| MadSci Network: Botany |
The first website cited describes an interesting idea that plants are green because purple Halobacterium evolved before chlorophyll-containing organisms. The photosynthetic Halobacterium absorbed green light so other organisms possibly evolved chlorophyll to absorb nongreen wavelengths and fill an ecological niche. The first two websites both argue that a black chlorophyll might absorb too much radiation and either overheat the plant or harm the plant by absorbing destructive UV and x-rays. It is important to remember that leaves often absorb more than half the green wavelengths and use them in photosynthesis. It is a widespread misconception that leaves reflect all green light. That misconception is based on looking at a chlorophyll absorption spectrum (see second website cited), which is obtained by extracting chlorophyll into an organic solvent, such as acetone, and measuring its absorption in a spectrophotometer at wavelengths between 400 and 700 nanometers. The chlorophyll absorption spectrum does show that chlorophyll in a test tube absorbs only about 2 to 3% of the green light. However, that is very artificial because a leaf is highly structured. Salisbury and Ross (1985) note that in the intact leaf, a green photon may not initially be absorbed by a particular chlorophyll molecule but it is reflected and then gets another chance to be absorbed, and perhaps another, and another, etc. within the complex leaf structure that does not exist in a test tube. Thus, each green photon has many opportunities to be absorbed in the leaf so the total absorption of green light by chlorophyll is much higher in the leaf than in a test tube of extracted chlorophyll. Accessory pigments, such as carotenoids, also absorb green light (see second website cited) and funnel the energy to chlorophyll. Instead of a chlorophyll absorption spectrum, people should be looking at a photosynthesis action spectrum (see third and fourth websites cited) which shows the amount of photosynthesis at each wavelength. Because plants do absorb substantial green and yellow wavelengths and use them in photosynthesis, plants are more efficient than they seem by assuming they only use the red and blue wavelengths that chlorophyll absorbs in a chlorophyll absorption spectrum. Photosynthesis is the most widely taught plant biology concept but it is often taught with numerous misconceptions. References Why Trees are Green Photosynthesis Pigments Photosynthesis Action Spectrum for Elodea Generalized Photosynthesis Action Spectrum Salisbury and Ross. 1985. Plant Physiology. Belmont, CA: Wadsworth.
Try the links in the MadSci Library for more information on Botany.