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Asked by to Aimee, Chris, Dave, Greig, Laurence on 16 Jun 2014. This question was also asked by .
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Aimee Hopper answered on 16 Jun 2014:
The colour of the leaf is due to a pigment in the leaf called chlorophyll. This is what takes in the sunlight and uses it to produce sugars.
Obviously the aim of the plant is to make as much sugar as possible, to try and get as much energy from the sun as possible.
Green leaves reflect away the green light, because for some reason the chlorophyll cannot use this particular wavelength of light. That reflected light then comes into our eyes, and we see the plant as “green”. It’s the same with the other colours.
In autumn, plants need to use this higher energy green, and so start absorbing that reflecting off yellow through to red, from shorter to longer wavelengths, so reflect off the lowest energy
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Greig Cowan answered on 16 Jun 2014:
Leaves are green due to a chemical called chlorophyll that exists in plant cells. This is a really special chemical that converts sunlight into energy that can be used by the plants through photosynthesis.
Sunlight is actually a mixture of all different colours of light. You can see this by placing a prism in a beam of sunlight and you will see a rainbow of colours. In fact, this is what happens when you see a rainbow in the sky after it has been raining: the droplets of water in the atmosphere are acting like a big prism!
The leaves are green since the chlorophyll absorbs all of the different colours of sunlight *except* the green colour. Colour is just a nice word for “wavelength”. Light moves in space as a wave and, just like sound or water waves, it has a wavelength, which is the distance between peaks of the wave. The green wavelength of light is reflected from the plant and comes to our eyes.
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Dave Jones answered on 16 Jun 2014:
Plants take their energy from sunlight (just like we take energy from food), but to be able to convert this energy into something they can use they need a chemical called chlorphyll. They use the sunlight and the chlorophyll in their leaves to make sugars which it can use later, this process is called photosynthesis.
Chlorophyll is green so the leaves appear green – different shades of green come from the presence of other chemicals in their leaves. The leaves then change colour in the autumn because the leaves are starting to die (that’s why they fall off afterwards). The chlorophyll begins to break down and the chemicals that are left over have a reddy, brown/yellow colour, so the leaves turn brown and then fall off. The next spring the tree will grow new leaves filled with fresh chlorophyll to start all over again!
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Laurence Perreault Levasseur answered on 17 Jun 2014:
All the answers given above are correct. Plants do take in light as an energy source to perform photosynthesis, and in this way produce the sugar that they need to grow and burn to keep their cells functioning.
It’s also true that in the fall, tree lose their leaves because the days become shorter and so leaves cannot produce enough energy anymore for it to be worth for them to maintaining their leaves. So trees block their sap supply, and they slowly decay, becoming progressively brown and finally falling down.
But I think your question is about during the summer. Why are some plants different shades of green, and some red, brown or yellow all the time? The reason is that there are actually many pigments that allow plants to do photosynthesis, not just chlorophyll! They actually have 6 different pigments that can do photosynthesis:
Carotene – an orange pigment
Xanthophyll – a yellow pigment
Phaeophytin a[1] – a gray-brown pigment
Phaeophytin b[1] – a yellow-brown pigment
Chlorophyll a – a blue-green pigment
Chlorophyll b – a yellow-green pigmentThe trick is that not all those pigments have the same sensitivity, and they are different colours because they absorb different wavelengths of light (and reflect others!). So the one at the bottom of the list is the least sensitive. This means it’s less likely to just burn up if you expose them directly to bright sunshine. So leaves on the top of trees are always bright, pale, almost a bit yellow, green. Same with full-sun plants, or the first leaves in the trees in the spring.
The next-to-least sensitive is chlorophyll-a, and it’s much darker green, so leaves at the bottom of trees have that darker green colour. That is great because in a forest, under the trees, there is still a lot of light, and this way trees can be sure to use as much of it as possible. Shadow plants are also darker green for that reason.
Even more sensitive are the yellow-brown, then yellow, then orange pigments. That’s why if you lock a plant up in a cupboard for a week it’ll turn yellow, because the less-sensitive chlorophyll died and now it’s trying to survive with more sensitive pigments! Similarly, it means that a carrot can actually do photosynthesis, but because it’s underground all the time, it has the colour of the most sensitive pigment!
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