Video Transcript
Which of the following is not a
primary function of carbohydrates in the human body? (A) Larger, complex carbohydrates
can be broken down into fatty acids that can help make hormones. (B) Carbohydrates provide
substrates that can be broken down in cellular respiration to release energy. (C) Carbohydrates act as storage
molecules to help regulate blood glucose concentrations. Or (D) simple carbohydrates can be
used to build larger, more complex carbohydrates that can be used as cell
components.
This question asks us about the
primary functions of carbohydrates in the human body. So let’s first look at what
carbohydrates are, and then we can review their key functions.
Carbohydrates are biological
molecules which are made of carbon, hydrogen, and oxygen, carbo- referring to carbon
and -hydrate referring to water. Many are macromolecules, macro-
meaning large. But their most basic building
blocks are monosaccharides, mono- meaning one and saccharide meaning sugar.
Glucose is the most abundant
monosaccharide in nature and is the principal substrate for cellular
respiration. During the first stage of cellular
respiration called glycolysis, glyco- referring to sugar and -lysis meaning to
split, glucose is broken down into smaller substrates called pyruvate. This stage occurs in the cytoplasm
of the cell.
In aerobic respiration, these
pyruvate molecules are then transported into the mitochondria and converted into
acetyl-CoA. These then enter the Krebs cycle,
and high-energy electron carriers are generated. The electron carriers deliver the
electrons to the electron transport chain to enable the synthesis of ATP, a
high-energy molecule.
Cellular respiration therefore
converts the chemical energy stored in the glucose molecule to chemical energy
stored in ATP molecules. This energy store is available for
immediate use for processes such as muscle contraction, active transport, nervous
conduction, and to build new molecules.
In addition to providing immediate
high energy molecules, we can store carbohydrates. When blood glucose levels rise,
insulin is released from the pancreas. This hormone enables liver and
muscle cells to take up the excess glucose and build it into the complex storage
polysaccharide, glycogen. When blood glucose levels fall
below normal, the pancreas releases glucagon. This hormone brings about
glycogenolysis, the breaking down of glycogen into glucose molecules. The glucose molecules can then be
released into the blood to restore the blood glucose levels to within the norm.
Monosaccharides, such as glucose
and fructose, can also be used to build larger molecules. The example we have already seen is
where glycogen is formed by joining glucose molecules. Glucose is soluble and would affect
the osmotic potential of the cell so cannot be used for storage. Glycogen, on the other hand, is
insoluble and inert so can just sit in the cell until it is needed. It is also highly branched, which
means it can be broken down rapidly to release glucose when it is needed.
Cellulose, used to build plant cell
walls, is also made up of repeating glucose molecules. This time, instead of branched
molecules, the glucose molecules are joined together to form long chains ideal for
structural support. Both cellulose and glycogen are
called complex carbohydrates as they are made up of many monosaccharides joined
together in a complex structure.
Now that we have reviewed the role
of carbohydrates in the human body, we are able to find an answer through the
process of elimination. Please note, our question asks for
a statement that is not a primary function of carbohydrates. Therefore, we are looking for an
answer that is false.
Answers (B), (C), and (D) are all
relevant functions of carbohydrates in the human body. They refer to energy production,
storage, and synthesis of larger macromolecules, respectively.
Looking at option (A),
carbohydrates do not break down into fatty acids. Lipids are broken down into fatty
acids and glycerol. However, when glucose is in excess
and it is converted to acetyl-CoA during glycolysis, the acetyl-CoA can be converted
into fatty acids by a process called lipogenesis, lipo- referring to lipids and
genesis meaning to make. The fatty acids can then be used in
steroid hormone generation.
Therefore, the correct answer is
(A). The option that is not a primary
function of carbohydrates in the human body is larger, complex carbohydrates can be
broken down into fatty acids that can help make hormones.
