Understanding the Correct Answer: Maltose
To answer the question, "Which sugar is a disaccharide: a glucose b galactose c maltose d dextrin?", we must look at the specific composition of each option. The correct answer is c) maltose. A disaccharide is a sugar composed of two monosaccharide units joined together by a glycosidic bond. Maltose, also known as malt sugar, is formed specifically from two molecules of alpha-D-glucose. This bonding occurs through a dehydration reaction, where a water molecule is removed as the two simple sugars join. The glycosidic linkage in maltose is typically an α(1→4) bond, connecting the first carbon of one glucose unit to the fourth carbon of the other. This is the key structural feature that defines it as a disaccharide and distinguishes it from the other options provided.
The Incorrect Options: A Deeper Look
Understanding why the other options are incorrect is crucial for a complete grasp of carbohydrate chemistry.
- a) Glucose: Glucose is a simple sugar, or monosaccharide, with the chemical formula $C6H{12}O_6$. As a single sugar unit, it serves as a fundamental building block for larger carbohydrates like maltose, as well as starches and cellulose. It is the most abundant monosaccharide and a vital energy source for most organisms.
- b) Galactose: Similar to glucose, galactose is also a monosaccharide. It is often found combined with glucose to form the disaccharide lactose, or milk sugar. It has the same chemical formula as glucose ($C6H{12}O_6$) but a different arrangement of atoms, making it an isomer.
- d) Dextrin: Dextrins are not a single type of sugar but a group of soluble polysaccharides. They are a byproduct of the hydrolysis of starch, meaning they consist of multiple glucose units linked together, though in shorter chains than full starches. Therefore, dextrin is a polysaccharide, not a disaccharide.
Comparison of Carbohydrate Types
To further clarify the differences, here is a comparison of monosaccharides, disaccharides, and polysaccharides.
| Feature | Monosaccharide (e.g., Glucose, Galactose) | Disaccharide (e.g., Maltose, Sucrose) | Polysaccharide (e.g., Dextrin, Starch) |
|---|---|---|---|
| Number of Sugar Units | One single sugar unit | Two monosaccharide units | Multiple sugar units (more than two) |
| Chemical Formula | $C6H{12}O_6$ for hexoses | $C{12}H{22}O_{11}$ (general formula) | $(C6H{10}O_5)n$ |
| Common Examples | Glucose, Fructose, Galactose | Maltose, Sucrose, Lactose | Dextrin, Starch, Cellulose |
| Formation | Cannot be hydrolyzed further into simpler sugars | Formed by dehydration synthesis of two monosaccharides | Formed by repeated dehydration synthesis of monosaccharides |
| Sweetness | Sweet | Generally sweet, but varies (e.g., maltose is less sweet than sucrose) | Not typically sweet |
Functions of Different Sugar Types
Each type of carbohydrate plays a distinct role in biological systems and food production.
Maltose in Brewing and Digestion Maltose is particularly important in the brewing industry, where enzymes break down starches in grains to produce it. Yeast then ferments this maltose into alcohol and carbon dioxide. In the human body, the enzyme maltase breaks down maltose into two glucose molecules in the small intestine, which are then absorbed into the bloodstream for energy.
Glucose as a Primary Energy Source Glucose is the body's primary and most readily available energy source. It circulates in the blood as 'blood sugar' and is used by cells to produce ATP. Excess glucose is stored in the body as the polysaccharide glycogen.
Galactose in Milk and the Body Galactose is a component of lactose, the sugar found in milk. It is also part of glycolipids and glycoproteins in nerve tissue, where it is sometimes referred to as 'brain sugar'. The body needs specific enzymes to metabolize galactose; a deficiency can lead to galactosemia, a serious genetic disorder.
Dextrins as Thickeners and Preservatives Because they are shorter chains of glucose than starch, dextrins are more water-soluble. This property makes them useful in the food industry as thickeners, gelling agents, and fat replacers in low-fat products. Maltodextrin, a common type of dextrin, is used to prevent instant foods from clumping.
Conclusion: The Final Answer Explained
In conclusion, when faced with the question of which sugar is a disaccharide among glucose, galactose, maltose, and dextrin, the definitive answer is c) maltose. Maltose is correctly identified as a disaccharide because it is formed from two glucose units joined by a glycosidic bond. Glucose and galactose are the single-unit monosaccharides, while dextrin is a more complex, multi-unit polysaccharide. This classification is foundational to understanding the chemical and biological functions of carbohydrates.
List of Carbohydrate Types
- Monosaccharides: Simple sugars like glucose, fructose, and galactose.
- Disaccharides: Double sugars like maltose, lactose, and sucrose.
- Polysaccharides: Complex carbohydrates with long chains, such as starch and dextrin.
Conclusion
Understanding the fundamental differences between monosaccharides, disaccharides, and polysaccharides is essential for anyone interested in nutrition or biochemistry. The case of maltose being the only disaccharide among the listed options provides a clear example of these classifications. Each sugar, from the simple glucose to the complex dextrins, plays a unique role, and knowing these roles helps in appreciating the complexity of the carbohydrate family.
Key Learnings
- Maltose is a disaccharide formed from two glucose molecules.
- Glucose and galactose are monosaccharides, or simple, single-unit sugars.
- Dextrin is a polysaccharide, a longer chain of glucose units.
- Disaccharides are formed when two monosaccharides join through a dehydration reaction.
- Enzymes are required to break down complex sugars like maltose into their simpler components for absorption.
FAQs
Q: What are the three most common disaccharides? A: The three most common disaccharides are maltose (glucose + glucose), lactose (glucose + galactose), and sucrose (glucose + fructose).
Q: How is maltose created in the body? A: Maltose is produced in the body during the digestion of starch, where the enzyme amylase breaks down the long starch chains into two-unit maltose molecules.
Q: Is maltose considered a reducing sugar? A: Yes, maltose is a reducing sugar because one of its glucose units has a free aldehyde group that can act as a reducing agent.
Q: Why do some people have trouble digesting lactose but not maltose? A: People with lactose intolerance lack the enzyme lactase, which is needed to break down lactose into glucose and galactose. While the body has other enzymes, such as maltase, to digest other sugars, the lack of lactase causes digestive issues.
Q: Where is dextrin used? A: Dextrin is commonly used in the food industry as a thickener and fat replacer, particularly in low-fat frozen desserts and coatings.
Q: What is the main difference between a monosaccharide and a disaccharide? A: A monosaccharide is a single sugar unit, while a disaccharide is composed of two monosaccharide units bonded together.
Q: Are all disaccharides equally sweet? A: No, disaccharides vary in sweetness. For example, sucrose (table sugar) is much sweeter than maltose.
Q: What is the chemical formula for maltose? A: The chemical formula for maltose is $C{12}H{22}O_{11}$.
Q: Where is maltose commonly found? A: Maltose is found in malted grains like barley and is a key component in beer brewing. It is also found in corn syrup.
Q: What happens when maltose is consumed? A: When you consume maltose, the enzyme maltase in your small intestine breaks it down into two glucose molecules, which are then absorbed into the bloodstream for energy.
