Video Transcript
Cellulose is formed by many repeats
of which carbohydrate? (A) 𝛼-glucose, (B) 𝛽-glucose, (C)
sucrose, or (D) lactose.
Cellulose is a polysaccharide. A polysaccharide is a type of
polymer that is made of monosaccharide monomers. By recalling this fact, we can
eliminate two of the answer choices right away. Both sucrose and lactose are
disaccharides. They are made of two
monosaccharides bonded together. They are not monomers that make up
polysaccharides.
So, we are left with 𝛼-glucose and
𝛽-glucose. 𝛼-glucose and 𝛽-glucose are
isomers of glucose. This means that they have the same
chemical formula but different chemical structures. Both 𝛼-glucose and 𝛽-glucose have
the chemical formula C6H12O6. And we number the carbon atoms in
the molecule like this.
The difference in structure is here
at the first carbon atom. In 𝛼-glucose, the hydroxyl group
attached to the first carbon atom is on the opposite side to the CH2OH group. In 𝛽-glucose, the hydroxyl group
attached to the first carbon is on the same side as the CH2OH group. This difference in structure makes
a difference in the structure of the polymers these different isomers will join to
make.
Both 𝛼- and 𝛽-glucose polymerize
by making what we call 1,4 bonds. This means that the atoms attached
to the first carbon molecule attach to the atoms attached to the fourth carbon
molecule, creating long chains. In polymers of 𝛼-glucose, this
makes flexible, sometimes coiled, and sometimes branched macromolecules. Polymers of 𝛼-glucose include
starches like amylose and amylopectin.
In polymers of 𝛽-glucose, the 1,4
bonds make rigid, stackable fibers. This makes 𝛽-glucose polymers more
suitable for functions related to structure. You may recall that cellulose is
the polysaccharide that gives the cell walls of plants some of their structural
rigidity.
Using this information, we can
conclude that cellulose is formed by many repeats of 𝛽-glucose.
