Essential Cell Biology 5th edition

Activated Carriers and Biosynthesis
101
ACTIVATED CARRIERS AND BIOSYNTHESIS
Much of the energy released by an energetically favorable reaction such as
the oxidation of a food molecule must be stored temporarily before it can
be used by cells to fuel energetically unfavorable reactions, such as the
synthesis of all the other molecules needed by the cell. In most cases, the
energy is stored as chemical-bond energy in a set of activated carriers,
small organic molecules that contain one or more energy-rich covalent
bonds. These molecules diffuse rapidly and carry their bond energy from
the sites of energy generation to the sites where energy is used either for
biosynthesis or for the many other energy-requiring activities that a cell
must perform (Figure 3−25). In a sense, cells use activated carriers like
money to pay for the energetically unfavorable reactions that otherwise
would not take place.
Activated carriers store energy in an easily exchangeable form, either as
a readily transferable chemical group or as readily transferable (“highenergy”)
electrons. They can serve a dual role as a source of both energy
and chemical groups for biosynthetic reactions. As we shall discuss
shortly, the most important activated carriers are ATP and two molecules
that are close chemical cousins, NADH and NADPH.
An understanding of how cells transform the energy locked in food molecules
into a form that can be used to do work required the dedicated
effort of the world’s finest chemists (How We Know, pp. 102–103). Their
discoveries, amassed over the first half of the twentieth century, marked
the dawn of the study of biochemistry.
The Formation of an Activated Carrier Is Coupled to an
Energetically Favorable Reaction
When a fuel molecule such as glucose is oxidized inside a cell, enzymecatalyzed
reactions ensure that a large part of the free energy released is
captured in a chemically useful form, rather than being released wastefully
as heat. When your cells oxidize the sugar from a chocolate bar,
that energy allows you to power metabolic reactions; burning that same
chocolate bar in the street will get you nowhere, warming the environment
while producing no metabolically useful energy.
In cells, energy capture is achieved by means of a special form of coupled
reaction, in which an energetically favorable reaction is used to drive an
energetically unfavorable one, so that an activated carrier or some other
useful molecule is produced. Such coupling requires enzyme catalysis,
which is fundamental to all of the energy transactions in the cell.
ENERGY
ENERGY
food
molecule
inactive carrier
new molecule
needed by cell
energetically
favorable
reaction
oxidized food
molecule
CATABOLISM
ENERGY
activated carrier
energetically
unfavorable
reaction
molecule
available in cell
ANABOLISM
Figure 3–25 Activated carriers can store
and transfer energy in a form that cells
can use. By serving as intracellular energy
shuttles, activated carriers perform their
function as go-betweens that link the
release of energy from the breakdown of
food molecules (catabolism) to the energyrequiring
biosynthesis of small and large
organic molecules (anabolism).