3.2
ATP: Energy Currency of the Cell
Thousands of reactions take place in living cells. Many reactions require the addi-
tion of energy for the assembly of complex molecules from simple reactants. These
reactions include DNA synthesis, protein synthesis, and the construction of cell walls
and other cellular structures. Other cell-driven actions—such as muscle contractions
in animals, the motion of flagella in bacteria (Figure 1), and the movement of sap
within a tree—also require a supply of energy.
Since so many cellular functions require energy, cells need a constant supply of
energy. Even though the species on Earth are very diverse, all cells in every organism
use the same energy carrier for almost all of their energy-driven actions. This
energy comes in the form of a compound called adenosine triphosphate, or ATP.
ATP directly supplies the energy that powers nearly every cellular function, and it is
considered the universal energy “currency.” The types of work that are carried out by
ATP include mechanical, transport, and chemical work (Table 1).
Table 1 Types of Work Performed by ATP
Mechanical work Transport work Chemical work
• beating of cilia or
movement of flagella
• contraction of muscle
fibres
• movement of chromosomes
during mitosis/meiosis
• process of pumping
substances across
membranes against their
concentration gradient
• process of supplying
chemical potential energy
for non-spontaneous,
endergonic reactions,
including protein synthesis
and DNA replication
Figure 1 A bacterium with flagella
ATP Hydrolysis and Free energy
Adenosine triphosphate (ATP) consists of three parts: a nitrogenous base called
adenine, which is linked to a five-carbon sugar called ribose, which in turn is linked
to a chain of three phosphate groups (Figure 2, next page). ATP contains large
amounts of free energy. The energy of the molecule is high because of its three nega-
tively charged phosphate groups. The phosphate groups crowd together, and their
close proximity creates a mutual repulsion of their electrons. The mutual repulsion
contributes to the weakness of the bond holding the groups together. The bonds of
ATP are easily broken by a catalyzed reaction with water—a process called hydrolysis
(Figure 2). The hydrolysis reaction results in the breaking off of the end (or terminal)
phosphate group and the formation of two products—adenosine diphosphate (ADP)
and an inorganic phosphate (Pi). In addition, an
H1
ion is released into the solution.
As bonds in these new products form, free energy is released.
ATP
1
H2O
S
ADP
1
Pi ∆G 5 –30.5 kJ/mol
Note that the H+ ion is not normally shown in the chemical equation, since it is
understood to be associated with the formation of Pi.
Recall, from Section 3.1, that during a chemical reaction, bonds in the reactants
break and new bonds in the products form. During ATP hydrolysis, bonds form when
a new —OH group attaches to the phosphorus atom of the phosphate group and when
an electron attaches to the oxygen that remains on the ADP molecule. Energy is also
released as the H+ ion interacts with water molecules. The bond rearrangements and
the change in entropy result in an overall free energy change of –30.5 kJ/mol. When
ATP splits into ADP and Pi within a cell, the phosphate group, rather than remaining
free in solution, often becomes attached to another molecule, which results in a dif-
ferent bonding arrangement.
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