Capsid vs. Envelope — What's the Difference?

A capsid is the protein coat surrounding the genetic material of a virus. It provides structural support and protection to the viral nucleic acids. Capsids are made up of protein subunits called capsomeres, which assemble into a geometrical shape, often icosahedral or helical. An envelope, on the other hand, is an additional lipid membrane that encases some viruses. This envelope is derived from portions of the host cell membranes, including the cell’s plasma membrane or internal organelle membranes. The envelope contains viral glycoproteins essential for the virus’s ability to infect host cells.

While the capsid serves as the primary protective layer for all viruses, the envelope is present only in certain types of viruses. Enveloped viruses include those like HIV and influenza, which rely on their envelopes for entry into host cells through fusion with the host’s cellular membrane.

Capsids are relatively rigid structures that provide resilience to the viral particle in harsh environmental conditions. In contrast, envelopes are more fragile, making enveloped viruses more susceptible to environmental factors such as detergents and desiccation.

The presence of an envelope affects the virus's mode of transmission. Non-enveloped viruses, relying solely on the capsid, tend to be more resilient and can survive longer outside a host, whereas enveloped viruses often require close contact for transmission.

Capsids and envelopes together define the structural complexity and infective strategies of viruses. While the capsid is a fundamental feature present in all viruses, the envelope adds an extra layer of complexity that enhances the infectivity and adaptability of certain viruses.