Cell Membranes are Selectively Permeable!

It's been awhile since I last posted, but we've been quite busy in biology class since then.  Now that the end of the second marking period has come and gone, I have a little time to catch up on what we've been doing so far this January...

Our main topic this month has been how the cell membrane allows cells to maintain homeostasis- a state of balance between a cell and its environment.  We talked about the molecules that make up the cell membrane, including phospholipids, proteins, and carbohydrates.  We discussed how these components allow the membrane to be strong, yet flexible, as well as selectively permeable, meaning that it allows some materials to pass through it into or out of the cell while other materials are not able to pass through it.

We did a short lab activity to demonstrate the selectively permeable aspect of the cell membrane.  Students set up a 2 beakers with different solutions.  The students measured water into one beaker and added several drops of iodine to the water, turning it a dark brown color. Students then mixed two spoonfuls of starch with water in a second beaker.  They then poured about half of the starch solution from the beaker into to a ziploc baggie.  They placed the baggie with the starch solution into the first beaker with the iodine water and let it sit overnight.  I reminded students of something we learned in a lab earlier in the year- that iodine is a starch indicator and will turn a black/purple color in the presence of starch, and asked them to make predictions about what will happen to the solutions over a period of 24 hours.

A student measures water to add to two beakers.  One beaker will contain an iodine solution, the other will contain a starch solution.

Starch solution- half was poured into the baggie and half was left in the beaker!

The beaker on the left contains starch solution and the beaker on the right contains iodine solution.  Immersed in the iodine solution is a baggie containing starch solution.

The next day, we returned to our two beakers.  Students added a few drops of iodine to the beaker that contained starch solution, just to observe the color change that occurs when starch and iodine come into contact.  They then removed the baggie containing the starch solution from the beaker with the iodine water to observe what had happened over 24 hours.  Students observed that the starch solution inside the baggie had become purple, while the iodine water remained yellowish-brown.  How can we explain these crazy results?!

The starch solution in the beaker turns purple with the addition of iodine- a starch indicator!

In this picture you can see the starch clump in the left corner of the baggie has turned purple.  The iodine solution left in the beaker has not turned purple.  Iodine was able to diffuse into the baggie to turn the starch purple, but starch was not able to diffuse out of the baggie to turn the iodine purple!

Another view of the beaker of iodine solution and the baggie of starch solution

We remembered that cell membranes are selectively permeable- some materials can pass through them, but not everything!  It turns out that the ziploc baggie in this experiment acted in a similar way- iodine molecules are small enough to diffuse through the tiny, tiny holes in the baggie, so over the 24 hour period, they diffused through the baggie and caused the starch solution inside the baggie to turn purple.  However, the starch molecules are too big to diffuse through the baggie, so they were not able to move outside the baggie- therefore, the iodine solution in the beaker did not turn purple.  Kind of cool!