Plants are composed mainly of carbohydrates like cellulose, sugar, and lignin. The element carbon is by far the greater part of carbohydrates [carbo(n)hydr(ogen)ates] by weight. Plants can readily manufacture carbohydrates in large quantities because carbon and hydrogen are derived from air (C02) and water (H2O), both substances being available to plants in almost unlimited quantities.
Sugar, manufactured by photosynthesis, is the simplest and most vital carbohydrate. Sugar is "burned" in all plant cells as the primary fuel powering all living activities. Extra sugar can be more compactly stored after being converted into starches, which are long strings of sugar molecules linked together. Plants often have starch-filled stems, roots, or tubers; they also make enzymes capable of quickly converting this starch back into sugar upon demand. We homebrewers and bakers make practical use of a similar enzyme process to change starches stored in grains back to sugar that yeasts can change into alcohol.
C/N of Various Tree Leaves/Needles
False acacia 14:1 Fir 48:1
Black alder 15:1 Birch 50:1
Gray alder 19:1 Beech 51:1
Ash 21:1 Maple 52:1
Birds's eye cherry 22:1 Red oak 53:1
Hornbeam 23:1 Poplar 63:1
Elm 28:1 Pine 66:1
Lime 37:1 Douglas fir 77:1
Oak 47:1 Larch 113:1
The protein content of tree leaves is very similar to their ratio of carbon (C) compared to nitrogen (N)
Sometimes plants store food in the form of oil, the most concentrated biological energy source. Oil is also constructed from sugar and is usually found in seeds. Plants also build structural materials like stem, cell walls, and other woody parts from sugars converted into cellulose, a substance similar to starch. Very strong structures are constructed with lignins, a material like cellulose but much more durable. Cellulose and lignins are permanent. They cannot be converted back into sugar by plant enzymes. Nor can most animals or bacteria digest them.
Certain fungi can digest cellulose and lignin, as can the symbiotic bacteria inhabiting a cow's rumen. In this respect the cow is a very clever animal running a cellulose digestion factory in the first and largest of its several stomachs. There, it cultures bacteria that eat cellulose; then the cow digests the bacteria as they pass out of one stomach and into another.
Plants also construct proteins, the vital stuff of life itself. Proteins are mainly found in those parts of the plant involved with reproduction and photosynthesis. Protein molecules differ from starches and sugars in that they are larger and amazingly more complex. Most significantly, while carbohydrates are mainly carbon and hydrogen, proteins contain large amounts of nitrogen and numerous other mineral nutrients.
Proteins are scarce in nature. Plants can make them only in proportion to the amount of the nutrient, nitrogen, that they take up from the soil. Most soils are very poorly endowed with nitrogen. If nitrate-poor, nutrient-poor soil is well-watered there may be lush vegetation but the plants will contain little protein and can support few animals. But where there are high levels of nutrients in the soil there will be large numbers of animals, even if the land is poorly watered and grows only scrubby grasses--verdant forests usually feed only a few shy deer while the short grass semi-desert prairies once supported huge herds of grazing animals.
Ironically, just as it is with carbon, there is no absolute shortage of nitrogen on Earth. The atmosphere is nearly 80 percent nitrogen. But in the form of gas, atmospheric nitrogen is completely useless to plants or animals. It must first be combined chemically into forms plants can use, such as nitrate (NO3) or ammonia (NH3). These chemicals are referred to as "fixed nitrogen."
