serves
a structural function.
Nucleus
The nucleus of eukaryotes is a structure surrounded by TWO membranes. It is therefore known as a
nuclear envelope rather than a nuclear membrane. It contains the genetic material (deoxyribonucleic acid,
or DNA) and is therefore the control center of the cell. Also present in the nucleus are one or more nucleoli
(singular, nucleolus), where the subunits of ribosomes are manufactured. These subunits are transported
across the nuclear envelope and the ribosomes are assembled in the cytoplasm.
Ribosomes
Ribosomes are the site of protein synthesis in both prokaryotic and eukaryotic cells. They are sometimes
free in the cytoplasm. However, if the protein is to be secreted from the cell they synthesize the protein on
internal cellular membranes. This membrane is known as the endoplasmic reticulum. Areas of the
endoplasmic reticulum that have many associated ribosomes are called “rough” endoplasmic reticulum.
1
Smooth endoplasmic reticulum lacks attached ribosomes. The vesicles formed by the endoplasmic
reticulum contain enzymes and other proteins produced by the ribosomes. The endoplasmic reticulum then
transports the proteins to another structure, the Golgi apparatus.
Golgi Apparatus
The Golgi apparatus (or complex) is also a system of membranes that form small vesicles or cisternae. It is
in these vesicles that the proteins are modified and transported. The proteins may be transported to the
plasma membrane for secretion.
Vacuoles
Some organelles consist of little more than a membranous sac. These are vacuoles. In plant cells, vacuoles
are numerous, and occupy most of the cell’s interior. In plants, vacuoles contain water, sugars, and salts,
and serve as storage sites as well as structural functions. In animal cells, vacuoles carry waste and food
molecules, as well as other macromolecules and water. You will examine the contractile vacuole in
Amoeba.
Chloroplasts
The organelle that performs photosynthesis in plants is called the chloroplast. Chloroplasts take the energy
from the sun and store it in organic molecules (carbohydrates) with the help of a pigment called chlorophyll.
The chlorophyll is found in membranous thylakoids (remember the cyanobacteria?) inside each chloroplast.
Mitochondria
Although animals lack chloroplasts, both animals and plants have mitochondria. Mitochondria are the
organelles that use the carbohydrates (produced during photosynthesis and ingested by the animals) to
release energy.
Cytoplasm
The cytoplasm of the cell includes everything enclosed by the plasma membrane, except the nucleus. The
fluid portion of the cytoplasm (everything outside of the membrane bounded organelles) is called the
cytosol. The cytosol contains different types of fibers called the cytoskeleton. The cytoskeleton
contributes to the cell’s shape, helps to move the organelles around within the cytoplasm (this movement is
called cytoplasmic streaming, or cyclosis), and plays an important role during cell division.
PROKARYOTIC CELLS
Kingdom Bacteria (Eubacteria)
(a) Lactobacillus
Yogurt is a nutrient-rich culture of the bacterium Lactobacillus. This bacterium is adapted to live
on lactose (milk sugar). The bacterium converts the milk to yogurt, which is acidic and keeps longer than
milk. This bacterium has historically been used by people who are lactase (the enzyme which breaks down
lactose) deficient.
1. Place a tiny dab of yogurt on a microscope slide.
2. Mix this drop of yogurt in a drop of water, coverslip, and dry any excess fluid with a kim wipe.
3. Performing the steps that we went through last week, examine the slide with the compound microscope,
using the 10X objective and finally the 40X.

What shape are the cells of Lactobacillus (coccus, bacillus, or spirillum)?

Draw the cells in the space below (label all visible structures below, eg. Plasma membrane
cytoplasm, etc.).
2
(b) Cyanobacteria
Cyanobacteria are often surrounded by a mucilaginous sheath. Oscillatoria occurs as a filament of
cells and Gleocapsa occurs as a loosely arranged colony.
1. Prepare wet mounts of Oscillatoria and Gleocapsa.
2. Examine with the 10X objective, then the 40 X.

Draw and label the cells of each genus below.

Can you see any nuclei within these cells?

How many cells are held within one sheath of Gleocapsa?

How does the size of Lactobacillus compare with that of Oscillatoria and Gleocapsa?
EUKARYOTIC CELLS
Kingdom Protista
The protists we will examine today are all unicellular. However, multicellular protists do occur,
such as the giant kelp you may have seen in the ocean.
(a) Amoeba
Amoeba is an irregularly shaped protist that moves by means of pseudopodia (false feet). It uses
its contractile vacuole to excrete waste products and expel water.
1. The Amoeba will be at the bottom of the vial, so make a wet mount from this area. You may want to
examine them with the dissecting microscope first.
2. Examine them with the compound microscope, but only with the 4X and/or the 10X objectives
(otherwise you will squash them!!!). Do not press on the coverslip!
3. Decrease the light intensity and observe them for a few minutes.
3

Draw the Amoeba below. Make sure you label as much of the cell as you can possibly see,
including the pseudopodia.

Describe how the pseudopodia form.
(b) Paramecium
Paramecium is a protist that moves very quickly using cilia. This protist has two nuclei, the
macro- and the micronuclei.
1. Make a wet mount of Paramecium and examine with the compound microscope.
2. Make a second wet mount of Paramecium, but place a drop of protoslo on the slide first. Place the drop
of Paramecium culture on top of the protoslo.

