The Chemical Reactions of Life

From Metabolism to Photosynthesis

Britannica Educational Publishing, Kara Rogers

• Publisher: Britannica Educational Publishing
• Year: 2010
• ISBN:
  9781615303878
• Topic: Biological Sciences
• Subtopic: Biochemistry
• Index: Biological Sciences

The development and evolution of all species can, in many ways, be traced to a few biochemical reactions that facilitate metabolic and/or photosynthetic changes in each life form. Indeed, advances in the field of biochemistry have intimately depended on the study of these processes and the way basic molecules fragment and synthesize to produce elements vital to the survival of each organism. This insightful volume considers the various types, causes, and results of different reactions that operate at the cellular level and beyond to sustain biological activity.

CHAPTER 1
Biochemical Reactions

The study of the chemical substances and processes that occur in plants, animals, and microorganisms has long formed a vital area of science. Biochemical reactions constitute the driving force behind the constant change of organisms, from changes that occur during development to changes that mark the evolution of life. The field of biochemistry deals with the chemistry of life, and as such it draws on the techniques of analytical, organic, and physical chemistry, as well as those of physiologists concerned with the molecular basis of vital processes. All chemical changes within the organism—either the degradation of substances (generally to gain necessary energy) or the buildup of complex molecules necessary for life processes—are collectively described by the term metabolism. These chemical changes depend on the action of organic catalysts known as enzymes, and enzymes, in turn, depend for their existence on the genetic apparatus of the cell. It is not surprising, therefore, that biochemistry enters into the investigation of chemical changes in disease, drug action, and other aspects of medicine, as well as in nutrition, genetics, and agriculture.

The term biochemistry is synonymous with two somewhat older terms: physiological chemistry and biological chemistry. Those aspects of biochemistry that deal with the chemistry and function of very large molecules (e.g., proteins and nucleic acids) are often grouped under the term molecular biology. The field of biochemistry emerged as an official area of science around 1900. Its origins, however, can be traced back much further. In fact, its early history is part of the early history of both physiology and chemistry.

HISTORICAL OVERVIEW OF BIOCHEMISTRY

---

English scientist Joseph Priestley designed this apparatus for observations on different kinds of gases. Hulton Archive/Getty Images

The particularly significant past events in biochemistry have been concerned with placing biological phenomena on firm chemical foundations. Before chemistry could contribute adequately to medicine and agriculture, however, it had to free itself from immediate practical demands in order to become a pure science. This happened in the period from about 1650 to 1780, starting with the work of Robert Boyle and culminating in that of Antoine-Laurent Lavoisier, the father of modern chemistry. Boyle questioned the basis of the chemical theory of his day and taught that the proper object of chemistry was to determine the composition of substances. His contemporary John Mayow observed the fundamental analogy between the respiration of an animal and the burning, or oxidation, of organic matter in air. Then, when Lavoisier carried out his fundamental studies on chemical oxidation, grasping the true nature of the process, he also showed, quantitatively, the similarity between chemical oxidation and the respiratory process. Photosynthesis was another biological phenomenon that occupied the attention of the chemists of the late 18th century. The demonstration, through the combined work of Joseph Priestley, Jan Ingenhousz, and Jean Senebier, that photosynthesis is essentially the reverse of respiration was a milestone in the development of biochemical thought.

Joseph Priestley

(b. March 13, 1733, Birstall Fieldhead, near Leeds, Yorkshire, Eng.—d. Feb. 6, 1804, Northumberland, Pa., U.S.)

English theologian, political theorist, and physical scientist Joseph Priestley conducted important work in the field of chemical reactions and change. He is considered the discoverer of nitrogen, carbon monoxide, ammonia, and several other gases. Priestley worked as a teacher and lecturer in various subjects before joining the ministry in 1767. His early scientific studies resulted in his History and Present State of Electricity (1767), which became a fundamental text in the field. His Essay on Government (1768) influenced later utilitarianism. In 1774 he identified oxygen, which had been discovered two years earlier by Swedish chemist Carl Wilhelm Scheele. However, it was Priestley’s report on this element that led Antoine Lavoisier to deduce oxygen’s nature and role. Lavoisier also named the element oxygen (oxygène) from the Greek words for “acid former.”

Priestley’s theological works include History of the Corruptions of the Christian Church (1782), burned as sacrilegious in 1785, and A General History of the Christian Church, 6 vol. (1790–1802). His nonconformist religious views and his political activities, particularly in support of the French Revolution, made him increasingly controversial in England, and he immigrated to the U.S. in 1794.

In spite of these early fundamental discoveries, rapid progress in biochemistry had to wait upon the development of structural organic chemistry, one of the great achievements of 19th-century science. A living organism contains many thousands of different chemical compounds. The elucidation of the chemical transformations undergone by these compounds within the living cell is a central problem of biochemistry. Clearly, the determination of the molecular structure of the organic substances present in living cells had to precede the study of the cellular mechanisms, whereby these substances are synthesized and degraded.

