How do plants with purple leaves photosynthesize?
The short answer
Plants with purple leaves are able to photosynthesize because they still contain chlorophyll, like green plants. But they appear purple because of anthocyanin, a special compound that absorbs green light.
The long answer
The grass isn't always greener on the other side ... sometimes it's purple.
But every photosynthesis diagram found in our biology textbooks shows a green plant. So it begs the question: How do purple plants photosynthesize?
Let's start with a refresher on photosynthesis.
A simple explanation of photosynthesis
"Photosynthesis en" by Nefronus is licensed under CC BY-SA 4.0 DEED.
Photosynthesis is a process plants use to convert solar energy, water, and carbon dioxide into chemical energy (sugar) and oxygen. It's a great deal: Plants get to eat and we get to breathe. Win-win.
Why are plants (normally) green?
Plants are typically green because of the presence of chlorophyll in their leaves. Chlorophyll is a natural compound found in the chloroplasts of plants' leaves that absorbs sunlight, kicking off the whole photosynthesis process.
This is a close-up image of plant cells with visible chloroplasts, found in a type of moss.
"Plagiomnium affine laminazellen" by Fabelfroh is licensed under CC BY-SA 3.0 DEED.
Chlorophyll is green because it is fantastic at absorbing all the light on the spectrum, with the exception of green light. As a result, green light gets reflected back to us, making plants appear green to our eyes. In other words, plants don't need to be green to photosynthesize, it's just a byproduct of chlorophyll absorbing all but green light.
But why wouldn't a plant want to absorb green light?
Plants evolved to reflect green light in order to protect themselves from taking into too much solar energy. During the middle of the day, solar energy is strongest at the green part of the light spectrum. By reflecting back most of this green light, plants are shielding themselves from damaging, excessive light exposure.
So it makes sense that most plants are green, but...
Why are some plants purple?
Purple plants contain a compound called anthocyanin, which absorbs green light. Because purple plants have a higher concentration of anthocyanin than chlorophyll, they appear as a deep red or purple to our eyes.
Purple plants are able to photosynthesize because they still contain chlorophyll, which kicks off the photosynthesis process. Anthocyanin is a bonus.
Having purple leaves has some advantages, including additional sun protection. Anthocyanin absorbs the excess green light photons that would otherwise hit chlorophyll. If a plant gets more light than it can use, its ability to photosynthesize can become less efficient and even damage the plant's cells.
Purple leaves act like a sort of sunscreen for the plant, but they're perfectly able to photosynthesize thanks to chlorophyll.
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Sources
Brookshire, B. (2020, October 28). Explainer: How photosynthesis works. Science News Explores. https://www.snexplores.org/article/explainer-how-photosynthesis-works
Dunedin Botanic Garden. (2017, December 7). Why do some plants have purple leaves?. Otago Daily Times Online News. https://www.odt.co.nz/lifestyle/home-garden/why-do-some-plants-have-purple-leaves
Khan Academy. (n.d.). Intro to photosynthesis. Khan Academy. https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/a/intro-to-photosynthesis
Kume, A. (2017). Importance of the green color, absorption gradient, and spectral absorption of chloroplasts for the radiative energy balance of leaves. Journal of Plant Research, 130(3), 501–514. https://doi.org/10.1007/s10265-017-0910-z
National Geographic Society. (n.d.). Photosynthesis. National Geographic. https://education.nationalgeographic.org/resource/photosynthesis/