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Why do the leaves change colours?


Every year, we see the summer leaves change into beautiful autumn colours but have you ever wondered how such colours are possible? To answer this question we have to look at the chemistry and structure of the leaves. 


Let’s start at the beginning. Leaves contain pigments, which gives them colours. Green leaves contain chlorophyll in the chloroplasts of the leaves cells - which is where photosynthesis occurs. The pigments absorb wavelengths from the sun, which is then used to convert water and carbon dioxide into sugar, carbohydrates and energy. This process is known as photosynthesis and is where oxygen is produced as a by-product! Green leaves contain chlorophyll which absorbs blue and red wavelengths but reflects green wavelengths, which is why these leaves appear green! 


Green leaves contain two types of chlorophyll - chlorophyll a and chlorophyll b. Both types absorb and reflect the same wavelengths, but have different functions in the plant. Chlorophyll A is the primary pigment used in photosynthesis, while chlorophyll b is the accessory pigment. The accessory pigment collects energy from the sun, which is then transferred to Chlorophyll A to continue with photosynthesis. 


Leaves also contain other pigments which absorb and reflect different wavelengths. Carotenoids are another pigment in leaves, which absorbs blue-green and violet wavelengths and reflects yellow  wavelengths. These pigments give leaves a yellow-orange colour. Carotenoids are also used in photosynthesis as another accessory pigment, transferring energy to Chlorophyll A and as a photoprotective agent. As a photoprotective agent, carotenoids help prevent harmful photodynamic reactions which can occur when there is an overexposure to direct light. Another pigment in leaves are anthocyanins, which reflect red wavelengths and give leaves their red-pink colour. 


Photosynthesis is a chemical reaction and occurs faster with lots of available sunlight and warmer temperatures. As fall approaches, temperatures drop and the days become shorter, limiting the ability of plants to continue photosynthesis. Most leaves don’t survive freezing well and in order to protect the nutrients that trees have made all summer, trees begin breaking down the chlorophyll stored in their leaves. Since the other pigments (carotenoids and anthocyanin) are only accessory pigments, they do not contain nutrients and are not broken down. As chlorophyll is broken down, other pigments become more visible, absorbing green wavelengths and reflecting other wavelengths. As these pigments become more visible, the leaves begin to change colour and become an ocean of different yellows, oranges and reds!