Metabolism refers to the highly integrated network of chemical reactions by which living cells grow and sustain themselves. This network is composed of two major types of pathways: anabolism and catabolism. Anabolism uses energy stored in the form of adenosine triphosphate (ATP) to build larger molecules from smaller molecules. Catabolic reactions degrade larger molecules in order to produce ATP and raw materials for anabolic reactions.

Together, these two general metabolic networks have three major functions: (1) to extract energy from nutrients or solar energy; (2) to synthesize the building blocks that make up the large molecules of life: proteins, fats, carbohydrates, nucleic acids, and combinations of these substances; and (3) to synthesize and degrade molecules required for special functions in the cell.

These reactions are controlled by enzymes, protein catalysts that increase the speed of chemical reactions in the cell without themselves being changed. Each enzyme catalyzes a specific chemical reaction by acting on a specific substrate, or raw material. Each reaction is just one in a sequence of catalyticsteps known as metabolic pathways. These sequences may be composed of up to20 enzymes, each one creating a product that becomes the substrate--or raw material--for the subsequent enzyme. Often, an additional molecule called a coenzyme is required for the enzyme to function. For example, some coenzymes accept an electron that is released from the substrate during the enzymatic reaction. Most of the water-soluble vitamins of the B complex serve as coenzymes;riboflavin (Vitamin B2) for example, is a precursor of the coenzyme flavine adenine dinucleotide, while pantothenate is a component of coenzyme A, an important intermediate metabolite.

The series of products created by the sequential enzymatic steps of anabolismor catabolism are called metabolic intermediates, or metabolites. Each steprepresents a small change in the molecule, usually the removal, transfer, oraddition of a specific atom, molecule, or group of atoms that serves as a functional group, such as the amino groups (-NH2) of proteins.

Most such metabolic pathways are linear, that is, they begin with a specificsubstrate and end with a specific product. However, some pathways, such as the Krebs cycle, are cyclic. Often, metabolic pathways also have branches thatfeed into or out of them. The specific sequences of intermediates in the pathways of cell metabolism are called intermediary metabolism.

Among the many hundreds of chemical reactions there are only a few that are central to the activity of the cell, and these pathways are identical in mostforms of life.

All reactions of metabolism, however, are part of the overall goal of the organism to maintain its internal orderliness, whether that organism is a singlecelled protozoan or a human. Organisms maintain this orderliness by removingenergy from nutrients or sunlight and returning to their environment an equal amount of energy in a less useful form, mostly heat. This heat becomes dissipated throughout the rest of the organism's environment.

According to the first law of thermodynamics, in any physical or chemical change, the total amount of energy in the universe remains constant, that is, energy cannot be created or destroyed. Thus, when the energy stored in nutrientmolecules is released and captured in the form of ATP, some energy is lost as heat. But the total amount of energy is unchanged.

The second law of thermodynamics states that physical and chemical changes proceed in such a direction that useful energy undergoes irreversible degradation into a randomized form--entropy. The dissipation of energy during metabolism represents an increase in the randomness, or disorder, of the organism's environment. Because this disorder is irreversible, it provides the driving force and direction to all metabolic enzymatic reactions.

Even in the simplest cells, such as bacteria, there are at least a thousand such reactions. Regardless of the number, all cellular reactions can be classified as one of two types of metabolism: anabolism and catabolism. These reactions, while opposite in nature, are linked through the common bond of energy.Anabolism, or biosynthesis, is the synthetic phase of metabolism during which small building block molecules, or precursors, are built into large molecular components of cells, such as carbohydrates and proteins.

Catabolic reactions are used to capture and save energy from nutrients, as well as to degrade larger molecules into smaller, molecular raw materials for reuse by the cell. The energy is stored in the form of energy-rich ATP, whichpowers the reactions of anabolism. The useful energy of ATP is stored in theform of a high-energy bond between the second and third phosphate groups of ATP. The cell makes ATP by adding a phosphate group to the molecule adenosinediphosphate (ADP). Therefore, ATP is the major chemical link between the energy-yielding reactions of catabolism, and the energy-requiring reactions of anabolism.

In some cases, energy is also conserved as energy-rich hydrogen atoms in thecoenzyme nicotinamide adenine dinucleotide phosphate in the reduced form of NADPH. The NADPH can then be used as a source of high-energy hydrogen atoms during certain biosynthetic reactions of anabolism.

In addition to the obvious difference in the direction of their metabolic goals, anabolism and catabolism differ in other significant ways. For example, the various degradative pathways of catabolism are convergent. That is, many hundreds of different proteins, polysaccharides, and lipids are broken down into relatively few catabolic end products. The hundreds of anabolic pathways,however, are divergent. That is, the cell uses relatively few biosynthetic precursor molecules to synthesize a vast number of different proteins, polysaccharides, and lipids.

The opposing pathways of anabolism and catabolism may also use different reaction intermediates or different enzymatic reactions in some of the steps. Forexample, there are 11 enzymatic steps in the breakdown of glucose into pyruvic acid in the liver. But the liver uses only nine of those same steps in thesynthesis of glucose, replacing the other two steps with a different set ofenzyme-catalyzed reactions. This occurs because the pathway to degradation ofglucose releases energy, while the anabolic process of glucose synthesis requires energy. The two different reactions of anabolism are required to overcome the energy barrier that would otherwise prevent the synthesis of glucose.

Another reason for having slightly different pathways is that the corresponding anabolic and catabolic routes must be independently regulated. Otherwise,if the two phases of metabolism shared the exact pathway (only in reverse) aslowdown in the anabolic pathway would slow catabolism, and vice versa.

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