The plant clans adorn their stem scaffoldings with a finery of leaves that spreads like an olive carpet over the earth. Within that green blanket, every single leaf is busy devouring carbon dioxide and expelling oxygen.
All together, at least 25 million square miles of leaf surface are engaged daily in the miraculous chemistry we call photosynthesis. Combined, these leaves produce not the hundreds of millions of tons of products like steel and concrete we humans churn out yearly but a mind-boggling 150 billion tons of sugar (carbohydrates) annually.
Photosynthesis Revealed
The importance of photosynthesis in the economy of nature was not fully recognized until comparatively recent times. Aristotle and other Greeks believed that plants derived their food from the soil.
More than 300 years ago, in one of the first carefully designed botanical experiments, the Belgian physician, Jan Baptista van Helmont, offered the first experimental evidence that soil alone does not nourish a plant. Van Helmont planted a small willow tree in an earthen pot with a carefully measured amount of soil, adding only water occasionally. At the end of five years, the willow had increased in mass dramatically but the soil had lost only a tiny fraction of its initial weight. From this, Van Helmont concluded, although wrongfully, that the substance of the tree came from the water he had applied.
Toward the end of the 18th century, the English clergyman and amateur chemist Joseph Priestly reported that he had “accidentally hit upon a method of restoring air that had been damaged by the burning of candles.” By placing sprigs of mint into air in which a candle had burned out, Priestly was able to “restore” the air so that a candle could burn in it once again.
Priestly’s experiments offered the first logical explanation of how air remained “pure” and able to support life despite the burning of countless fires and the breathing of many animals. Priestly’s “restored air” (or “dephlogisticated air” as he called it) would later be named oxygen by Antoine Lavoisier.
Later, the Dutch physician Jan Ingenhousz confirmed Priestly’s work and showed that the air was “restored” only in the presence of sunlight and only by the green parts of plants. Further experiments by the Swiss Nicholas Theodore de Saussure helped to finally organize the whole process into a chemical formula—the formula for photosynthesis:
Leaves are Living Food Factories
Humans consume an estimated 375 billion tons of food each year, with the bulk of it coming from the sugars, starches, and oils that plants synthesize out of air, soil minerals, and water using the energy of sunlight. The remainder of human provisions comes from animal products, which in turn are derived from plants.
The importance of photosynthesis, however, extends far beyond the sheer weight of these products. Without this flow of energy from the sun, channeled through green plant cells, the pace of life on this planet would swiftly diminish and virtually cease altogether eventually.
In the process of photosynthesis, plants convert the energy of sunlight (radiant energy) into the energy in the chemical bonds of carbohydrates such as sugars and starches. These carbohydrates can then be used as:
- An immediate food source. The main type of carbohydrate produced by plants through photosynthesis is glucose—a six carbon sugar. Glucose can be broken down by the plant (and also by animals) to yield needed energy. The blood sugar in human blood is glucose derived by digesting plant material.
- Food storage. If the glucose produced during photosynthesis is not needed immediately, it can be stored. However, glucose does not function well as a storage molecule so chemical reactions in the plant bond (bind) the glucose molecules together into long chains known as starches. Starch is a very stable and functions well as a food storage molecule. Animals in turn eat these starches and digest them back into glucose. In some plants with large fleshy leaves, such as lettuce, cabbage, and spinach, excess food is stored within the leaf itself.
- Structural (building) components. Wood is composed of the same material—glucose—that is coursing through animal blood vessels providing food (energy) for animal cells. How can that be? When one eats a plant (or plant parts), the digestive system breaks the starches in that plant back into glucose and sends that glucose out into the bloodstream where it eventually reaches the cells and satisfies their energy needs.
In a plant those same small glucose molecules can be bound together into long, very stable chains of cellulose. The cellulose chains can then be connected together to form that hard, extremely durable material we know as wood.
As scientists continue to unravel the mysteries of photosynthesis, it becomes clear that leaves not only feed the plant they are attached to but that collectively they feed all animal life on the planet as well.
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