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.
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.
We will look at prokaryotic cells in this exercise but we will examine details of the structure of eukaryotic cells.
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.
Visit the Cells Alive web page (http://www.cellsalive.com/)
Select "Cell Models", then click "Take me to the ANIMATION".
Select animal cell. Click on cell structures in the drawing or click on the name of the structure to view information for that structure.
After you finish reviewing the structure of animal cells, select "Plant Cell" and review the structures.
After completing animal and plant cells, click "Cell Models" and then scroll down to "Take me to the BACTERIAL CELL".
Review the structure of a bacterial cell.
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.
The exercises below require the use of a microscope. Click here for instructions on using the microscope.
Examine a slide of bacteria (suggested slides: typical bacillus (rod-shaped bacteria) or typical spirilla (spiral-shaped bacteria) under high power (400 X). The bacillus cells are often attached end-to-end forming a long, threadlike structure composed of many cells. Draw a bacillus or spirillum cell. If you draw more than one cell (bacillus) identify a single cell. Write the name of the slide next to your diagram.
- Cyanobacteria are photosynthetic prokaryotes and may be connected in chains or filaments. Examine a slide of cyanobacteria such as Anabaena under high power (400X). Draw representative cyanobacteria in your notebook.
Below: Typical bacilli, typical spirilla, and anabaena (400X). Click on the photographs to view an enlargement.
We will examine an organism called amoeba as an example of a eukaryotic cell.
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.
Identify the pseudopodia. If you are also viewing a prepared slide that has been stained, you should also be able to see the nucleus. 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.
Draw an Amoeba in your notebook 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.
This part should be done last in case you run out of time to do the other parts of this lab. It can be completed outside of class if you run out of time.
- State the function of each structure. If you are unsure, click on the word below to find the answer.
rough endoplasmic reticulum
smooth endoplasmic reticulum
Draw a typical plant cell. Label the first 11 structures listed in the table above.
Draw and label a typical animal cell. Nine of the items listed above are found in animal cells. Draw and label these nine items.
Observation of a Living Plant Cell
Prepare a wet mount of an Elodea leaf. View the cell under low and high power. Use the fine focus to focus up and down on a cell. Cells above and below your cell may interfere with your viewing. Identify the cell wall, and chloroplasts. If your specimen is fresh, you should be able to see the chloroplasts moving within the cell.
Notice that there are few chloroplasts in the center of the cell. This space is occupied by the central vacuole.
Draw an Elodea cell and state the magnification used.
Below: Elodea 100X and 400X.
Click on the photographs to view enlargements.
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.
Identify the nucleus.
How do these animal cells differ from the Elodea (plant) cells? See your drawings of typical plant and animal cells to help with the answer to this question.
Draw a cheek cell.
Below: Human cheek cells 100X. Click on the photograph to view an enlargement.
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 in a Liquid
Obtain a Petri dish and add enough water to cover the bottom.
Place the dish on a ruler so that the metric scale crosses the center of the dish.
Allow the water to remain still for one minute, then add a crystal of potassium permanganate to the center of the dish.
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).
Calculate the rate of diffusion per hour.
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.
Obtain one of these previously-prepared tubes and measure how far the dye diffused.
Record the number of hours that the dye has been diffusing.
Calculate the rate of diffusion per hour.
Obtain a tube that had been prepared last semester. What happened?
The tube on the top was prepared at 9:00 AM on 3/5/07.
The bottom tube was prepared at 1:00 PM on 10/10/07.
The photograph was taken at 11:17 AM on 10/11/07.
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
Cut a piece of dialysis tubing approximately 15 cm in length.
Moisten the tube with water and then clamp one end. Plastic or foam clamps may be used. Instructions for using the foam clamps are below. Click an image to view an enlargement.
Twist the end of the dialysis tubing several times as shown in the photograph.
Next, fold the twisted area as shown Insert the folded end into the foam clamp.
Rinse the tubing under water so that it can be opened.
Fill the tube 1/2 full with 50% molasses solution.
Clamp the other end of the tube. Be sure that you leave plenty of room in the tube for water to enter.
Rinse the tube under water and let it drip for about 10 seconds to remove excess moisture.
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.
Place the bag in a beaker containing distilled water.
Weigh the bag again 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.
Record your data in a table in your notebook.
Plot your results using a drawing in your notebook. If your notebook does not have a grid, you can use graph paper or you can use a computer graphing program such as Create A Graph.
A) Be sure that you put the independent variable on the X axis.
B) A line graph is appropriate for continuous data. A bar graph is more appropriate for data that fall into categories with no intermediate points. In this case, time is continuous. We measured it in five increments but it is possible to have measurements in between the five.
Did the rate of gain appear to be constant? You can answer this question by seeing if the graph is a straight line.
What do you predict would happen to the bag after one day?
Cut two strips of potato about the size of a French fry. They should be no thicker than 0.5 cm.
Put one of the strips in a test tube that contains enough 10% NaCl to cover the potato.
Put the other strip in a test tube that contains enough distilled water to cover the potato.
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 your notebook.
Explain your observations. Be sure to mention where the concentration of water molecules is greater (and salt is less) and where the concentration of water molecules is less (salt is greater).
Below: The potato strip on the left was in fresh water and the strip on the right was in NaCl for 60 minutes.
Plasmolysis in Elodea
Prepare a wet mount of Elodea using 10% NaCl instead of water.
Observe the cells under scanning and low power immediately after you prepare the slide.
Let the slide sit for 10 minutes and observe the cells again. It may be helpful to use a brighter light to view the cells. As the cell shrinks, the chloroplasts will appear to clump together.
Describe what happened to the cells.
Why did this happen? To help you answer this question, consider where the concentration of water molecules is greatest and where it is least.
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.
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.
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.
Put a cover slip on each and observe the cells under high power beginning with the 0.9% NaCl slide..
Record your observations and an explanation why they occurred.
Below: Sheep blood in 0.9% NaCl. Notice that the cells appear to be spherical.
Below: Sheep blood in 10% NaCl. The cells appear to be irregular in shape due to the loss of water. You may need to use the fine focus and focus up and down to see the irregular shape of the cells.