Click here for a blank answer sheet.
Cells
Time
The part of this laboratory dealing with osmosis in potato cells takes 60 minutes after the experiment is set up. You may wish to begin this part before doing other parts of the lab. Click the link to go to that section of the lab.
The artificial cell part takes 40 minutes after it is set up.
Parts of this exercise can be done at home. It is recommended that you watch your time and, if necessary, work on these parts after completing the rest of the lab exercises.
Introduction
All organisms are composed of cells. Cells are the smallest structures that are living; they are the unit of life. Two very different kinds of cells exist in nature. Prokaryotes are the simplest kind of organisms (example: bacteria). Their cells lack many of the structures (organelles) typically found in more complex cells. All other organisms contain cells that are considerably more complex. These organisms include all of the plants, animals, fungi, and protists.
The plasma membrane is vitally important in regulating the passage of materials into and out of the cell. We will see that small cells have a large surface to volume ratio, thus, more plasma membrane to service it's contents.
The plasma membrane is differentially permeable, that is, some molecules such as water can pass through but others cannot. We will study some characteristics that result from this property.
On the Web
Both of the web sites below will help you learn about the structure of cells.
Visit The Cell Page and explore the various parts of the cell by clicking on the structure name. (http://sun.menloschool.org/~cweaver/cells/index.html)
Visit the Cells Alive web page (http://www.cellsalive.com/)
Select Cell Biology.
Select Plant and Animal Cell Models, then select Animal Cell.
Click on cell structures in the drawing or click on the name of the structure to view information for that structure.
Prokaryotic and Eukaryotic Cells
The diagram below shows the evolutionary relationship between prokaryotes (bacteria and archaea) and eukaryotes (protists, fungi, plants, and animals).
Click here to go to the Biology 102 chapter on prokaryotes.
Examination of Eukaryotic Cells
We will examine an organism called amoeba as an example of a eukaryotic cell.
1. Prepare a slide of live Amoeba. Use a dropper to obtain a sample from the bottom of the culture jar. There may be a wheat seed on the bottom of the jar. Try to obtain a drop from the bottom near the seed. If live Amoeba are not available, observe a prepared slide of Amoeba.
2. Identify the nucleus and pseudopodia. Note that the cell is much larger than the prokaryotic cells (above) and is filled with numerous organelles. The functions of some of these organelles will be discussed later.
3. Draw an Amoeba below and indicate the magnification used.
Below: Amoeba proteus, 200X. The second photograph was taken several minutes after the first. Click on the photographs to view enlargements.
Click on the photographs to view enlargements.
Structure of Eukaryotic Cells
1. Use drawings of a typical animal cell in your text book to identify the structures in the list below. In the table on the answer sheet, state the function of each structure. If you are unsure, click on the word below to find the answer.
2. Draw and label a typical animal cell in the space provided on the answer sheet.
Cheek Cells
1. If you have not observed human cheek cells in a previous laboratory exercise prepare a wet mount by using the following procedure.
Scrape the inside of your cheek with a toothpick and rub it on a dry slide.
Add one drop of methylene blue to stain the cells. This will make them easier to see.
Place a cover slip on the slide as described above and observe the cells under low power then high power.2. Identify the nucleus.
3. Draw a cheek cell on the answer sheet.
Below: Human cheek cells 100X. Click on the photograph to view an enlargement.
Diffusion and Osmosis
Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. The movement is due to molecular collisions, which occur more frequently in areas of higher concentration.
Diffusion
Diffusion in a Liquid
1. Obtain a Petri dish and add enough water to cover the bottom.
2. Place the dish on a ruler so that the metric scale crosses the center of the dish.
3. Allow the water to remain still for one minute, then add a crystal of potassium permanganate to the center of the dish.
4. Measure how far the molecules diffused after 10 minutes. Distance should be measured from the crystal to the edge of the purple area (or diameter of the purple area/2).
5. Calculate the rate of diffusion per hour.
6. Observe the dish at the end of the laboratory period. Did the potassium permanganate diffuse throughout the entire dish by the end of the laboratory period?
Diffusion in a Gelatin
Several drops of dye have been added to tubes containing a clear gelatin.
