Download Lab 3 – The Cell

Lab 3 – The Cell
Learning Objectives
1.
Use a microscope properly to identify various types of cells.
2.
Identify numerous structures and organelles within specified cells.
3.
Distinguish plant from animal cells and describe their similarities and differences.
4.
Recognize that cells demonstrate a relationship between their structure and function.
Procedures
The cell is the basic unit of life. It is the cell with its intricate organization that possesses all the
properties and processes that we call "living." A functioning organism may be a single cell (such
as bacteria or protozoa), or an extremely complex organization of millions of specialized and
highly coordinated cells.
It is important to recognize the wide diversity of form among cells, and how closely the structure
of each cell is related to its function. In other words, the structure of a cell can help one identify
what function it performs. In turn if one knows the function performed, the structure of the cell
can be predicted. In these exercises, you will look at a variety of different kinds of cells, and
attempt to correlate their structure with the various functions they perform.
You will also examine the internal structures in cells. The cell is a complex organization of subcellular structures called organelles which perform particular functions. As you go through the
lab, take note of which organelles are unique to plant or animal cells, and which they have in
common.
PLANT CELLS
A.
Onion cells
Cut or peel off one layer of onion from an onion bulb. Break the layer in half and peel off a
single, thin piece of epidermis from the inner surface. The outer surface may contain only
dead cells. Place the epidermal tissue on a slide, cover it with 1-2 drops of iodine and lay a
coverslip over it. Examine the cells under all three magnifications of your microscope.
The onion cells are unspecialized, non-green plant cells. They are bordered by a cellulose
cell wall, which surrounds an inner plasma membrane. This plasma membrane encloses the
granular-looking cytoplasm, nucleus, and one or more vacuoles. Although membranebound vacuoles take up most of the internal space, they are not easily visible.
While observing the tissue under high power, sketch three adjacent cells and make your
drawings of each cell at least 3 cm in length. Label the cell wall, cytoplasm, nucleus,
nucleolus and indicate the position of the plasma membrane with an arrow.
1. What do you think is the function of these cells? (hint: onion cells
contain starch)
B.
Elodea cells
Elodea is an aquatic flowering plant. Using forceps, remove a leaf from near the tip of the
stem. Place it on a slide, add 1-2 drops of water and cover with a coverslip. Observe the
rectangular-shaped cells under all three magnifications and draw one cell under high power
(at least 3 cm long). The oval green structures you see are chloroplasts. Label them as well
as the cell wall. Indicate the location of the plasma membrane with an arrow.
2. What is the function of chloroplasts?
3. Why don't onion cells contain chloroplasts?
In some cells, you may be able to observe the circulating cytoplasm moving the chloroplasts
around the perimeter of the cell. This process is called cytoplasmic streaming or cyclosis.
4. What might be the function of cytoplasmic streaming?
C. Guard cells
A unique plant cell is present in the epidermis of many leaves. Take a geranium leaf and
strip off a small piece of the lower epidermis by tearing the leaf at an angle. Prepare a wet
mount of the epidermis using water and a coverslip. First observe the cells under low power.
You will see two types of cells: (1) epidermal cells that are irregularly shaped like pieces of a
jigsaw puzzle and (2) very small, doughnut-shaped structures that are each composed of two
guard cells. Each guard cell is shaped like a kidney bean, thus each cell forms half of the
“doughnut.”
Now switch your microscope to the high power objective. Notice that the doughnut-shaped
feature produced by the two guard cells has an opening in the center. This opening is
actually a hole in the epidermis and is called a stoma (plural: stomata).
Draw a pair of guard cells (large drawing) that form the “doughnut” and a few epidermal
cells that surround them. Label the guard cells, stoma, chloroplasts and epidermal cells.
5. Describe one way in which the leaf benefits by having stomata.
6.
D.
Occasionally the guard cells collapse in the center and temporarily close the
stomata. Describe how the leaf benefits from having closed stomata.
Potato cells
You have already observed one type of plant cell plastid called a chloroplast (i.e. green
plastid). Potato cells contain plastids called leucoplasts (i.e. colorless plastid). Remember
that plastids are organelles not cells. You can observe leucoplasts by carrying out the
following procedures: (1) add 1-2 drops of water to a slide, (2) using a scalpel or razor blade,
cut a very thin slice of potato, the thinner the slice, the better your results will be, (4) cover
with a cover slip and (5) observe the leucoplasts with high power.
As you view the leucoplasts, carefully focus your microscope using fine adjustment only. You
should be able to see that some leucoplasts contain structures that have concentric rings.
These rings are evidence that the leucoplasts have starch grains.
Diagram a potato cell, and label the cell wall and leucoplasts
Draw iodine under the cover slip by placing a drop at one edge of the cover slip, and placing
a piece of paper towel against the opposite edge of the cover slip
7.
What change occurred in the color of the leucoplasts, and why?
8.
Would you say that the function of the potato cell is more like the Elodea cell or
the onion cell? Explain your answer.
ANIMAL CELLS
The cells which you will use in the rest of this exercise are animal cells, both human and those of
other animals. In the following exercises, make special note of how animal cells differ from the
plant cells you have already studied. Also note the basic shape of the cells and try to relate the
form of the cell to the function it performs.
A.
Human blood cells
Examine the prepared human blood slides under both low and then high power. Observe
both red and white blood cells - the red are round and lack a nucleus, white ones are
irregularly shaped and have a nucleus.
Make a drawing of a red blood cell, and label the plasma membrane.
Also draw a white blood cell, and label the plasma membrane and the nucleus. Use the
chart in the lab to identify the type of white blood cell you have drawn, and give its function
1.
Which are larger, red or white blood cells, and which are more numerous?
2. What is the function of human red blood cells?
3. What is the general function of human white blood cells?
B.
Human Epithelial Cells - cheek cells
Prepare a wet mount of a few of the cells that line your mouth. Gently scrape the inside of
your cheek with a toothpick and spread the scrapings across the surface of a slide. Add a
drop of iodine and cover with a coverslip.
Make a drawing of a cheek cell, and label the plasma membrane, nucleus and cytoplasm.