Problem 9 - AP Biology Premium Prep 2021

Question

Maltotriose is a trisaccharide composed of three glucose molecules linked through \(\alpha-1,4\) glycosidic linkages formed via dehydration synthesis. What would the formula be for maltotriose? (A) \(\mathrm{C}_{18} \mathrm{H}_{36} \mathrm{O}_{18}\) (B) \(\mathrm{C}_{18} \mathrm{H}_{10} \mathrm{O}_{15}\) (C) \(\mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16}\) (D) \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

The formula for maltotriose is (C) \(\mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16}\).

Step 1: Determine the total number of atoms in three glucose molecules

In order to determine the molecular formula for maltotriose, first, we will find out the total number of carbon, hydrogen, and oxygen atoms in three glucose molecules. Since the molecular formula for glucose is \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\), for three glucose molecules, it would be: Carbon atoms: \(6 \times 3 = 18\) Hydrogen atoms: \(12 \times 3 = 36\) Oxygen atoms: \(6 \times 3 = 18\) So, the combined formula for three glucose molecules is \(\mathrm{C}_{18} \mathrm{H}_{36} \mathrm{O}_{18}\).

Step 2: Subtract water molecules lost due to dehydration synthesis

Since maltotriose is formed by linking three glucose molecules with \(\alpha-1,4\) glycosidic linkages formed via dehydration synthesis, it involves the loss of water molecules. For every linkage formed, one water molecule is lost. As there are two linkages in maltotriose (to connect three glucose molecules), there will be a loss of two water molecules (\(\mathrm{H}_{2} \mathrm{O}\)). Subtract the lost water molecules from the combined formula of three glucose molecules: Carbon atoms remain the same: \(18\) Hydrogen atoms: \(36 - 2 \times 2 = 32\) Oxygen atoms: \(18 - 2 = 16\)

Step 3: Write the molecular formula for maltotriose

The molecular formula for maltotriose after removing the water molecules lost due to dehydration synthesis is: \(\mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16}\) Hence, the correct answer is (C) \(\mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16}\).

Key Concepts

Polysaccharides

Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units. These chains can vary in length and be branched or unbranched. In biochemistry, polysaccharides serve numerous roles:

• Energy Storage: Polysaccharides like starch and glycogen are used to store energy in plants and animals, respectively.
• Structural Support: Cellulose, a type of polysaccharide, provides structural support in plant cell walls.
• Recognition and Signaling: Some polysaccharides are involved in cellular recognition and signaling processes.

Polysaccharides are formed by linking monosaccharide units through a process called dehydration synthesis, which we'll explore next. This linkage not only dictates the structural properties of the polysaccharide but also its biological function.

Dehydration Synthesis

Dehydration synthesis is a chemical reaction that involves the joining of two molecules, resulting in the loss of a water molecule. This process is fundamental in forming complex carbohydrates like polysaccharides from simpler units such as monosaccharides. Here's how it works:

• Removing Water: When two glucose molecules join, a hydroxyl group (OH) from one glucose and a hydrogen (H) from another combine to form water (H₂O).
• Formation of Linkages: The remaining oxygen atom forms a covalent bond between the two glucose molecules, creating a glycosidic linkage.

In the case of maltotriose, each glycosidic linkage results in the removal of one molecule of water. Since maltotriose consists of three glucose units, two water molecules are lost during its formation.

Glycosidic Linkage

A glycosidic linkage is the covalent bond formed between two sugar molecules during the dehydration synthesis process. These linkages are crucial in the formation of disaccharides and polysaccharides.

• Types of Linkages: The most common glycosidic linkages are alpha (α) and beta (β). The difference lies in the orientation of the bonds relative to the glucose ring structure.
• Alpha Linkages: In alpha linkages, the bond is formed below the plane of the glucose rings—such as the α-1,4 linkages in maltotriose.
• Significance: The type of glycosidic linkage can influence the digestibility, solubility, and overall structure of the carbohydrate.

Understanding glycosidic linkages helps us comprehend how sugars interact and how their properties can change when linked together.

Chemical Formula

A chemical formula represents the composition of a compound, showing the elements present and the number of atoms of each element. For example, glucose is represented by the formula \( \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \). When calculating the formula of a compound like maltotriose:

• Start with Individual Units: Multiply the formula of glucose by the number of glucose molecules involved.
• Account for Reactions: Subtract atoms lost or gained during chemical reactions, such as those lost in dehydration synthesis.

The final formula for maltotriose, \( \mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16} \), reflects adjustments based on the formation of two glycosidic linkages resulting in the loss of two water molecules.

Glucose

Glucose is a simple sugar and a primary source of energy for living cells. It is a monosaccharide with the chemical formula \( \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \) and plays several critical roles:

• Energy Supply: Glucose is metabolized to produce ATP, a molecule essential for energy transfer within cells.
• Building Block: It serves as a building block for more complex carbohydrates like starch, glycogen, and cellulose.
• Regulation of Blood Sugar: In humans, insulin and other hormones regulate blood glucose levels to maintain homeostasis.

Understanding glucose's role and properties is crucial when exploring more complex carbohydrates like maltotriose, which are formed by the polymerization of glucose units.