activated Carrier molecules are essential for Biosynthesis
The energy released by the oxidation of food molecules must be stored temporarily before it can be channeled into the construction of the many other molecules needed by the cell. In most cases, the energy is stored as chemical-bond energy in a small set of activated “carrier molecules,” which contain one or more energy-rich covalent bonds. These molecules diffuse rapidly throughout the cell and thereby carry their bond energy from sites of energy generation to the sites where the energy will be used for biosynthesis and other cell activities (Figure 2–31).
The activated carriers store energy in an easily exchangeable form, either as a readily transferable chemical group or as electrons held at a high energy level, and they can serve a dual role as a source of both energy and chemical groups in biosynthetic reactions. For historical reasons, these molecules are also sometimes referred to as coenzymes. The most important of the activated carrier molecules are ATP and two molecules that are closely related to each other, NADH and NADPH. Cells use such activated carrier molecules like money to pay for reactions that otherwise could not take place.
The formation of an activated Carrier is Coupled to an energetically favorable Reaction
Coupling mechanisms require enzymes and are fundamental to all the energy transactions of the cell. The nature of a coupled reaction is illustrated by a mechanical analogy in Figure 2–32, in which an energetically favorable chemical reaction is represented by rocks falling from a cliff. The energy of falling rocks would normally be entirely wasted in the form of heat generated by friction when the rocks hit the ground (see the falling-brick diagram in Figure 2–17). By careful design, however, part of this energy could be used instead to drive a paddle wheel that lifts a bucket of water (Figure 2–32B). Because the rocks can now reach the ground only after moving the paddle wheel, we say that the energetically favorable reaction of rock falling has been directly coupled to the energetically unfavorable reaction of lifting the bucket of water. Note that because part of the energy is used to do work in Figure 2–32B, the rocks hit the ground with less velocity than in Figure 2–32A, and correspondingly less energy is dissipated as heat.
Similar processes occur in cells, where enzymes play the role of the paddle wheel. By mechanisms that we discuss later in this chapter, enzymes couple an
Figure 2–31 energy transfer and the role of activated carriers in metabolism.
By serving as energy shuttles, activated carrier molecules perform their function as go-betweens that link the breakdown of food molecules and the release of energy (catabolism) to the energy-requiring biosynthesis of small and large organic molecules (anabolism).
Figure 2–32 a mechanical model illustrating the principle of coupled chemical reactions. The spontaneous reaction shown in (a) could serve as an analogy for the direct oxidation of glucose to Co2 and h2o, which produces heat only. in (B), the same reaction is coupled to a second reaction; this second reaction is analogous to the synthesis of activated carrier molecules. The energy produced in (B) is in a more useful form than in (a) and can be used to drive a variety of otherwise energetically unfavorable reactions (C).
energetically favorable reaction, such as the oxidation of foodstuffs, to an energetically unfavorable reaction, such as the generation of an activated carrier molecule. In this example, the amount of heat released by the oxidation reaction is reduced by exactly the amount of energy stored in the energy-rich covalent bonds of the activated carrier molecule. And the activated carrier molecule picks up a packet of energy of a size sufficient to power a chemical reaction elsewhere in the cell.
aTp is the most Widely Used activated Carrier molecule