Dandelion Curls

It's spring in the northeast and that means (at least in my yard): Dandelions!

Did you know you can use dandelion stems to teach a simple (and pretty fun) lesson in osmosis as well as introducing the terms hydrophilic and hydrophobic?

Separate the stem from the flower and pull the stem into long strings.

Drop the strings into a tub of water and watch the stems curl up into all kinds of fun shapes! 

 If you drop the stem pieces one at a time, you can actually watch the curling process take place within just a minute or two.  Or you can dump a whole bunch in and have fun sorting through the results!

 What's happening?

The inside of the stem is hydrophilic, which is sometimes referred to as water-loving.  It's the part of the plant that absorbs the water.  And when it's placed into a tub of water, there's a whole lot of water to absorb!  The water moves into the cells through the process of osmosis. 

The outside of the stem is hydrophobic - it repels water.

The cells that make up the inside of the stem absorb so much water that they swell up.  The cells on the outside of the stem stay the same size.  The increasing size of the cells on the one side of the stem forces the stem into curls of various shapes.  

There's definitely something fun about sitting outside on a warm day and watching the curls form!  And if you can't be outside, grab some dandelions on your way to school and bring a bit of the outdoors in for your students.   

PS The idea of one side expanding more than the other side is similar to the way a bimetallic strip in a thermostat works.  The expansion is caused by temperature instead of water movement and it isn't as drastic as this, but it's conceptually similar. 

Osmosis: Should I Salt My French Fries Before or After Cooking?

Cut a potato into 4 sticks, about 1/2 cm thick (5 mm or about 1/4 inch).  You could try other shapes as well.

Fill two beakers or small bowls with water.

Dissolve salt in one of the beakers of water, about 1 tablespoon per cup of water.

Place 2 potato slicks in each beaker and allow to sit for about an hour (you can leave it overnight if need be).

After the time has elapsed, pick up the potato slices and observe.

You should find that the potato slices that were in water are limp and bend easily, which the ones in pure water are still rigid and crisp. 

Through osmosis, water moved out of the potato and into the salt water. 

Comparing Photosynthesis and Cellular Respiration

Understanding photosynthesis and cellular respiration are major concepts middle school students should master.

Here is a way to provide students with the information in yet another way, using foldables.  Two foldables are pictured, but they contain the same information, they care just oriented differently.

Each foldable uses two pieces of paper, folded so there are flaps.

Cut through the top three layers, at the middle of the book.

The top layer gets a title, "Cellular Respiration" on one side, "Photosynthesis" on the other side.

The next layer is labeled "What Cells" or "Where". 

The next layer is labeled "Ingredients" or "Reactants".

The bottom layer is labeled "Products". 

The corresponding information is filled in on each layer, for each process.  When finished, students have a nice study guide for comparing and contrasting photosynthesis and cellular respiration.

Gummy Bear Lab

This is one of my all-time favorites!

I use this lab at the beginning of the school year, when we're reviewing measurement.  However, it is equallly apprpopriate for the study of osmosis.  (As fate would have it, I first did this lab with my 7th graders who go on to study life science, including osmosis.  When we got to osmosis, they made the connection back to our measurement study.  It was great, and I've never considered doing it any other way, or with any of my other classes).

The procedure is simple enough....

Each student gets a gummy bear*.  The gummy bear gets measured thoroughly: length, width, height and mass.  Volume and subsequently density can be determined.

The gummy bear then spends a night in a cup of water.

When the students return the next day, the bears get measured once more (after students get over the shock of seeing their newly enlarged gummy bear).

Conclusions are drawn.

In my experience, this lab leads to all kinds of questions for further experimentation... What if I place my gummy bear in Coke/tea/milk/etc?  What if I leave my gummy bear in the water for 2 days?  What if I allow my newly enlarged gummy bear to sit out for a day?  What if I use a gummy worm instead of a gummy bear?  If you have the time and resources, it's a great opportunity for students to design their own experimental process and carry it out. 


*I tried to use a gummy worm one time, when I had some at home.  It didn't work, it completely fell apart.  Fortunately, it was just me playing around at home.  For that reason, always do a test run on the gummy bears you plan to use with your students.  You really want a gummy product that's going to hold up, at least for the initial experiment.

