Video Transcript
In this video, we will learn how to
describe the structure of the essential energy-carrying molecule, ATP. We will investigate how ATP is
synthesized and how it’s hydrolyzed to release energy that cells can use. We will also take a look at the key
properties of the molecule that make it so useful in vital cellular processes.
All living things require a
continual supply of energy in order to function. Adenosine triphosphate, which is
better known by its initials ATP, is the primary molecule that’s responsible for
energy storage and energy transfer in cells. No matter what goes into an
organism as a fuel source, whether it’s fats, carbohydrates, or proteins, it is
ultimately used to generate ATP in order to supply all of the immediate power needs
of a living cell.
Our bodies make and break down our
body weight in ATP every day. So, if you weigh about 50
kilograms, in one day, you’d use about 50 kilograms of ATP. Although ATP is a small and
relatively simple molecule, within its bonds, there is enough energy to perform all
sorts of cellular work. This is why ATP is commonly
referred to as the energy currency of cells. Because much in the same way as
money is the currency that people exchange for the things that they need, ATP is
used to store the energy for reactions in a cell, which can be exchanged into a more
useful form when needed. And ATP can be stored in cells like
money can be stored in a bank. Even though ATP can be used to
store energy in the cell, because it is being constantly broken down and then
reformed, it is more of an immediate energy source rather than a long-term one.
Let’s take a look at the structure
of ATP in some detail. ATP is a nucleotide, which may seem
surprising as we often hear the word “nucleotide” when discussing genetics and
molecules like DNA. However, this is a bit limiting as
nucleotides are more than just DNA. Nucleotides are the building blocks
or more precisely the monomers of polymers called nucleic acids. Nucleotides are the basic units
that can be joined together to make these more complex molecules. In fact, DNA is a nucleic acid,
which is a polymer that’s made up of many nucleotide monomers joined together.
You may have even noticed that the
structure of this particular nucleotide that’s found in DNA is not all that
different from the structure of ATP. That’s because all nucleotides have
the same basic structure composed of three subunits: a nitrogen-containing base,
sometimes called a nitrogenous base; a five-carbon sugar, which is sometimes called
a pentose sugar; and at least one phosphate group.
Given the full name of ATP,
adenosine triphosphate, and the typical structure of a basic nucleotide, what can
you deduce about the structure of ATP? You might remember that adenine is
a nitrogenous base that’s also found in DNA. And notice that the start of the
word “adenosine” is fairly similar. This is because adenine serves as
the nitrogenous base in an ATP molecule.
So what does the word
“triphosphate” tell us? It indicates the number of
phosphate groups in an ATP molecule. The prefix tri- means three. So adenosine triphosphate has three
phosphate groups. The pentose sugar of a nucleotide
may either be deoxyribose or ribose. While the pentose sugar in
deoxyribonucleic acid, more commonly known as DNA, is deoxyribose, the pentose sugar
in other nucleic acids, like ribonucleic acid, otherwise known as RNA, or the
nucleotide of adenosine triphosphate, otherwise known as ATP, is ribose. So, as we can see, a nitrogenous
adenine base, a five-carbon ribose sugar, and three phosphate groups forms the
structure of an ATP molecule.
The three phosphate groups in an
ATP molecule are linked to each other by high-energy bonds that can be easily
broken. It’s within these bonds between the
three phosphate groups that the actual power source of ATP is stored. When energy is needed immediately,
the covalent bond between the outermost phosphate groups of ATP is broken. Let’s take a closer look at the
reaction that breaks down bonds between these phosphate groups.
When energy is needed immediately
in the cell and the bond between the second and third phosphate groups of ATP is
broken, this converts ATP into adenosine diphosphate, which is better known by the
abbreviation ADP, and an inorganic phosphate group, which is often represented by
the abbreviation Pi. The breakdown of the bond between
the two outer phosphate groups and an ATP molecule is called hydrolysis.
