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Biochemistry is the study of the molecular basis of life.The chemical energy of ATP is then utilized for nearly all the functions of the cell, such as synthesis of its component molecules, active transport of substances into and out of the cell, and also osmotic or mechanical work, for example contraction in muscles and motility in single celled organisms.The integration of molecular genetics and protein biochemistry has also had a profound influence in medicine, and has shed much light on the underlying molecular lesions causing inherited diseases such as sickle cell anemia, cystic fibrosis, and hemophilia.In the field of medical diagnostics, DNA probes are being used in the precise diagnosis of genetic disorders and in industry, genetically engineered bacteria, containing recombinant DNA, are producing important proteins such as insulin, growth hormones, and many others.All reactions in living cells are catalyzed by enzymes and certain basic metabolic pathways, such as glycolysis, the citric acid cycle, and beta-oxidation of fats, are ubiquitous.

Biochemistry is the study of the molecular basis of life. It investigates the chemical substances and processes that occur within plants, animals, and microorganisms at all levels of organization, from single cells to tissues and organs. It is a biological discipline that attempts to answer biological questions, but its methods frequently draw upon the techniques of analytical, organic, and physical chemistry. Today biochemistry is synonymous with two somewhat older terms: physiological chemistry and biological chemistry. It is also closely attached to another modern discipline known as molecular biology. The latter specifically deals with the chemistry and function of macromolecules such as nucleic acids and proteins. Since the beginning of the nineteenth century, the growth of biochemistry has been explosive and, with all its sub-disciplines, it is now one of the most active of all biological sciences. Today, its scope of investigation is very broad and includes the analysis of the structural components of living organisms, the chemical reactions involved in the metabolic interconversion of substances, the mechanisms of energy production and utilization, and the chemical basis of the storage and transfer of genetic information.

From a thermodynamic point of view, living organisms are open systems, meaning that they exchange matter and energy with their environment. Their chemistry approximates that of a steady state in which the rates of influx and efflux of matter and energy are equal. In biochemical reactions, the reactants reach steady state concentrations that are different from the equilibrium concentrations in the classical thermodynamic sense. Within this context, living cells are often considered as isothermal (operating at a constant temperature) chemical machines that absorb energy from the environment and convert it to chemical energy, for example in the form of adenosine triphosphate (ATP). The chemical energy of ATP is then utilized for nearly all the functions of the cell, such as synthesis of its component molecules, active transport of substances into and out of the cell, and also osmotic or mechanical work, for example contraction in muscles and motility in single celled organisms.

The chemical mechanisms of many central processes in living cells are now understood, and common molecular patterns underlie the diverse expressions of life on Earth. There are, for example, structural similarities in that all cells are enclosed by membranes constructed as lipid bilayers. It is also of enormous interest that the genetic material of all organisms consists of the nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), as this suggests that all life originated from a common ancestor. In most cases the ability of living organisms to reproduce themselves lies in the complementary structures of the two strands of the DNA molecules. DNA contains information coded into its chemical components and this is required for the synthesis of both structural proteins and the catalytic proteins known as enzymes. The interactions of these proteins with other classes of substances (fats, carbohydrates, etc.) create and maintain the structures of the organism. All reactions in living cells are catalyzed by enzymes and certain basic metabolic pathways, such as glycolysis, the citric acid cycle, and beta-oxidation of fats, are ubiquitous.

Biochemistry is profoundly concerned with the metabolism of living organisms. Metabolic reactions are not independent of each other, but are organized into pathways that may consist of as much as 20 or more steps. The pathways themselves are also not isolated but constitute a coherent web of interdependent reactions. Those metabolic pathways serving to generate ATP from the breakdown of fuel substances are called catabolic pathways. Examples include the glycolytic pathway that degrades glucose to pyruvate or lactate. Other pathways that synthesize cell components are called anabolic pathways and examples include the biosynthesis of glucose, fatty acids, or proteins. There are also amphibolic pathways having both catabolic and anabolic functions. Some metabolic pathways function as cycles, an example of one of these being the citric acid or tricarboxylic acid (TCA) cycle. Taken as a whole, the network of interconnected metabolic pathways channel energy and some of the matter taken from the environment into the synthesis of cell components, and ultimately into the growth and replication of organisms. The coupling of many reactions into this metabolic network is achieved by several universal levels of regulation. Examples of regulatory mechanisms include feedback inhibition, such as the inhibition of synthetic pathways by their end product, and the activation or inactivation of enzymes by chemical modification such as phosphorylation. There are also mechanisms of regulating the amounts of enzymes synthesized in different metabolic situations by the repression and depression of genes.

The rapid development of molecular biology in recent years has had a large influence on the field of biochemistry and most biochemical investigations today rely in some way on recombinant gene technology. In the field of medical diagnostics, DNA probes are being used in the precise diagnosis of genetic disorders and in industry, genetically engineered bacteria, containing recombinant DNA, are producing important proteins such as insulin, growth hormones, and many others. Research is also providing much scope for the rational design of new drugs. The integration of molecular genetics and protein biochemistry has also had a profound influence in medicine, and has shed much light on the underlying molecular lesions causing inherited diseases such as sickle cell anemia, cystic fibrosis, and hemophilia. Such knowledge has aided in the discovery and implementation of more effective therapies.