Virus Capsid and Envelope: Structure and Function

Capsid is the protein coat around the viral genome. It differs in size, shape and symmetry. See classification based on viral symmetry.
Individual protein units are called capsomeres
Capsid proteins are encoded by the viral genome

Functions:
1. Protects nucleic acid from host nuclease degradation
2. Helps in introduction of viral genome to the host cell
3. Determines the antigenic specificity of virus

Viral envelope
In some cases apart from capsid, a membranous envelope may be present. Such viruses are called as enveloped viruses.
It is made of lipid and proteins rarely glycoproteins
Sometimes it may be a modified host plasma membrane or internal membranes
Eg: Herpes simplex, chickenpox virus, Influenza virus etc
Projections from the envelope are known as spikes or peplomers.
Function: attachment of the virus to the host cell.
HIV virus uses its spikes for this purpose.
See the difference between enveloped and non-enveloped viruses