What is the Difference Between Primary and Secondary Lysosomes

The main difference between primary and secondary lysosomes is that primary lysosomes are the small vesicles formed from the Golgi apparatus, whereas secondary lysosomes are the primary lysosomes fused with phagosomes or pinosomes. Furthermore, primary lysosomes contain inactive digestive enzymes, while secondary lysosomes contain active digestive enzymes. Besides, a single phospholipid layer surrounds primary lysosomes while secondary lysosomes are large and are capable of releasing their content. 

Primary and secondary lysosomes are two stages of lysosomes. Generally, they are the main digestive compartments of the cell. 

Key Areas Covered 

1. What are Primary Lysosomes
     – Definition, Structure, Role
2. What are Secondary Lysosomes
     – Definition, Structure, Role
3. What are the Similarities Between Primary and Secondary Lysosomes
     – Outline of Common Features
4. What is the Difference Between Primary and Secondary Lysosomes
     – Comparison of Key Differences

Key Terms 

Endosomes, Exocytosis, Lysosome, Primary Lysosomes, Secondary Lysosomes

What are Primary Lysosomes 

Primary lysosomes are the small vesicles released from the Golgi apparatus. Generally, they contain acid hydrolases or hydrolytic enzymes in the inactive form. Moreover, the content of the primary lysosomes is surrounded by a single phospholipid layer. 

Figure 1: Lysosome Structure

Furthermore, the main function of primary lysosomes is to fuse with endosomes such as phagocytes and pinosomes to from secondary lysosomes.  

What are Secondary Lysosomes 

Secondary lysosomes are the vesicles formed with the fusion of primary lysosomes to the endosomes. Here, endosomes, including phagosomes and pinosomes, contain materials to be digested. Generally, phagosomes either contain food particles or pathogens. On the other hand, pinosomes contain extracellular materials.

Figure 2: Lysosome Function

Moreover, primary lysosomes contain the inactive form of digestive enzymes. However, with the fusion of the endosomes, these enzymes become active. Furthermore, the activity of the enzymes depends on the pH inside the lysosome. Generally, acidic pH activates the enzymes to digest different types of macromolecules. After the digestion, secondary lysosomes eliminate unwanted materials inside it through exocytosis. 

Similarities Between Primary and Secondary Lysosomes 

• Primary and secondary lysosomes are two stages of lysosomes. 
• Their main function is to serve as the main digestive compartments, digesting different types of macromolecules. 
• Moreover, they occur in animal cells and some plant cells. 
• Both contain hydrolytic enzymes. 

Difference Between Primary and Secondary Lysosomes 

Definition 

Primary lysosomes refer to a membrane-bound sac, which buds from Golgi apparatus while secondary lysosomes refer to a lysosome formed by the combination of a primary lysosome and an endosome. Thus, this is the main difference between primary and secondary lysosomes.

Size 

While primary lysosomes are small, secondary lysosomes are large. 

Digestive Enzymes 

Primary lysosomes contain inactive digestive enzymes in the form of granules while secondary lysosomes contain active digestive enzymes. 

Activity 

Another difference between primary and secondary lysosomes is that the former does not undergo digestion while the latter undergoes digestion. 

Elimination 

Primary lysosomes are unable to eliminate their content to the outside while secondary lysosomes can eliminate their content to the outside of the cell. 

Conclusion 

Basically, primary lysosomes are the small vesicles formed from the Golgi apparatus. Generally, they contain inactive digestive enzymes in the form of granules. On the other hand, secondary lysosomes are formed when primary lysosomes are fused with the endosomes such as phagosomes or pinosomes. Here, this fusion also activates enzymes in the lysosomes. Additionally, secondary lysosomes are capable of eliminating materials inside the lysosome into the outside. Therefore, the main difference between primary and secondary lysosomes is their structure and function. 

References:

1. “Lysosomes – Types, Morphology, Function, Process and Microscopy.” MicroscopeMaster, Available Here.

Image Courtesy:

1. “Lysosome” By lumoreno – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “Phagocytosis2” By GrahamColm at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia