Biology: You must be this small to ride this ride

What does the cell membrane have in common with filters, sifters, and colanders? All of the above are SEMI-PERMEABLE, meaning some substances can pass through them, while others can't.



Today we used a plastic bag to model the cell membrane. We filled the bag with corn starch solution and submerged the bag in a jar of iodine. The idea was to see which if any molecules could pass in or out of the bag. Since iodine is an indicator for starch, a color change inside or outside the bag would demonstrate which molecules had traveled through the bag.

Turns out bigger isn't always better. It's the smaller molecules, like iodine that can pass through the bag, while a large molecule like starch is going nowhere fast! We concluded by relating this to the importance of digestion. Our cells can't process large molecules like starch, so we need to break them down into smaller ones, like sugar, that can be transported more easily through the cell membrane.