How Carbohydrates, Lipids and Proteins are Chemically Broken Down by Digestive Enzymes AND WHY
Metabolism refers to biochemical processes that occur with any living organism - including humans - to
maintain life. These biochemical processes allow us to grow, reproduce, repair damage, and respond to
our environment.
Majority of our food can be categorized as carbohydrates, lipids (fats), or proteins. These groups are
polymers which must be broken down into smaller subunits before they can be absorbed into the cell.
The process of digestion is simply the breaking up of long chains of polymers so that they can be
incorporated into the system cell and rearranged to produce the molecules you specifically need. This is
done through the help of biological catalysts known as enzymes. Without these chemicals, our body
cannot absorb the large molecules of food, and you cannot rearrange them according to the
‘ingredients’ you need to create the ‘recipe’ of you.
Digestion starts in the mouth where the food is mechanically broken down into tidbits. It is inside the
stomach, where is it temporarily stored, that preliminary chemical digestion begins. Partially digested
food then goes to the small intestine where much of the digestion and absorption of nutrients occur.
The large intestine simply serves for the further absorption of water.
Chemical digestion of proteins starts in the stomach. The stomach produces HCl
(hydrochloric acid) which converts pepsinogen to pepsin. Pepsin is a kind of
enzyme which splits the internal peptide bonds in the protein molecules. That
is, it breaks the primary structure of the protein. Protein digestion is a
concerted effort of all these enzymes.
Lipid or fat digestion occurs only in the small intestine. They are broken into
fatty acids and glycerol by the enzyme lipase, and are then absorbed in the
intestine. The components are used to produce the cell membrane
(phospholipids), provide a source of stored energy (triglycerides) and produce
waxes such as ear wax or oil for your skin.
Carbohydrate digestion begins in the mouth with the breaking up of starch (polymer for used by plants
to store extra glucose) and glycogen (polymer used by animals to store extra glucose) into smaller
sugars. Each disaccharidase (sugar enzyme) is specific to a particular disaccharide. For instance, maltase
breaks down maltose. Monosaccharides are then absorbed in the area where the final breakdown
occurred.
Anabolism is the building up of things - a succession of chemical reactions that constructs or synthesizes
molecules from smaller components, usually requiring energy in the process.
Catabolism is the breaking down of things - a series of degradative chemical reactions that break down
complex molecules into smaller units, and in most cases releasing energy in the process.
Anabolism - builds things and consumes energy - making bigger things out of smaller things and using
up energy in the process. Anabolism, or biosynthesis, allows the body to grow new cells and maintain all
the tissues. The body uses simple molecules to create complex ones. In the same way a builder will use
simple building blocks, such as bricks, to create a house. Anabolic reactions in our bodies utilize a few
simple chemicals and molecules to manufacture (synthesize) a vast array of finished products. The
growth and mineralization of bone and increases in muscle mass are examples of anabolism.
The anabolic process uses monomers to build polymers. A polymer is a large complex molecule made of
many small molecules that are similar to each other. Those small molecules are called monomers. For
example:
Amino acids, which are simple molecules (monomers)
…through a series of anabolic chemical reactions build…
Proteins, which are large and complex molecules (polymers
Catabolism - breaks things down and gives out energy. Using bigger things to make smaller things and
releasing energy in the process. Catabolism provides the energy our bodies need for physical activity,
from a cellular level right up to whole body movements.
The catabolic chemical reactions in the living cell break down polymers into their constituent
monomers. For example:

Polysaccharides are broken down into monosaccharides
Complex carbohydrates, such as starch, glycogen, and cellulose are polysaccharides.

Nucleic acids are broken down into nucleotides
Nucleic acids are the chemical basis of life and heredity - they encode our genetic information.
They serve as transmitters of genetic information. Examples are RNA (Ribonucleic acid) and DNA
(Deoxyribonucleic acid).
Nucleic acids are broken down to purines, pyrimidines and pentoses, which among other things,
are involved in our body's energy supply.

Proteins are broken down into amino acids
The amino acids produced by catabolism may be directly recycled, used to make new amino
acids, or be converted to other compounds. Sometimes protein is broken down into amino acids
to make glucose, which appears in the blood.
When we eat our body breaks down the organic nutrients - this breaking down process releases energy,
which is stored inside molecules of adenosine triphosphate (ATP) in the body.
Questions to study:
1. What does metabolism allow us to do?
2. How does anabolism and catabolism compare to each other?
3. When we eat, what are the primary nutrients we obtain from food?
4. Why must they be broken down into the smaller subunits?
5. What are proteins broken into?
6. How does the primary structure of a protein compare to the tertiary structure?
7. What are lipids broken into? When fat is eaten, in what molecules can they rearrange
themselves?
8. What would happen if your body could not catabolize (break down) polymers?
9. Explain how nucleic acids provide energy for your body.
10. What do you think is the relationship between proteins and nucleic acids?
Catabolism that breaks
organic molecules
down to give us energy
and provide the
building blocks for
other molecules.
Lipids produce:
Nucleic acids produce:
Proteins produce:
Carbohydrates produce
__________________
rearranges the
molecules to build
specific molecules just
for us. Requires an
input of _________.