Cell Processes: Modeling Endocytosis: The Jelly Bean Problem

I just refer to this activity as modeling endocytosis, but you might not want to refer to it that way, as it may give too much away to your students.  

It comes from the Access Excellence collection and is named The Jelly Bean Problem.

In short, the students are trying to get a handful of candy into a plastic bag following these rules:

• The candy must enter through a solid part of the bag.
• The inside of the bag may not be directly open to the external environment.
• The candies entering the bag must remain clustered together.
• Students may work with their hands in the bag to act as the inside of a cell.
• The candy may be eaten only if it enters the bag "cell" under the specified conditions. 

Check out the original version for a nice illustration of how to accomplish this goal.

It's fun, candy is always an attention-getter, and it relates directly to cells.

Cell Processes: Acting Out Mitosis

In this activity, students use simple props to carry out the process of mitosis.  It takes a little time to get the props ready, but they provide a valuable lesson and help students understand this sometimes "mysterious" process.  All materials can be picked up at a hardware store. 

These mitosis props were based on the instructions found in Chromosome Shuffle.  Please refer to those instructions for more complete information.

You will need to make at least 2 pairs of chromosomes.

For each pair of chromosomes, you'll need two dowels cut to the same length.  Wrap a length of self-adhesive Velcro (the soft side) around the middle of each dowel (or use a hot clue gun to attach regular Velcro).  Slip a length of tubing over the dowel*.  Mark the genes using colored tape (or paint). Screw in the eye half of a hook & eye closure to the middle of the dowel. 

Keep the rough side of the Velcro handy for connecting the chromosomes.

To make the spindles... attach the hook part of the hook & eye closure to the end of a 6 foot length of string.

Two lengths of rope to create the nuclear membranes.  You might also wish to use a larger length of rope to make the cell.  I tend to skip this part - I find the extra rope gets in the way more than it helps.   

Start with a cell with the nuclear membrane in tact.  Paired chromosomes are in the nucleus.

Nuclear membrane is removed.

Chromosomes line up.

Spindles attach to the chromosomes.

Chromosomes are pulled apart, to opposite sides.

Nuclear membranes reform. 

You could also make the corresponding chromosomes to use when studying meiosis. 

*In the orginal instructions, she uses two sizes of tubing - one that goes directly on the dowel, othe other, a larger size that slips over the first tubing.  It is the largest tubing that gets the genes.  This allows you to demonstrate things such as crossing over, etc.  I skipped the larger tubing, because my classes don't go into that kind of detail.  As a result, I could have skipped the tubing altogether and just painted the stripes on the dowels, but I didn't consider that at the time. 

Mitosis Line Up

Find a set of sketches that illustrates mitosis in as much or as little detail as you'd like.

Cut apart the images and glue each one on an index card (I actually used half an index card for each). 

Have students place the cards in order. 

To allow students to self-check, number the back of the cards. 

I really try to focus on the process of mitosis and the order in which things happen.  We talk about the names for each of the phases, but it's most important to me that my students know what needs to happen first, second, etc. 

Students can  have head-to-head races to see who can put their cards in order first. 

Students can also play a game in which they need to be the first one to collect all six cards and put them in the proper order.  Two students will need two sets of cards to play.  Shuffle the two sets together.  The first student draws the top card and places it in front of him.  The second student follows suit.  The first student then draws another card.  He then has to decide whether it goes before or after the first card he drew.  If he draws a card he already has, it goes to the bottom of the stack and it's the second player's turn.  If a player decides that he's placed the cards in the wong order, he can use a turn to move one card (he does not draw a card during that turn). 

Osmosis: Egg-speriment

A classic demonstration of osmosis using a large cell: an egg.

Place an egg in a beaker filled with vinegar for a day or two (over a weekend works well). The vinegar will remove the shell of the egg, leaving the cell membrane. Additionally the process of osmosis will begin with the water found in the vinegar.


Place the egg in a variety of other substances, for one night each.


