Photosynthesis in leaves that aren’t green

by on July 01, 2008

 

 

Q: How does photosynthesis occur in plants that are not obviously green, such as ornamental plum trees with deep purple-colored leaves? [Paul, Santa Cruz]

A: Photosynthesis (which literally means “light put together”) is that very elegant chemical process that jump-started life as we know it some 4 billion years ago. So to answer your question, we’ll need a short chemistry lesson. Basically six molecules of water (H2O) plus six molecules of carbon dioxide (CO2) in the presence of light energy produce one molecule of glucose sugar (C6H12O6) and emit six molecules of oxygen (O2) as a by-product. That sugar molecule drives the living world. Animals eat plants, then breathe 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’s life, in a nutshell.

All photosynthesizing plants have a pigment molecule called chlorophyll. This molecule absorbs most of the energy from the violet-blue and reddish-orange part of the light spectrum. It does not absorb green, so that’s reflected back to our eyes and we see the leaf as green. There are also accessory pigments, called carotenoids, that capture energy not absorbed by chlorophyll. There are at least 600 known carotenoids, divided into yellow xanthophylls and red and orange carotenes. They absorb blue light and appear yellow, red, or orange to our eyes. Anthocyanin is another important pigment that’s not directly involved in photosynthesis, but it gives red stems, leaves, flowers, or even fruits their color.

Many plants are selected as ornamentals because of their red leaves— purple smoke bush and Japanese plums and some Japanese maples, to name just a few. Obviously they manage to survive quite well without green leaves. At low light levels, green leaves are most efficient at photosynthesis. On a sunny day, however, there is essentially no difference between red and green leaves’ ability to trap the sun’s energy. I have noticed the presence of red in the new leaves of many Bay Area plants as well as in numerous tropical species. The red anthocyanins apparently prevent damage to leaves from intense light energy by absorbing ultraviolet light. There is also evidence that unpalatable compounds are often produced along with anthocyanins, which may be the plant’s way of advertising its toxicity to potential herbivores. So red-leaved plants get a little protection from ultraviolet light and send a warning to leaf-eating pests, but they lose a bit of photosynthetic efficiency in dimmer light.

Botanists have been wondering about red versus green leaves for the past 200 years and there is still much research to be done in this arena. So you are in good company, Paul.

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15 comments:

QWERTY on September 3rd, 2012 at 2:25 am

You started by explaining in detail how photosynthesis works, and then just said that there are also other molecules than chlorophyll and… that’s it. How do they convert solar energy into sugar molecules? Do they? Is the chlorophyll always present? Is photosynthesis possible without it?

Dan Rademacher on September 4th, 2012 at 1:24 pm

Hi QWERTY. Good questions. We can’t answer them all, but here’s a decent primer on photosynthesis. According to that page, there are some halobacteria (salt-loving critters) that perform photosynthesis without chlorophyll.

EVOLS on December 21st, 2012 at 3:24 pm

The article mentions Norway maples as having red leaves, which is not a feature of the Norway maple. Their leaves are green. Maybe you were thinking of the Crimson King maple?

Dan Rademacher on December 21st, 2012 at 3:27 pm

Not sure what we meant there, to be honest. That was a few years ago. Still wrong though! I’d say we meant some Japanese maples, but the Crimson King would work too obviously.

LILWAYNE on January 16th, 2013 at 2:55 pm

why are green leaves chosen?

DC on April 8th, 2013 at 6:20 am

cool page helped me with my biology homework

diana on April 8th, 2013 at 6:26 am

nice stuff you have written, saved me from the trouble of going to the local library

Bryan on August 28th, 2013 at 3:02 pm

‘Crimson King’ IS a cultivar of Norway maple, for what it’s worth.

Mike Dalbey on April 4th, 2014 at 2:33 pm

Plants that have obviously red/purple leaves rather than green leaves (think red cabbage vs green cabbage) typically have BOTH chlorophyll AND abundant anthocyanin pigments that “mask” the green chlorophyll in the sense that the presence of the chlorophyll is no longer apparent to visual inspection. These often turn out to be “ornamental” varieties of plants whose wild populations are largely comprised of green-leaved varieties.

The dual presence of anthocyanins and chlorophylls can be demonstrated experimentally by grinding the leaf in an aqueous solution, which extracts the anthocyanins, leaving the green, non water-soluble, chlorophylls behind in the leaf.

Raluca on May 3rd, 2014 at 12:42 am

” That SUGAR molecule DRIVES THE LIVING WORLD. Animals eat plants, then breathe 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’s life, in a nutshell.” And this is proof that life is always sweet :P

Paul Reeve on June 8th, 2014 at 1:48 pm

Do not presume that the red-leafed plants are so much less efficient at photosynthesis than are the green-leafed plants. Plants employ a chemical process of energy transfer from anthocyanins and other colored pigments to the chlorophylls in the leaves, and in many species the balance of the other pigments (distribution and percentages) changes through the seasons. I first encountered research on this in the 1950s when in high school and wondering about the subject. The best collection of research on the subject I found was in a book by Rabinowitz, and may have had a title along the lines of Plant Pigments and Energy Transfer. At the time, while I was in high school, I was wondering if it would be possible to synthesize the pigments on a large scale and use them in a liquid state with anodes and cathodes to create photovoltaic cells to collect d.c. solar energy. I did read a decade later that some Japanese scientists were attempting just that, and follow-up reading indicated minor successes, but that the pigments had to be replaced in uneconomically short order. Perhaps somebody will be able to create economically efficient PV cells someday that are not so energy-intensive in their manufacture and maintenance as today’s solid state cells.

Paul Reeve on June 8th, 2014 at 1:57 pm

Remember, too, dear readers, that plants also store the solar energy they process as starches and proteins. I think that animals find those as much or more useful as or than the sugars stored by the plants. Some plants also store sugar, and convert it concurrently or later to starch. The leaf temperatures are also factors in what energy can be used directly, what reserves of water must be diverted to cooling leaves when they get too warm, etc. Pretty complex set of functions in leaves, roots, stems, storage organs, be they fruit or tubers. Consider, too, that there are plants that photosynthesize in their exposed roots when those roots are wet, but that those roots may be reflective and not so active in photosynthesis when dry…even performing all of the photosynthesis for the plant in some lithophytic and epiphytic plants under usual conditions.

muin arkom on August 8th, 2014 at 4:04 pm

how to identify the anthocyanin compounds????

Ntapo lebelo on October 9th, 2014 at 2:42 pm

Can photosynthesis take place in a green stem?

It’s not easy being blue: Why we don’t see many blue-leafed plants in nature | Visionlearning Blog on October 10th, 2014 at 4:04 am

[…] Learn about photosynthesis in leaves that aren’t green at Bay Nature. […]

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