Nutrition in Green Plants

Although people have known and used plants for as long as the human race has existed, the first records of an understanding of plant nutrition date back to the Greek philosopher Aristotle (384-322 B.C.) and his student Theophrastus of Eresus (371-285 B.C.) in Athens. Aristotle was not a scientist. He did not perform experiments to test his ideas. Since plants grow from the soil, it seemed evident to Aristotle that their substance must come from the soil. He wrote, "As many flavors as there are in the flesh of different fruits, so many, it is plain, prevail also in the earth". Aristotle regarded the soil as equivalent to a vast stomach that prepares and supplies the food of plants. This view later became known as the humus theory of plant nutrition. Humus refers to the organic matter in the soil.

Aristotle's limited contributions to botany are scattered through his works on other subjects. But his student, Theophrastus, concentrated on the investigation of plants. Toward the end of his life, his knowledge of plants was written down in two treatises called Enquiry into Plants and Cases of Plants. Since these are the first known works deveted systematically to plants, Theophrastus is properly regarded as the "father of botany". Theophrastus did not advance ideas about the nature of plant growth that differed in any important way from Aristotle's. His most interesting contribution was to point out how different kinds of plants grow in, and are characteristic of, different environments or habitats. With these ideas, Theophrastus appears to have been the first ecologist.

Few records exist from the intervening period. The first person known to have actually performed experiments related to plant nutrition is the Belgian chemist Johann Baptista Van Helmont (1578-1644). His experiments showed that most of the weight of a plant did not come from the soil, but from water. An Englishman, John Woodard, published results of an experiment in 1699 that showed that something from the earth other than water was important to plant growth.

The Italian Marcello Malphigi (1628-1694) and the Englishman Nehemiah Grew (1641-1721) made careful microscopic studies of the internal structure of plants in the 1670's. The anatomical features that they discovered were a revelation to scientists of the times, but did not really show how plants feed themselves. Mainly on intuitive grounds, however, Malpighi advanced the idea that the leaves of a plant are its nutritive organs. He described one experimental result in support of his proposal. When the first green leaves of a squash seedling were cut off, the seedling did not grow, even though its roots still had free access to water and soil. In 1727, the English scientist Stephen Hales published Vegetable Staticks (staticks meant what we now call physics). In his book, Hales described many investigations of the flow of sap in roots and stems of plants and the evaportation of water from their leaves. At the time no one knew that air is not a single substance. Hale attempted to show that plants absorb air, but he was not successful because he didn't realize that plants give off another gas as rapidly as they absorb the component of air that is used in its nutrition.

In 1772 another Englishman, Joseph Priestly (1733-1804), reported the results of an important experiment. He found that a sprig of mint would not die when placed in air that had been "spent" by burning a candle in it. To the contrary, in such air the plant would grow and the air would then, to his astonishment, again support a candle flame. Priestly inferred that a plant can "purify" the air of the harmful material that combustion releases. His conclusion that the earth's vegetation constantly restores that air that human and animal respiration, and combustion, has rendered unfit for breathing excited immediate attention. But other scientists, attempting to duplicate Priestly's experiment, obtained contridicting results, and a dispute began. The dispute was resolved in 1779. The Dutch physician, Jan Ingenhousz, published Experiments on Vegetables, putting forth experimental evidence that plants purify the air only in sunlight. He showed that only the green parts of plants, especially the leaves, have this capacity.

In 1784 the French scientist Antoine Lavouisier, the father of modern chemistry, arrived at an understanding of combustion and respiration as processes that consume oxygen from the air. It then became evident that the "pure air" that plants release is oxygen. A complete elementary picture of plant nutrition was first achieved in 1804 by the Swiss scientist Theodore de Saussure (1767-1845). In his book Chemical Researches on Vegetation, de Saussure showed conclusively that green plants produce oxygen only when they consume carbon dioxide from the air, and that its uptake adds carbon to the plant. His experiments also showed that plants convert water into weight as well. His conclusion was that plants convert water, along with carbon dioxide from the air into dry matter in their food-making process.

The final concept needed for an elementary understanding of the nature of plant nutrition was to understand the role of light as an energy source for the food-making process of plants. That was recognized in 1845 by the German biologist Robert Mayer. He wrote, "plants absorb one form of energy, light, and put forth another, chemical". By this time the green pigment found in green plants had been named chlorophyll. The role of light energy lead to the food-making process in plants being called photosynthesis.

Photosynthesis link to an Internet Website

6CO2 + 6H2O C6H12O6 + 6O2

chlorophyll is found only in the chloroplasts

While the equation above looks like a simple one step reaction, there are actually quite a few steps between the reactants and products. This complex reaction can be broken down into the following two reaction systems:

Chlorophyll: the chemical that makes it all possible.

Chlorophyll is a very large molecule with a chemical formula of to a local picture This molecule is called a pigment because it absorbs certain wavelengths of light. Its color represents the colors of light that it reflects, not absorbs. Therefore, green light is not useful to chlorophyll. Red and blue wavelengths of light are absorbed and provide the energy for photosynthesis.

Types of chlorophyll:

Chloroplasts: the sites of photosynthesis.

Chlorophyll is only found in chloroplasts, cell organells, never in cell cytoplasm. The structure of chloroplasts is quite complex, but these are the major structures:

an electron microscope picture of a chloroplast inside a plant cell

Reaction centers: link to an Internet Website

Only 1 in 250 chlorophyll molecules actually converts quanta, units of light energy, into usable energy. These molecules are called reaction-center chlorophyll. The other molecules absorb light energy and deliver it to the reaction-center molecule. These bulk chlorophyll molecules are known as antenna pigments because they collect and channel energy. A unit of several hundred antenna pigment molecules plus a reaction center is called a photosynthetic unit.

The large number of antenna pigment molecules in each photosynthetic unit enables its reaction center to be constantly supplied with quanta of energy.

Factors determining the rate of photosynthesis:

Photosynthetic efficiency:

To get some idea about just how well photosynthesis changes light energy into chemical energy, follow this process:

This is an absolutely maximum value, good only for red light and completely optimal conditions, including completely ignoring photorespiration, which substantially reduces photosynthesis by most plants under field conditions. Most practical agricultural and forestry measurements in the field give efficiencies at or below 1%.

Not all plants have the same photosynthetic process.

Significance of photosynthesis:

Photosynthesis is responsible for the conversion of carbon from carbon dioxide into organic compounds in plants. It allows the plant to make organic building blocks, new cells, starch, and proteins. Without this process, life as we know it would not exist on earth. Plants provide, directly or indirectly, food for all animals and all of our atmospheric oxygen.

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