V

Waste products

(acids, carbon dioxide)

Nutrients

(source of nitrogen, sulfur, etc.)

Figure 6.1 The Relationship Between Catabolism and Anabolism

Catabolism encompasses processes that harvest energy released during disassembly of compounds, using it to synthesize ATP; it also provides precursor metabolites used in biosynthesis. Anabolism, or biosynthesis, includes processes that utilize ATP and precursor metabolites to synthesize and assemble the subunits of the cell.

ics. The first law of thermodynamics recognizes that the energy in the universe can never be created or destroyed; however, it can be changed from one form to another. In other words, while energy cannot be created, potential energy can be converted to kinetic energy and vice versa, and one form of potential energy can be converted to another. For example, hydroelectric dams unleash the potential energy of water stored behind a dam, creating the kinetic energy of moving water; this can then be used to generate an electrical current, which can then be used to charge a battery.

Consistent with the laws of thermodynamics, cells do not create energy but instead convert available energy in the universe into a form that can be used by the cell. They do this in a variety of different ways. Photosynthetic organisms harvest the energy of sunlight, using it to power the synthesis of organic compounds such as glucose (figure 6.3). In other words, they convert the kinetic energy of photons to the potential energy of chemical bonds. In contrast, chemoorganotrophs obtain energy by degrading organic compounds such as glucose, releasing the potential energy of their chemical bonds. Thus, most chemoorganotrophs ultimately depend on solar energy harvested by photosynthetic organisms, because this is what is used to power the synthesis of glucose. ■ chemoorganotroph, p. 91

The amount of energy that can be released and used by degrading compounds can be explained by the concept of free energy. This is the amount of energy that can be gained by breaking the bonds of a compound. A chemical reaction, which breaks some bonds and forms others, results in a change in free energy. Energy is released in a chemical reaction if the reac-tants, or starting compounds, have more free energy than the products, or final compounds (figure 6.4a). Such a reaction is said to be exergonic. In contrast, if the products have more free

Figure 6.2 Forms of Energy Potential energy is stored energy, such as water held behind a dam. Kinetic energy is the energy of motion, such as the movement of water from behind the dam.

Radiant energy

Photosynthetic organisms

(harvest energy of sunlight and use it to synthesize organic compounds from CO2)

Photosynthetic organisms

(harvest energy of sunlight and use it to synthesize organic compounds from CO2)

Radiant energy converted by photosynthetic organisms

Organic compounds

(including glucose)

Organic compounds degraded by chemoorganotrophs

Chemoorganotrophs

(generate ATP by degrading organic compounds))

Chemoorganotrophs

(generate ATP by degrading organic compounds))

Figure 6.3 Most Chemoorganotrophs Depend on the Radiant Energy Harvested by Photosynthetic Organisms Photosynthetic organisms use the energy of sunlight to power the synthesis of organic compounds; chemoorganotrophs can then use those organic compounds as an energy source.

6.1 Principles of Metabolism 131

energy than the reactants, then the reaction requires an input of energy and is termed endergonic (figure 6.4b).

The change in free energy for a given reaction is the same regardless of the number of steps involved (figure 6.4c). For example, converting glucose to carbon dioxide and water in a single step by combustion releases the same amount of energy as degrading it in a series of steps. Cells exploit this fact to slowly release free energy from compounds, harvesting energy released at each step. A specific energy-releasing reaction is used to power an energy utilizing reaction.

Components of Metabolic Pathways

Metabolic processes often occur as a series of sequential chemical reactions, which constitute a metabolic pathway (figure 6.5). A series of intermediates are produced as the starting compound is gradually converted into the final product, or end product. A metabolic pathway may be linear, branched, or cyclical, and like the flow of a river that is controlled by dams, its activity may be modulated at certain points. In this way, a cell can regulate certain processes, ensuring that specific molecules are produced in precise quantities when needed. If a metabolic step is blocked, all products "downstream" of that blockage will be affected.

The intermediates and end products of metabolic pathways are sometimes organic acids, which are weak acids. Depending on the pH, these may exist primarily as either the undissociated form or the dissociated (ionized) form. Biologists often use the names of the two forms interchangeably, for example pyruvic acid and pyruvate. Note, however, that at the near-neutral pH inside the cell, the ionized form predominates, whereas outside of the cell, the acid may predominate. ■ pH, p. 23

To recognize what metabolic pathways accomplish, it is helpful to first understand the critical components—enzymes, ATP, the chemical energy source, electron carriers, and precursor metabolites (table 6.1).

The Role of Enzymes

A specific enzyme facilitates each step of a metabolic pathway. Enzymes are proteins that function as biological catalysts, accelerating the conversion of one substance, the substrate, into another, the product. Without enzymes, energy-yielding reactions would still occur, but at a rate so slow it would be imperceptible.

An enzyme catalyzes a chemical reaction by lowering the activation energy of that reaction (figure 6.6). This is the initial energy it takes to break chemical bonds; even exergonic chemical reactions have an activation energy. By lowering the activation energy barrier, enzymes allow chemicals to undergo rearrangements.

The Role of ATP

Adenosine triphosphate (ATP) is the energy currency of a cell, serving as the ready and immediate donor of free energy. It is composed of the sugar ribose, the nitrogenous base adenine, and three phosphate groups (see figure 2.11). Its counterpart, adenosine diphosphate (ADP), can be viewed as an acceptor

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