Question

What is a polysaccharide? What monomer unit makes up the common polysaccharides starch and cellulose?

Short answer

A polysaccharide is a complex carbohydrate comprising long chains of monosaccharide units, such as glucose, linked together by glycosidic bonds. They serve various functions in living organisms, including energy storage and structural support. The monomer unit that makes up the common polysaccharides starch and cellulose is glucose, with different linkages (α-1,4-glycosidic bonds in starch and β-1,4-glycosidic bonds in cellulose) giving them distinct properties.

Step-by-step solution

Definition of Polysaccharide

A polysaccharide is a complex carbohydrate composed of long chains of monosaccharide units (simple sugars) linked together by glycosidic bonds. Polysaccharides perform various functions in living organisms, such as energy storage (e.g., starch in plants) and providing structural support (e.g., cellulose in plant cell walls).

Monomer Unit of Starch and Cellulose

The monomer unit that makes up the common polysaccharides starch and cellulose is glucose. Glucose is a monosaccharide, and it serves as the building block for both starch and cellulose. However, the way glucose molecules are linked together and organized in these two polysaccharides is different, which gives them distinct properties. In starch, glucose molecules are joined by α-1,4-glycosidic bonds, forming linear chains (amylose) or branched structures (amylopectin). In cellulose, glucose molecules are connected through β-1,4-glycosidic bonds, resulting in straight chains that can form strong intermolecular hydrogen bond networks, giving cellulose its structural role in plant cell walls.

Key concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. Often referred to as simple sugars, they are the basic building blocks for more complex carbohydrates like disaccharides and polysaccharides. These simple sugars typically have the general formula \(C_nH_{2n}O_n\). Glucose, fructose, and galactose are common examples of monosaccharides.

Monosaccharides have several key characteristics:

• They are usually colorless, crystalline solids.
• They are water-soluble.
• They possess a sweet taste.

In biological systems, monosaccharides play crucial roles. They provide energy, form the backbone of nucleic acids, and are precursors for other essential biomolecules.

Glycosidic Bonds

Glycosidic bonds are the critical linkages that connect monosaccharides to form more complex carbohydrates, such as disaccharides and polysaccharides. These bonds are covalent connections established during a condensation reaction, where a molecule of water is released.

The type of glycosidic bond, whether \(\alpha\) or \(\beta\) , determines the structure and function of the polysaccharide:

• \(\alpha\)-glycosidic bonds make the chains flexible, as seen in starch, allowing it to serve as an energy reserve.
• \(\beta\)-glycosidic bonds create rigid and straight structures, making cellulose excellent for structural support.

For example, in starch, the glucose units are connected via \(\alpha\)-1,4 bonds for amylose and \(\alpha\)-1,6 bonds in amylopectin branches, while cellulose consists of glucose linked by \(\beta\)-1,4 bonds.

Starch

Starch is a polysaccharide that serves as a main energy reserve in plants. It is composed of numerous glucose units.

Starch has two components:

• Amylose, which is mostly a linear molecule formed by \(\alpha\)-1,4-glycosidic bonds.
• Amylopectin, a branched polysaccharide that also contains \(\alpha\)-1,6-glycosidic bonds in addition to \(\alpha\)-1,4 bonds.

These structures enable starch to be packed densely and be easily hydrolyzed when energy is needed.

In human diets, starch is a vital carbohydrate source. Our digestive systems break it down into glucose units, which cells then use for energy.

Cellulose

Cellulose is a major component of the plant cell wall. This polysaccharide consists of glucose monomers, similar to starch, but it plays a structural role rather than an energy storage one.

Key properties of cellulose include:

• It is made of glucose units connected by \(\beta\)-1,4-glycosidic bonds.
• The straight chains allowed by these bonds form strong fibers due to intermolecular hydrogen bonding.
• It is not digestible by humans because we lack the necessary enzyme to break its \(\beta\)-glucosidic linkages.

However, cellulose acts as dietary fiber, aiding in digestive processes by promoting healthy bowel movement.

Glucose

Glucose is a primary energy source for living organisms and a fundamental building block for various polysaccharides.

Important characteristics of glucose include:

• It is a monosaccharide with the chemical formula \(C_6H_{12}O_6\).
• Exists naturally in alpha (\(\alpha\)) and beta (\(\beta\)) anomeric forms.

In nature, it combines with other glucose molecules to form essential polysaccharides such as starch and cellulose.

In energy metabolism, glucose's breakdown through glycolysis and its further conversion in cellular respiration are crucial for powering cellular activities. Its versatile roles underscore its importance in biological processes.