Video Transcript
In some metabolic reactions, large molecules are made from smaller ones, as shown in
the simple diagram provided. Which monomer subunits join together to form starch in a metabolic reaction? (A) Glucose, (B) glycogen, (C) cellulose, or (D) amino acids.
This question asks us about a metabolic reaction, in which smaller molecules are
joined together to form larger ones. Metabolism describes all of the chemical reactions that occur within living organisms
to maintain life. You might have learned that metabolic reactions can be grouped into those that are
catabolic and those that are anabolic.
Catabolic reactions are those that break the bonds in large, complex molecules,
breaking these molecules down into smaller subunits. The breaking of bonds in catabolic processes releases energy, which is temporarily
stored in cells in the high-energy bonds of a molecule called ATP so it can be
easily accessed when needed. In contrast, anabolic reactions form bonds between small molecules, joining them
together to form larger molecules. The formation of these bonds requires an input of energy, which is supplied by
ATP.
This question presents us with an example of an anabolic reaction. A large molecule, starch, is formed from multiple smaller molecules. Starch is a polymer made up of many monomers. The word “monomer” contains the word mono- which means one, while the prefix poly-
means many, referring to the fact that polymers are made up of many repeating
subunits of monomers. Starch is a large carbohydrate made of many repeating units of a much smaller
carbohydrate, or sugar, monomer.
Amino acids are the monomers of proteins, not carbohydrates. So, we can eliminate option (D). Cellulose and glycogen are also carbohydrate polymers made of many repeating units of
simple sugars. So, these cannot be the monomer subunits of starch either. Glucose is a small, simple sugar molecule. Many plants produce their own sugars in the form of glucose through the process of
photosynthesis. They can then convert glucose into a large, insoluble polymer called starch, which
makes it easier to store as it does not interfere with the balance of water in
cells.
Knowing this, we can deduce the correct answer to this question. The monomer subunits that can join together to form starch in a metabolic reaction
are (A) glucose.
