In carbohydrate chemistry, a disaccharide is formed by the chemical bonding of which types of sugar units?

Introduction / Context:
Carbohydrates are classified based on the number of sugar units present. Monosaccharides are single sugar units, disaccharides contain two sugar units, and polysaccharides contain many units. This question asks specifically how a disaccharide is formed, which is important for understanding common sugars such as sucrose, lactose, and maltose, all of which are widely discussed in basic biology and chemistry.

Given Data / Assumptions:

• A monosaccharide is the simplest sugar, for example glucose or fructose.
• A disaccharide consists of exactly two monosaccharide units joined together.
• A polysaccharide consists of many monosaccharide units.
• The options offer different combinations of mono and polysaccharides.

Concept / Approach:
Disaccharides are formed when two monosaccharides react together in a condensation reaction, usually releasing a molecule of water and forming a glycosidic bond. For example, glucose plus fructose forms sucrose, glucose plus galactose forms lactose, and two glucose molecules form maltose. Polysaccharides such as starch or cellulose are much larger and are built by repeated addition of monosaccharides, not by joining two polysaccharides. Therefore, the core definition of a disaccharide must be based on two monosaccharides joined together.

Step-by-Step Solution:
Step 1: Recall the definition of monosaccharide. A monosaccharide is a single sugar unit such as glucose, fructose, or galactose. Step 2: Recall the definition of disaccharide. A disaccharide is formed when two monosaccharide molecules join through a glycosidic linkage. Step 3: Consider polysaccharides. Polysaccharides are long chains of many monosaccharides and not involved in forming disaccharides directly. Step 4: Match the correct description with the options.

Verification / Alternative check:
Textbooks commonly show equations like glucose + fructose → sucrose + water to illustrate disaccharide formation. They also present lactose as glucose plus galactose and maltose as glucose plus glucose. Each example supports the rule that disaccharides consist of two monosaccharides. There is no standard example where a disaccharide is formed by bonding two polysaccharides or a monosaccharide with an entire polysaccharide, so the alternative descriptions do not match the definition.

Why Other Options Are Wrong:
Option B: Two polysaccharides joined together would form an even larger polysaccharide, not a disaccharide. Option C: One monosaccharide and one polysaccharide would again produce a complex carbohydrate, not a disaccharide of exactly two units. Option D: All the above cannot be correct because only the first option matches the definition of a disaccharide.

Common Pitfalls:
Students sometimes think that any combination of sugar molecules might be called a disaccharide as long as it involves a bond, but the name specifically refers to two sugar units. Confusing the number of units in di and poly can also cause mistakes. A simple memory aid is that mono means one, di means two, and poly means many, and these prefixes directly tell you how many sugar units are present in the carbohydrate.

Final Answer:
A disaccharide is formed by the chemical bonding of two monosaccharides.