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Chapter 18: Problem 27

How are cellulose and starch similar? How do they differ from each other?

Short Answer

Expert verified
Cellulose and starch are both carbohydrates composed of glucose units and are polysaccharides. However, they differ in function, type of bonds, structure, and digestibility. Cellulose provides structural support to plant cell walls with linear, unbranched structures and beta (尾) glycosidic bonds, while starch acts as an energy storage molecule with alpha (伪) glycosidic bonds and two types of structures - amylose (linear, unbranched) and amylopectin (branched chain). Humans can digest starch but not cellulose due to differences in bond types.

Step by step solution

01

Introduction to Cellulose and Starch

Cellulose and starch are both carbohydrates, or complex sugars, that play various roles within plants. Cellulose is the main structural component of plant cell walls, providing strength and rigidity to the plant. Starch is a storage polysaccharide found in plant cells, mainly used for energy storage.
02

Similarities between Cellulose and Starch

1. Both are made up of glucose molecules: Cellulose and starch are both composed of glucose units, which are simple sugar molecules. 2. Both are polysaccharides: They are both long chains of sugar molecules bonded together.
03

Differences between Cellulose and Starch

1. Function: Cellulose provides the structural support to plant cell walls, while starch is used as an energy storage molecule. 2. Type of Glucose Bonds: The glucose molecules in cellulose are connected by beta (尾) glycosidic bonds, while the glucose molecules in starch are bonded by alpha (伪) glycosidic bonds. 3. Structure: Cellulose has a linear, unbranched structure with parallel chains that form strong hydrogen bonds, resulting in a rigid structure. Starch, on the other hand, has two types of structures - amylose, which is a linear, unbranched chain, and amylopectin, which is a branched chain. This makes starch a more compact and easily accessible energy storage molecule. 4. Digestibility: Humans and animals can digest and take energy from starch because our bodies can break the alpha (伪) glycosidic bonds. However, we cannot break down cellulose due to the lack of a specific enzyme needed to break the beta (尾) glycosidic bonds. Some animals like cows and termites have cellulose-digesting microbes in their digestive systems, allowing them to break down cellulose into usable sugars. In conclusion, cellulose and starch are similar in that they are both composed of glucose units and are polysaccharides. Their differences lie in their function, type of bonds, structure, and digestibility.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Polysaccharides
Polysaccharides are essentially long chains of monosaccharides, mostly glucose, linked together through glycosidic bonds. They are macromolecules that serve various functions in both plants and animals.

In plants, polysaccharides like cellulose are crucial in forming the rigid structure of cell walls, which helps to maintain cell shape and protect against mechanical stress. Starch, another polysaccharide, serves as a storage form of energy that can be broken down when the plant requires sugar.

Plant Energy Storage and Structure

  • Cellulose: forms the structure of cell walls, offering support.
  • Starch: stores energy, ready for conversion back into glucose.
This dual role showcases the versatility and essential nature of polysaccharides within plant biology, and indeed in broader ecological systems.
Glycosidic Bonds
Glycosidic bonds are the bonds that link sugar molecules together in polysaccharides. The two common types are alpha (伪) and beta (尾) glycosidic bonds. Alpha-glycosidic bonds are formed in a way that allows the bonded glucose molecules to be easily broken down by enzymes in the human digestive tract, making nutrients readily available. This is why we can digest starch, which has 伪-glycosidic bonds.

Structural Variation

  • Alpha (伪) bonds: easily digested, found in starch.
  • Beta (尾) bonds: resistant to digestion, found in cellulose.
Beta-glycosidic bonds, however, are oriented differently, offering a challenge to our digestive enzymes, thus making cellulose resistant to digestion. This distinction is important for understanding how the same glucose units can serve vastly different functions based on their bonding.
Plant Cell Structures
Plant cells are unique in their structure, primarily because of the presence of a cell wall made of cellulose. This strong and rigid wall differentiates plant cells from animal cells, which do not possess such a structure.

The cell wall serves several pivotal roles; it defines the shape of the cell, offers protection, and provides the pressure necessary to keep plants upright. Starch granules, often found in plastids such as chloroplasts, are crucial for energy storage and are carefully packaged within the cell to maintain balance and functionality.

Key Plant Cell Components

  • Cell Wall: Composed of cellulose, providing structure.
  • Chloroplasts: Where starch granules are often stored.
Understanding these components of plant cell structures allows us to appreciate how polysaccharides are intricately woven into the life of plants.
Carbohydrate Digestibility
The digestibility of carbohydrates such as starch and cellulose in humans depends on the presence of specific enzymes that can break down glycosidic bonds. Amylase is the enzyme in our saliva and pancreas that cleaves alpha-glycosidic bonds in starch, making the glucose available for absorption.

Cellulose, on the other hand, contains beta-glycosidic bonds which human digestive enzymes cannot break down. As a result, cellulose passes through our digestive system as fiber, aiding in digestive health. Some herbivorous animals have symbiotic relationships with microbes that produce cellulase, the enzyme required to break down cellulose.

Carbohydrate Digestion Factors

  • Amylase: Enzyme that breaks down starch into glucose.
  • Cellulase: Enzyme produced by microbes in some herbivores, not humans, to digest cellulose.
The biological availability of carbohydrates is therefore influenced by both the type of glycosidic bonds present and the enzymes that can interact with them.

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Most popular questions from this chapter

Define monosaccharide, disaccharide, and polysaccharide and give an example of each.

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Proteins, like DNA, often adopt a coiled-like structure, known as an a-helix. This helical structure is stabilized by the formation of intramolecular hydrogen bonds between the backbone amino group of one amino acid and the \(\mathrm{C}=\mathrm{O}\) group of another amino acid a bit further down the chain. The amino acid proline is often referred to as a "helix breaker" because when it is part of a protein molecule, the \(\alpha\) -helix begins to unravel at its position. What is so different about this particular amino acid that would cause this disruption of the helix structure and earn it that name?

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