Question

(a) Explain why humans can use glycogen but not cellulose for energy. (b) Why can cows digest cellulose?

Short answer

Humans lack cellulase to digest cellulose; cows have microorganisms that produce cellulase to digest cellulose.

Step-by-step solution

Understanding Glycogen and Cellulose

Glycogen and cellulose are both carbohydrates, but they have different structures. Glycogen is a storage form of glucose in animals, while cellulose is a structural component in plants.

Structure of Glycogen vs. Cellulose

Glycogen has a branched structure, which makes it easily accessible for enzymes to break down into glucose. Cellulose has a linear structure with beta-linkages, which makes it difficult for human enzymes to break down.

Human Digestive Enzymes

Humans have the enzyme amylase which breaks down alpha-linkages found in glycogen but lack cellulase, the enzyme required to break beta-linkages in cellulose.

Cow Digestive System

Cows are ruminants and have a specialized digestive system that includes a rumen. The rumen contains microorganisms that produce cellulase, which breaks down cellulose into glucose that cows can absorb and use for energy.

Summary

Humans can't digest cellulose because they lack the enzyme cellulase, whereas cows can digest cellulose due to their microorganisms that produce cellulase.

Key concepts

Glycogen

Glycogen plays a crucial role in energy storage for humans and many animals. It is a type of carbohydrate, specifically a polysaccharide composed of glucose molecules. This energy reserve is primarily stored in the liver and muscles.
The structure of glycogen is highly branched, allowing rapid release of glucose when energy is needed. When we eat, the body converts excess glucose into glycogen for storage. Later, if the body requires energy, enzymes break glycogen down back into glucose through a process known as glycogenolysis. This breakdown ensures a steady supply of energy, especially important during activities like exercise or fasting.
The branching in glycogen is key. It allows more points for enzymes to attach and break down the glycogen, making it efficient and quick as an energy source.

Cellulose

Cellulose is a major structural component in the cell walls of plants. It gives plants their rigidity and strength. This polysaccharide is made up of glucose molecules linked together. However, unlike glycogen, cellulose features beta-glucose linkages that form a linear chain.
Due to these beta-linkages, the structure of cellulose is incredibly stable. The linear chains can pack closely together, creating a strong fibrous structure. In humans, the digestive system cannot break these linkages. We lack the enzyme cellulase, which is necessary to break down the beta-linkages in cellulose. As a result, cellulose passes through the human digestive tract mostly undigested and is often referred to as dietary fiber, which aids in digestive health by promoting bowel movement.
While we cannot use cellulose directly for energy, it is vital for maintaining intestinal health.

Enzymes in Digestion

Enzymes are essential for the digestion of carbohydrates, including glycogen and cellulose. These biological catalysts speed up chemical reactions. In the human digestive system, different enzymes target specific types of linkages within carbohydrate molecules.
Humans possess the enzyme amylase, which acts on alpha-linkages commonly found in glycogen. This allows us to efficiently break down glycogen into glucose for energy. However, when it comes to cellulose, humans cannot digest it because we lack the enzyme cellulase. Cellulase specifically targets the beta-linkages in cellulose.
This absence of cellulase highlights the limitation in human carbohydrate digestion, emphasizing the specialized role of enzymes in the digestive process.

Ruminant Digestive System

Ruminants like cows have a highly specialized digestive system to break down tough plant materials like cellulose. Unlike humans, ruminants have a multi-chambered stomach, which includes the rumen. This compartment hosts a variety of microorganisms, particularly bacteria, capable of producing the enzyme cellulase.
These microorganisms ferment cellulose, breaking down the beta-linkages into simpler sugars that the ruminant can absorb as a source of energy. This symbiotic relationship enables ruminants to digest cellulose efficiently, even though the animals themselves do not produce cellulase.
The rumination process, which involves regurgitating and re-chewing partially digested food, also aids in the thorough breakdown of plant material. This digestive system adaptation allows ruminants to access nutrients in plant-based diets that are beyond human capability.

Human Digestive System

The human digestive system is optimized for the breakdown of readily accessible nutrients, particularly those with alpha-linkages like glycogen. It starts in the mouth, where enzymes in saliva begin to break down carbohydrates.
As food travels through the digestive tract, different enzymes are secreted at various stages to enhance digestion. The stomach deals with proteins, while the small intestine continues to break down carbohydrates, proteins, and fats with the help of enzymes from the pancreas and intestinal lining.
Although efficient, the human digestive system cannot break down certain complex carbohydrates like cellulose due to the absence of specific enzymes like cellulase. As a result, these indigestible carbohydrates pass through mostly unchanged, contributing important dietary fiber that aids in intestinal health.