Altering Ideas In The Light Of New Evidence

What Is The Color Of Chlorophyll?

Marc Avery Bellow

Overview

This is not a trivial question like, "What is the color of George Washington’s white horse?" In this exercise, your students’ powers of observation are challenged and an introduction to the interactions of chlorophyll and light are examined. Additionally your students will be faced with having to alter their conceptual point of view based upon their changed physical vantage point.

Biological Concept

Science is the systematic questioning of what is not known. It is also the continuous questioning of the currently held dogma.

Students frequently can learn the facts of science. Here is an opportunity to ask them to reevaluate their thinking and to form testable hypotheses. [The appropriateness of the aforesaid two goals depends of course on the aperceptive knowledge base of your students.]

The "color" of chlorophyll depends upon which wavelengths of light it reflects.

Chlorophyll reflects wavelengths of the electromagnetic spectrum that we see as green.

Chlorophyll absorbs light energy in the areas of the electromagnetic spectrum that we sense as red and blue.

Energy from light is absorbed by electrons in chlorophyll. Eventually that energy becomes available to organisms as chemical bond energy.

See Hard Copy for Diagram

diagram of exercise I The color of chlorophyll diagram of exercise II The setup to have your students

illuminated by different wavelengths of light view a chlorophyll extract from two positions.

Method

EXERCISE I

• A sheet of white copy paper is stained over about ¹/₂ of its surface.

• A 35 mm slide projector is used to project the seven major spectral colors on the chalkboard.

• Ask your students to predict (in writing) what color the chlorophyll will be in each of the seven differently illuminated areas.

• Starting with green, actually place the sheet of paper in each colored field. Have your students write the name of the observed color of the chlorophyll. Have them write an explaination of why the chlorophyll appeared to be that color.

• This should be followed up by asking students to read and to discuss their results. Class discussion in response to your directed questions can elicit some characteristics of color, reflection, and absorption observed in the course of the lesson.

EXERCISE II

• The class is divided so that they can view the illuminated acetone extract from only one or the other of the two vantage points. Each group can see only the light coming from one of the slits.

• Each student is asked to write a description of what he or she sees. They are then asked to describe what the chemical might be and what properties are being illustrated.

• After three minutes the students are asked to change their seats to the opposite side of the room.

They are enjoined not to speak or to remark on what they now see. Again each is asked to write an observation first and then to write an explanation.

The development of the lesson can also include the following:

• What is obvious may not be true.

• A change in ones observational or conceptual bias may reveal different results.

• The strength of the scientific method is to correctly resolve these inconsistencies.

Class Time

15 to 20 minutes for each of the two exercises

Background Information

• The transmitted light (at a slight angle so that the glare of the projector is not a factor) results from a series of absorptions of certain wavelengths and a series of reflections of other wavelengths of light. The summation of all of these leaves green.

• The light viewed at the more acute angle is a dark wine-red color. This results from the re-emission of energy from the chlorophyll molecule as the "excited" electron falls back to a lower energy level. (The acetone extracted chlorophyll molecules are not membrane bound. The excited electrons cannot be transferred to other chemical pathways as they would in an intact chloroplast.)

Your students should certainly be able to observe the following:

• Chlorophyll interacts with light by absorbing energy in complex interactions. You may choose to extend the biochemistry and physical aspects of the situation to any level desired. • The second student outcome should be the same as in the previous exercise. Perception of sensual reality or of scientific "truth" can be an artifact of one’s physical and mental stance.

Materials

• A 35 mm slide projector

• diffraction grating or a 35 mm projection slide of the spectrum

• saturated 35 mm projection slides of each of the major colors of the spectrum

• a piece of white paper that has been stained over 1/2 of its surface by the chlorophyll extract

• acetone extract of chlorophyll The strength of a concentrated acetone extract from chlorophyll is adjusted downward until the experimental set up behaves as described.

About The Author

Marc Avery Bellow is a teacher of biology and AP psychology at Benjamin Cardozo H. S., 5700 223 Street, Bayside, NY 11364. The school’s fax number is 718 631-7880. He can also be contacted by e-mail at jam13@ix.netcom.com

The Woodrow Wilson National Fellowship Foundation webmaster@woodrow.org
CN 5281, Princeton NJ 08543-5281 Tel:(609)452-7007 Fax:(609)452-0066