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Bio10 Lab 2: Cells
Cells are the smallest living things–and all living things are composed of cells. They are
able to perform all necessary metabolic functions as well as specialized tasks such as moving,
feeding and reproducing. Despite their minute size, cells are amazingly complex and often very
beautiful.
There are two general types of cells:
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Prokaryotic cells
o Lack a nucleus, but they do contain DNA
o Also lack other organelles, such as mitochondria, chloroplasts, etc.
o Are simple unicellular organisms like bacteria.
o They represent earth’s oldest organisms, having evolved about 4 billion years
ago
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Eukaryotic cells
o Do contain a membrane-bound nucleus and other organelles
o Most organisms are eukaryotic–all protists, fungi, plants and animals are in this
group.
Using your text and the cell models and posters in the lab, sketch an animal cell and a plant
cell on the group results sheet.
Activity 1: Looking at selected eukaryotic cells
Microscope
Eyepieces
Objective
lenses
Coarse focusing
(outer) knob
Stage
Fine focusing
(inner) knob
Illuminator
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Focusing the Microscope:
1. Position slide on stage; use the clips to hold the slide in place.
2. Using the 4x objective lens and the coarse focus knob, bring the specimen into focus
3. Center the specimen and switch to the 10x objective lens. Focus using the fine focus
knob
** Note: the course focus knob can only be used with the 4x objective lens!**
4. Center the specimen and switch to the 40x objective lens. Focus using the fine focus
knob
Calculating the total magnification
• To determine the total magnification, multiply the magnification of the eyepiece lens
(10x) by the magnification of the objective lens (4x, 10x, or 40x).
Examine an animal cell (your cheek cell) under the microscope
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Gently scrape the inside of your mouth with a toothpick. Smear the cells on a clean slide
and stain them with a drop of methylene blue (being careful not to stain your fingers).
Place a coverslip on top.
Discard the toothpick and slide in the biohazard container.
Beginning on low power (4X), locate some cheek cells. Center them in the field of view
and increase the magnification to 10X and then to 40X. You should be able to see some
flat cells with a tiny, dark speck inside the cell – this is the nucleus. Are we prokaryotic or
eukaryotic organisms?
Draw your cheek cells on the Group Results Sheet.
Examine a plant cell under the microscope
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Make a wet-mount slide of Elodea leaves (an aquatic plant often used in fish tanks).
Take just a small sample from the growing tip of the Elodea, place it in a clean drop of
water and add a coverslip. Do NOT add stain.
Begin on low power … you know the drill.
Sketch some Elodea cells on the Group Results Sheet.
Activity 2: Osmosis and Water Balance
In this activity, you will be looking at the diffusion of water (aka osmosis). Remember that
passive transport is driven by a concentration gradient and will continue until equilibrium is
reached provided the solute (e.g. salt) does not encounter any barriers it cannot pass through
(like a cell membrane)
Salt exists in water as sodium ions (Na+) and chloride ions (Cl-). Charged ions like these are
unable to diffuse through most biological membranes. However, water molecules can diffuse
through biological membranes so salinity imbalances are corrected by the diffusion of water
across the membrane to equalize salt concentrations on both sides of the membrane. The
diffusion of water across a selectively permeable membrane is known as osmosis.
Consider the case of a membrane separating two solutions with different concentrations of a
solute–say salt (NaCl). Solutions with equal solute concentrations are said to be isotonic to
each other. A solution with a higher concentration of solute is hypertonic to the other solution.
The solution with a lower solute concentration is hypotonic to the other solution.
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Note that the hypotonic solution has the lower solute concentration, but also the higher water
concentration. Therefore water will diffuse across the membrane along its concentration
gradient from an area of higher water concentration (the hypotonic solution) to one of lower
water concentration (the hypertonic solution).
Water balance in animal cells. In order to survive, marine and freshwater organisms must
have a way to balance an excessive uptake or excessive loss of water. Organisms living in
seawater must have a means of preventing the loss of water from their body to the highly saline
environment. Freshwater organisms must deal with the opposite problem of preventing
excessive amounts of water from the freshwater environment entering their more saline bodies.
The control of water balance is called osmoregulation.
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The POTATO Activity
1. In your lab groups (4 students), core out 4 equally sized pieces of potato. Keep the
potato(s) that you take the cores from at your table.
2. Take 3 empty beakers and fill them with the salt water concentrations provided (0.5%,
3.5%, 7.5%). Fill them enough to cover the potato.
3. Place one piece of potato into each filled beaker.
4. Wait 10-20 minutes, and while you’re waiting go to the Elodea activity (below)
5. Remove the potato cores from the solutions and try to reinsert them back into the potato
that you removed them from.
6. Based on how easy it is to reinsert the core, complete the data sheet below.
"Feel" of
Reinsertion
What do you
think happened
to the cells?*
Is the solution hypo,
iso, or hypertonic to the
potato cells?
Solution A
(0.5%)
Solution B
(3.5%)
Solution C
(7.5%)
* Are the potato cells larger? Did they shrink? Did they stay the same size?
The Elodea Activity
1. Each lab group should take a single leaf of Elodea and place it on a microscope slide.
Place a few drops of freshwater on the leaf and place a coverslip over it.
2. Observe it under the microscope using the 40X objective. Draw it on the Results Sheet.
3. Take a few drops from the 7.5% salt water beaker and place them at one edge of the
cover slip. At the same time, place a piece of paper towel at the adjacent edge of the
cover slip. This will cause the salt water to flow through the cells of the Elodea plant and
get soaked up by the paper towel.
4. After 1-2 minutes, observe the Elodea cells and draw your observations on the Group
Results Sheet.
5. Repeat steps 3 & 4 using regular tap water instead of the salt solution.
6. After 1-2 minutes, observe the Elodea cells and note any changes that you observe.
Illustrate.
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Names ___________________________
___________________________
Lab 2 Group Results Sheet
1. In the space below, diagram two eukaryotic cells–an animal cell and a plant cell. Using
your text as well as the cell models and posters in the lab, label the following structures
on your drawings.
Cell membrane
Mitochondria
Cell wall
Nucleus
Chloroplast
Central vacuole
Animal cell
Plant cell
2. Which of the structures listed above are only found in plant cells?
3. In the space below draw your cheek cells using the 40X objective. Label the cell
membrane, nucleus and cytoplasm.
a. What is the total magnification? TM= _____X
4. Sketch some Elodea cells in the space below. Label the cell wall, central vacuole and
chloroplasts.
a. TM= _____X
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5. What do you think happened to the potato cells in each of the solutions? Did they get
bigger? Shrink? Say the same? Why?
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Solution A (0.5%)
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Solution B (3.5%)
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Solution C (7.5%)
Which solution is hypertonic?
6. What would happen to a red blood cell placed in a hypotonic solution? Why?
7. Based on your results, do you think potatoes can osmoregulate successfully? Why or
why not?
8. Elodea Cell Drawings (Please note the total magnification for each image)
Original wet mount
After 2 drops of 7.5% salt
water
After 2 drops of tap water
9. Did water move into or out of the Elodea cell when the salt water solution was applied?
Explain your reasoning.
10. Did water move into or out of the Elodea cell when the fresh water solution was applied?
Explain your reasoning.
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