Difference Between Capsid and Envelope

Main Difference – Capsid vs Envelope

Capsid and envelope are two protective structures of a virus. Viruses are self-replicative structures. A virus is not considered as a living organism. It is just the genetic material protected by a protein coat called capsid. Some viruses consist of another protective coat called the envelope. The capsid is made up of proteins. The envelope is made up of proteins and phospholipids. The main difference between capsid and envelope is that capsid is the protective coat of the genetic material of the virus whereas envelope is a protective covering of the protein capsid. Viruses that consist of an envelope are called enveloped viruses. Viral envelope allows the virus to invade the host cell by attaching to the cell membrane of the host cell.

Key Areas Covered

1. What is a Capsid
     – Definition, Characteristics, Function
2. What is an Envelope
      – Definition, Characteristics, Function
3. What are the Similarities Between Capsid and Envelope
      – Outline of Common Features
4. What is the Difference Between Capsid and Virus
      – Comparison of Key Differences

Key Terms: Capsid, Capsomeres, Envelope, Glycoproteins, Icosahedral, Helical, Phospholipids, Prolate, Proteins, Virus

What is a Capsid

A capsid is a protective coat of protein that protects the genetic material of the virus. Capsids can be identified in three different shapes: icosahedral, helical, and prolate. Most viruses are icosahedral and helical in shape. But, some viruses such as bacteriophages have more complicated shapes. The capsid is made up of proteins. The protein subunits that form the capsid are called capsomeres.

Figure 1: Viral capsid

The main function of the viral capsid is to protect the contents of the virus. Capsids protect the virus from extreme temperatures, pH differences, radiation, chemicals, and enzymes. A helical-shaped viral capsid is shown in figure 1.

What is an Envelope

An envelope is the outer structure of some viruses that encloses the capsid of the virus. The envelope is derived from the host’s cell membrane. Therefore, the envelope is mainly composed of phospholipids and proteins. The envelope also consists of viral glycoproteins. Glycoproteins are involved in the attachment of the virus to the receptors of the host’s cell membrane. After attaching to the cell membrane, the envelope fuses with the host’s cell membrane and capsid is released into the cytoplasm of the host. The capsid protects the genetic material of the virus from enzymatic degradation inside the host cell. The viral envelope is shown in figure 2.

Figure 2: Viral Envelope

Similarities Between Capsid and Envelope

Both capsid and envelope are protective layers of a virus.
Both capsid and envelope are made up of proteins.

Definition

Capsid: A capsid is a protein shell that protects the genetic material of a virus.

Envelope: An envelope is the outer structure of some viruses that encloses the capsid.

Composition

Capsid: Capsid is made up of proteins.

Envelope: Envelope is made up of proteins and phospholipids.

Enclose

Capsid: Capsid encloses the genetic material of the virus.

Envelope: Envelope encloses the capsid.

Presence

Capsid: Capsids are present in all viruses.

Envelope: Envelopes are present in only some viruses .

Role

Capsid: Capsid protects the genetic material of the virus inside the host.

Envelope: Envelope allows the virus to invade the host cell by fusing with the host’s cell membrane.

Conclusion

Capsid and envelope are the two protective layers of the content of a virus. The capsid is made up of proteins and it protects the genetic material of the virus. Viral envelope is derived from the host’s cell membrane. It is made up of phospholipids and proteins. The viral capsid is protected by the viral envelope. Not all viral capsids are enclosed by viral envelopes. The main difference between capsid and envelope is the composition and function of the each protective layer in the virus.

Reference:

1.”Capsid: Definition, Function & Structure.” Study.com. N.p., n.d. Web. Available here. 05 Aug. 2017.
2.”Viral envelope.” Wikipedia. Wikimedia Foundation, 02 Aug. 2017. Web. Available here. 13 Aug. 2017.

Image Courtesy:

1. “Helical capsid with RNA” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Virus stucture simple” By GrahamColmTalk – (CC BY-SA 3.0) via Commons Wikimedia