The word “hydrolysis” contains the
prefix hydro-, which means water, and the suffix -lysis, which means separation. During hydrolysis, water is split,
resulting in the release of a hydrogen atom and a hydroxyl group. Hydrolysis breaks the bond between
the outer phosphate groups and consumes a water molecule in the process. With the addition of water, the
hydrolysis of ATP produces ADP, an inorganic phosphate group, and free energy that
the cell can use. The structure of ADP is very
similar to that of ATP, except that ADP has one less phosphate group attached to the
end. In fact, the prefix di- means two,
indicating that there are two phosphate groups in an ADP molecule.
The removal of the phosphate group
from ATP is catalyzed by an enzyme called ATP hydrolase. This is also nice and easy to
remember. As ATP is the molecule being broken
down, hydro- indicates water and hydrolysis and the -ase indicates that this is an
enzyme. This process can be so quick that
it takes only a few seconds for half of the ATP molecules in a cell to be converted
into ADP, showing how efficient molecules of ATP can be as an energy source.
When ATP hydrolysis occurs, unless
the energy released is used quickly, much of it will be lost as heat or thermal
energy. To avoid this loss, ATP hydrolysis
is often coupled to other energy-requiring reactions in cells. This way, rather than being lost as
heat, the energy can be used to power up cellular reactions. Instead of being left floating in
the cell, the inorganic phosphate group that’s produced through ATP hydrolysis can
also be put to good use. The inorganic phosphate group can
be added to other molecules in a cell in a process called phosphorylation.
Phosphorylation can make other
molecules more reactive. And phosphorylated molecules might
be useful to drive metabolic reactions, many of which include phosphorylation of a
protein. Some of these proteins might be
enzymes. They can be activated by the
binding of an inorganic phosphate group, thus allowing them to proceed in catalyzing
an enzyme-controlled reaction.
Let’s take a quick look at a
specific example of protein phosphorylation. One such important example is
involved with the proteins that allow our DNA to be packaged so tightly and
efficiently into our cells’ nuclei. These proteins are called
histones. And when they associate with DNA,
they form chromatin. When DNA becomes damaged, histones
are often phosphorylated. This changes the structure of
chromatin by freeing up spaces around the damaged DNA segments, providing space for
other proteins and factors to repair the damaged DNA effectively.
Although ATP is continuously broken
down in hydrolysis to provide this free energy that can be used in reactions and a
phosphate group that can be used for phosphorylation, it is also constantly being
replenished. The regeneration of ATP is
important because cells tend to use it very quickly and rely on this constantly
replenished ATP supply to power up the cell. ATP is easily resynthesized in a
condensation reaction that adds an inorganic phosphate group to ADP.
Generally, a condensation reaction,
which is also called dehydration synthesis, is a reaction that joins two molecules
with a chemical bond and results in the formation and release of a water
molecule. You might find this easier to
remember by thinking about the other contexts where the word “condensation” is
used. For example, clouds are made of
water vapor, and when they condense, liquid water falls from them. This reminds us that condensation
reactions produce a water molecule. Although water is lost through ATP
hydrolysis, water is reformed and then released when a third phosphate group is
added to an ADP molecule, thus reforming ATP.
This reaction is catalyzed by an
enzyme called ATP synthase, which is nice and easy to remember as synth- reminds us
that this is synthesis or making ATP and the suffix -ase reminds us that this is an
enzyme. If the addition of another
phosphate group to an ADP molecule to form ATP sounds like phosphorylation, that’s
because it is. As it’s formed from the addition of
a phosphate group to ADP, ATP can be thought of as a phosphorylated nucleotide. In plants, ATP are synthesized
through phosphorylation during photosynthesis. And in both animal and plant cells,
ATP can be regenerated during cellular respiration.
While ATP can help to power up
reactions through the free energy that it releases, it is not a long-term storage
molecule for chemical energy. Although six-carbon sugars like
glucose are considered excellent long-term storage sites of energy for a cell, they
take a long time and a lot of energy to break down. So, instead, to provide them with
quick access to energy, cells can convert glucose into ATP through cellular
respiration in order to provide an immediate access to stored energy.
