NAME: ______DATE: ______BLOCK: _____

OBJECTIVE:

v  To observe transport across the membrane of yeast cells and to differentiate between the types of transport taking place.

PROCEDURE:

1.  Put approximately 20 drops of yeast suspension into each of 2 test tubes.

2.  Add four drops of Congo Red Stain to each tube of yeast and mix by swirling the test tube.

3.  Place one of the tubes containing yeast and Congo Red in the hot water bath and boil for 10 minutes.

4.  While you are waiting for the yeast to boil, use a plastic pipet to put one drop of unboiled yeast/Congo red mixture on a slide. Cover the drop with a cover slip and observe the yeast mixture. Record the color of the yeast cells in Table 1. After the yeast have boiled for 10 minutres, repeat these steps and make a slide with one drop of the boiled yeast/Congo red mixture.

5.  Observe and compare the boiled and unboiled yeast cells under medium and then high power of the microscope. (Yeast cells appear as very tiny circles. Large circles with dark outlines are air bubbles!)

6.  Record the color of the yeast cells in Table 1 on the next page.

NOTE:

This lab concerns the transport of materials across a cell membrane. The lab has nothing to do with the movement of the yeast cells. Yeast cells have no means of movement. They appear to be moving only because the water molecules are moving. In the boiling water, the water molecules and therefore the yeast cells are moving faster due to the heating. This movement has nothing to do with this lab! FOCUS ON THE COLOR OF THE YEAST CELLS IN THE TWO TUBES!

TABLE 1

Sample / Observations (color of cells)
Yeast
Boiled Yeast

QUESTIONS:

1.  What does boiling do to the live yeast cells? (What would boiling do to most living organisms?)

2.  How do the boiled yeast appear different from the unboiled yeast cells? Describe the difference in color between boiled and unboiled yeast.

3.  Are the cell membranes of the yeast permeable to Congo Red? ______

How do you know?

4.  What are the unboiled cells able to do that the boiled cells could not do (in terms of Congo Red)?

5.  Was this lab an example of active or passive transport? Think carefully about this question and explain your answer.