Video Transcript
In this video, we will learn what
metabolism is and how metabolic reactions are essential to our survival. We will also look at the difference
between the key terms anabolism and catabolism and some examples of each reaction in
the human body. One specific example that we will
focus on is the role of the liver in removing toxic lactic acid.
There are many misconceptions
surrounding metabolism in humans. For example, you might have heard
that slim people have a really high metabolism. This is not necessarily correct, as
metabolism is not how fast someone digests food nor how little weight they will
gain. Instead, metabolism is the sum of
all chemical reactions that occur in a cell or body.
Metabolic reactions occur in all
living organisms. This sunflower has metabolic
reactions occurring within it, as does a single-celled bacterium, and as do humans
like you and I. Metabolic reactions are occurring
in all of your cells all of the time. And your metabolism describes all
the chemical reactions that occur within you that help you to maintain your
life.
There are two types of metabolic
reactions: anabolic reactions and catabolic reactions. Let’s look at anabolic reactions
first. Anabolism is when metabolic
reactions construct large molecules from smaller units. Anabolic reactions require energy
that is stored in the form of a molecule called ATP. When certain bonds in an ATP
molecule are broken, this releases the energy needed for anabolic reactions to
occur. Let’s explore catabolism next. Catabolism is when metabolic
reactions break down large molecules into smaller units. Catabolic reactions release energy,
some of which is stored in cells in the form of ATP molecules. This means that the energy stored
in the bonds of the ATP molecules is readily available for anabolic reactions.
Let’s look at some of the examples
of anabolic and catabolic reactions to understand these processes better, starting
with catabolic reactions. When we consume food like this
chicken sandwich, for example, that is rich in proteins and carbohydrates, these
large molecules are digested by enzymes in our digestive system and are broken down
into smaller molecules.
By zooming in on the reactions
occurring in the small intestine, we can see that these are examples of catabolic
reactions as large proteins are broken down into smaller amino acid subunits and
large carbohydrates, such as the starch in bread, are broken down into smaller,
simple sugars, like glucose. Both of these processes release
energy. And the glucose, specifically can
either be stored, or it can be further metabolized in a process called aerobic
respiration.
This diagram is of a mitochondrion,
the site of aerobic respiration in human cells. Aerobic respiration breaks down
glucose to release energy, which is stored as ATP. So, this process is another example
of a catabolic reaction. This diagram is of the liver, an
organ which carries out a vast number of metabolic functions. One catabolic reaction that’s
carried out by the liver is converting excess proteins into a substance called
urea. This process of breaking down
proteins releases energy, some of which will be used to convert the product into
urea, which can then be excreted from cells to exit the body as part of urine.
Let’s look at some examples of
anabolic reactions next. If we think back to our first
catabolism example where proteins and starch were broken down into amino acids and
glucose, respectively, you may have wondered what happens to these smaller molecules
once they have been formed. Simple molecules like these can be
used to build up a variety of other larger protein and carbohydrate molecules that
have a function within an organism’s body. For example, these amino acids can
be combined in diverse sequences to form multitudes of different proteins, while the
glucose molecules can be reformed into starch in plant cells or into glycogen in
animal cells. As they are anabolic, both of these
processes require an input of energy, which will be provided by the breaking of
certain bonds in ATP.
As the liver is such an important
organ when it comes to discussing metabolism, let’s explore some other metabolic
functions that it carries out, one of which is metabolism of lactic acid, sometimes
called lactate. Lactic acid is produced by lactic
acid fermentation, which is sometimes called anaerobic respiration. Humans are an example of aerobic
organisms, as our cells usually carry out the oxygen-fueled aerobic respiration to
release energy. Anaerobic respiration is the
process by which anaerobic organisms that live in low-oxygen environments release
energy as it can occur even when oxygen is absent.
Sometimes human cells can also
carry out anaerobic processes, such as lactic acid fermentation. For example, if someone was
sprinting in a race and their muscle cells demanded more energy than the
oxygen-fueled aerobic respiration can deliver, anaerobic respiration occurs. Lactic acid fermentation provides
the muscle cells with energy, and it also produces lactic acid. Lactic acid is toxic to humans, so
it must be broken down into a less toxic substance. It is commonly believed that a
buildup of lactic acid is linked to the immediate muscle pain you may feel while
doing intense exercise.
The organ that detoxifies lactic
acid, among many other toxic substances, is the liver. The liver has two options, one of
which is converting lactic acid back into glucose. You can see from its molecular
formula that lactic acid is effectively half a molecule of glucose, which means that
the conversion of lactic acid to glucose would be building up a larger molecule from
a smaller one. This reaction is therefore anabolic
and would require ATP. The other option the liver has is
to break down lactic acid by reacting it with oxygen to form carbon dioxide and
water, which are the same as the product of aerobic respiration. Carbon dioxide and water are both
smaller molecules than lactic acid. So, this reaction is catabolic, as
it involves breaking down lactic acid to release ATP.
Let’s see how much we can remember
about metabolism and the liver by having a go at a practice question.
Which of the following statements
correctly relates energy to catabolic and anabolic reactions? Anabolic reactions require energy
to form new molecules, whereas catabolic reactions release energy as molecules are
broken down. Or catabolic reactions require
energy to form new molecules, whereas anabolic reactions release energy as molecules
are broken down.
Let’s investigate some of the key
terms in the question to help us figure out how to answer it. Catabolic and anabolic reactions
are both types of metabolic reactions that occur within the cells of living
organisms to help them survive. Let’s take a look at catabolic
reactions first. Catabolic reactions break down
large molecules into smaller units. The breaking of bonds in the large
molecule releases energy, which can be used in the cell for other reactions. For example, proteins are broken
down into amino acids in our digestive system. This is an example of a catabolic
reaction, as a large molecule is forming several smaller molecules and the process
overall releases energy.
Anabolic reactions construct larger
molecules from individual units, and this process requires energy to form the bonds
between these subunits. For example, these amino acids are
being joined together to form a large protein, and it requires an input of
energy. The energy is used to form these
bonds between the individual amino acids. So, this is an anabolic
reaction. Therefore, our correct answer is
that anabolic reactions require energy to form new molecules, whereas catabolic
reactions release energy as molecules are broken down.
Let’s have a look at some of the
key points that we have addressed in this video. Metabolism describes all the
reactions occurring in the cells of a single living organism. And these metabolic reactions help
us all to survive. Metabolic reactions can either be
anabolic, where small molecules are built up into larger molecules using an input of
energy. Or the reactions can be catabolic,
where large molecules are broken down into smaller ones releasing energy. The liver is a major organ that
carries out various metabolic reactions involved in detoxification and waste
removal. For example, the liver cells are
responsible for detoxifying the lactic acid produced in lactic acid fermentation, an
example of an anaerobic process which occurs in human muscle cells.
