![]() ![]() These molecules have characteristic and intense orange and red colours that are responsible for the colour of carrots, pumpkins and maple leaves. As a result, plants developed a chemical defence mechanism to hide these yellow leaves: carotenoids and anthocyanins. This is because yellow foliage attracts aphids and other parasites that can damage it. While we may think yellow leaves are pretty, they’re a problem for the plant. Xanthophylls are molecules with a lot of conjugated carbon–carbon double bonds that reflect yellow light and are also responsible for the colour of egg yolks and sunflowers. As plants breakdown all their chlorophyll as winter approaches, their leaves gradually turn yellow thanks to a family of molecules called xanthophylls that are found in leaves all year round. As a result many plants stop producing the pigment altogether. This is because chemical reactions proceed more slowly when it’s cold and the shorter days and more diffuse light mean little energy is harvested. Why do leaves turn yellow or red in autumn?Īs it gets colder and darker as autumn arrives, having leaves packed with chlorophyll costs plants a lot of energy for little return. Chlorophyll absorbs the rest of the visible spectrum and is capable – with an array of biomolecular machinery – of transforming this energy into carbohydrates in a process known as photosynthesis. The combination of magnesium with the chlorophyll molecule means that it reflects green light. ![]() This lollipop shaped molecule is very similar to the haem group that carries oxygen in the blood, but instead of having iron at its centre it has magnesium. During spring and summer, the most abundant pigment in plant leaves is chlorophyll. Our eyes see the colours corresponding to the reflected wavelengths. A pigment is a chemical that absorbs some wavelengths of light but reflects others. ![]() The colour of leaves comes from molecules called pigments. But what is the chemistry behind autumn’s colours? Why do some plants lose their leaves? What role do enzymes play in all these processes? First things first. Nothing says autumn like comforting, warm, orange and red tones: sweet potatoes, carved pumpkins, fallen leaves, a night by the fireplace. ![]()
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