While I am teaching the biology lab, I set this post to show up automatically at the same time. It describes what we do today, the same stuff we did back on March 26, 2006:
This week we had a busy lab, which means I did not have time for much inpired talking like I did last time. We did some exercises together as a group, while some other exercises were set as stations arund the room and each student did them alone, at their own time.
First, the students used the staining technique they learned last week to find out what kinds of organisms live on their fingers. They saw bacteria from store-bought colonies last week. This week they saw their own cocci and baccili. They also saw quite a lot of molds (and I placed on other microscopes some ready-made slides of Aspergillus, Rhizopus and Pennicilium for them to compare).
Of course, their first reaction is “Yeeew!” and comparisons who had dirty fingers and who did not. This was a nice entry for me to talk about all the symbiotic microorganisms that live on our body, as well as those that live inside of our bodies, mainly in the digestive system. I told them about the initiative to make the Human Genome Project complete by adding the complete sequqnces of all the microorganisms that live inside of our bodies. Without them, we are only half-human. They have co-evolved with us for millions of years and have taken on a number of roles that we are incpabale of doing ourselves, from defense to absorption of some vitamins. Also, it is useful to think of the bacteria in our intestines as an ecosystem. If you get sick and take lots of antibiotics, the bacterial flora is wiped out. Just like an island after a volcanic eruption, there is an orderly succession process that follows. There are species that come first and pave the way for the introduction of other species etc. Over time, the ecosystem changes a number of times until it reaches the mature, balanced stage.
The second big exercise was an experiment that tested essentially two things: 1) which of the three possible catalysts (sand, MnO2 or catalase) is the best at breaking down hydrogen peroxyde into water and oxygen; and 2) what intercellular conditions need to be met for catalase to work properly. Each test tube was a model of a cell. Just like in Week One, when we moved a salt-lake into a rubber hose, this week we moved a cell into a glass test-tube. That way, we can control all the factors one at a time and eliminate the complexities of the real world.
Manganese Dioxide actually worked too well – I think the students put too much in the tube! Sand was pretty slow. Catalase worked great this time around (last time it sat outside the fridge for a couple of days and got stale because nobody told me it has arrived!).
Catalase worked well on room temperature in water at pH=7. It did not work at all at pH=3 and pH=10, which I connected to the mechanisms of pH control I talked about at length last week (when I was talking about homeostasis and rheostasis, I used calcium control and pH control as two examples of processes where the limits are so narrow, there is no daily rhythm at all). Hot water denatured catalase (which is an enzyme, thus a protein). In ice-cold water, there was no reaction at first, but as the water warmed up to room temperature we could observe the reaction (oxygen forming bubbles).
Then I explained in quite a lot of detail what happens in the mitochondria, i.e., starting with food being digested and broken down to glucose, glucose being broken down via glucolysis and Krebs cycles, the electron transfer cascade from one cytochrome to the next with the final recepient being oxygen, and the resulting production of ATP. AS no machine is 100% efficient, there is some wobble in this mechanism as well, resulting in production of free radicals, one of which is hydrogen peroxyde. Free radicals are implicated in cell damage and perhaps the process of aging. Catalase is the enzyme that neutralizes free radicals and protects the cell from damage.
As every machine that transforms one form of energy into another is less than 100% efficient, some of the energy gets lost, mostly in the form of heat. Heat generated by the mitochondria in this process is what warms up our bodies and keeps our core body temperature more or less constant. Hormones, like thyroxine, can modulate the efficiency of the electron transfer, thus modulate the amount of heat produced by the cells in out body, thus controlling thermoregulation.
In the second half of the lab, students went around the lab and got familiarized with various types of plants, including mosses and ferns. They worked as a team to identify tree species from small disks. They made slides from leaves of Zebrina (a terrestrial plant) and Elodea (an aquatic plant) and found stomata in the former but not in the latter, and we discussed how stomata work and why a submerged plant would not need to have them.
We got a Venus flytrap to close its leaves (trick: do not use a pencil or a needle – use the corner tip of a paper towel) and discusssed the mechanism by which the leaves close at such a high speed.
Finally, the students looked at a number of animal specimens laid out in jars (mostly filled with alcohol, only a few in formaldehyde). Their job was to identify at least the Phylum for each specimen, which was, in some cases quite hard. If they managed to do that, they should also have tried to go down the taxonomic levels and try to identify the Class, Order, Family, Genus and perhaps even species (that last one was possible in only a couple of specimens, e.g., flunder and snapping turtle). This is an exercise I like a lot because it gives me an opportunity to give little tidbits about various animals, to tell some cool stories (e.g., how it was discovered here in NC, at Greensboro College, that sponges actually move along the surface), and to dispell some myths that people tend to have about some kinds of animals. This also reinforces the evolutionary message of the course – all those things are related and we explored their exact relationships. Homework: a worksheet – answering ten questions about vertebrate evolution.