There are few sharp boundaries in science, and the boundaries between organic and physical chemistry, on the one hand, and biochemistry, on the other, have always shown much overlap. Biochemistry has borrowed the methods and theories of organic and physical chemistry and applied them to physiological problems. Progress in this path was at first impeded by a stubborn misconception in scientific thinking—the error of supposing that the transformations undergone by matter in the living organism were not subject to the chemical and physical laws that applied to inanimate substances and that consequently these “vital” phenomena could not be described in ordinary chemical or physical terms. Such an attitude was taken by the vitalists, who maintained that natural products formed by living organisms could never be synthesized by ordinary chemical means. The first laboratory synthesis of an organic compound, urea, by Friedrich Wöhler in 1828, was a blow to the vitalists but not a decisive one. They retreated to new lines of defense, arguing that urea was only an excretory substance—a product of breakdown and not of synthesis. The success of the organic chemists in synthesizing many natural products forced further retreats of the vitalists. It is axiomatic in modern biochemistry that the chemical laws that apply to inanimate materials are equally valid within the living cell.

At the same time that progress was being impeded by a misplaced kind of reverence for living phenomena, the practical needs of humans operated to spur the progress of the new science. As organic and physical chemistry erected an imposing body of theory in the 19th century, the needs of the physician, the pharmacist, and the agriculturalist provided an ever-present stimulus for the application of the new discoveries of chemistry to various urgent practical problems.

The work of two outstanding figures of the 19th century, Justus von Liebig and Louis Pasteur, emphasized the significance of the application of chemistry to the study of biology. Liebig studied chemistry in Paris and carried back to Germany the inspiration gained by contact with the former students and colleagues of Lavoisier. He established at Giessen a great teaching and research laboratory, one of the first of its kind, which drew students from all over Europe.

Besides putting the study of organic chemistry on a firm basis, Liebig engaged in extensive literary activity, attracting the attention of all scientists to organic chemistry and popularizing it for the layman as well. His classic works, published in the 1840s, had a profound influence on contemporary thought. Liebig described the great chemical cycles in nature. He pointed out that animals would disappear from the face of Earth if it were not for the photosynthesizing plants, since animals require for their nutrition the complex organic compounds that can be synthesized only by plants. The animal excretions and the animal body after death are also converted by a process of decay to simple products that can be re-utilized only by plants.

In contrast with animals, green plants require for their growth only carbon dioxide, water, mineral salts, and sunlight. The minerals must be obtained from the soil, and the fertility of the soil depends on its ability to furnish the plants with these essential nutrients. But the soil is depleted of these materials by the removal of successive crops; hence the need for fertilizers. Liebig pointed out that chemical analysis of plants could serve as a guide to the substances that should be present in fertilizers. Agricultural chemistry as an applied science was thus born.

In his analysis of fermentation, putrefaction, and infectious disease, Liebig was less fortunate. He admitted the similarity of these phenomena but refused to admit that living organisms might function as the causative agents. It remained for Pasteur to clarify that matter. In the 1860s Pasteur proved that various yeasts and bacteria were responsible for “ferments,” substances that caused fermentation and, in some cases, disease. He also demonstrated the usefulness of chemical methods in studying these tiny organisms and was the founder of what came to be called bacteriology.

Later, in 1877, Pasteur’s ferments were designated as enzymes, and, in 1897, German chemist Eduard Buchner clearly showed that fermentation could occur in a press juice of yeast, devoid of living cells. Thus a life process of cells was reduced by analysis to a nonliving system of enzymes. The chemical nature of enzymes remained obscure until 1926, when the first pure crystalline enzyme (urease) was isolated. This enzyme and many others subsequently isolated proved to be proteins, which had already been recognized as high-molecular-weight chains of subunits called amino acids.

The mystery of how minute amounts of dietary substances known as the vitamins prevent diseases such as beriberi, scurvy, and pellagra became clear in 1935, when riboflavin (vitamin B₂) was found to be an integral part of an enzyme. Subsequent work has substantiated the concept that many vitamins are essential in the chemical reactions of the cell by virtue of their role in enzymes.

In 1929 the substance adenosine triphosphate (ATP) was isolated from muscle. Subsequent work demonstrated that the production of ATP was associated with respiratory (oxidative) processes in the cell. In 1940 Fritz Albert Lipmann proposed that ATP is the common form of energy exchange in many cells, a concept now thoroughly documented. ATP has been shown also to be a primary energy source for muscular contraction.

The use of radioactive isotopes of chemical elements to trace the pathway of substances in the animal body was initiated in 1935 by two U.S. chemists, Rudolf Schoenheimer and David Rittenberg. That technique provided one of the single most important tools for investigating the complex chemical changes that occur in life processes. At about the same time, other workers localized the sites of metabolic reactions by ingenious technical advances in the studies of organs, tissue slices, cell mixtures, individual cells, and, finally, individual cell constituents, such as nuclei, mitochondria, ribosomes, lysosomes, and membranes.

