Investigation 11
How Do Molecules Pass Into and Out of
Cells?
Introduction
You have discovered that single-celled organisms are surrounded by a cell membrane that serves as a barrier
between the cell’s interior and the external environment. For the organism to survive, however, food and water
molecules must pass through the cell membrane into the interior, and waste molecules must pass out. The same
situation holds true in a multi-cellular organism. Your digestive system is designed primarily to break up food
into molecules small enough to pass into your body cells through their cell membranes. Likewise, waste
molecules must be able to leave cells to be carried away by the circulatory system. But how do molecules
actually pass through cell membranes?
In this investigation, you will test the effect of two independent variables (molecular size, and concentration of
molecules) on the passage of molecules through a cell membrane and attempt to create a theory of membrane
transport.
Objectives
1. To investigate variables that influence the passage of molecules into and out of cells.
2. To create a theory to explain the passage of molecules through cell membranes.
Materials
various concentrations of starch solution
droppers
various glassware
water
cellulose tubing
balances
distilled water
Lugol’s Iodine
Procedure
1. A water molecule is made up of two hydrogen atoms and one oxygen atom as shown below. A sugar
molecule is made up of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms- which is know as glucose.
When glucose molecules join with other glucose molecules, they form long chains that loop around. These
long, looping chain molecules are known as starches.
Notice that a single molecule of glucose is about ten times the size of a single molecule of water; thus, a single
molecule of starch is many, many times larger than a molecule of water. Do you believe that the size of a
molecule and/or its concentration might affect its movement through a cell membrane? If so, why? Record
your ideas.
2. Design two experiments to test your ideas, using the materials provided. You may wish to discuss your
ideas with your teacher or classmates before you proceed. In any event, using the dialysis tubing, you will need
to construct a “model cell”. Keep in mind that when starch solutions mix with iodine they turn purple- so you
may use this indicator to judge molecular movement. Record your experiments below: (10pt)
3. What alternative hypotheses are you going to test? (20pt)
4. What are your expected (predicted) results? (10pt)
6. What are the independent and dependent variables in your experiments? (10pt)
7. Conduct your experiments. Record your results below: (10pt)
8. Following the class discussion, summarize your results and those of others. Did a consistent pattern emerge
from the data? If so, what was it? (10pt)
9. Describe the appearance of a hypothetical cell membrane that is consistent with your experimental
observations from this lab. What is the best answer your data will allow you to give to the question What
causes molecules to pass into and out of cells? (20pt)
Part II
1. Obtain a microscope, slide, cover slip, and the appropriate liquids/droppers you wish to use (salt and fresh
water).
2. As demonstrated, place a thin sheet of onion skin on the microscope slide. This represents a single layer of
cells. Draw the onion skin (in color) below: (20pt)
3. While looking through the scope, apply a drop of salt water to the skin. You need to be watching exactly
when this happens, so you might need your partner to do this for you. Draw what you see (in color) below.
(20pt)
4. Now, repeat the procedure, using distilled (fresh) water. Once again, you need to be watching as this
happens. Have a partner help if you might have difficulty. Draw what you see (in color) below: (20pt)
Part III
1. Observe the three tubes at the front of the classroom. They contain blood, which is made up of thousands of
tiny red cells. Your instructor will be placing one of three types of water in each tube: distilled, drinking, and
salt water. Use the space below to draw what you predict each tube will look like once mixed with each type of
water. Use color (30pt)
2. Use the space below to draw (in color) what each tube actually looked like: (30pt)
Study Questions
1. Given what you know about diffusion, would you expect lightweight or heavy molecules to move faster?
Explain. (10pt)
2. Drinking salt water will make a person thirsty. If a person continues to drink salt water, he or she will die.
Explain why a person gets thirsty when drinking salt water. (10pt)
3. What effect do you think a rise in temperature would have on the rate of diffusion? Explain. (10pt)
4. What happens when red blood cells are placed in distilled water? Explain. (10pt)
5. Molecules tend to move from an area of high concentration to an area of low concentration. Why? (20pt)