Winter is the season in which the countryside slows down and appears to sleep. Then, with the first blink of spring, life seems to move into gear and the world’s most prolific chemical factories, the plants, start cranking into production. The first sign of this is the almost imperceptible greening of the brown land and a dusting of buds on trees and bushes. Within a few days, green becomes the dominant hue, overwhelming the drabness of the dead months, to be followed, almost in an instant, by the profligacy of colour brought by the flowers.
The ubiquity of green is due to the presence of chlorophyll in the leaves of plants. This pigment absorbs light radiation from the red end of the sun’s spectrum. The energy thus gained is used to break carbon dioxide and water, taken in through the leaves and roots, into fragments from which the plant can build up the sugars and starch which are to be its stores of food, and the cellulose from which its tissues are constructed. The by-product of this process, called photosynthesis is oxygen, which is released into the air.
The early atmosphere of the earth contained no oxygen, so the first living organisms obtained their energy by means of chemical reactions that did not involve the use of this highly reactive gas. It was only with the evolution of the first green plants that free oxygen appeared in the atmosphere. Though oxygen was toxic to many of the early life forms, it opened the door to the emergence of creatures that were able to fulfiltheir energy needs by the more efficient aerobic burning of foods.
There are, in fact, two forms of chlorophyll to be found in leaves, both based on a similar chemical structure. The basic unit of this structure is a pentagon-shaped molecule, consisting of four carbon atoms joined to a nitrogen atom, which is known as a pyrrole ring. Four of these rings are linked together to form a square molecule with a hole in the centre, called a porphyrin. In chlorophyll, the hole is occupied by an atom of the metal, magnesium.
Porphyrins are quite widespread in nature, hinting at an evolutionary link between plants and animals. Haemoglobin, the red pigment in blood, contains porphyrin molecules in which the central hole is occupied by iron rather than magnesium, while the blood equivalent in certain sea creatures and some insects contains copper. Vitamin B12, essential to the prevention of pernicious anaemia, has a cobalt atom in the centre of the porphyrin molecule.
If a spot of a solution containing the juices of green leaves is placed on some absorbent paper, and a suitable solvent allowed to soak into the paper, in a manner that carries the juices with it, the spot is seen to separate out into a variety of colours. This technique is known as chromatography. It reveals that green leaves contain not only the two chlorophylls, but yellow and orange pigments as well. These are the carotenoids.
The orange pigment, carotene, itself exists in a number of variations and is responsible for the colours of carrots and oranges. One of the variants, beta-carotene, is broken down by mammalian digestive systems into vitamin A, each molecule of carotene forming two molecules of the vitamin. Alpha-carotene, in contrast, produces only half as much. Vitamin A, as such, is not found in plants, but is stored in the fats of creatures that have eaten carotene.
Another variant, lycopene, is responsible for the red colour in such fruits as tomatoes. Similar in structure to the carotenes, but with oxygen atoms attached, are the yellow carotenoid pigments known as the xanthophylls.
Carotene and the carotenoids absorb ultra-violet radiation that might otherwise damage chlorophyll. They give colour to orange and yellow lichens and also to plants that lack chlorophyll, such as fungi.
Insects concentrate carotenoids in their bodies from eating plants and green leaves. Birds, in turn, eat insects, with the result that carotenoids are largely responsible for the colours of their plumage, particularly red, orange and yellow feathers. When pollution reduces insect populations, the plumage of many birds becomes much duller because of a lack of carotenoids in the birds’ diets.
As summer comes to an end, and decreasing sunlight means that trees lose water at a faster rate than they can convert it to sugars, production begins to slow down. The carotenoids start to show their colours and the chlorophylls break down to compounds of equally vivid hue, giving rise to the glories of autumn woodlands.
Chemical activity, however, does not cease. Red, blue and violet pigments, called anthocyanins, shade the fruits that have been developing all summer. These can change colour according to acidity. Blackberry and elderberry juice, for example, turn red in acid and yellow in alkali. Beetroot and red cabbage show similar effects, so that these pigments may be used to distinguish between acids and alkalis.
Together with carotenoids, the anthocyanins have coloured the flowers throughout the spring and summer, to attract the insects necessary to pollinate the seeds. They now serve to attract birds and animals to the fruits, so that the ripened seeds can be spread around the countryside. There they lie dormant through the winter months, ready to begin the new greening of the land with the arrival of the next spring.