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.
6CO2 + 6H2O C6H12O6 + 6O2
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:
- Light reactions: H2O O2 + ATP + NADPH2
- This system depends on sunlight for activation energy.
- Light is absorbed by chlorophyll a which "excites" the electrons in the chlorophyll molecule.
- Electrons are passed through a series of carriers and adenosine triphosphate or ATP (energy) is produced.
- Water is split, giving off oxygen.
- Dark reactions: ATP + NADPH2 + CO2 C6H12O6
- While this system depends on the products from the light reactions, it does not directly require light energy.
- Energy from the light reaction is used to convert carbon dioxide into a series of carbon sugars.
- The ultimate product is glucose.
Chlorophyll: the chemical that makes it all possible.
Chlorophyll is a very large molecule with a chemical formula of C55H70MgN4O6. 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:
- Chlorophyll a - a bright green pigment that is indispensable to photosynthesis.
- Chlorophyll b - an olive-green pigment that contributes to photosynthesis.
- Chlorophyll c - a yellow-green pigment that is only an accessory pigment.
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:
- The organelle is surrounded by a double membrane.
- Inside the inner membrane is a complex mix of enzymes and water called stroma.
- Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum.
- Each of the flattened sacs which make up the granum is called a thylakoid.
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:
- Light intensity:
- light-limited - At low light intensities photosynthesis is starved for energy. The system uses most of the quanta the pigments capture and is therefore maximally efficient, but because there are few quanta, the rate is low. Under these conditions the rate may only slightly exceede the respiration rate, so the net photosynthetic production by the cells is actually very poor.
- light saturation - As the light intensity is raised, the rate of photosynthetic production increases. However, a plateau is reached at about one-fourth the intensity of full sunlight. Light saturation does not result from any limitation in the capacity of chlorophyll to absorb light. It represents the maximum rate at which the dark reactions of photosynthesis can use energy from chlorophyll. A further increase in the energy supply becomes excess energy and it converted to heat and wasted.
- The light reactions of photosynthesis are not temperature dependent.
- The dark reactions of photosynthesis are temperature dependent enzymatic processes.
- These reactions do have an optimum temperature. Photosynthesis by most plants increases only up to about 25o C, 77o F. The rate levels out and then actually declines as the temperature approaches or exceeds human body temperature. This seems odd because we normally think of human body temperature as a physiological optimum temperature.
- Other factors:
- length of day
- amount of carbon dioxide available
- level of air pollution
To get some idea about just how well photosynthesis changes light energy into chemical energy, follow this process:
- At least 10 moles of quanta are required in photosynthesis.
- Red light contains about 40 kcal/mole of quanta, therefore 10 moles of quanta would contain 400 kcal of energy.
- One mole of glucose (C6H12O6) is known to contain 686 kcal of stored energy.
- One-sixth mole of glucose can be formed photosynthetically from one mole of CO2. This amount contains 114 kcal of stored energy.
- The maximum efficiency of photosynthetic energy conversion is therefore 114/400, or about 28.5%.
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.
- C3 Plants:
- Most plants are C3 type.
- These plants form 3-carbon organic acids as their first stable products.
- C4 Plants:
- These plants include sugar cane, corn, and sorghum.
- The leaf mesophyll produces 4-carbon organic acids as their first stable products.
- These plants are more efficient in producing sugars because they have little or no sugar loss during respiration.
- The efficiency of photosynthetic energy conversion in C4 plants can be as much as 50% higher than in C3 plants.
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.