Q: How does photosynthesis occur in plants that are not obviously green, such as ornamental plum trees with deep purple-colored leaves?  What other chemicals are involved in photosynthesis besides chlorophyll? –Paul, Santa Cruz

A: Photosynthesis is that very elegant chemical process begun 4 billion years ago that jump started all life as we know it on our planet. The word literally means photo = light + synthesis = to put together. Basically six molecules of water plus six molecules of carbon dioxide in the presence of light energy produces one molecule of glucose sugar and emits six molecules of oxygen as a by-product. That sugar drives the living world. Animals eat plants, then breath in oxygen which is used to metabolize the sugar, releasing the solar energy stored in glucose and giving off carbon dioxide as a by product. That is it.

GREEN PLANTS DO THIS: 6 CO2(g) + 6 H2O(l) + light → C6H12O6(aq) + 6 O2(g)

ANIMALS DO THIS: C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + energy

All photosynthesizing plants have a molecule called Chlorophyll a. This molecule absorbs most of the energy from the violet-blue and reddish-orange part of the spectrum. It does not absorb green; which is reflected back to our eyes. There are also accessory pigments which absorb energy that Chlorophyll a does not. These are chlorophyll b and carotenoids. There are at least 600 known carotenoids; they are split into two classes, xanthophylls and carotenes. They absorb blue light. Xanthophylls are yellow and carotenes are red and orange. Anthocyanin while not directly involved in photosynthesis is an importan pigment that gives stems, leaves, flowers or even fruits their red color.

Many ornamental plants are selected because of their red leaves – Japanese plums, Norway maples, purple smoke bush to name just a few. Obviously they manage to survive quite well without green leaves. At low light levels leaves with chlorophyll a and b are most efficent at photosynthesis. On a sunny day however there is essentially no difference between red and green leaves in trapping the suns energy. I have especially noticed the presence of red in brand new leaves and in many tropical plants. Anthocyanins apparently prevent damage to leaves from ultra intense light energy by absorbing UV light. There is also evidcence that unpalatable compounds are often produced along with Anthocyanins which may indicate to potential herbivores the presence of toxins.

There is still much research to be done in this arena and botanists have been wondering about red vs green leaves for the past 200 years! So you are in good company, Paul

Michael Ellis – Ask the Naturalist – Bay Nature Magazine

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November 5, 2010