Tuesday, December 13, 2005

A Final Fiasco

I am tearing my hair out.

What is wrong with me, my students, and my textbook?

My final in my General Biology: Molecules and Cells (required first Biology for majors) is optional. It is cumulative and can be used to replace a lower earlier test score.

Some topics are harder for students than others. Photosynthesis, how plants make sugars using water, carbon dioxide, and energy from the sun, is one of the harder ones. Nonetheless, there a number of points of general interest in the topic. These are things I would think that a student would remember, because it explains what you see around you.

At the time I was lecturing on photosynthesis, the leaves were turning. Our falls colors are beautiful here in North Eastern Ohio.

So, first I talked about pigments. We see colors on objects because they have pigments that absorb part of the visible spectrum of light. Our eyes only get what is reflected. The light absorbed is trapped in the pigment molecules and never comes back to our eyes.

A black T-shirt is black because it absorbs all of the visible wavelengths of photons (energy particles of light), so our eyes see darkness, no light. A red T-shirt is red because it absorbs wavelengths other than red. The red light reflects off and comes to our eyes. And green leaves are green because chlorophyll absorbs blue and red and orange wavelengths. Green gets reflected so we see green.

Light is energy, so pigments gain energy in absorbing photons. That is why a black car seat gets so very hot on a sunny summer day, and why white clothes are cooler on such a day. Chlorophyll traps light energy. That energy collects and eventually pops an electron from chlorophyll entirely out of orbit and it goes to another molecule, setting off a cascade of energy transfering reactions of a type called electron transport. That is how the whole process begins.

Because chlorophyll does not get energy from green wavelengths, thrifty plants make additional pigments that absorb green light, instead reflecting yellow, orange, and red. These accessory pigments include carotenoids, of which beta-carotene is probably the most famous. These pigments are more stable than chlorophyll. When the weather gets cold, deciduous trees shut down the metabolism in their leaves. Chlorophyll is relatively unstable and rapidly degrades and vanishes. The carotenoids linger on for a while. So the green of chlorophyll vanishes, leaving instead yellow, orange and red carotenoids that you normally don’t see because of the more prevalent green chlorophyll overpowering them.

I spent most of an entire lecture on this, with lots of pictures, spectrums, pictures of trees before and after turning color. And, as I said above our trees were turning at that time. The text has two pages on pigments in general, with spectrums and wavelength diagrams.it then has a page each on chlorophyll and carotenoids. There are pictures of trees when green then turned orange and pictures of colored leaves on the carotenoid page. The book then spends six pages talking about the displacement of an electron by absorbed light energy, and it going into electron transport. That is a complicated process.

I also talked about where chlorophyll is in the plant cells, with pictures. It is located in the innermost parts of chloroplasts in structures called the thylakoid membranes. Again there are many digrams of plant leaf structure on my slides and in the book to show this.

The students missed a question about chlorophyll and fall colors on the test that included this subject. So, I put another one on the final. The students know to study from their previous quizzes for my final. So I put the following question on the final.

Which of the following is true of chlorophyll?
a) It absorbs green light.
b) It has electrons that can be shifted out of orbit by photons.
c) It changes from green to yellow or red in the fall
d) It is localized in the chloroplast outer membrane
e) none of the above

The students answered as follows on the group that had the Monday afternoon final:
18 picked c)
11 picked a)
9 picked d)
2 picked e)

And one, only ONE picked b) – the right answer

What is wrong? What am I doing? What are THEY doing?