Science Questions

Why do some plants have purple leaves?

Sat, 8th May 2010

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Question

Hugh McDonald via Twitter asked:

Why do some plants have purple leaves? Do they not need green leaves to photosynthesise?

Answer

Dave -  I had a go at this doing chromatography on chlorophyll on Kitchen Science a few weeks ago.  Although most of the pigment in the plant is actually purple, if you actually do the chromatography, you do see there is still some green pigment in there, and so there is some green chlorophyll there in purple plants to do the photosyntesising, they just have other pigments as well which absorb green and yellow light to make them a deep purple colour.

In things like algae and sea weeds, they canhave a different colour of chlorophyll, a completely different chlorophyll which absorbs blue light better than red light, because blue light travels through water a lot better than red light.

Chris -   Yes, red light doesn’t - that’s why blood looks black under water because all the red light has been soaked up by the upper layers of water.

Dave -   Yes.

Chris -   And there’s nothing to reflect off the red blood, so it looks black.

Dave -   So red sea weeds have just got a different form of chlorophyll and I think that purple plants just have some green chlorophyll in there as well as a purple pigment which doesn't photosynthesise.

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