We inflated two balloons. In the first, we poured about 10 anise seeds. In the second, we added about five drops of anise extract. We sealed both balloons. After a short time, we sniffed the balloons. The balloon with the anise seeds didn't smell like anything other than the balloon plastic. But the balloon with the anise extract had a strong scent of anise coming from it. We wondered: why can we smell the anise extract through the ballon, but not the anise seeds? We tried thinking about it from the perspective of the "smell particles", the little molecules bouncing around in the air that enter our noses and tell our brains what kind of smell to decode. When we light a punk stick or incense in one corner of the classroom, it doesn't take very long at all – just a few seconds – before everyone on the other side of the room can smell the smoke. Why? And how does it relate to the balloons?
This was our introduction to diffusion, an important concept related to digestion. It actually gets to the reason why we have to break down foods in the first place. To understand diffusion, we have to understand how gas molecules move – and we can go back to our learning in Catastrophic Events, when we learned about land and sea breezes, to revisit how gas molecules behave. It will help us understand how diffusion works.
Our second investigation in Lesson 6 had to do with a model for cell membranes, or the outside walls of our body's cells.Investigation 6.1 Worksheet
We used a synthetic membrane that looked like a plastic tube-shaped bag. We tied one end shut tightly with twine, and left the other end open so we could add liquids to it.
We used two membranes and two test tubes. In the first setup, we added a measured amount of sugar solution to the membrane bag. We then added distilled water to a test tube and lowered the bag into the test tube so it was sitting on the bottom. The open top of the bag was draped over the side of the test tube. We repeated this for a second setup, but instead of sugar solution, we used cornstarch solution.
We let these sit for a time, then used pipettes to remove samples of the water from the test tube (not from the membrane bags). We tested these for sugar and starch. We found that there was sugar in the water outside the sugar bag, but nothing in the water outside the starch bag. This was evidence that the sugar could pass through the membrane bag, but the starch could not. We referred back to the balloon exercise, comparing our results, and tried to determine a characteristic of the sugar molecules that would give them the ability to pass right through the membrane, while the starch molecules could not. It helped to think back to what each molecule looked like: Starch is a complex carbohydrate, made up of simple sugars. Amylase breaks down this carbohydrate into smaller, simple sugars.
This is covered in more detail in the readings in Lesson 4 and Lesson 6 (available here on the blog!). The reading in Lesson 6 also covers the concept of active transport, which is how our body absorbs molecules and substances that are too large to pass through the membranes by diffusion (which is also called passive transport). Please let us know if you have any questions about this – it gets to the reason why our bodies break down nutrients during digestion!
