Cell Membrane Experiments
Introduction:
Get ready to amaze your students and yourself with this experiment! Not only can you remove the shell of an egg without hard boiling it first but also you can make the egg shrink and swell! ! Besides all the fun and excitement, though, this experiment demonstrates a very important process that occurs all the time in our bodies to keep us alive. Our bodies are made up of cells that are responsible for every job our body does. But for our cells to perform all these jobs properly, they need to bring in certain particles from outside the cell (just the way that an individual person usually needs help from other people to do a big project). One way for particles to enter the cell and help it do its job is to simply pass through the cell membrane. All living cells are surrounded by a membrane which separates the stuff inside the cell (water, nucleus, etc.) from the stuff outside the cell (water, other cells, nutrients, etc.) This membrane decides what can enter the cell and what should stay outside. One way that it prevents certain particles from entering is by having holes only large enough to allow small particles to enter. It is therefore called a semipermeable membrane. An analogy could be made to a basketball net. Only certain size balls will get through. That is, a tennis ball will go through the hoop, a basketball will go through the hoop, but a big beach ball will not. The same holds true with cell membranes. Many particles, such as sugars and salts that are dissolved in the water surrounding the cell try to enter the cell, but they are too big and therefore cannot pass through the membrane. To demonstrate this principle to the students, do the following activity as a warm-up (taken from The Human Body for Every Kid, p. 17):
WARM-UP #1:
Materials:
Another concept that must be understand before attempting to comprehend the main experiment is diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration until the concentration in both areas is uniform. A simple yet educational demonstration of this principle is described below:
WARM-UP #2:
Materials:
Procedure:
EGG-SPERIMENT:
At last to the main part of the experiment -- here an egg will be the model for a cell. First, the shell must be removed to expose the cell membrane. Vinegar takes care of this job very easily because it is an acid that reacts with the calcium in the shell to remove the shell. (Evidence of the reaction is provided by the bubbles that form when the egg is dropped in vinegar). Once the shell is removed, the concepts of diffusion and osmosis can be illustrated. The egg will be transferred from the beaker with vinegar to a beaker containing corn syrup or some other highly concentrated solution such as honey. In a few days, the egg will actually shrink because all the water inside the cell wants to move to an area of lower concentration (i.e. outside the cell). (When I did this experiment, the egg actually shrunk from 17.7 centimeters to 16.5 centimeters from Wednesday night to Saturday morning. But even more dramatic was how the egg felt before and after being placed in the corn syrup.) Once the egg shrinks, the egg can be put into a new beaker containing water. Here, the egg will swell up again because now there is a higher concentration of water outside the cell than inside. The new size is even bigger than the original size of the egg. Incidentally, the reason that only the water diffuses across the membrane while the sugar in the corn syrup does not is that the sugar particles are too big to cross the membrane. That is what the first warm-up experiment showed. Examples of the relevance of osmosis will be given in the "Extensions" section.
Objectives:
Vocabulary:
cell
cell membrane
semipermeable membrane
concentration
diffusion
osmosis
circumference
Materials:
for each group of students:
Procedure:
Make sure the students can define the vocabulary given above.
Also have them answer the following questions to turn in. They should try to work individually at first. Then they can discuss their observations and conclusions with their group or the whole class.
BONUS: if you want to take this one step further, check this out!
Introduction:
Get ready to amaze your students and yourself with this experiment! Not only can you remove the shell of an egg without hard boiling it first but also you can make the egg shrink and swell! ! Besides all the fun and excitement, though, this experiment demonstrates a very important process that occurs all the time in our bodies to keep us alive. Our bodies are made up of cells that are responsible for every job our body does. But for our cells to perform all these jobs properly, they need to bring in certain particles from outside the cell (just the way that an individual person usually needs help from other people to do a big project). One way for particles to enter the cell and help it do its job is to simply pass through the cell membrane. All living cells are surrounded by a membrane which separates the stuff inside the cell (water, nucleus, etc.) from the stuff outside the cell (water, other cells, nutrients, etc.) This membrane decides what can enter the cell and what should stay outside. One way that it prevents certain particles from entering is by having holes only large enough to allow small particles to enter. It is therefore called a semipermeable membrane. An analogy could be made to a basketball net. Only certain size balls will get through. That is, a tennis ball will go through the hoop, a basketball will go through the hoop, but a big beach ball will not. The same holds true with cell membranes. Many particles, such as sugars and salts that are dissolved in the water surrounding the cell try to enter the cell, but they are too big and therefore cannot pass through the membrane. To demonstrate this principle to the students, do the following activity as a warm-up (taken from The Human Body for Every Kid, p. 17):
WARM-UP #1:
Materials:
- 1/2 cup salt
- 1/2 cup dry beans
- jar with lid (quart or liter)
- colander
- large bowl
- Pour salt and dry beans into jar. Secure with lid and shake back and forth several times to thoroughly mix the contents.