1. Obtain one of these previously-prepared tubes and measure how far the dye diffused.
2. Record the number of hours that the dye has been diffusing.
3. Calculate the rate of diffusion per hour.
4. Obtain a tube that had been prepared last semester. What happened?
Osmosis
The center of cell membranes contains the nonpolar fatty acid tails of phospholipid molecules. Because of this large nonpolar area, charged particles and large polar molecules cannot diffuse across the membrane. Small polar molecules such as water can diffuse across the membrane.
Osmosis is the diffusion of water across a differentially permeable membrane (see "Diffusion" above).
It occurs when a solute (example: salt, sugar, protein, etc.) cannot pass through a membrane but the solvent (water) can. Water always moves from where it is most concentrated (has less solute) to where it is less concentrated.
In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.
In the container on the left side of the diagram, water will enter the cell because it is more concentrated on the outside. In the center drawing, water is more concentrated inside the cell, so it will move out. If the solute concentration is the same inside as it is out, the amount of water that moves out will be approximately to the amount that moves in.
Osmotic pressure is the force of osmosis. In the diagram above, the cell on the left will swell. The pressure within the cell is osmotic pressure.
An Artificial Cell
1. Cut a piece of dialysis tubing approximately 10 cm in length.
2. Clamp one end of the tube.
3. Rinse the tubing under water so that it can be opened.
4. Fill the tube 1/2 full with 50% molasses solution.
5. Clamp the other end of the tube. Be sure that you leave plenty of room in the tube for water to enter.
6. Rinse the tube under water and let it drip for about 10 seconds to remove excess moisture.
7. Place a plastic weighing tray on the scale and zero the scale. Place the bag in the plastic and record its weight to the nearest 0.1 g. Do not place the bag directly on the metal weighing pan of the scale and do not drip liquids on the scale because this could damage the scale.
8. Place the bag in a beaker containing distilled water.
9. Weigh the bag again every 10 minutes (after 10, 20, 30 and 40 minutes). Be sure to use a plastic weighing tray and to zero the scale before placing the bag in the tray.
10. Record your data in the table in the answer sheet.
11. Plot your results using a computer graphing program such as Create A Graph.
12. Did the rate of gain appear to be constant? You can answer this question by seeing if the graph is a straight line.
13. What do you predict would happen to the bag after one day?
Potato
1. Cut two strips of potato about the size of a French fry. They should be no thicker than 0.5 cm.
2. Put one of the strips in a test tube that contains enough 10% NaCl to cover the potato.
3. Put the other strip in a test tube that contains enough distilled water to cover the potato.
4. Remove the strips from the test tubes after about 60 minutes and examine the potatoes. Is one of them limp? Is one firm? Record your observations in the table on the answer sheet.
5. Explain your observations on the answer sheet.
Plasmolysis in Elodea
1. Prepare a wet mount of an Elodea leaf using 10% NaCl instead of water.
2. Observe the cells under scanning and low power immediately after you prepare the slide.
3. Keep the slide under the microscope and observe it periodically for 10 minutes. It may be helpful to use a brighter light to view the cells. As the cells contents shrink, the chloroplasts will appear to clump together.
4. Describe what happened to the cells.
5. Why did this happen?
Below: Elodea cells in 10% NaCl. 400X, 200X. Click on the photographs to view enlargements. Notice that the contents of the cells have shriveled into a ball. The cell wall is rigid and cannot shrink. Click here for further explanation.
Blood
1. Use a toothpick to place a small amount (approx. 1/2 drop) of sheep blood or rabbit blood on each of three slides. Be careful not to use too much blood because there will be too many cells to see anything under the microscope.
2. Add a drop of 0.9% NaCl to one of the slides, a drop of 10% NaCl to a second slide and a drop of distilled water to the third.
3. Put a cover slip on each and observe the cells under high power beginning with the 0.9% NaCl slide..
4. Record your observations and an explanation why they occurred in the table in the answer sheet.
Rabbit blood mixed with 0.9% NaCl X 400. Rabbit blood mixed with 10% NaCl X 400. Notice that the cells appear to be caved in or otherwise distorted due to a loss of water.
Review Questions
Complete the review questions on cells.