Biological Molecules: Question 10

Worked solution

Part (a): Classifying glucose, maltose and glycogen

• Glucose is a monosaccharide: a single sugar unit, with the molecular formula C6H12O6.
• Maltose is a disaccharide: it is formed when two monosaccharide units (two glucose molecules) join together by a condensation reaction, forming one glycosidic bond.
• Glycogen is a polysaccharide: it is a large polymer made of many hundreds or thousands of glucose monomers joined together by many glycosidic bonds.

Part (b): Counting glycosidic bonds and water molecules

When monomers are joined one after another into an unbranched chain, each new bond joins one monomer to the growing end of the chain, so the very first monomer in the chain is not itself joined by a bond to anything before it. A chain of $n$ monomers therefore has $n-1$ bonds linking them together.

For this amylose molecule, $n = 600$:

$$\text{number of glycosidic bonds} = 600 - 1 = 599$$

Each glycosidic bond is formed by a condensation reaction, which releases exactly one molecule of water. So the number of water molecules released is also:

$$\text{number of water molecules released} = 599$$

Part (c): Breaking the amylose molecule down

Breaking a glycosidic bond requires the addition of a water molecule, the reverse of condensation, so this is a hydrolysis reaction. In cells, this reaction is catalysed by a carbohydrase enzyme; because this particular substrate is amylose (a component of starch), the specific enzyme involved is an amylase.

Final answers

• (a) Glucose = monosaccharide (one sugar unit); maltose = disaccharide (two glucose units, one glycosidic bond); glycogen = polysaccharide (many glucose units, many glycosidic bonds).
• (b) 599 glycosidic bonds formed; 599 water molecules released.
• (c) Hydrolysis, catalysed by a carbohydrase (amylase).