SciWhys: Why are plants green?

Every month OUP editor and author Jonathan Crowe answers your science questions in the monthly SciWhys column. Got a burning question about science that you’d like answered? Just email it to us, and Jonathan will answer what he can. Today: Why are plants green?

By Jonathan Crowe

After the greyness of winter, the arrival of spring is heralded by a splash of colour as plants emerge from the soil, and trees seemingly erupt with leaves. Soon, much of the countryside has moved from being something of a grey, barren wasteland to a sea of verdant green. But why is it that so much vegetation is green? Why not a sea of red, or blue? To answer this question let me take you on a colourful journey from the sun to within the cells of plant leaves.

As humans, our waking hours are punctuated by mealtimes: we must consume food on a regular basis if our bodies are to be able to generate the energy we need to survive. Plants, however, fuel their survival in an altogether different way. Plants don’t need to ‘eat’ as such; instead, they generate their own food supply. This manufacturing of food is powered by energy that plants capture from sunlight.

It might seem odd to say that sunlight contains energy, but our everyday experience shows this to be so: if we sit outside on a sunny day, our skin quickly becomes warm. This warmth is the result of our skin cells absorbing the energy contained in the rays of sunlight.

Sunlight is made up of millions of individual rays containing a huge range of different energies. Imagine standing on a beach, watching the waves rolling in. These waves aren’t identical in size: there will be some small waves that contain little energy, which fall gently on the sand, and other much larger ones that come crashing down; these contain a lot more energy. And so it is with the rays of sunlight: some rays have a small amount of energy; other rays have much more.

This leads us on to another, perhaps surprising, phenomenon: we see rays of light of different energies as different colours, as beautifully illustrated by rainbows, which we’ll all have seen from time to time (most often on a typical British summer afternoon, as bright sunshine is suddenly replaced by a downpour of rain). When sunlight – a mix of rays of different energies – hits drops of rain, the individual rays are separated according to their energy. Suddenly, a mix of rays that, to us, have no obvious colour become transformed into the characteristic colours of the rainbow: red, orange, yellow, green, blue, indigo, violet. We call this range of colours the visible spectrum.

Sunlight also contains rays that fall outside of the visible spectrum, none of which are visible to us, but which are visible to other creatures. For example, many insects can view ultra-violet light, which has greater energy than the violet light at the one extreme of our visible spectrum. In fact, some flowers look much more visually interesting to insects than they do to us: insects can see extra marks and patterning that show up in ultra-violet light, but which are simply invisible to us. (You can see a couple of examples here and here.)

But what has this to do with the colour of plants? Why do we see plants as green? The cells of a plant leaf contain compartments called chloroplasts, which house some special machinery that enables the plant cells to capture the energy in sunlight and to use it to power the formation of food. Chloroplasts contain a substance called chlorophyll, which is an example of a pigment – a substance with an obvious colour. This pigment isn’t there just to make the plant look pretty to the human eye: it is the component of the chloroplast that actively absorbs energy from sunlight. However, it can only absorb rays of light that have particular energies – those corresponding (in broad terms) to red and blue light. By contrast, the rays of light that fall in between red and blue light in the visible spectrum cannot be absorbed. Instead, they bounce off the surface of the leaf and into our eyes (if we’re looking at them). And what colour lies between red and blue in our spectrum? You’ve guessed it: green. The reason we see plants as green is because green is the colour of the rays of light that get bounced back by the chlorophyll in the plant cell.

In fact, this concept holds true for any colour that we see. The reason that tomatoes appear red is that they don’t absorb red light: it is bounced back from the surface of the tomato to be detected by our eyes. Tomatoes exhibit their red colour thanks to the pigment lycopene, which absorbs blue and green light, but bounces back red.

Let’s end this journey with a brief flit across the Atlantic to see why plant leaves don’t always appear green. Parts of the Eastern seaboard, in the US and Canada, are famed for the fiery displays of colour by trees in the autumn. This dramatic change in colour is a consequence of chlorophyll being degraded as air temperatures drop with the change in season, leaving behind other pigments that had previously been masked by the chlorophyll. As chlorophyll is degraded, the greenness fades, to be replaced with vivid reds and oranges. It’s now the turn of other light rays to be bounced back – giving us something truly to feast our eyes on.

Jonathan Crowe is Editor in Chief for Natural, Health & Clinical Sciences in the Higher Education Department at Oxford University Press. A biochemistry graduate, he manages OUP’s undergraduate textbook publishing programme across a range of science and science-related disciplines. He is also an author of Chemistry for the Biosciences, now in its second edition, and was a runner-up in the Daily Telegraph/BASF Young Science Writer Awards (in 2001, when he was still classed as being ‘young’). You can read more about OUP’s science books here.

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