Energy Transfer and ATP
Cells harness the energy from the energy-yielding nutrients in food by using a series of
hydrolysis reactions. These reactions involve the oxidation of the energy-yielding nutrients. A precise
series of redox reactions results in the formation of ATP. ATP connects many energy-conversion
reactions during metabolism and is called the “energy currency” of living cells.
Imagine foreign tourists who arrive in the United States with no American dollars, only money
from their home. The tourists must pay a fee to change their money into dollars, which they can use to
pay for their activities. Similarly, cells transform or change the chemical energy in various organic
compounds into the chemical energy of ATP. ATP then “pays” for activities that require energy in cells.
The “fee” is the energy lost as heat during the conversion.
ATP is constantly being made and broken down in cells, forming a cycle. When a molecule of
ATP (adenosine triphosphate) gives up one phosphate group, it becomes ADP (adenosine diphosphate)
and releases energy. This energy is available for cellular work. When ADP combines with a phosphate
group it forms ATP. This step requires energy. Thus, ATP acts as an energy carrier. It can be used to
release energy (ATP breaks down) and store energy (ATP forms).
Cells need to use chemical energy for many things. They need it for biosynthesis and to remove
wastes. They need it to take in nutrients and to move ions in and out of cells. Cells also need energy to
move from place to place, to move their internal structures such at chromosomes or vesicles around, as
well as for many other processes. ATP supplies much of the energy for this work.
The chemical energy in the energy-yielding nutrients in food is converted to chemical energy in
ATP. Other molecules exist that have similar energy-transferring properties. These energy carriers could
function similarly to ATP, but all known living cells use ATP as an energy carrier. This includes the cells of
fungi, plants, humans, bacteria, and even microorganisms known as archaea. The first organisms on
Earth most likely used ATP as an energy source.
Stop and Think
1. All organisms on Earth use ATP instead of other similar energy carriers. Explain how this fact
provides strong evidence that all the organisms on Earth share one common ancestor.
2. When ATP breaks down, ADP forms. Does ADP possess potential energy? Explain.