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AS Chemistry (Edexcel International)
  • Unit 1: Structure, Bonding and Introduction to Organic Chemistry
  • Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols
  • Unit 3: Practical Skills in Chemistry I
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The Chemistry Behind Autumn Leaf Colours

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The Chemistry Behind Autumn Leaf Colours

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Chlorophyll: The Green Pigment

During spring and summer, leaves appear green because of chlorophyll, the pigment that absorbs sunlight for photosynthesis. Chlorophyll absorbs red and blue light but reflects green, giving leaves their bright green colour. It’s constantly being produced and broken down in summer.

But in autumn, cooler temperatures and less daylight trigger a slowdown in chlorophyll production. As chlorophyll fades away, other pigments that were already inside the leaf start to shine through.

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Other Pigments Take Over

When the green disappears, other pigments reveal their colours:

  • Carotenoids – These give yellow and orange colours (like in carrots and pumpkins). They were always in the leaf, but hidden by chlorophyll. Carotenoids are more stable, so they last longer when chlorophyll breaks down.

  • Anthocyanins – These produce red, pink and purple colours (seen in red maples and some fruits like apples). Interestingly, unlike carotenoids, anthocyanins are made in the leaf during autumn, especially in bright, cool conditions. They're thought to protect leaves from UV light and metal ions, and help trap sugars.

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Why Do Leaves Fall?

As days get colder, trees begin to prepare for winter. At the base of each leaf stalk, a separation layer (called the abscission layer) forms. This cuts off the flow of water and nutrients between the leaf and the tree. Eventually, the leaf dies and falls.

What about the brown colour? That’s due to tannins, waste products that remain in the leaf after other pigments have broken down. Tannins give leaves their final brown appearance.

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