Make a labeled diagram of Paramecium below.

Describe how these cells move (contrast it to the movement of Amoeba).
(c) Euglena
Euglena is a photosynthetic protist that moves via flagella.
1. Make a wet mount of Euglena and examine with the compound microscope.
4

Make a labeled diagram of Euglena below.

Describe how these cells move.

What color are these cells? Why?
Kingdom Animalia
Human Cheek Cells
Examine your own cheek cells using the method below.
1. Place a drop of water on a clean slide.
2. Gently scrape the inside of your cheek with a clean applicator stick to harvest several dozen (probably
hundreds) of cells
3. Stir the end of the toothpick in the water on your slide, add a coverslip and examine with the compound
microscope.
4. Dispose of the toothpick in the trash – DO NOT LEAVE THEM LYING AROUND!

Adjust the aperture diaphragm. What effects do these adjustments have on your image?
You can improve the contrast of the specimen by making adjustments to the aperture disk. Another
way to improve contrast is to use a stain such as methylene blue or iodine. Because cytoplasm is usually
clear or transparent, stains are added to improve visibility and increase contrast between structures.
Different stains are absorbed by different kinds of cells and/or specific organelles.
5
Remove the slide from the stage, and place one drop of stain at the edge of the coverslip. Using a
Kimwipe or a piece of paper towel, draw the stain under the slide by absorbing water from the edge of the
coverslip opposite the stain.

Describe any changes, other than the color, in the appearance of the cells.

Try adjusting the aperture diaphragm again. Does this adjustment have a larger effect on stained
cells?

Draw and label the stained cells under high power.
Kingdom Plantae
(a) Elodea
You will now examine a typical plant cell, from the leaf of the aquatic plant Elodea (Canadian pond weed).
1. Gently remove one leaf from near the tip of an Elodea stalk.
2. Place the leaf on a clean slide, and add a drop of water and a coverslip.
3. Return to your microscope and examine the leaf tip under low power.
Once you have centered the tip of the leaf in the field of view under low power, switch to the next highest
power objective.

How do the cells of the plant differ from the human cheek cells? Try to find the cell wall (a rigid
structure that surrounds the cell and encloses the cell membrane), and the chloroplasts (organelles
that contain the green pigment, chlorophyll, and perform photosynthesis).

Are the chloroplasts moving? If not, move towards the center of the leaf and watch for
movement of the chloroplasts (you may have to focus up and down through the leaf until you find
some moving chloroplasts....remember that the leaf is 3-dimensional and the microscope has a
limited depth of focus).
If you can find some chloroplasts that are moving, you are seeing an example of cytoplasmic streaming
(also called cyclosis). Cytoplasmic streaming is the movement of cytoplasm from one part of the cell to
another part of the same cell. It serves to transport different molecules to all parts of the cell, maintain
optimal light and temperature conditions, and (in some cases, although not in plants) cytoplasmic streaming
serves to help the cell move. All cells exhibit cytoplasmic streaming.
6
Note that the chloroplasts are moving around the outside of the cell, instead of moving around near the
interior. This is because most plant cells have a large central vacuole filled mainly with water. You will
probably not be able to see this vacuole directly, but you can infer its existence by noting the position of the
chloroplasts. Additionally, because of the central vacuole, the nucleus of plant cells is not located in the
approximate center of the cell, as it was in your cheek cells. Although you may not be able to see the
nucleus either (because it requires staining to see easily) you might be able to find its location by noting an
area in the cell where the moving chloroplasts seem to slide over an invisible “bump” next to the cell wall.

Draw and label a cell below. Include labels showing the central vacuole, the chloroplasts, the
plasma membrane, cell wall, and the area where the nucleus lies (if you can find it). Additionally,
include the magnification you used, and estimate the size of the plant cells (both length and width),
as well as the size of the central vacuole and chloroplasts.
Like cheek cells, plant cells are also bound by a plasma membrane. In normal plant cells, this
membrane is pressed tightly against the inside of the cell wall (because of the pressure from the water in the
central vacuole), and is impossible to pick out. However, if we can cause water to move out of the central
vacuole, the volume inside the cell will decrease and the cell membrane will pull away from the cell wall.
We will now try it.
 Take your slide and put a drop of 10% saline solution (salt water) on the edge of the coverslip just like
you did with the stain and your cheek cells. Now, like before, pull the saline solution under the
coverslip with a piece of Kimwipe. Return to your microscope and examine the slide under 100X.

Where are the chloroplasts located?

What do you think has happened? Can you hypothesize about what has caused the change in
appearance?
Adjust the aperture disk until you get an excellent image. Can you see the plasma membrane, or infer its
presence from the distribution of the chloroplasts?
7

Draw and label your observation of Elodea cells in 10% saline solution. Include the
magnification, the approximate sizes of the cells, the central vacuole, and the chloroplasts.
(b) Onion
1. Cut a piece of onion with a razor blade. Snap the piece backwards, then cut a small square of inner
epidermis and use forceps to remove it.
2. Place the epidermis in a drop of water on a microscope slide and cover slip.

Draw and label an onion cell below.
3. Stain the onion tissue with either a drop of iodine or methylene blue solution by placing a small drop of
stain at the edge of the coverslip.
 Do onion cells have chloroplasts? Why or why not?
8