Although cellulose and starch are identical in terms of stored chemical energy, our ability to harvest the energy from these two polysaccharides differs considerably. What is responsible for this difference?

Verified step by step guidance

1. Understand the structure of cellulose and starch: Both are polysaccharides composed of glucose units, but they differ in the type of glycosidic bonds that link the glucose molecules.
2. Identify the type of glycosidic bonds: Starch is composed of α(1→4) and α(1→6) glycosidic bonds, while cellulose is composed of β(1→4) glycosidic bonds.
3. Consider the enzyme specificity: Human digestive enzymes, such as amylase, can break down α-glycosidic bonds in starch but cannot break down β-glycosidic bonds in cellulose.
4. Analyze the structural implications: The β(1→4) bonds in cellulose result in a linear, fibrous structure that is resistant to enzymatic digestion, whereas the α(1→4) bonds in starch create a helical structure that is more accessible to enzymes.
5. Conclude the reason for the difference: The inability of human enzymes to digest cellulose is due to the presence of β-glycosidic bonds, which are not recognized by these enzymes, unlike the α-glycosidic bonds in starch.

Key Concepts

Polysaccharide Structure

Cellulose and starch are both polysaccharides made up of glucose units, but their structural configurations differ significantly. Starch consists of amylose and amylopectin, which are branched and can be easily broken down by enzymes. In contrast, cellulose has a linear structure with β-1,4-glycosidic bonds, making it resistant to enzymatic degradation in humans and many animals.

Enzymatic Breakdown

The ability to harvest energy from polysaccharides relies on specific enzymes that can break down their glycosidic bonds. Humans possess enzymes like amylase that can effectively hydrolyze starch, releasing glucose for energy. However, we lack the necessary enzymes, such as cellulase, to break down cellulose, which is why we cannot utilize its energy despite its abundance in plant materials.

Digestibility and Energy Harvesting

Digestibility refers to how well a substance can be broken down and absorbed by an organism. Starch is digestible and provides a readily available energy source, while cellulose is largely indigestible for humans. This difference in digestibility is crucial for understanding why we can efficiently harvest energy from starch but not from cellulose, despite their similar energy content.