The properties of ATP highlight why
this molecule is so important for living organisms. Let’s take a look at some of these
key properties and why they’re so useful. ATP is a fairly small molecule. This means that ATP, which we can
see being released from this mitochondrion as these tiny green dots, can easily
diffuse to the different parts of the cell where it’s needed. ATP is soluble in water. Generally, cell cytoplasm, the
space within many organelles, and much of the extracellular space surrounding cells
are aqueous environments, which means that they contain water molecules. The water solubility of ATP makes
it very handy, as it means that it can be used in reactions occurring in all of
these aqueous environments.
Even though ATP is small, ATP
hydrolysis releases just enough energy in small, manageable quantities. This means that it can power up
cellular reactions without producing waste. And as we know, this same reaction
also releases an inorganic phosphate group, which can make other molecules more
reactive through phosphorylation.
Finally, given the importance of
ATP, the fact that it’s constantly being broken down and regenerated quickly is very
useful. This means that ATP can be quickly
remade to provide a constant supply of energy to our highly energy-demanding
cells. It’s these properties of ATP that
make it an excellent resource for powering up reactions in cells. So ATP can serve as a shuttle,
delivering energy to all the places in the cell where energy-consuming activities
are taking place.
Let’s take a quick look at the
three general types of tasks in cells where ATP is required to release energy to
power these processes, for example, to drive metabolic reactions that cannot occur
automatically through phosphorylation and activation of a molecule by the inorganic
phosphate. ATP can also be useful for
transporting substances like molecules or ions that are needed in cells across the
plasma membrane from an area of low to high concentration against or up a
concentration gradient. ATP can also supply the energy for
mechanical work, such as providing the energy for muscle contraction.
Let’s review how much we’ve learned
about energy and ATP by applying our knowledge to a practice question.
Which of the following best
describes the structure of an ATP molecule? (A) An ATP molecule is composed of
a ribose sugar, an adenine nitrogenous base, and three phosphate groups. (B) An ATP molecule is composed of
a deoxyribose sugar, an adenine nitrogenous base, and two phosphate groups. (C) An ATP molecule is composed of
a hexose sugar, three adenine nitrogenous bases, and a phosphate group. Or (D) an ATP molecule is composed
of a glucose molecule, three adenine nitrogenous bases, and a phosphate group.
In order to answer this question,
we’re going to need to see a diagram of ATP. So let’s remove our answer options
for now. Adenosine triphosphate, which is
the full name for the molecule ATP, is a nucleotide that stores chemical energy in
living organisms. All nucleotides are made up of the
same basic structure: a nitrogenous base; a pentose sugar, which means that it
contains five carbons; and one or more phosphate groups.
Given the full name of ATP, we can
work out a few things about its structure. The prefix aden- in adenosine
triphosphate tells us that the nitrogenous base it contains is adenine, while the
tri- in triphosphate tells us that there are three phosphate groups in an ATP
molecule. But what about the pentose
sugar? As we now know, there are five
carbons in a pentose sugar, while a hexose sugar like glucose would have six
carbons, as the prefix hex- means six, while the prefix pent- means five.
While we can’t necessarily deduce
this from its name, the pentose sugar in adenosine triphosphate is ribose. So we’ve deduced that the structure
of an ATP molecule includes a ribose sugar, a nitrogenous adenine base, and three
phosphate groups. Let’s bring back our answer
options. So we’ve worked out that an ATP
molecule is composed of a ribose sugar, an adenine nitrogenous base, and three
phosphate groups.
Now it’s time for us to summarize
our knowledge about energy and ATP into some key points from the video. Adenosine triphosphate, often known
by ATP, is an immediate energy source in cells. When energy is needed immediately,
water is used to convert ATP into ADP and an inorganic phosphate group in a process
known as hydrolysis that releases energy, catalyzed by the enzyme ATP hydrolase. ATP is easily resynthesized from
ADP and an inorganic phosphate in a condensation reaction catalyzed by the enzyme
ATP synthase. The properties of ATP make it an
excellent resource for powering up different functions in the cell, such as driving
metabolic reactions through phosphorylation, transporting substances across
membranes, and to do mechanical work.