- Hold colander over bowl as a helper student opens the jar and pours its contents into the colander.
- Gently shake the colander up and down several times over the bowl.
- Observe the contents of the bowl and colander.
Another concept that must be understand before attempting to comprehend the main experiment is diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration until the concentration in both areas is uniform. A simple yet educational demonstration of this principle is described below:
WARM-UP #2:
Materials:
- ( 1 ) beaker or clear cup
- 1/2 cup of water
- 3-4 drops of food coloring
- paper and pencil to record observations
Procedure:
- Fill the beaker 1/2 way with the water.
- Add 3-4 drops of food coloring. Just let the jar sit. Do not swirl.
- Observe the beaker for a few minutes while recording what is happening to the color in the water.
EGG-SPERIMENT:
At last to the main part of the experiment -- here an egg will be the model for a cell. First, the shell must be removed to expose the cell membrane. Vinegar takes care of this job very easily because it is an acid that reacts with the calcium in the shell to remove the shell. (Evidence of the reaction is provided by the bubbles that form when the egg is dropped in vinegar). Once the shell is removed, the concepts of diffusion and osmosis can be illustrated. The egg will be transferred from the beaker with vinegar to a beaker containing corn syrup or some other highly concentrated solution such as honey. In a few days, the egg will actually shrink because all the water inside the cell wants to move to an area of lower concentration (i.e. outside the cell). (When I did this experiment, the egg actually shrunk from 17.7 centimeters to 16.5 centimeters from Wednesday night to Saturday morning. But even more dramatic was how the egg felt before and after being placed in the corn syrup.) Once the egg shrinks, the egg can be put into a new beaker containing water. Here, the egg will swell up again because now there is a higher concentration of water outside the cell than inside. The new size is even bigger than the original size of the egg. Incidentally, the reason that only the water diffuses across the membrane while the sugar in the corn syrup does not is that the sugar particles are too big to cross the membrane. That is what the first warm-up experiment showed. Examples of the relevance of osmosis will be given in the "Extensions" section.
Objectives:
- To understand one way that materials enter a cell
- To understand the function of the cell membrane
- To understand the principles of diffusion and osmosis
- To record careful and detailed observations of a scientific experiment
Vocabulary:
cell
cell membrane
semipermeable membrane
concentration
diffusion
osmosis
circumference
Materials:
for each group of students:
- (3) 250 ml beakers or big clear cups
- (1) egg (be careful handling the egg!)
- 250 ml vinegar
- 250 ml corn syrup or honey
- 250 ml water approximately
- 12" piece of string ruler paper and pencil to record observations
Procedure:
- Place egg into one of the beakers. (Be careful -- do not drop the egg in.)
- Fill beaker almost to the top with vinegar.
- Record observations immediately. What do you notice? Continue to record observations over the next three days. Be sure to note how the egg feels when you touch it each day. Don't squeeze too hard! It may seem disgusting to touch, but it actually feels quite amazing!
- Once the shell is removed (i.e. the egg is soft), gently rinse off the egg. Measure the circumference of the egg by wrapping the string gently around the middle of the egg. Hold your finger on the spot where the end of the string meets the rest of the string. Measure the distance between the end of the string and your finger with the ruler. Record measurement. Also record how the egg feels at this point. Transfer the egg to a new beaker.
- Fill this beaker almost to the top with the corn syrup or honey.
- Record the properties of the syrup (i.e. is it thick to touch? slow to pour? etc.).
- Each day for the next 3-4 days, record observations. First note if the syrup has changed any. Also. note how the egg feels. Compare these observations to those made in step 4 and step 6?
- Next, gently take the egg out of the beaker and rinse off. Remeasure the circumference as directed in Step 4. Record measurement. Place the egg into a new beaker and fill this one almost to the top with water.
- In several hours (it's okay to wait until the next class period), come back and record observations again. How does the egg feel this time? How does it compare to the observation in step 7 and step 4?
- Remeasure the egg as directed above. Record measurement. How does this measurement compare to the measurements in step 4 and step 8?
Make sure the students can define the vocabulary given above.
Also have them answer the following questions to turn in. They should try to work individually at first. Then they can discuss their observations and conclusions with their group or the whole class.
- Why was the shell removed first?
- How did the syrup change (see steps 6 and 7)? Explain why this change occurred?
- How did the egg change from the beginning of the experiment to the middle to the end. Explain why using the vocabulary above.
- What do you think would happen if the egg was placed in orange juice instead of corn syrup? Why?
- What do you think would happen if you repeated step 8 a couple of times except you used new corn syrup each time instead of the water? Why?
- What would happen to a plant if it was placed in a high concentration of salt water? Why?
BONUS: if you want to take this one step further, check this out!