Energy Metabolism in Animals and Plants
Energy Metabolism in Animals and Plants
Metabolism focuses on the sum total of all physical and chemical changes that take place within an organism. Derived from the Greek word metabole, meaning "change," it includes all energy and material transformations that occur within a living cell.
Energy is a fundamental feature of life. Processes at the cellular level are the same whether they are in animals, plants, fungi, or bacteria. Living matter is made up of large molecules called proteins, which are assembled from some 20 amino acids.
At the beginning of the nineteenth century, however, people held to traditional beliefs that special biological laws or forces controlled life. This belief, called vitalism, was well entrenched in the romantic thinking prevalent in that period. It is understandable that people who had no knowledge of atoms and molecules would explain life by adopting such ideas to account for the apparent miracle of life.
Scientists began to apply the principles of chemistry and physics to biological functions during the nineteenth century. Friedrich Wöhler (1800-1882) shocked the academic world by making an organic compound out of inorganic materials. This direct challenge attacked the precepts that non-living things are made only from non-living things and living things come from only living things. Henri Dutrochet (1776-1847) studied a variety of living things and argued that processes are similar in all living things. He believed life can be explained in terms of physical and chemical forces. German physiologist Max Rubner (1884-1932) determined that certain foods have energy value. He opened the door for the study of comparative nutrition. Max von Pettenkofer (1818-1907) and Carl von Voit (1831-1908) created measuring devices to study the physiology of metabolism.
As the century progressed, these investigators, along with many others, helped render vitalism obsolete and established the foundations of modern biology.
It is hard for people today to put themselves in the state of mind of scientists at the beginning of the nineteenth century. Modern scientists agree that both living and non-living materials are made of molecules. But in the 1800s, many prominent scientists did not accept molecules as real. Likewise, they believed living and non-living things were made differently. Living things were endowed with a special, or "vital," life force that was completely separate and different from inorganic things. The legacy of alchemists—pseudoscientists who sought to turn lead into gold—also left a magical or mystical aura around chemistry.
John Dalton (1726-1824), Amadeo Avogadro (1776-1856), and others changed how chemicals were made in the laboratory. Dalton had proposed that every element is made of tiny particles called atoms, which can neither be divided nor destroyed. Every atom of each element is identical. Scientists began to argue that living things also were made of the same substances as non-living things. A great debate of "mechanism versus vitalism" ensued. During the nineteenth century the debate played out in experiments that determined the fundamental chemistry and physics of life. It seemed amazing that the real vital force could be found in chemistry, determined by the various ways in which certain atoms are arranged. In fact, it was shown that these arrangements of atoms determine organic molecules such as proteins, amino acids, and nucleic acids.
Metabolism involves the chemical activities essential to life and is a process of building up and breaking down complex substances. These substances are needed by the living organism to grow, repair or replace cells, and give energy. Today, the term "organic" is used to refer to chemistry of carbon compounds, but in the early nineteenth century it referred to all things made of living matter.
Fredrich Wöhler, a German physician, lived in Heidelburg when he came under the influence of Leopold Gmelin, one of Germany's most prominent physicians. Gmelin recognized Wöhler's talent and encouraged him to go into chemistry. Wöhler went to Stockholm to study with the leading chemist of Europe, Jöns Berzelius (1779-1848). There he absorbed his professor's new techniques and enthusiasm for finding new elements. While working in the municipal technical school in Berlin, Wöhler made two of his major discoveries. In 1828 he accidentally synthesized urea—the chief compound used by the body to excrete nitrogenous wastes. Fascinated by the synthesis, he pursued and found that urea had the same chemicals as ammonium cyanate. The formula for ammonium cyanate is NH4CNO, while the formula for urea is CO(NH2)2. The two compounds contain exactly the same number of atoms of the same elements, though their properties are very different. This was the first discovery of an isomer. The significance of the discovery was that he was able to synthesize urea, the product of a living process, from the inorganic compound. This showed inorganic materials could be made into organic substances. This realization stunned the scientific world.
After the death of his wife, Wöhler befriended German chemist Justus von Liebig (1803-1873). The two investigated the theory of radicals—groups of atoms that work together as one atom—and struggled to comprehend the nature of organic compounds. Wöhler has been credited as the father of organic chemistry. With the synthesis of urea, he discredited the notion of the vital force once and for all. Older historians hailed this as a deliberate attempt by Wöhler to smash vitalism; however, most recent historians contend that he was much more interested in urea and its compounds than in philosophical statements.
French physiologist Henri Dutrochet noticed that the chemical and physical processes in plants and animals were very similar. While investigating cellular processes, he discovered and named the process of osmosis. Osmosis is the passage of substances from a place of high concentration to low concentration across a barrier or membrane. He was the first to investigate respiration in plants and found that green pigment in plants used carbon dioxide. He studied light sensitivity and geotropism, the response to gravity. Constructing an osmometer, a device that measures osmotic pressure, he recognized the role of internal plant transport by diffusion across these semipermeable membranes. He was one of the first to reorganize that individual cells are very important to the functioning of the organism.
Dutrochet insisted that physical and chemical forces explain the processes of living things and that all living things are similar. In 1824 he advanced the understanding of the cell principle when he declared that all organic tissues are actually globular cells of exceeding smallness. These precepts are his most important contribution to science.
Justus von Liebig made several exciting discoveries relating to organic chemistry. He organized the compounds into the various groups. He was one of the first to apply chemistry to biology in the field of biochemistry. He became a university professor in 1824 and was very dedicated to chemistry education, setting up a design for laboratory instruction that made possible the great advances in chemistry in Germany in the nineteenth century. He had many outstanding students, such as Max Rubner and Max von Pettenkofer. Liebig's work with isomers of cyanic and fulmeric acids influenced Wöhler's work. In 1838 Liebig shifted his attention to the chemistry of animals and plants, studying their metabolism. He rejected the idea that plants get their food from humus and made great contributions to practical agriculture.
Rubner added to the knowledge of metabolism by discovering that the rate is proportional to the surface of the body. He made a respiratory apparatus into a calorimeter. Placing a dog inside, he measured the heat production of the dog, relating size to diet. The experiment showed that heat in warm-blooded animals is related to the energy supply of nutritive materials. In 1884 he extended the work of Liebig by making quantitative determinations of the energy values of certain foods. We now know these values as calories. His work made possible a scientific explanation for metabolism and a basis for the study of comparative nutrition.
Max von Pettenkofer, a German hygienist and chemist, is recognized as the father of experimental hygiene. He used specially designed chambers for analyzing food and air consumed by an animal, and also the products exhaled and excreted. He added much information to the process of metabolism.
Carl von Voit, a German physiologist, measured metabolism in humans and other mammals and founded modern nutritional science. Voit was a student of both Liebig and Wöhler at the University of Munich. He later served as a professor there and conducted experiments to determine how animals use proteins, fats, and carbohydrates. In 1882 he collaborated with Pettenkofer to build a respiration chamber that could hold human subjects. They tested subjects during various states of activity, rest, and fasting by measuring how much food was taken in and how much waste was excreted. They also measured consumption of oxygen and how much carbon dioxide and heat were given off. For 11 years they measured calorie intake in relation to energy requirements. Their experiments illustrated the laws of conservation of energy in living animals and showed that metabolism is actually carried out in the cells rather than in the blood, as some had proposed.
Together, the work of these scientists made important strides toward revealing the complex processes of molecular activity and metabolism. They laid the foundation for the amazing breakthroughs in cell respiration and molecular biology in the twentieth century.
EVELYN B. KELLY
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