foglia piante natura fotosintesi clorofilliana

The colour of the leaves

Chlorophyll photosynthesis and the internal relationship between sunlight and plants.

by Luca Longo
12 February 2021
4 min read
by Luca Longo
12 February 2021
4 min read

We know that excess carbon dioxide (CO2) in the atmosphere is the main cause of climate change. We understand that the Earth is warming up because in the past few centuries, we burned lots of fossil fuels that took hundreds of millions of years to form and accumulate in the depths of our planet.

Fortunately, on the surface of the Earth there are CO2 hunters that spend their lives mercilessly capturing carbon dioxide. They break it down, release some of the oxygen, and then mix together the remnants with a little water and a pinch of other substances to produce seeds, fruit, leaves and other good things.

For the past two billion years, plants –our CO2 hunters– have been learning to do something that all the animals on Earth have not yet learned to do. That includes us, who, in recent (geological) times, have been dominating the planet. To understand this process we need to start with the definition of chlorophyll photosynthesis.

What is chlorophyll photosynthesis?

The trick lies in photosynthesis, which is a rather complicated mechanism that allows chloroplasts, or tiny specialised organelles in leaves, to convert sunlight into energy. The photons from the sun that manage to break through the atmospheric barrier have a wavelength between 380 and 750 nanometres (nm; one nanometre is one billionth of a metre). The range of the solar light spectrum that plants are able to use for photosynthesis ranges from 400 nm (blue) to 700 nm (red).

Chloroplasts under the microscope

How does chlorophyll photosynthesis work?

Unlike animals, plants use the sun's rays of the right wavelength to excite electrons in special molecules. The sun energy converted into the excitation energy of these electrons undergoes numerous other chain transformations and eventually produces carbohydrates. These carbohydrates can be sugars, starches, cellulose, lignin or glycogen.

In practice, plants use the sun to create order from disorder. They convert disordered substances (carbon dioxide dispersed into the atmosphere, water and a handful of other elementary substances scattered in the soil) into highly ordered structures capable of storing a lot of energy within their chemical bonds. Thanks to plants’ help, when we need it we can burn the energy from sugar and starch to make sweets and pasta or burn cellulose and lignin to build wooden objects or warm ourselves in front of the fire. Of course, when we are in the mood for steak, eggs or milk, we can also use these concentrated stores of energy to raise animals. And let's not forget that the byproduct of all the plants’ work is the oxygen that allows us to breathe!

Chlorophyll molecules

All plants, from clover to oak trees, use two special molecules that can interact with light for photosynthesis: Chlorophyll A and Chlorophyll B. Green algae work almost in the same way, while red algae, together with the ubiquitous Chlorophyll A, also use Chlorophyll D instead of B. Lastly, brown algae prefer Chlorophyll C. These different molecules are built to work best when hit by rays of light in a precise colour, meaning electromagnetic waves in a certain wavelength range.

In particular, Chlorophyll A absorbs light at around 435 nm (blue violet) and 670-680 nm (red). B, Instead, prefers to work with the wavelengths of 480 nm (blue) and 650 nm (orange). In addition to chlorophyll, carotenoids and lycopene also contribute to the absorption of visible light (especially in the green area), with the former responsible for the orange of carrots and the latter the red in tomatoes. So, taking all the various components into account, plants absorb violet/blue and red/orange light, while they are unable to use light in the yellow and green wavelengths. Therefore, leaves reflect all yellow/green rays of light.

And this is why leaves are green.