Video Transcript
In this video, we will learn about
metabolism in the body and a two different types of metabolic reactions, anabolic
reactions and catabolic reactions. We will go over some examples of
each of these types of reactions and understand how energy is involved in these
processes.
All of the systems in our body work
together to provide the body with the energy, oxygen, and nutrients it needs. Each of these systems are
constantly performing chemical reactions in the body to support and maintain
life. For example, when you do some form
of exercise like running or playing sports, you may notice that you breathe heavily
for a short while afterwards. When you exercise, your body cells
need energy to keep you going.
In order to release energy, cells
need to perform cellular respiration. This is a chemical reaction that
occurs most efficiently in the presence of oxygen. It also explains why your breathing
becomes faster and heavier so that your body can acquire the oxygen that it
needs. Cellular respiration is just one
example, but there are actually millions of chemical reactions that happen in our
body every second. Molecules are constantly being
built up and broken down within our various organ systems.
We use the term metabolism to
describe the set of all of these chemical reactions that take place within living
organisms to maintain life. We can consider these metabolic
chemical reactions to be of two broad types. In some cases, large molecules in
the body are broken down into smaller ones. In other cases, small molecules are
put through chemical reactions that combine them into larger molecules. We call the breakdown of large
molecules into smaller ones catabolic reactions. And the production of large
molecules from smaller ones are called anabolic reactions. Both anabolic and catabolic
reactions involve energy in the form of a tiny molecule called ATP, which is short
for adenosine triphosphate. Let’s take a look at the structure
of ATP.
It consists of a sugar called
ribose attached to a nitrogenous base called adenine and three phosphate groups, as
we can see here. We can deduce a lot about the
structure of ATP from its name. Aden- tells us that the
nitrogen-containing base in this molecule is adenine. And tri- means three, telling us
that there are three phosphate groups in an ATP molecule. ATP is often said to be the energy
currency of the cell. It is broken down when the cell
needs energy to carry out certain reactions and formed when different reactions
within the cell release energy.
So how is this energy stored in an
ATP molecule? The bond between the two outer
phosphate groups in an ATP molecule is said to carry this chemical energy. When the cell needs energy for its
reactions, this bond is broken and energy is released along with the outermost
phosphate group, which we call an inorganic phosphate and represent with the
initials P i. The resulting molecule only has two
phosphate groups, which is why it’s called ADP, or adenosine diphosphate, as the
prefix di- means two. If a cell’s reactions release
energy, this energy can be used to bind ADP to an inorganic phosphate, reforming
ATP. In this way, the energy that’s
released by certain cellular reactions can be captured and stored in the form of
ATP, ready to be broken down again whenever it’s needed to release this energy once
more.
So what is the relationship between
the different metabolic reactions and ATP energy? As we learned earlier, anabolic
reactions construct large molecules out of smaller ones. To form these chemical bonds that
make up a larger molecule, anabolic reactions need energy which they get by breaking
down ATP molecules. Catabolic reactions, on the other
hand, break down large molecules into smaller molecules. By breaking the chemical bonds in
the larger molecules, catabolic reactions release energy which is stored as ATP
molecules. In short, we say that anabolic
reactions require ATP, while catabolic reactions release ATP.
Now let’s take a look at some
examples of metabolic reactions that take place in the human body, starting with
catabolic reactions. Catabolic reactions play a major
role in breaking down the food that we eat into the nutrients that our body utilizes
to function. Foods like bread, rice, and
potatoes are rich in carbohydrates. One of the most common forms of
carbohydrates that all these foods contain is starch. Starch is called a polymer because
it’s made of multiple repeating units of a smaller sugar called glucose. We’ll represent glucose as a
hexagon because it has six carbon atoms. Thousands of glucose molecules can
join together to form a single starch molecule. So a portion of a starch molecule
might look something like this.
Our body needs glucose for the
process of cellular respiration, which we briefly mentioned earlier. But if all these glucose molecules
are connected together in starch molecules, they’re not of much use to the cell. These long molecules of starch are
therefore broken down into glucose through the action of enzymes in our digestive
system. This is a simple example of a
catabolic reaction. So now that we have glucose, this
simple sugar can be used in cellular respiration, which as we mentioned earlier is
another example of a catabolic reaction. In cellular respiration, glucose is
broken down to release carbon dioxide, water, and energy in the form of ATP. In our body, this reaction most
commonly takes place in the presence of oxygen. This is called aerobic cellular
respiration.
We’ve discussed what happens to the
carbohydrates that we eat in our food. But what about other molecules like
proteins? Foods like lentils, nuts, and meat
and dairy products like milk are usually rich in proteins. Proteins are also broken down in
the body through catabolic reactions using enzymes that are present in the stomach
and the small intestine. Proteins are long chains of
individual units called amino acids, which are joined together and folded into a
specific shape to perform a function. When enzymes in the stomach and
small intestine act on these proteins, they can break them down into amino
acids. These free amino acids can then be
used to make other proteins that are essential in the body. As with all catabolic reactions,
the breakdown of proteins into amino acids releases energy in the form of ATP.
