Difference Between Intracellular and Extracellular Digestion

Main Difference – Intracellular vs Extracellular Digestion

Heterotrophic organisms obtain energy through the ingestion of energy-rich food. The ingested food should be digested into small compounds to absorb as nutrients. Intracellular and extracellular digestion are the two methods of digestion of food in the above-mentioned organisms. Intracellular digestion mainly occurs in unicellular organisms such as protozoans. In addition, extracellular digestion occurs in animals with a digestive system and in fungi. The main difference between intracellular and extracellular digestion is that intracellular digestion occurs inside the food vacuoles within the cell whereas extracellular digestion occurs outside the cell in the lumen of the alimentary canal or on the decaying organic materials.

Key Areas Covered

1. What is Intracellular Digestion
     – Definition, Type, Place of Occurrence, Mechanism
2. What is Extracellular Digestion
     – Definition, Type, Place of Occurrence, Mechanism
3. What are the Similarities Between Intracellular and Extracellular Digestion
     – Outline of Common Features
4. What is the Difference Between Intracellular and Extracellular Digestion
     – Comparison of Key Differences

Key Terms: Alimentary Canal, Animals, Autophagic Digestion, Extracellular Digestion, Food Vacuoles, Fungi, Heterophagic Digestion, Intracellular Digestion, Protozoans

What is Intracellular Digestion

Intracellular digestion refers to a form of digestion where the break down of materials into small components takes place inside the cell. The hydrolytic enzymes stored in the lysosomes are responsible for the chemical digestion of the food particle. Intracellular digestion can be categorized into two types as heterophagic digestion and autophagic digestion.

Heterophagic Digestion

The heterophagic digestion is the breaking down of molecules brought into the cell by endocytosis. The degradation of ingested food during intracellular digestion occurs in a process known as phagotrophy. The endocytic vesicle or the food vacuole is fused with a lysosome and the chemical digestion occurs inside the food vacuole. The nutrients diffuse to the cytoplasm through the walls of the vesicle. The indigestible materials are excreted through exocytosis.

Figure 1: Amoeba Phagocytosis

The heterotrophagic digestion in amoeba is shown in figure 1.

Autophagic Digestion

Autophagic digestion occurs inside the cell to digest internal molecules and organelles. Autophagy maintains energy sources in the cell by recycling the damaged proteins, aggregates, and organelles in the cell. The end products of the degradation can be used as building blocks for the replacement of the depleted cellular components. Thereby, autophagy promotes the survival of the cell during stress by balancing the cellular energy levels. It allows the clearing of unwanted components from the cell as well. Hence, autophagy is pro-survival and capable of undergoing cellular stress like nutrient deprivation. But, autophagy lets the cell die by destroying active organelles in it like mitochondria.

What is Extracellular Digestion

Extracellular digestion refers to a form of digestion where the breakdown of materials into smaller components takes place outside the cell. Thus, the hydrolytic enzymes are secreted on the food materials via the cell membrane. In animals, extracellular digestion occurs inside the lumen of the alimentary canal. The alimentary canal of animals is differentiated into different regions such as mouth, esophagus, stomach, small intestine, large intestine, and anus. Different regions possess different functions during the digestion of food. The initial regions are involved in the mechanical digestion of food while the lattermost regions are involved in the chemical digestion as well as the absorption of nutrients. The salivary, gastric, pancreatic, and intestinal glands secrete digestive enzymes to the lumen. The components of the alimentary canal of humans are shown in figure 2.

Figure 2: Alimentary Canal of Humans

However, in fungi, the hydrolytic enzymes are secreted on the decaying organic matter. The digested simple nutrients are absorbed through the cell wall. As the digestion of food does not take place inside the cells in fungi, this type of digestion is considered as extracellular. Bacteria also undergo extracellular digestion same as fungi. Thus, both fungi and bacteria are referred to as decomposers, possessing an important role in the recycling of nutrients in ecosystems.

Similarities Between Intracellular and Extracellular Digestion

• Intracellular and extracellular digestion are two types of mechanisms involved in the digestion of food.
• Enzymes are involved in the digestion of both intracellular and extracellular digestion.
• Both intracellular and extracellular digestion break down complex compounds into simple compounds.
• Both intracellular and extracellular digestion facilitate the absorption of nutrients.

Difference Between Intracellular and Extracellular Digestion

Definition

Intracellular Digestion: Intracellular digestion refers to a form of digestion where the break down of materials into small components takes place inside the cell.

Extracellular Digestion: Extracellular digestion refers to a form of digestion where the break down of materials into smaller components takes place outside the cell.

Place of Occurrence

Intracellular Digestion: Intracellular digestion occurs inside food vacuoles within the cell.

Extracellular Digestion: Extracellular digestion occurs outside the cell in the lumen of the alimentary canal or on the decaying organic materials.

Ingestion

Intracellular Digestion: Ingestion occurs through a phagocytic vesicle in intracellular digestion.

Extracellular Digestion: Ingestion occurs through the mouth in extracellular digestion.

Mechanism

Intracellular Digestion: The digestive enzymes in the lysosomes are secreted into the food vacuole in intracellular digestion.

Extracellular Digestion: The glands of the alimentary canal secrete digestive enzymes into the lumen in extracellular digestion. Fungi secrete digestive enzymes on the decaying organic materials.

Type of Digestion

Intracellular Digestion: Only the chemical digestion of food occurs during intracellular digestion.

Extracellular Digestion: Both mechanical digestion and chemical digestion occur in extracellular digestion in animals.

Method of Absorption

Intracellular Digestion: The nutrients diffuse into the cytoplasm through the membrane of the vacuole in intracellular digestion.

Extracellular Digestion: The nutrients are absorbed into the blood through the gut epithelia in extracellular digestion in animals. In fungi, nutrients are absorbed through the cell wall.

Excretion of the Indigestible Materials

Intracellular Digestion: The indigestible materials are excreted through exocytosis in intracellular digestion.

Extracellular Digestion: The indigestible materials are excreted through the anus in extracellular digestion

Complexity

Intracellular Digestion: Intracellular digestion is a simple mechanism of digestion.

Extracellular Digestion: Extracellular digestion is a complex mechanism of digestion.

Components

Intracellular Digestion: Vesicles are involved in the intracellular digestion.

Extracellular Digestion: Organs and glands are involved in the extracellular digestion.

Examples

Intracellular Digestion: Intracellular digestion occurs in protozoans.

Extracellular Digestion: Extracellular digestion occurs in bacteria, fungi and in animals with an alimentary canal.

Conclusion

Intracellular and extracellular digestion are the two types of digestion of the ingested food materials in animals and protozoans. In protozoans, the ingested food particles are digested inside a food vacuole by intracellular digestion. Besides in animals with an alimentary canal, the digestion occurs within the lumen of the alimentary canal by extracellular digestion. The main difference between intracellular and extracellular digestion is the location and complexity of each type of digestion mechanisms.

Reference:

1. Anderson, O. Roger. “Intracellular Digestion.” The American Biology Teacher, vol. 32, no. 8, 1970, pp. 461–467. JSTOR, JSTOR, Available here.
2.McMahon, Mary, and Nancy Fann-Im. “What Is Extracellular Digestion?” WiseGEEK, Conjecture Corporation, 7 Dec. 2017, Available here.

Image Courtesy:

1. “Digestive system diagram en” By Mariana Ruiz Villarreal(LadyofHats) – Own work (Public Domain) via Commons Wikimedia
2. “Amoeba phagocytosis” By Miklos – Wikimedia Commons (Public Domain) via Commons Wikimedia

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