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132 Chapter 6 Metabolism: Fueling Cell Growth

Figure 6.4 Energetics of Chemical Reactions (a) Exergonic reactions release energy. (b) Endergonic reactions consume energy. (c)The change in free energy for a given reaction will be the same, regardless of the number of steps involved.

(a) LINEAR METABOLIC PATHWAY

Starting compound

Intermediate.

Intermediateb

End product

(b) BRANCHED METABOLIC PATHWAY

Intermediate^

End product^

Starting compound

Intermediatea

Intermediateb2

End product2

(c) CYCLICAL METABOLIC PATHWAY

Starting compound

Starting compound

Figure 6.5 Metabolic Pathways Metabolic processes often occur as a series of sequential chemical reactions that convert starting compounds into intermediates and then, ultimately, into end products. A metabolic pathway can be (a) linear, (b) branched, or (c) cyclical.

6.1 Principles of Metabolism

Table 6.1 Components of Metabolism

Component

Function

Enzymes

Biological catalysts. A specific enzyme facilitates each step of a metabolic pathway by lowering the activation energy of a reaction that converts a substrate into a product.

Adenosine triphosphate (ATP)

Serves as the energy currency of the cell. Hydrolysis of its high-energy phosphate bonds can be used to power endergonic reactions.

Substrate-level phosphorylation

Synthesis of ATP from ADP and inorganic phosphate (PJ using the energy released in an exergonic reaction.

Oxidative phosphorylation

Synthesis of ATP from ADP and inorganic phosphate (PJ using the energy of a proton motive force; in oxidative phosphorylation, chemical energy is harvested to drive the formation of the proton motive force.

Photophosphorylation

Synthesis of ATP from ADP and inorganic phosphate (PJ using the energy of a proton motive force; in photophosphorylation, radiant energy is harvested to drive the formation of the proton motive force.

Chemical energy source

The compound that is oxidized to release energy; also called an electron donor.The cell harvests that energy to synthesize ATP.

Electron carriers

Molecules including NAD+, NADP+, and FAD; the reduced forms, NADH, NADPH, and FADH2, carry the electrons that are removed during the oxidation of the energy source.The reduced forms are called reducing power. Depending on the carrier, the reducing power can be used either to generate proton motive force or in biosynthesis.

Precursor metabolites

Metabolic intermediates that can either be used to make the subunits of macromolecules, or be oxidized to generate ATP.

of free energy. An input of energy is required to add an inorganic phosphate group (Pi) to ADP, forming ATP; energy is released when that group is removed from ATP, yielding ADP (figure 6.7).

The phosphate groups of ATP are arranged in tandem (see figure 2.11). Their negative charges repel each other, making the bonds that join them unstable. The bonds are readily hydrolyzed, releasing the phosphate group and a sufficient amount of energy to power an endergonic reaction. Because of the relatively high amount of free energy released when the bonds between the phosphate groups are hydrolyzed, they are called high-energy phosphate bonds, denoted by the symbol ~ ■ hydrolysis, p. 25

Cells constantly turn over ATP, powering biosynthetic reactions by hydrolyzing the high-energy phosphate bond, and then exploiting energy-releasing reactions to form it again. Two different processes are used by chemoorganotrophs to provide the energy necessary to form the high-energy phosphate bond.

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