Next let’s look at some examples of
anabolic reactions in the human body. As we just mentioned, the amino
acids that we get from the proteins we eat can be used to synthesize other proteins
that our body needs. This is an example of an anabolic
reaction as a large molecule is being constructed out of smaller units. Amino acids are joined together
through peptide bonds which need ATP to form. This creates a chain of amino acids
called a polypeptide chain. The polypeptide chain can then fold
into a specific shape to perform a specific function in the body as we mentioned
earlier. One example of a protein that can
be formed in this process is hemoglobin, the protein that carries oxygen in our red
blood cells.
Another example of an anabolic
reaction in the body is the formation of glycogen through a process called
glycogenesis. Earlier, we talked about glucose, a
simple sugar molecule that we get from foods that contain carbohydrates. If there’s too much glucose in the
blood, this can actually lead to problems with the heart, kidneys, or vision. To prevent this, glucose is
sometimes converted into glycogen. Glycogen is a branched molecule
made of repeating units of glucose. It acts as a storage molecule for
glucose in the body. Since the large molecule is being
constructed out of smaller ones, this is another example of anabolic reaction. Let’s apply what we’ve learned
about metabolism to some practice questions.
A buildup of amino acids can be
toxic to the human body. To prevent this, amino acids have
their amine group removed, which is then converted to a relatively nontoxic molecule
and excreted. The removal of an amine group is an
example of what type of metabolic reaction, anabolism or catabolism?
Let’s start by going over some of
the key terms that have been used in the question. Metabolism is the term used to
describe all the chemical reactions that take place in the body to support and
maintain life. Metabolic reactions can either be
anabolic, in which large molecules are constructed from smaller molecules, or
catabolic reactions, in which large molecules are broken down into smaller
molecules. This question talks about amino
acids and how their buildup can be toxic to the human body. Amino acids are the individual
units that make up proteins. Here we can see the simple
structure of an amino acid with the amine group circled.
As mentioned in the question, to
prevent toxicity to the body, the amine group is removed from the amino acid. This process is called
deamination. Since deamination involves the
breakdown of amino acids to separate the amine group, this is an example of a
catabolic reaction. So we’ve deduced that deamination
is an example of catabolism.
Let’s have a go at another question
together.
A student reads about metabolism in
humans. They conclude that the cells of
humans require larger amounts of energy to break down complex molecules into smaller
ones than they require to synthesize complex molecules from small ones. Are they correct? (A) No, it is the synthesis of
complex molecules that requires a relatively larger amount of energy. Or (B) yes, it is the breaking down
of complex molecules that requires a relatively larger input of energy.
Metabolism refers to all of the
chemical reactions that take place within the body to support and maintain life. There are two types of metabolic
reactions, anabolic reactions and catabolic reactions. Let’s take a look at each of these
types of reaction. In anabolic reactions, small
molecules are linked together through chemical bonds to form larger, more complex
molecules. The formation of these chemical
bonds requires energy which cells obtain from a molecule called ATP. In catabolic reactions, on the
other hand, large and complex molecules are broken down into smaller units. This process involves breaking
chemical bonds, and so it releases energy in the form of ATP.
Let’s take another look at the
conclusion that the student in our question has made. They’ve stated that larger amounts
of energy is required to break down large complex molecules into smaller ones than
is needed to synthesize large molecules from small ones. This is in fact the opposite of
what we’ve learned about anabolic and catabolic reactions. While catabolic reactions that
break down large complex molecules into multiple smaller ones release energy in the
form of ATP, anabolic reactions that synthesize large complex molecules from
multiple smaller ones require an input of ATP. So the correct answer is therefore
“No, it is the synthesis of complex molecules that requires a relatively larger
amount of energy.”
Let’s review the key points that we
have learned about metabolism in this video. Metabolism describes all the
chemical reactions that take place within a cell or body of a living organism that
allow them to survive. Anabolism and catabolism are the
two types of metabolic reactions. Anabolic reactions require an input
of energy to synthesize large molecules from multiple smaller ones. The formation of polypeptides from
individual amino acids is an example of an anabolic reaction. This requires an input of energy
which is supplied by a molecule called ATP.
Catabolic reactions, on the other
hand, break down large molecules into smaller ones, releasing energy in the process
which is stored in the form of ATP. An example of a catabolic reaction
is the breakdown of starch in carbohydrate-rich foods into simple sugars like
glucose. The breaking of bonds in the large
starch molecules is what releases energy.