In 1869 a substance was isolated from the nuclei of pus cells and was called nucleic acid, which later proved to be deoxyribonucleic acid (DNA), but it was not until 1944 that the significance of DNA as genetic material was revealed, when bacterial DNA was shown to change the genetic matter of other bacterial cells. Within a decade of that discovery, the double helix structure of DNA was proposed by biochemists James D. Watson and Francis Crick, providing a firm basis for understanding how DNA is involved in cell division and in maintaining genetic characteristics. Advances have continued since that time, with such landmark events as the first chemical synthesis of a protein, the detailed mapping of the arrangement of atoms in some enzymes, and the elucidation of intricate mechanisms of metabolic regulation, including the molecular action of hormones.

Jean Senebier

(b. May 6, 1742, Geneva, Switz.—d. July 22, 1809, Geneva)

Swiss botanist and naturalist Jean Senebier demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light.

After completing studies in theology and serving as city librarian of Geneva, Senebier began a study of botany. In 1787 he became a staff member of the Encyclopédie méthodique, with the task of producing a section on plant physiology. His most important contribution to botany, however, appeared in Mémoires physicochimiques sur l’influence de la lumière…. (1782; “Physico-Chemical Memoirs on the Influence of Light”), the Recherches sur l’influence de la lumière solaire … (1783; “Research on the Influence of Sunlight”), and Expériences sur l’action de la lumière solaire dans la végétation (1788; “Experiments on the Action of Sunlight on Vegetation”). In 1800 he completed a major work, Physiologie végétale, in which he demonstrated that light is the agent responsible for the fixation of carbon dioxide and that oxygen is liberated only in the presence of carbon dioxide. This work was fundamental to subsequent research in photosynthesis.

THE CHEMISTRY AND REACTIONS OF CELLS

---

A description of life at the molecular level includes a description of all the complexly interrelated chemical changes that occur within the cell—i.e., the processes known as intermediary metabolism. The processes of growth, reproduction, and heredity, also subjects of the biochemist’s curiosity, are intimately related to intermediary metabolism and cannot be understood independently of it. The properties and capacities exhibited by a complex multicellular organism can be reduced to the properties of the individual cells of that organism, and the behaviour of each individual cell can be understood in terms of its chemical structure and the chemical changes occurring within that cell.

CHEMICAL COMPOSITION OF LIVING MATTER

In addition to water and salts or minerals, every living cell also contains a large number of organic compounds, substances composed of carbon combined with varying amounts of hydrogen and usually also of oxygen. Nitrogen, phosphorus, and sulfur are likewise common constituents. In general, the bulk of the organic matter of a cell may be classified as (1) protein, (2) carbohydrate, and (3) fat, or lipid. Nucleic acids and various other organic derivatives are also important constituents. Each class contains a great diversity of individual compounds. Many substances that cannot be classified in any of the above categories also occur, though usually not in large amounts.

Proteins are fundamental to life, not only as structural elements (e.g., collagen) and to provide defense (as antibodies) against invading destructive forces but also because the essential biocatalysts are proteins. The chemistry of proteins is based on the researches of the German chemist Emil Fischer, whose work from 1882 demonstrated that proteins are very large molecules, or polymers, built up of about 24 amino acids. Proteins may vary in size from small—insulin with a molecular weight of 5,700 (based on the weight of a hydrogen atom as 1)—to very large—molecules with molecular weights of more than 1,000,000. The first complete amino acid sequence was determined for the insulin molecule in the 1950s. By 1963 the chain of amino acids in the protein enzyme ribonuclease (molecular weight 12,700) had also been determined, aided by the powerful physical techniques of X-ray-diffraction analysis. In the 1960s, Nobel Prize winners John Cowdery Kendrew and Max Ferdinand Perutz, utilizing X-ray studies, constructed detailed atomic models of the proteins hemoglobin and myoglobin (the respiratory pigment in muscle), which were later confirmed by sophisticated chemical studies. The abiding interest of biochemists in the structure of proteins rests on the fact that the arrangement of chemical groups in space yields important clues regarding the biological activity of molecules.

This plasticine model, made by Sir John Kendrew, was the first ever ...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: Biochemical Reactions
• Chapter 2: Classifying and Studying Chemical Reactions
• Chapter 3: The Chemical Reactions of Cells
• Chapter 4: The Fragmentation of Complex Molecules
• Chapter 5: The Combustion of Food Materials
• Chapter 6: The Biosynthesis of Cell Components and Regulation of Metabolism
• Chapter 7: The Transformation of Light Into Chemical Energy
• Chapter 8: The Process of Photosynthesis
• Chapter 9: Bioluminescence and Light Reactions
• Glossary
• Bibliography
• Index