Cells, up close and personal

This week we continued our investigations with microscopes. Having already gotten some experience last week with the ins and outs of using one, we focused on cells. The kids got a chance to compare plant and animal cells by extracting human cheek cells (sounds much more painful than it actually is) and onion cells (a little smelly, but otherwise no tears). We talked about how animal cells are more "blob-like," and how plant cells have a rigid exterior, which makes them look like boxes or bricks. 

Afterward the kids looked at other specimens under the microscope including: a mosquito (the wings are really interesting up close), a spider (much hairier than you'd think) , a louse (even scarier magnified 100X), a blade of grass, and various leaves.

Biology: Why are cells so small?

I've been getting some push back from the kids on all the tiny stuff we've been studying: microscopes, cells, chemistry, etc. So before we move on to bigger and better things, I thought we should at least address one last itsy bitsy, but not unimportant question: Why are cells so darn small? Even elephants, whales, and oak trees, big as they are, are made of of tiny cells, the same size as the unicellular organisms we found in drops of pond water.

To answer this question we looked at potatoes. The potatoes represented cells, and we submerged these potato cells in iodine, which stood for oxygen, food, waste or whatever stuff needed to be moved in or out of the cell. We cut up potatoes into 2 different-sized cubes: teeny tiny (c. 0.5 cm3) and large (c. 5 cm3). We placed these in a jar of iodine and waited. After about 20 minutes, we removed the potatoes, now black after reacting with the iodine, and sliced them in half. The idea was to see how far the iodine penetrated the potato (Picture shows similar experiment done with agar cubes absorbing phenolphthalein). In both teeny tiny and large potatoes the iodine moved less than a millimeter in. However, the big difference was when we considered what percentage of the potato had been exposed to the iodine.

We discussed how the surface area of cell is a limit on how much stuff can be moved in and out of a cell, while the volume of the cell determines the demand on imports (food, O2, etc) and exports (waste products). In large cells, there is simply too much volume for the surface area to handle, and the cell cannot move stuff in and out of the cell fast enough to meet the cell's needs.

Biology: How small is small?

This week we learned a technique for measuring specimens under the microscope. The kids measured the lengths of ants and even actual cells. You can be sure we weren't measuring in meters or even centimeters. The kids got some practice with micrometers, 1000 of which make a millimeter.

How did we do this? Using a ruler, the kids measured the field of view. Then they converted this number to micrometers (so much easier to work with when you're measuring cells and such). They estimated how many times a specimen would fit within the field of view, did some simple math and came up with a measurement that was within 100 micrometers (.1mm of the accepted answer). Not bad for a day's work!