Vinegar – water flows into the egg, increasing its size.
Water – water flows into the egg, increasing its size (but not as dramatic after a day in the vinegar)
Colored Water – water and dye particles flow into the egg, increasing its size and changing its color
Corn syrup – water flows out of the egg, decreasing its size


You can make the experiment as complex or simple as you wish. You can present this activity as a demonstration, or have students complete it as a lab experiment. Have students measure and record the egg’s dimensions by wrapping a string around the egg’s circumference. Students could also keep track of the volume of liquid that is placed in the beaker and how much remains the following day. After students have recorded data, they can graph it.

Make sure you have antibacterial cleaning supplies ready and available if you take on this lab, especially if you have students working with the eggs – some will break!

Diffusion: Food Dye in Water

A very simple way to illustrate diffusion. Place a clear container filled with water on a table in the front of the room. Add a drop of two of food dye to the water. Point out to the students how dark and concentrated the dye is initially. The dye will spread out until it has reached an equal concentration throughout the entire container.

Cellular Respiration: Respiration in Yeast

Place some yeast, sugar, and warm water in a flask (or bottle with small neck). Quickly place a balloon over the flask opening and allow it to sit for the class period (or longer).

At the end of the period, you will find the balloon has inflated. It is filled with carbon dioxide released during cellular respiration.

You can prove that it’s carbon dioxide and not oxygen: light a wood splint or popsicle stick on fire and then blow it out so that it’s just glowing. Release the contents of the balloon onto the glowing splint. If it’s oxygen, the splint will return to burning; if it’s carbon dioxide, it will go out.

A note about my pictures... In the picture of the initial set-up, the bottle only has a little water in it. It wasn't enough - the yeast respirated, but the carbon dioxide they emitted took up the remaining space in the bottle and didn't make it to the balloon. I redid the experiment, filling the bottle much fuller, which resulted in an inflated balloon; but I didn't retake the set-up picture.

Photosynthesis: Photosynthesis Races

Teams of students (or individuals) race against each other to assemble the equation for photosynthesis. As students master the equation, they move from words to chemical symbols.


Carbon dioxide + Water + Light--> Oxygen + Sugar
6 CO2 + 6H2O + Light --> 6O2 + C6H12O6



Use index cards to create the equation components. You will need a set for each team you plan to have compete at one time.

Set one:
Carbon dioxide
Water
Light (Sun)
Oxygen
Sugar
-->
+ (x3)

Set two:
CO2
H2O
Light (Sun)
O2
C6H12O6
-->
+ (x3)

Depending upon the level of your students or how much of a challenge you wish to present, you can also provide the coefficients for students to put in the correct places.

I like to have each team begin with the words. The teams each work together to properly assemble their equation (remember, it doesn’t matter which order the reactants and products are in, just that they are on the proper side of the arrow).

As soon as a team claims to have completed the equation, I check it for accuracy. While I am checking, the other team can continue to assemble their equation in case the first team is incorrect. If they are incorrect, both teams continue to assemble the equation.


Once a team has correctly assembled the equation, the winning team moves on to chemical equations while the losing team continues to use the words.

This game can be played very quickly and is a good way for student to learn the equation for photosynthesis. By providing one additional card – ATP – and removing one card – Light – you can have students create the equation for cellular respiration. You could set up a whole tournament for your class to crown a photosynthesis champion!

Photosynthesis: The Big Green Mixing Bowl

A demonstration to magnify the process of photosynthesis.

You’ll need:
A large green mixing bowl
A flashlight
2 zip-top bags
1 labeled “O2”
1 labeled “CO2”
Sugar
Water (in a cup)
Large spoon

Before class begins, inflate each of the bags (blow them up and quickly seal them). Place the bag of O2 and the sugar inside the green mixing bowl. Don’t let the students see the inside of the bowl.

For the demonstration:
Photosynthesis takes place in the chloroplasts, represented by the large green bowl. Ask for volunteers to name the “ingredients” needed for photosynthesis: CO2 and water. As the reactants are named, add each to the mixing bowl (I pour the water in and empty the contents of the CO2 bag into the bowl). Photosynthesis also requires the presence of light, so shine the flashlight into the bowl while you give it a stir. Ask for volunteers to name the products made during photosynthesis: O2 and sugar. As each of the products are named, pull them out of the mixing bowl.